Heat Exchanger Design and Selection

Heat Exchanger GeneralMany types of heat exchanger are employed in such varied installations as steam power

plants, chemical processing plants, building heating, air conditioning, refrigeration systems, and mobile power plants for automotive, marine, and aerospace vehicles. The principal types of equipment employed in these applications are reviewed and illustrate the problems with which is concerned and to clarify the nomenclature.

In almost any  chemical, electronic, or mechanical system, heat must be transferred from one place to another or from one fluid to another. Heat exchangers are used to transfer heat from one fluid to another. A basic understanding of the mechanical components of a heat exchanger is important to understanding how they function and operate

A heat exchanger is a component that allows the transfer of heat from one fluid (liquid or gas) to another fluid. Reasons for heat transfer include the following:

1. To heat a cooler fluid by means of a hotter fluid

2. To reduce the temperature of a hot fluid by means of a cooler fluid

3. To boil a liquid by means of a hotter fluid

4. To condense a gaseous fluid by means of a cooler fluid

5. To boil a liquid while condensing a hotter gaseous fluid

Regardless of the function the heat exchanger fulfills, in order to transfer heat the fluids involved must be at different temperatures and they must come into thermal contact. Heat can flow only from the hotter to the cooler fluid.

In a heat exchanger there is no direct contact between the two fluids. The heat is transferred from the hot fluid to the metal isolating the two fluids and then to the cooler fluid.

General ApplicationHeat exchangers are found in most chemical, electrical or mechanical systems. They serve

as the system's means of gaining or rejecting heat. Some of the more common applications are found in heating, electronic equipment, ventilation and air conditioning (HVAC) systems, radiators on internal combustion engines, boilers, condensers, and as preheaters or coolers in fluid systems. The associated webages will review some specific heat exchanger applications. The intent is to provide several specific examples of how each heat exchanger functions in a system, not to cover every possible applicaton.

Fluid Flow Arrangement Most heat exchangers may be classified as being in one of several categories on the basis of

the configuration of the fluid flow paths through the heat exchanger. The four most common types of flow path configuration are parallel flow, counter flow, single-pass crossflow, multippass crossflow. In cocurrent, or parallel-fiow, units the two fluid streams enter together at one end, flow through in the same direction, and leave together at the other end where as in countercurrent, or counterflow, units the two fluid streams move in opposite directions. In single-pass crossflow units one fluid moves through the heat transfer matrix at right angles to the flow path of the other fluid. In multipass crossflow units one fluid stream shuttles back and forth across the flow path of the other fluid stream, usually giving a crossflow

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approximation to counterflow.The most important difference between these four basic types lies in the relative amounts of

heat transfer surface area required to produce a given temperature rise for a given temperature difference between the two fluid streams where they enter the heat exchanger.

Heat comparison of heat exchanger types

Each of the three types of heat exchangers (Parallel, Cross and Counter Flow) has advantages and disadvantages. But of the three, the counter flow heat exchanger design is the most efficient when comparing heat transfer rate per unit surface area. The efficiency of a counter flow heat exchanger is due to the fact that the average T (difference in temperature) between the two fluids over the length of the heat exchanger is maximized, Counter Flow. Therefore the log mean temperature for a counter flow heat exchanger is larger than the log mean temperature for a similar parallel or cross flow heat exchanger. (See the Thermodynamics, Heat Transfer, and Fluid Flow Fundamentals for a review of log mean temperature).

In actuality, most large heat exchangers are not purely parallel flow, counter flow, or cross flow; they are usually a combination of the two or all three types of heat exchangers. This is due to the fact that actual heat exchangers are more complex than the simple components shown in the idealized figures used to depict each type of heat exchanger. The reason for the combination of the various types is to maximize the efficiency of the heat exchanger within the restrictions placed on the design. That is, size, cost, weight, required efficiency, type of fluids, operating pressures, and temperatures, all help determine the complexity of a specific heat exchanger.

One method that combines the characteristics of two or more heat exchangers and improves the performance of a heat exchanger is to have the two fluids pass each other several times within a single heat exchanger. When a heat exchanger's fluids pass each other more than once, a heat exchanger is called a multi-pass heat exchanger. If the fluids pass each other only once, the heat exchanger is called a single-pass heat exchanger. See Figure 6 for an example of both types. Commonly, the multi-pass heat exchanger reverses the flow in the tubes by use of one or more sets of "U" bends in the tubes. The "U" bends allow the fluid to flow back and forth across the length of the heat exchanger. A second method to achieve multiple passes is to insert baffles on the shell side of the heat exchanger. These direct the shell side fluid back and forth across the tubes to achieve the multi-pass effect.

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Heat exchangers are also classified by their function in a particular system. One common classification is regenerative or nonregenerative. A regenerative heat exchanger is one in which the same fluid is both the cooling fluid and the cooled fluid, That is, the hot fluid leaving a system gives up its heat to "regenerate" or heat up the fluid returning to the system. Regenerative heat exchangers are usually found in high temperature systems where a portion of the system's fluid is removed from the main process, and then returned. Because the fluid removed from the main process contains energy (heat), the heat from the fluid leaving the main system is used to reheat (regenerate) the returning fluid instead of being rejected to an external cooling medium to improve efficiency. It is important to remember that the term regenerative/nonregenerative only refers to "how" a heat exchanger functions in a system, and does not indicate any single type (tube and shell, plate, parallel flow, counter flow, etc.).

Pre-Heater Application Heat ExchangerIn large steam systems, or in any process requiring high temperatures, the input fluid is

usually preheated in stages, instead of trying to heat it in one step from ambient to the final temperature. Preheating in stages increases efficiency and minimizes thermal shock stress to components, as compared to injecting ambient temperature liquid into a boiler or other device that operates at high temperatures. 

In the case of a steam system, a portion of the process steam is tapped off and used as a heat source to reheat the feedwater in preheater stages. As the steam enters the heat exchanger and flows over and around the tubes, it transfers its thermal energy and is condensed. Note that the steam enters from the top into the shell side of the heat exchanger, where it not only transfers sensible heat (temperature change) but also gives up its latent heat of vaporization (condenses

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steam into water). The condensed steam then exits as a liquid at the bottom of the heat exchanger. The feedwater enters the heat exchanger on the bottom right end and flows into the tubes. Note that most of these tubes will be below the fluid level on the shell side.

Cross flowCross flow, as illustrated below, exists when one fluid flows perpendicular to the second

fluid; that is, one fluid flows through tubes and the second fluid passes around the tubes at 90° angle. Cross flow heat exchangers are usually found in applications where one of the fluids changes state (2-phase flow). An example is a steam system's condenser, in which the steam exiting the turbine enters the condenser shell side, and the cool water flowing in the tubes absorbs the heat from the steam, condensing it into water. Large volumes of vapor may be condensed using this type of heat exchanger flow.

Counter flowCounter flow exists when the two fluids flow in opposite directions. Each of the fluids

enters the heat exchanger at opposite ends. Because the cooler fluid exits the counter flow heat exchanger at the end where the hot fluid enters the heat exchanger, the cooler fluid will approach the inlet temperature of the hot fluid. Counter flow heat exchangers are the most efficient of the three types. In contrast to the parallel flow heat exchanger, the counter flow heat exchanger can have the hottest coldfluid temperature greater than the coldest hot-fluid temperatue.

Parallel flowParallel flow, as illustrated below, exists when both the tube side fluid and the shell side

fluid flow in the same direction. In this case, the two fluids enter the heat exchanger from the same end with a large temperature difference. As the fluids transfer heat, hotter to cooler, the temperatures of the two fluids approach each other. Note that the hottest cold-fluid temperature is always less than the coldest hot-fluid temperature.

TYPES OF APPLICA TION

Heat exchangers are often classified on the basis of the application for which they are intended, and special terms are employed for major types. These terms include boiler, steam generator, condenser, radiator, evaporator, cooling tower, regenerator, recuperator, heater, and cooler. The specialized requirements of the various applications have led to the development of many types of construction some of which are unique to particular applications. Typical units are described in subsequent sections to illustrate the characteristics and features of the principal types.

Boilers

Steam boilers have been used to produce power for over two-hundred years, and constitute one of the earliest subjects for the application of engineering principles to heat exchanger design. There is an en or mo us variety of boilers ranging from many sm all, relatively simple units for space he at ing applications to the huge, complex, and expensive boilers in modern central stations; in these, the boiler is thoroughly integrated with the furnace so that heat losses from the furnace walls are minimized. Layers of tubes are installed in the walls

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surrounding the combustion zone, and tremendous amounts of surface area in the form of great banks of tubes are exposed to the hot gases. A typical unit is about 200 ft high and costs about $10,000,000.

The term steam generator is often applied to boilers in which the he at source is a fluid stream other than the hot products of combustion. A typical steam generator for a pressurized water nuclear reactor power plant has a high-temperature, high-pressure water from the reactor is circulated through 0.5 in. diameter U-tubes coaxial with the casing. Steam is generated outside the tubes as the boiling water circulates by thermal convection upward through the tube matrix, where it forms steam at a temperature somewhat lower and a pressure very much lower than in the high-pressure water circuit through the reactor. A completely different type of steam generator unit for a large, low pressure, gas-cooled reactor has multipass, counterflow units are employed with each of the four reactors at the Hinkley Point power plant in England. Note that the heat is extracted from the gas in seven stages. This unusual design resulted from limitations on the reactor operating temperature which made it advantageous to employ two steam systems-one at a moderately high pressure and one at a lower pressure-in order to extract as much heat as possible from the gas circulated through the reactor.

Condensers James Watt more than tripled the thermal efficiency of the steam engin es of his day by

applying condensers so that the steam would be condensed outside rather than inside the engine cylinders. The steam condensers for large modern turbines are built in much the same way as Watt's early units except that they are enormously larger. A typical modern unit employs nearly 1,000,000 ft of tubing in 21,850 tubes.

Shell-and- Tube Heat Exchangers

A host of units known as shell-and-tube he at exchangers are built of round tubes mounted in cylindrical shells with their axes parallel to that of the shell. These are employed as heaters or coolers for a variety of applications that include oil coolers in power plants and process heat exchangers in the petroleum-refining and chemical industries. Many variations of this basic type are available; the differences lie mainly in the detailed features of construction and provisions for differential thermal expansion between the tubes and the shell.

CoolersThe atmosphere is a convenient heat sink for applications where the heat is to be rejected at

temperature IOO°F or more above the ambient air temperature or where ample supplies of cooling water are not available. Such applications include petroleum refineries in arid regions, power plants in the arctic (where freezing is a problem), and mobile power plants. These units may be mounted in air ducts inside the plant or -where large amounts of heat must be rejected-they may be mounted out in the open. In the second case a cooling fan arranged to discharge the air vertically upward, gives a low-cost installation with a minimal pumping power requirement. Furthermore, such an installation is insensitive to wind velocity or direction. Units of the type are commonly used as coolers or heaters in buildings and ship's air conditioning systems, and for industrial processes where air or gases must be cooled or heated in the temperature range up to about 500°F.

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Radiators

The term radiator is commonly applied to a variety of heat exchangers employed to dissipate heat to the surroundings. Automotive radiators of the type are crossflow units in which the temperature change in either fluid stream is small as compared to the temperature difference. Units of essentially the same construction are employed as condensers in refrigerators or air conditioning units and, with fans, as heaters for large, open rooms. Aircraft oil coolers have much the same function as automotive radiators, but the premium placed on light-weight construction and compactness has led to the development of the different types of construction.

For this application heat can be dissipated only by thermal radiation to space which is at an effective temperature of absolute zero or to the earth which has a me an temperature of 520 R. In either case the condenser has been designed to operate red hot in ord er to dissipate the waste heat of the thermodynamic cycle from a reasonable size and weight of surface.

Cooling Towers

In locations where the supply of water is limited, heat may be rejected to the atmosphere very effectively by means of cooling towers. A fraction of the water sprayed into these towers evaporates, thus cooling the balance. Because of the high he at of vaporization of water, the water consumption is only about 1 % as much as would be the case if water were taken from a lake or a stream and heated 10 or 20°F. Cooling towers may be designed so that the air moves through the m by thermal convection or fans may be employed to provide forced air circulation. To avoid contamination of the process water, shell-andtube heat exchangers are sometimes employed to transmit he at from the process water to the water recirculated through the cooling tower.

Regenerators and RecuperatorsThe thermal efficiency of both steam- and gas-turbine power plants can be greatly

increased if heat can be extracted from the hot gases that are leaving the steam boiler or the gas turbine and added to the air being supplied to the furnace or the combustion chamber. For a major gain in thermal efficiency it is necessary to employ a very large amount of heat transfer surface area. This is particularly noticeable in gas-turbine plants where even with counterflow the size of the heat exchanger required for good performance is inclined to be large compared to the sizes of the turbine and compressor. This characteristic can be observed even in the small, portable gas-turbine plant (about 3 ft in diameter). Note that in this plant the hotcombustion gases leave the radial in-flow turbine wheel at the right end of the shaft and enter a set of heat exchanger cores arranged in parallel around the central axis at the right end of the plant. In each core the hot gases from the turbine flow roughly radially outward through one set of gas passages. Air from the centrifugal compressor wheel at the center of the shaft flows to the right through the space just inside of the outer casing and axially into the other set of gas passages through the core. The air being heated makes two passes, flowing first to the right in the outer portion of the core and then back to the left through the inner portion, thus giving a two-pass crossflow approximation to counterflow.

The heat exchanger core is constructed of alternate layers of flat and corrugated sheets. The flat sheets separate the hot and cold fluid streams while the corrugated sheets act as fins that roughly triple the heat transfer surface area per unit of volume. Note also that the axis of the corrugations is at right angles in alternate layers to provide a crossflow pattern for the two fluid streams.

One of several recuperator units to be mounted in parallel in a much larger gas turbine plant the hot exhaust gas from the turbine enters vertically at the bottom, flows upward

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through the heat transfer matrix, and discharges vertically from the top. The air from the compressor enters a large circular port at the top at the right end, flows vertically downward in pure counterflow, and leaves a second circular port at the bottom to flow to the combustion chamber.

The sectional view through the top corner at the air inlet end to show the internal con-struction. The hot exhaust gas passages are formed by corrugated sheets sandwiched between fiat plates that extend all the way from the bottom to the top of the unit. The air to be heated flows horizontally from the long plenum at the top into the spaces between the walls of the exhaust gas passages. Curved spacer strips guide the air through a 90° bend and then downward between the heated walls. A similar headering arrangement is used at the bottom. Note that both the fiowpassage area and the heat transfer surface area for the hot exhaust gas are about three times as great as the corresponding values for the air being heated. This comes about because the two fiuid streams differ in density by a factor of about four.

The air preheaters in steam power plants are usually quite different from the units just described for gas turbines. Rotary regenerators of the type are often used. These consist of a cylindrical drum filled with a heat transfer matrix made of alternately fiat and corrugated sheets. The drum is mounted so that a portion of the matrix is heated by the hot gas as it passes from the furnace to the stack. The balance of the matrix gives up its stored heat to the fresh air enroute from the forced draft fans to the furnace. The ducts are arranged so that the two gas streams move through the drum in counterflow fashion while it is rotated, so that the temperature of any given element of the metal matrix fluctuates relatively little as it is cycled from the hot to the co Id gas streams.

In the steam- and gas-turbine power plant fields a distinction is sometimes made between air preheaters that involve a conventional heat transfer matrix with continuous flow on both sides of a stationary heat transfer surface and those through which the fluids flow periodically, the hot fluid alternately heating one section of the matrix while the cold fluid is removing heat from another section. Where this distinction is made, the term regenerator is applied to the periodic flow-type of he at exchanger, since this term has long been applied to units of this type employed for blast furnaces and steel furnaces, while the term recuperator is applied to units through which the flow is continuous.

Plates and PanelsWhere it is desirable to build heat transfer surfaces into the walls of a compartment as in

refrigerators, steam chests, or environmental test chambers, one of the simplest and least expensive arrangements is provided by units formed of sheet steel stampings seam-welded to form panels with integral fluid passages. The panels of this sort used to line the interior of a cylindrical compartment.

Immersion Heaters and Coolers

Immersion heaters and coolers provide a convenient means for controlling the temperature of baths or pools. Panel-type heat transfer surfaces arranged in banks of closely-spaced par all el panels are of ten used. Axially finned tubes arranged in banks, and mounted with the tube axes vertical constitute another type of heat transfer surface well suited to this purpose. Natural thermal convection usually induces sufficient circulation in the bath to maintain it within the desired temperature limits.

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Double-Pipe Heat ExchangersTwo concentric pipes with one fluid in the inner pipe and the other in the annulus between

them give a simple heat exchanger construction well suited to some applications. For smaIl laboratory heat exchangers such a unit can be made up of two lengths of copper tubing that fit inside each other with a standard reducing Tee copper tube fitting at either end. Asimilar construction is of ten used in large units. if a fluid with a poor heat transfer coefficient, such as oil or air, is to be cooled by water, an axially finned tube can be placed inside of alarger pipe to give the construction. Units of this sort can be mounted in both series and parallel to give any desired capacity and heating or cooling effectiveness, so that special requirements can be met by assembling a bank of stock commercial units. This type of construction is particularly advantageous where one or both of the fluids is at high pressure causing the shell wall thickness and cost to be large if a conventional shell-and-tube heat exchanger were employed.

Evaluation and Selection of Heat Exchangers

Control of process-solution temperature is essential in metal products finishing. Therefore, the heat exchanger that adds or removes heat from the finishing tank is critical equipment.

Solution temperature affects many finishing factors, including rates of deposition, activation and decomposition of delicate chemistries, the corrosion resistance of equipment and the ability of deposits to adhere to surfaces.

So even when other finish variables are under control—including solution-chemical content and concentration, basis-metal preparation, electrical current and voltage, and rack design—consistent solution temperature is still required for best results.

Choosing the Right Exchanger

Exchanger size and choice of material of construction are critical details in heat-exchanger selection. Correct decisions will assure low-cost, effective plating-process operation.

Immersion heaters used in the finishing industry today include immersion heaters in the form of serpentine coils, "U" coils, pipe coils, grid of plate-style coils and electric heaters. External exchangers come as shell-and tube as well as plate-and frame designs.

Immersion exchangers offer the simplest answer to heat-exchange requirements. Such units are economical, easily installed and maintained, and are popular in both large and small systems. Immersion exchangers are readily available in a wide variety of sizes and shapes. The chief drawback of these units is that they occupy space in the process tank. Thus this type of exchanger can occasionally interfere with tank processes.

Good process and tank design can help you avoid these problems and prevent the exchanger from being snagged or bumped while in use. Immersion exchangers can be mounted anywhere in the tank—on sides, front or bottom—to reduce interference with work flow.

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External exchangers typically are chosen only by finishers with large systems, where in-tank space is a vital concern. External units require a pump to circulate often-corrosive liquids from the process tank through the exchanger and back to the tank.

External exchangers may be of either the shell-and-tube style, which can be mounted as a space-saving vertical unit, or the newer plate-and-frame type. This type also minimizes the exterior mounting space required.

Asking and answering a few questions during the design stage will help us properly size our heat exchanger

Most water-cooled heat exchangers can cool air to within 10°F of the water inlet temperature.

Heat exchanger manufacturers need answers to a number of questions to correctly size a heat exchanger for a specific application. The questions attempt to address the three basic points that drive heat exchanger design:

Why a heat exchanger is needed. What the exchanger needs to do. How the exchanger can do the job.

But, how does a user provide that information? Answering the following series of questions when requesting a heat exchanger quotation can help you get a quote for a correctly sized unit.

1. What Is the Process Flow? This question sounds simple, but it is important. Is the process flow air, nitrogen or something else? This also is the time to address composition for multicomponent flows. When cooling conveying air, this can be as simple as considering the humidity in the ambient air at the job site. If the application is more complicated - condensing chemicals venting off a storage tank, for example - the answer to this question should include the amount of each component in the process stream.

Summer conditions usually are considered when sizing a heat exchanger that will cool ambient air used for conveying because this is when air will be at its warmest temperature. In general, if the exchanger can provide the desired cooling in the middle of summer, it will perform even better on a cooler day.

2. What Is the Flow Rate? The flow rate can be expressed as a volumetric or mass flow rate. Blower manufacturers usually express performance curves in volumetric flow rates. Most heat transfer calculations are based on standard cubic feet per minute (scfm). If you are

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providing actual cubic feet per minute (acfm) or inlet cubic feet per minute (icfm), make sure you know the temperature and pressure at which this value is measured.

3. What Must Be Done to the Flow? Fully define the process by answering a series of questions about the flow. What is the de-sired outlet temperature for the process flow? Does it require cooling? If so, is the goal merely to cool the process medium, or is there a component in the gas stream that also needs to be condensed?

4. What Is Available to Provide the Required Cooling? In order to cool process flow, another medium that is colder than the desired process outlet temperature is required. Depending on what must be done to the flow, ambient air may be able to provide the desired cooling. If not, a water source or another cooling medium may be required. A good rule of thumb is to have a cooling medium that is at least 10°F colder than the desired process outlet temperature.

5. Are There Any Other Heat Exchanger Requirements? Once the process flow is defined, consider the exchanger itself. Does the process flow require certain materials of construction such as stainless steel to withstand a corrosive environment? Are there restrictions on the maximum pressure drop across the exchanger? Can the blower withstand another psi of pressure drop or only a few more inches of water column?

In addition, for air-cooled exchangers, consider: Does the environment require specific electrical requirements? Some harsh environments may require an explosion proof motor. Other countries use different voltages or frequencies than the United States. If this is known at the outset, the correct motor can be included in the heat exchanger design.

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Construction Basics of Shell and Tube Heat Exchangers

Although there exists a wide range of designs and materials, some components are common in all shell and tube designs. Shell and tube heat exchangers represent the most widely used vehicle for heat transfer in process applications. They frequently are selected for duties such as:

Process liquid or gas cooling. Process or refrigerant vapor or steam condensing. Process liquid, steam or refrigerant evaporation. Process heat removal and preheating of feedwater. Thermal energy conservation efforts and heat recovery.

Although there exists a wide range of designs and materials, some components are common to all. In all shell and tube heat exchangers, the tubes are mechanically attached to tube sheets, which are contained inside a shell with ports for inlet and outlet fluid or gas. They are designed to prevent the liquid flowing inside the tubes from mixing with the fluid outside the tubes. Tube sheets can be fixed to the shell or allowed to expand and contract with thermal stresses. In the latter design, an expansion bellows is used or one tube sheet is allowed to float inside the shell. The nonfixed tube sheet approach allows the entire tube bundle assembly to be pulled from the shell to allow cleaning of the shell circuit.

Fluid Stream Allocations

There are a number of practical guidelines that, if followed, can lead to the optimum design of a given heat exchanger. Remembering that the heat exchanger's primary responsibility is to perform its thermal duty with the lowest cost yet provide in-service reliability, the selection of fluid stream allocations should be of primary concern to the designer. When designing, keep the following points in mind:

The higher pressure fluid normally flows through the tube side. With their small diameter and nominal wall thicknesses, the tubes are better able to accept high pressures, and this approach avoids having to design more expensive, larger diameter components for high pressure. If it is necessary to put the higher pressure fluid stream in the shell, it should be placed in a small diameter, long shell.

All other items being equal, place corrosive fluids in the tubes. It is much less expensive to use the special alloys designed to resist corrosion for the tubes than for the shell. Other tube-side components can be clad with corrosion-resistant materials or epoxy coated.

Flow higher fouling fluids through the tubes.Tubes are easier to clean using common mechanical methods than the shell.

Many possible designs and configurations - affecting tube pitch, baffle use and spacing, and multiple nozzles, to name a few - can be used when laying out the shell circuit. Because of this, it is best to place fluids requiring low pressure drops in the shell circuit.

The fluid with the lower heat transfer coefficient normally goes in the shell circuit. With this setup, low-fin tubing, which will increase available surface area, can be used to offset the low heat transfer rate.

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Tubes

Heat exchangers with shell diameters of 10 to more than 100" typically are manufactured to the standards set forth by the Tubular Exchangers Manufacturers Association. Generally, the 0.625 to 1.5" tubing used in TEMA-sized exchangers is made from low carbon steel, copper, Admiralty, copper-nickel, stainless steel, Hastalloy, Inconel, titanium or other materials.

Tubes are either drawn and seamless, or welded. High quality electroresistance welded tubes exhibit good grain structure at the weld. Extruded tube with low fins and interior rifling is specified for certain applications. Surface enhancements are used to increase the available metal surface or aid in fluid turbulence, thereby increasing the effective heat transfer rate. Finned tubing is recommended when the shell-side fluid has a substantially lower heat transfer coefficient than the tube-side fluid. Finned tubing is not finned in its landing areas, where it contacts the tube sheets. Also, the outside diameter of the finned portions of this tube design is slightly smaller than the unfinned areas. These features allow the tubes to be slid easily through the baffles and tube supports during assembly while still minimizing fluid bypass.

U-tube designs are specified when the thermal difference between the fluids and flows would result in excessive thermal expansion of the tubes. U-tube bundles do not have as much tube surface as straight tube bundles due to the bending radius, and the curved ends cannot be easily cleaned. Additionally, interior tubes are difficult to replace and often requiring the removal of outer layers or simply plugging the tubes. To ease manufacturing and service, it is common to use a removable tube bundle design when specifying U-tubes.

Tube Sheets

Tube sheets usually are made from a round, flat piece of metal. Holes are drilled for the tube ends in a precise location and pattern relative to one another. Tube sheets are manufactured from the same range of materials as tubes. Tubes are attached to the tube sheet by pneumatic or hydraulic pressure, or by roller expansion. If needed, tube holes can be drilled and reamed, or they can be machined with one or more grooves. This greatly increases tube joint strength.

The tube sheet is in contact with both fluids, so it must have corrosion resistance allowances and metallurgical and electrochemical properties appropriate for the fluids and velocities. Low carbon steel tube sheets can include a layer of a higher alloy metal bonded to the surface to provide more effective corrosion resistance without the expense of using the solid alloy.

The tube hole pattern, or "pitch," varies the distance from one tube to the other as well as the angle of the tubes relative to each other and to the direction of flow. This allows the fluid velocities and pressure drop to be manipulated to provide the maximum amount of turbulence and tube surface contact for effective heat transfer.

Where the tube and tube sheet materials are joinable weldable metals, the tube joint can be further strengthened by applying a seal weld or strength weld to the joint. In a strength weld, a tube is slightly recessed inside the tube hole or slightly extended beyond the tube sheet. The weld adds metal to the resulting lip. A seal weld is specified to help prevent the shell and tube liquids from intermixing. In this treatment, the tube is flush with the tube sheet surface. The weld does not add metal but rather fuses the two materials. In cases where it is critical to

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avoid fluid intermixing, a double tube sheet can be provided. In this design, the outer tube sheet is outside the shell circuit, virtually eliminating the chance of fluid intermixing. The inner tube sheet is vented to atmosphere, so any fluid leak is detected easily.

Shell Assembly

Figure 1. Machining grooves in the tube will increase joint tube strength.

The shell is constructed either from pipe up to 24" or rolled and welded plate metal. For reasons of economy, low carbon steel is in common use, but other materials suitable for extreme temperature or corrosion resistance often are specified. Using commonly available shell pipe to 24" dia. results in reduced cost and ease of manufacturing, partly because they generally are more perfectly round than rolled and welded shells. Roundness and consistent shell inner diameter are necessary to minimize the space between the baffle outside edge and the shell, as excessive space allows fluid bypass and reduces performance. Roundness can be increased by expanding the shell around a mandrel or double rolling after welding the longitudinal seam. In extreme cases, the shell can be cast and then bored to the correct inner diameter.

In applications where the fluid velocity for the nozzle diameter is high, an impingement plate is specified to distribute the fluid evenly to the tubes and prevent fluid-induced erosion, cavitation and vibration. An impingement plate can be installed inside the shell, eliminating the need to install a full tube bundle, which would provide less available surface. Alternatively, the impingement plate can be installed in a domed area (either be reducing coupling or a fabricated dome) above the shell. This style allows a full tube count and therefore maximizes utilization of shell space (figure 2).

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End Channels and Bonnets

Figure 2. An impingement plate distributes the fluid to the tubes and prevents fluid-induced erosion, cavitation and vibration.

Used to control the flow of the tube-side fluid in the tube circuit, end channels or bonnets typically are fabricated or cast. They are attached to the tube sheets by bolting with a gasket between the two metal surfaces. In some cases, effective sealing can be obtained by installing an O-ring in a machined groove in the tube sheet.

The head may have pass ribs that dictate whether the tube fluid makes one or more passes through the tube bundle sections (figure 3). Front and rear head pass ribs and gaskets are matched to provide effective fluid velocities by forcing the flow through various numbers of tubes at a time. Generally, passes are designed to provide roughly equal tube-number access and to ensure even fluid velocity and pressure drop throughout the bundle. Even fluid velocities also affect the film coefficients and heat transfer rate so that accurate prediction of performance can be readily made.

Designs for up to six tube passes are common. Pass ribs for cast heads are integrally cast, then machined flat while pass ribs for fabricated heads are welded into place. The tube sheets and tube layout in multipass heat exchangers must have provision for the pass ribs. This requires either removing tubes to allow a low cost straight pass rib, or machining the pass rib with curves around the tubes, which is more costly to manufacture. Where a full bundle tube count is required to satisfy the thermal requirements, the machined pass rib approach may prevent having to consider the next larger shell diameter.

Cast head materials typically are used in smaller diameters to around 14" and are made from iron, ductile iron, steel, bronze or stainless steel. Typically, they have pipe-thread connections. Cast heads and tube side piping must be removed to service tubes. Fabricated heads can be made in a range of configurations, including metal cover designs that allow servicing the tubes without disturbing the shell or tube piping. Heads can have axially or tangentially oriented nozzles, which typically are ANSI flanges.

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Baffles

Figure 3. The head may have pass ribs that dictate whether the tubeside fluid makes one or more passes through the tube bundle sections.

Baffles serve two important functions. First, they support the tubes during assembly and operation and help prevent vibration from flow-induced eddies. Second, they direct the shell-side fluid back and forth across the tube bundle to provide effective velocity and heat transfer rates.

A baffle must have a slightly smaller inside diameter than the shell's inside diameter to allow assembly, but it must be close enough to avoid the substantial performance penalty caused by fluid bypass around the baffles. Shell roundness is important to achieve effective sealing against excessive bypass. Baffles can be punched or machined from any common heat exchanger material compatible with the shell side fluid. Some punched baffle designs have a lip around the tube hole to provide more surface against the tube and eliminate tube wall cutting from the baffle edge. The tube holes must be precise enough to allow easy assembly and field tube replacement yet minimize the chance of fluid flowing between the tube wall and baffle hole.

Baffles do not extend edge to edge but have a cut that allows shell-side fluid to flow to the next baffled chamber (figure 4). For most liquid applications, the cuts areas represent 20 to 25% of the shell diameter. For gases, where a lower pressure drop is desirable, baffle cuts of 40 to 45% are common. Baffles must overlap at least one tube row in order to provide adequate tube support. They are spaced somewhat evenly throughout the tube bundle to provide even fluid velocity and pressure drop in each baffled tube section.

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Figure 4. Baffles support the tubes during assembly and operation, help prevent vibration and direct the shelf-side fluid back and forth across the tube bundle.

Single-segmental baffles force the fluid or gas across the entire tube count, where it changes direction as dictated by the baffle cut and spacing. This can result in excessive pressure loss in high velocity gases. To effect heat transfer yet reduce the pressure drop, double-segmental baffles can be used. This approach retains the structural effectiveness of the tube bundle yet allows the gas to flow between alternating tube sections in a straighter overall direction, thereby reducing the effect of numerous direction changes. This approach takes full advantage of the available tube surface. But, reduced performance should be expected due to a reduced heat transfer rate. Because pressure drop varies with velocity, cutting the velocity in half using double-segmental baffles results in roughly one-quarter of the pressure drop seen in a single-segmental baffle space over the same tube surface.

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Plate-and-Frame Heat Exchangers Designing

Considering available heat exchanger Technologies at the outset of process design (at the process synthesis stage) is not general practice. In fact, procedures established in some companies preclude it. For instance, some purchasing departments’ “nightmare” is having to deal with a single supplier — instead they want a general specification that can be sent to all equipment vendors, in the mistaken belief that they are then operating on a “level playing field.”This omission is both unfortunate and costly. It results in unnecessary capital expenditure and in reduced energy efficiency. It also hinders the development of energy saving technology. Pinch analysis is the key tool used by engineers to develop flowsheets of energy intensive processes, where heat exchanger selection is particularly important.Yet, this tool is hindering the adoption of a more-progressive approach because of the way it is restricted to traditional heat exchangers. Numerous articles have been published regarding the advantages of compact heat exchangers. Briefly, their higher heat-transfer coefficients, compact size, cost effectiveness, and unique ability to handle fouling fluids make them a good choice for many services. A plate-and-frame heat exchanger (Figure 1) consists of pressed, corrugated metal plates fitted between a thick, carbon-steel frame. Each plate flow channel is sealed with a gasket, a weld or an alternating combination of the two. It is not uncommon for plate-and-frame heat exchangers to have overall heat transfer coefficients that are three to four times those found in shell-and-tube heat exchangers. This article discusses some general aspects of plate-and-frame heat exchangers, outlines a procedure for accurately estimating the required area, and shows how these units can be used to simplify processes.

Specifying plate-and-frame heat exchangersEngineers often fail to realize the differences between heat transfer technologies when preparing a specification to be sent to vendors of different types of heat exchangers. Consider the following example. A process stream needs to be cooled with cooling water before being sent to storage. The stream requires C276, an expensive high-nickel alloy, to guard against corrosion; thismetallurgy makes the stream a candidate for the tubeside of a shell-and-tube heat exchanger. The cooling water is available at 80°F and must be returned at a temperature no higher than 115°F. The process engineer realizes that with the water flow being placed on the shellside, larger flowrates will enhance the heat-transfer coefficient. The basis for the heat exchanger quotation was specified as shown in the table. According to the engineer’s calculations, these basicparameters should result in a good shell-and-tube design that uses a minimum amount of C276 material. A typical plate-and-frame exchanger designed to meet the specification would have about 650 ft square of area, compared to about 420 ft square for a shell-and-tube exchanger. A plate-and frame unit designed to the above specification is limited by the allowable pressure drop on the cooling water. If the cooling water flow is reduced to 655 gal/min and the outlet watertemperature allowed to rise to 115°F, the plate-and-frame heat exchanger would contain about 185 ft square of area. The unit is smaller and less expensive, and it uses less water. Theload being transferred to the cooling tower is the same. With shell-and-tube heat exchangers, increasing water flow will minimize heat-transfer area. However, with compact technologies, the effect is exactly the opposite. The larger water flow actually drives up the cost of the unit.Rather than supplying a rigid specification to all heat exchanger manufacturers, the engineer should have explained the goal for the process stream. This could have been in the form of the following statement: “The process stream is to be cooled with cooling water. Up to 2,000

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gal/min of water is available at 80°F. The maximum return temperature is 115°F.” This simple statement could result in vastly different configurations compared with the designsthat would result from the original specification.

Design charts for plate-and-frame exchangersWhen it comes to compact heat-transfer technology, engineers often find themselves at the mercy of the equipment manufacturers. For example, limited literature correlations are available to help in the preliminary design of plate-and-frame heat exchangers. This article introduces a series of charts (Figures 2–7) that can be used for performing preliminary sizing of plate and frame exchangers. Examples will help clarify their use.The following important points should be noted regarding the charts and their use: 1. The heat-transfer correlations apply to single-phase, liquid-liquid designs. 2. These charts are valid for single-pass units with 0.50 mm-thick plates. The accuracy of the charts will not be compromised for most materials of construction. 3. Wetted-material thermal conductivity is taken as 8.67 Btu/h-ft-°F (which is the value for stainless steel). 4. The following physical properties for hydrocarbonbased fluids were used for the basis: thermal conductivity (k) = 0.06 Btu/h-ft-°F, density () = 55.0 lb/ft3, heat capacity (Cp) = 0.85 Btu/lb-°F. The following physical properties for water-based fluids were used for the basis: thermal conductivity = 0.33 Btu/h-ft-°F, density = 62.0 lb/ft3, heat capacit = 0.85 Btu/lb°F. 5. Accuracy should be within ±15% of the service value for the overall heat-transfer coefficient, assuming a nominal 10% excess heat-transfer area.6. For fluids with viscosities between 100 and 500 cP, use the 100 cP line on the graphs. For fluids in excess of 500 cP, consult equipment manufacturers. Equations 1–3 are used to calculate the log-mean temperature difference (LMTD) and number of transfer units (NTU) for the hot and cold streams. After the local heattransfercoefficients (h) are read from the charts, the overall heat-transfer coefficient (U) is calculated by Eq. 4.

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Using the chartsConsider the following example. 150,000 lb/h of water is being cooled from 200°F to 175°F by 75,000 lb/h of SAE 30 oil. The oil enters the exchanger at 60°F and leaves at 168°F. The average viscosity of the water passing through the unit is 0.33 cP and the average viscosity of the oil inthe unit is 215 cP. The maximumallowable pressure drop through the plate heat exchanger is 15 psi on the hot and cold sides. Step 1: Calculate the LMTD. From Eq. 1, LMTD = [(200 – 168)– (175 – 60)]/ln[(200 – 168)/(175 – 60)] = 64.9°F. Step 2: Calculate NTUhot and NTUcold. From Eqs. 2 and 3, NTUhot = (200 – 175)/64.9 = 0.38 and NTUcold = (168 – 60)/64.9 = 1.66.

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Step 3: Read hhot from the appropriate chart. Use Figure 5, the chart for hydrocarbons when 0.25 < NTU < 2.0. Although there is not a viscosityline for 215 cP, the line representing 100 cP can be used for viscositiesup to about 400–500 cP. The heat exchanger will be pressure drop limited and the heat-transfer coefficient will not change appreciably over this viscosity range forplate-and-frame exchangers. Reading from the chart, a pressure drop of 15 psi corresponds to hhot ≈ 50 Btu/h-ft2-°F.

Step 4: Read hcold from the chart. Use Figure 2, which applies to waterbased liquids when 0.25 < NTU < 2.0. Again, the exact viscosity line needed for pure water (0.33 cP) in this case is not available. However, the 1.0 cP line provides a very good estimate ofthe heat-transfer coefficient for pure water. Reading from the chart, a pressure drop of 15 psi corresponds tohcold ≈ 3,000 Btu/h-ft2-°F. Step 5: Calculate U. Assume a stainless steel plate with a thickness of 0.50 mm is being used. conductivity of 8.67 Btu/h-ft- °F. Then from Eq. 4, 1/U = (1/50 + 0.000189 + 1/3,000) andU = 49 Btu/h-ft2-°F. Now let’s consider another example.150,000 lb/h of water is being cooled from 200°F to 100°F by 150,000 lb/h of NaCl brine. Thebrine enters the exchanger at 50°F and leaves at 171°F. The average viscosity of the water passing through the unit is 0.46 cP and the average viscosity of the brine in the unit is 1.10 cP. The maximum-allowable pressure drop through the plate heat exchanger is 10 psi on the hot

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(water) side and 20 psi on the cold (brine) side. The LMTD is calculated to be 38.5°F. NTUhot and NTUcold are 2.59 and 3.14, respectively. From the charts for 2.0 < NTU < 4.0 (waterbased), hhot ≈ 2,000 Btu/h-ft2-°F and hcold ≈ 2,500 Btu/h-ft2-°F. Although the material of choice may be titanium or palladium-stabilized titanium, the properties for stainlesssteel are used for preliminary sizing. U is calculated to be 918 Btu/h-ft2-°F.

Implications of size reductionAlternative technologies offer significant size advantages over shell-and-tube heat exchangers.Let’s now consider the implications of this. The individual exchangers are smaller, and the spacing between process equipment can be reduced. Thus, a smaller plot is needed forthe process plant. If the plant is to be housed in a building, the building can be smaller. The amount of structural steel used to support the plant can be reduced, and because of the weight saving, the load on that structure is also reduced. The weight advantage extends to thedesign of the foundations used to support the plant. Since the spacing between equipment is reduced, piping costs are lower. However, we stress again that the savings associated with size and weight reduction can only be achieved if these advantages are recognized and exploited at the earliest stages of the plant design.

Reduced plant complexityThe use of alternative heat exchanger technologies can significantly reduce plant complexity byreducing the number of heat exchangers through improved thermal contacting and multi-streaming. This adds to the savings associated with reduced size and weight and also has safety implications. The simpler the plant structure, the easier it is for the process operator to understand the plant. In addition, plant maintenance will be safer, easier and more straight-forward. Mechanical constraints play a significant role in the design of shell-and-tube heat exchangers. Forinstance, it is common to find that some users place restrictions on tube length. Such a restriction can have important implications for the design. In the case of exchangers requiring large surface areas, the restriction drives the design toward large tube counts. If such large tube counts lead to low tubeside velocity, the designer is tempted to increase the number of tubeside passes in order to maintain a reasonable tubeside heat-transfer coefficient. Thermal expansion considerationscan also lead the designer to opt for multiple tube passes, because the cost of a floating headis generally lower than the cost of installing an expansion bellows in the exchanger shell. The use of multiple tube passes has four detrimental effects. First, it leads to a reduction in the number oftubes that can be accommodated in a given size shell, thereby leading to increased shell diameter and cost. Second, for bundles having more than four tube passes, the pass-partition lanes introduced into the bundle give rise to an increase in the quantity of shellside fluid bypassingthe tube bundle and a reduction in shellside heat-transfer coefficient. Third, it results in wastedtubeside pressure drop in the return headers. Finally, and most significantly, the use of multiple tube passes results in the thermal contacting of the streams not being pure counter-flow, which reduces the effectivemean temperature driving force and possibly produces a temperature cross (i.e., where the outlet temperature of the cold stream is higher than the inlet temperature

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of the hot stream, as shown in Figure 8). If a temperature cross occurs, the designer must split the duty between multiple heat exchangers arranged in series.

Many of the alternative heat-exchanger technologies allow the application of pure countercurrent flow across all size and flow ranges. This results in better use of the available temperature driving force and the use of single heat exchangers.

Multi-streamingThe traditional shell-and-tube heat exchanger handles only one hot stream and one cold stream. Some heat exchanger technologies (most notably plate-fin and printed circuit exchangers) can handle many streams. It is not uncommon to find plate-fin exchangers transferring heat between ten individual processes. (The principles behind the design of multi-stream exchangers and theoperability of such units are discussed in Ref. 1.) Such units can be considered to contain a whole heat exchanger network within the body of a single exchanger. Distribution and recombination of process flows is undertaken inside the exchanger. The result is a major reduction in piping cost.Engineers often overlook the opportunities of using a plate-and-frame exchanger as a multi stream unit. As mentioned earlier, this is a common oversight when exchanger selection is not made until after the flowsheet has been developed. A good example of multi-streaming is a plate heat exchanger that serves as a process interchanger on one side and a trim cooler on the other. This arrangement is particularly useful for product streams that are exiting a process and must be cooled for storage. Another popular function of multi-streaming is to lower material costs. Some streams, once they are cooled to a certain temperature, pose much less of a corrosion risk. One side of the exchanger can be made of a moreexpensive corrosion-resistant alloy while the other side can utilize stainless steel or a lower alloy. Figure 9 shows a plate-and-frame unit applied to three process streams. A single exchanger with 1,335 ft2 of effective surface area is used. Figure 10 is the equivalent shell-and-tube solution to avoid temperature crosses, six individual exchangers are needed: the cooler having two shells in series (each with 1,440 ft2 of effective surface area) and the heat recovery unit having four shells in series (each with 2,116 ft2 of surface area). So, the plateand frame design involves the use of 1,335 ft2 of surface area in a single unit, whereas the equivalent shell-andtube design has 11,344 ft2 of surface area distributedacross six separate exchangers.

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Budget pricing correlationsFor plate-and-frame heat exchangers with a design pressure up to 150 psi and a design temperature up to 320°F, the following cost equations can be used to estimate the purchased cost.For areas (A) less than 200 ft2: C = 401 A 0.4887 for Type 316 stainless steel (5)C = 612 A 0.4631 for Grade 1 titanium (6) For areas larger than 200 ft2: C = 136 A 0.6907 for Type 316 stainless steel (7) C = 131 A 0.7514 for Grade 1 titanium (8) Typical installation factors for plate-and-frame heat exchangers can range from 1.5 to 2.0, depending on the size of the unit.

Being able to estimate the area and prices of plate and frame heat exchangers is an important first step in including alternative heat-transfer technology in process synthesis.

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The Spiral Heat Exchanger

In applications prone to high fouling, consider using a spiral heat exchanger. Its single-channel design minimizes fouling and erosion and helps ensure high flow velocities even with heavy process slurries.

Figure 1. A spiral heat exchanger consists of two long flat plates wrapped around a mandrel or center tube, creating two concentric sprial channels.

Heat exchanger fouling is a major source of maintenance costs and lost production time. It has been estimated that fouling costs U.S. industry more than $5 billion annually. Over time, as material builds up on the heat transfer surfaces of a typical heat exchanger, an insulating layer is formed that reduces the heat transfer rate and increases pressure drop through the exchanger. Eventually, the heat exchanger must be cleaned to restore the heat transfer rates and pressure drops required by the process. With many traditional heat exchanger designs, cleaning is time-consuming and costly, and it may need to be performed frequently. A spiral heat exchanger can help processors avoid these problems.

A spiral heat exchanger is a useful alternative to shell and tube designs for many applications prone to fouling and plugging problems. For more than 60 years, it has been used in difficult services ranging from PVC slurry coolers to asphalt heaters. Its flow-channel geometry and single-channel design induce highly turbulent flow, so the compact exchanger can operate reliably with low fouling rates even in heavy fouling, fibrous or slurry duties. It can be opened quickly and easily for inspection, cleaning and maintenance.

Construction and Operating Principles

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Figure 2. In a spiral heat exchanger, the hot fluid flows into the center of the unit and spirals outward toward the outer plates while at the same time, the cold fluid enters the periphery and spiral inward, exiting at the center.

A spiral heat exchanger is composed of two long, flat plates wrapped around a mandrel or center tube, creating two concentric spiral channels (figure 1). The channels are seal-welded on alternate sides to provide a sturdy barrier between the fluids. A cover is fitted on each side, and a full-faced gasket is positioned between each cover and spiral element to prevent fluid bypassing and leakage to the atmosphere. Access to the hot and cold heat transfer surfaces is gained by removing the respective covers. Connections are installed in the center of each cover and on the peripheral pockets.

Countercurrent flow maximizes heat transfer. The hot fluid flows into the center of the unit and spirals outward in the long, flat, rectangular channel toward the periphery (figure 2).

Figure 3. In a spiral heat exchanger, each fluid flows through a single channel. If suspended solids settle on the heat transfer surface, the increased fluid velocity creates a scrubbing effect that removes the deposits.At the same time, the cold fluid enters at the periphery and spirals inward, exiting at the center. Channel spacing can be varied from approximately 0.25 to 1" to optimize channel velocities and heat transfer coefficients.

The spiral heat exchanger can handle heat loads up to 20,000 kBTU/hr and flow rates of 3,000 gal/min, with heat transfer surface areas up to 6,000 ft2 in a single unit. Many applications with higher heat loads and flow rates -- ammonia liquor cooling in coke plants or foul condensate interchangers in pulp mills, for example -- are handled with multiple spiral units installed in parallel. Capable of handling pressures up to 500 psi and design temperatures up to 1,500oF (816oC), a spiral heat exchanger can be constructed of carbon steel, stainless steel, titanium or any other metal that can be cold formed, rolled and welded.

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Fouling and Plugging

A spiral heat exchanger can be cleaned by removing the cover and spraying the interior with a pressure washer or by using other mechanical means. Most spiral designs are only 6' deep, so it is easy to access the entire heat transfer surface area.

In a spiral heat exchanger, the single, curving channel and presence of spacer studs create a rigorous flow path that ensures turbulent flow regimens even at low velocities. There are no dead spots, so velocity is uniform throughout the channel. Its curved design causes the flow to continuously impinge upon the heat transfer surface, creating high shear rates and preventing solids from clinging to the wall.

Suspended solids present a major problem in most heat exchangers in applications such as pellet water coolers as well as catalyst slurry heaters and coolers. If the solids begin to settle on the heat transfer surface, the channel's cross-sectional area is reduced. In multiple-channel heat exchangers such as the shell and tube design, this creates flow distribution problems as flow is diverted away from partially fouled channels to nonfouled channels. As a result, flow velocity in the fouled channel is reduced. With lower velocity, the tube continues to collect solids until it eventually plugs, eliminating all flow through the tube. Once many tubes are plugged, the unit must be cleaned before it will operate effectively.

A spiral heat exchanger has a single channel for each fluid. If solids settle onto the heat transfer surface, the cross-sectional area of the channel is decreased, yet the fluid has no alternative channel in which to flow. In areas with solid deposits, the local velocity and shear rates are increased, creating a scrubbing effect that removes the solids from the wall

Spiral heat exchangers are used in many processes prone to heavy fouling and erosion.

For example, if 0.25" particles settle in a spiral heat exchanger channel that is 0.5 high by 24" wide, the channel geometry is reduced to 0.25" by 24". Because the fluid has no alternative channels in which to flow, the channel velocity doubles and the shear rate increases by a factor of four, causing the solids to be resuspended and flushed out of the exchanger.

Spiral heat exchangers are designed with fouling factors between one-quarter to one-eighth of standard TEMA fouling factors. Even with these lower fouling factors, the spiral heat exchanger generally can be expected to operate effectively three to four times longer than a shell and tube exchanger before cleaning is required.

Another problem associated with heavy fouling applications is erosion. Erosion occurs when the local velocity in a heat exchanger becomes excessive and begins to wear away its

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walls. With many process slurries, a fouling problem can lead to erosion if the local velocity cannot be controlled effectively. Some examples of erosive services are TiCl4 slurry cooling in titanium dioxide plants and bauxite slurry heating in alumina plants. If the flow is not evenly distributed or if fouling diverts a large portion of the flow, local velocities may vary significantly within the heat exchanger. This can increase fouling in low velocity areas and accelerate erosion in higher velocity areas. With a spiral heat exchanger, fouling and variations in local flow velocities are reduced and erosion can be minimized.

Eventually, every heat exchanger must be cleaned. A spiral heat exchanger's heat transfer surfaces can be fully accessed for inspection and cleaning by removing a cover and cleaning with high-pressure water or other mechanical means (figure 4). Held in place with hookbolts, the covers can be removed and replaced easily. And, because the spiral channel is a maximum of 6' deep, all heat transfer surfaces can be easily reached. The covers are often fitted with hinges or davits to facilitate opening and closing the units. In general, a spiral heat exchanger can be opened, cleaned and resealed within 4 to 6 hours.

Spiral heat exchangers can be used in processes where erosion is expected. Because a spiral heat exchanger is less prone to fouling, local fluid velocities are subject to less variation.

Spiral heat exchangers can be used in most applications in the chemical process industry. In difficult services where fouling is a concern, life cycle cost should be considered when a heat exchanger is designed. Calculating the installation, operation and maintenance costs up front will provide a perspective on the equipment's total cost.

In many difficult applications at where fouling and plugging are problems, a standard shell and tube design may not be effective. While a spiral heat exchanger often has a higher initial cost, it may provide a lower life cycle cost due to lower fouling rates and ease of maintenance

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Air Cooled Heat Exchanger

Purpose

The purpose of this paper is to provide some general information on air-cooled heat exchangers and answer some of the commonly heard questions. This is a mixture of fact and opinion. Wherever the opinion is obvious to me, I have attempted to show it by use of italics.

Why use an air-cooled heat exchanger?

Air-cooled heat exchangers are generally used where a process system generates heat which must be removed, but for which there is no local use. A good example is the radiator in your car. The engine components must be cooled to keep them from overheating due to friction and the combustion process. The excess heat is carried away by the water/glycol coolant mixture. A small amount of the excess heat may be used by the car's radiator to heat the interior. Most of the heat must be dissipated somehow. One of the simplest ways is to use the ambient air. Air-cooled heat exchangers (often simply called air-coolers) do not require any cooling water from a cooling tower. They are usually used when the outlet temperature is more than about 20 deg. F above the maximum expected ambient air temperature. They can be used with closer approach temperatures, but often become expensive compared to a combination of a cooling tower and a water-cooled exchanger.

How are they constructed?

Typically, an air-cooled exchanger for process use consists of a finned-tube bundle with rectangular box headers on both ends of the tubes. Cooling air is provided by one or more fans. Usually, the air blows upwards through a horizontal tube bundle. The fans can be either forced or induced draft, depending on whether the air is pushed or pulled through the tube bundle. The space between the fan(s) and the tube bundle is enclosed by a plenum chamber which directs the air. The whole assembly is usually mounted on legs or a piperack.

The fans are usually driven be electric motors through some type of speed reducer. The speed reducers are usually either V-belts, HTD drives, or right angle gears. The fan drive assembly is supported by a steel mechanical drive support system. They usually include a vibration switch on each fan to automatically shut down a fan which has become imbalanced for some reason.

What standards air used for Air-Cooled Exchangers?

First, almost all air coolers are built to Sect. VIII of the ASME Code, since they are pressure vessels. For refinery and petrochemical services most customers include API 661 (Air-Cooled Heat Exchangers for General Refinery Service) in their specifications. This API spec is very good since it includes all the necessary information to properly specify a cooler and provides for a high level of minimum quality in the design and fabrication of the cooler. In the back it has a very good checklist where a customer can decide exactly what type construction is needed and what options are important. These include such items as galvanizing vs. painting, types of headers, maintenance walkways and platforms, controls, and external loads on the cooler. The following details refer mostly to the API specifications.

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What kinds of finned tubes are used?

The tubes can be of virtually any material available, such as carbon steel, stainless steel, Admiralty brass, or more exotic alloys. The minimum preferred outside diameter is one inch. Some manufacturers sometimes use smaller tubes, but most of the process coolers have tubes which are 1.0", 1.25", or 1.5" OD. The minimum tube wall thicknesses vary with the material. In some cases the design pressure and design temperature of the exchanger govern the minimum thickness.

The fins are almost always of aluminum material. The most common type of fin is the helically wrapped, L-footed type. These are used where the process temperatures are below about 350 deg. F. The API specification calls for cast zinc bands at the ends of the tubes to prevent the fins from unwrapping. Some of the better manufacturers also use cast zinc bands at the tube supports. For higher process temperatures, most customers prefer either embedded or extruded fins. The embedded fins have the highest temperature capabilities. They are made by a process which cuts a helical groove in the OD of the tube, wraps the fin into the groove, then rolls the upset metal from the tube back against the fin to lock it into place. The tube wall must be thicker with embedded fins because of the groove.

In some applications customers often prefer extruded fins. Extruded fins are made by putting an aluminum sleeve (sometimes called a muff) over the tube, then passing the tube through a machine which has rollers which squish the aluminum out to form fins. The process is similar to a thread-rolling machine. The end result is a fin which has extremely good contact with the tube, and no crevices to allow corrosion to start on the tube OD. Extruded fins are often used in coastal locations or on offshore platforms for this reason.

Some manufacturers make some rather startling claims for their "special" finned tubes. These modifications usually involve some kind of wrinkles or cuts in the fins to enhance air turbulence. We believe this is a lot of baloney. The cost of this extra turbulence is increased static pressure for the fan(s) to overcome. These claims are sometimes just too fantastic to be considered seriously.

What are headers?

Headers are the boxes at the ends of the tubes which distribute the fluid from the piping to the tubes.

How are headers constructed?

Almost all headers on air-cooled exchangers are welded rectangular boxes. A vast majority of the headers are of the plug type. This means that there is a shoulder plug opposite each tube which allows access for inspection and cleaning of individual tubes. They can also be used to plug a leaking tube. The plug holes are used in the manufacturing process for access to roller expand the tubes into the headers.

The other common type of header is the cover plate or bonnet type. These are usually used in low pressure applications (say below 150 PSIG) where complete tube access is desired. This usually means applications where fouling is a potential problem and the tube bundle may require occasional internal cleaning. As the name implies, these have a removable plate on the back side of the header opposite the tubes. The cover plate is attached to the header by a set of studs or through-bolts to a flange around the perimeter of the header. A bonnet header is

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similar, but opposite in construction. The whole header or bonnet bolts to the tubesheet and comes off. Bonnet headers are sometimes used where the corrosion potential of the process fluid is very high and the tubesheet material is some kind of expensive exotic alloy, such as titanium.

Headers are usually constructed of carbon steel or stainless steel, but sometimes more exotic alloys are used for corrosion resistance. The selection of materials is usually made by the customer.

Why are some coolers forced draft and some induced draft? Which is better?

It depends. The majority of air-cooled exchangers is of forced draft construction. Forced draft units are easier to manufacture and to maintain. The tube bundle is mounted on top of the plenum, so it can be easily removed and replaced. The fan shaft is short, since it does not have to extent from the drive unit through the tube bundle and plenum to the fan, as in an induced draft design. Forced draft units require slightly less horsepower since the fan are moving a lower volume of air at the inlet than they would at the outlet. If the process fluid is very hot, the cooling air is hot at the outlet. This could cause problems with some fans or fan pitch actuators if the fan is exposed to very hot exhaust air. Since forced draft coolers do not have the fans exposed to hot exhaust air, they are a better choice in such cases. (API 661 par. 4.2.3.15&16 offer some guidelines for this.)

However, induced draft units have some advantages, too. A common problem with forced draft coolers is accidental warm air recirculation. This happens when the hot exhaust air is pulled back in to the fans. Since a forced draft cooler has a low air velocity at the exhaust from the bundle and a high velocity through the fan, a low pressure area is created around the fan, causing the hot air to be pulled over the side or end of the bay. For this same reason, there should never be a small space between the bays of a bank of forced-draft cooler. Induced draft cooler have a high exhaust air velocity through the top-mounted fan, and a lower velocity into the face of the tube bundle below. This tends to minimize the probability of accidental air recirculation. Also an induced draft plenum does not have to support the tube bundle so some weight can often be saved in this area.

Painted or Galvanized?

This is usually a matter of customer preference. However, the costs are roughly the same if a multiple coat paint system is specified. Often the painted units are more expensive. There seems to be a trend toward more galvanized structures because they require virtually no maintenance. Painted structures require touch-up after installation and they often rust anyway. We recommend galvanized units wherever possible.

Plenums, dispersion angle, and fan coverage:

The API specification includes a number of paragraphs about fan coverage and dispersion angle. This is for a very good reason. The actual air coming from a fan does not distribute itself evenly at first. The most air flow is seen around the fan tip area. If you measure the air flow across the face of a tube bundle, it is often very different around the fan blade tip as opposed to the center of the fan or the corner of the bundle. However, as the plenum becomes deeper, this localized effect is diminished as the air becomes more evenly distributed. All of the heat transfer programs assume that the air is distributed perfectly evenly.

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The fan coverage is the ratio of the fan area to the bundle face area. The higher this ratio, the better the fan coverage. The API minimum is 40% with a 45 degree maximum dispersion angle from the fan ring to the middle of the tube bundle at the middle of the sides or the middle of the ends of each fan chamber. More fan coverage or a lower dispersion angle can improve the air distribution.

A few manufacturers actually improve on this idea one step more, by using rounded and eased fan rings. Rounded and eased rings offer two advantages compared to the conventional fan rings. First, they enhance the distribution of the air. Secondly, they reduce the air pressure drop through the fan ring, slightly reducing the fan brake horsepower. When designing their coolers, some cooler manufacturers base their fan designs on the use of rounded and eased rings, even though they don't build them this way.

What kinds of controls are used?

As one might expect the best kind of control scheme depends on the application. Does the process require a very tight control on the process outlet temperature, or is it better to allow the process temperature to go down with the ambient air temperature. Is there a possibility of freezing the process? Is there a pour-point problem? Is the cost of operating the fan motors a significant factor? The following is a list of some of the commonly used control devices for air coolers, but in no particular order.

1. Manually operated louvers.

2. Electrically or pneumatically operated louvers.

3. Pneumatically actuated automatic variable-pitch fans.

4. Variable-frequency fan drives.

5. Warm-air recirculation systems for freezing/pour point control in cold climates.

6. Steam coils

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General Design Considerations And ApproachesThe selection of most heat exchangers follows routine pattern established by many years of

heat exchanger design and operating experience, but changes in operating conditions or completely new types of application may justify or require a fresh approach. In some seemingly conventional applications, the amount of money involved or the unusual requirements may call for an extensive engineering effort to work out and evaluate new designs. It is especially concerned with such cases below, and with the selection of criteria and their use in the development of new designs.

DELINEATION OF REQUIREMENTS

The first step in preparing the design for a new type ofheat exchanger is a dear statement of the requirements. Where possible, the relative

importance of each of the various factors and requirements should be established, and the areas in which a wide latitude of choice exists should be indicated. Attention should be directed toward those areas in which special care must be taken to effect good compromises, and the incentives for improving particular performance characteristics should be established as well as possible.

Heat Transfer Performance.

The specification of the inlet and outlet temperatures for each of the two fluid streams is the first step in establishing the requirements. Where a range of temperatures is under consideration, the incentives to reach the more desirable end of each range should be indicated. Once the inlet and outlet temperatures are defined, the heat exchanger effective-ness can be estimated. This is important since it will give a good indication of the flow passage length diameter ratios required and the feasibility of using parallel or crossflow units as opposed to counterflow units.

Fluid-flow rates must then be established for each of the two fluid streams. Since liquid velocities are ordinarily kept between 2 and 20 ft/sec, and gas velocities between 10 and 100 ft/sec (with each usually near the middle of the range given), the flow rates give a good indication of the flow passage cross-sectional area required for each of the two fluid streams. it is sometimes necessary to restrict the fluid velocity to avoid difficulties with such problems as erosion, tube vibration, flow stability (as in boiler tubes), or noise (as in air conditioning units).

Sludge or other deposits form on the heat transfer surfaces in some types of application. The extent and thickness of these deposits should be estimated together with their effects on the heat-transfer coefficient and fluid friction factor. Such allowances may significantly affect the size of the heat exchanger required because, if heavy deposits are anticipated, they may require that rather low unit heat fluxes be employed to avoid excessive temperature drops. The tube diameter is also influenced since it is ordinarily impracticable to use small tubes if heavy deposits are anticipated. This last consideration will also determine in substantial measure the geometry of the heat exchanger. if periodic mechanical cleaning of the tubes will be required, provisions for such cleaning must be made. if chemical solvents or special cleaning compounds are to be employed to remove deposits, the need for these may affect the choice of the materials of construction. For example, it may be necessary to use stainless steel to withstand an acid cleaning solution even though plain carbon steel would serve for the process fluids.

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Size RestrictionsIt is of ten important to limit the length, height, width, volume, or weight of a heat

exchanger because of requirements peculiar to a particular application. These limitations may apply not only to the heat exchanger itself but also to provisions for maintenance. For example, it may be essential that the he at exchanger casing be installed in such a way that individual tubes or the entire tube bundle be removable simply by opening a flange at one end of the heat exchanger. The space available is of ten such that the length of the tube bundle that can be handled is limited. The fluid inventory is likely to be an important consideration for expensive, toxic, or combustible fluids. it may also be necessary to impose special requirements for drainage, vertical removal of the tubes or the tube bundle, or the like.

Cost FactorsCost factors are of ten dominant in the choice of a heat exchanger. These include not only

the initial capital cost of the heat exchanger but also the costs of plant operation and maintenance. These effects may be complex as, for example, in a chemical processing plant where the value of one or more products may be affected. The situation in a steam power plant is somewhat simpler, because the capital cost of the equipment can be related directly to the efficiency of the power plant and hence to the costs of producing electrical power.

An interesting example showing one way of coping with the problems involved is given by a study of steam generators for a gas-cooled reactor power plant in which materials considerations limited the gas outlet temperature from the reactor. Note that, as the heat exchanger size is increased to increase the outlet steam temperature, the cost of the steam generator per kw of plant output drops a little at first because of increasing plant efficiency, and then levels out and begins to increase rapidly as the steam outlet temperature approaches the temperature of the hot gas coming from the reactor.

To achieve minimum overall costs it is necessary to balance operating costs against capital charges. The problem can be illustrated by considering a particular case. Once a basic heat-transfer matrix geometry is selected on the basis of fabrication, performance, and maintenance considerations, the amount of heat that will be transferred from one fluid to the other for a given set of fluid temperatures in a given size of heat exchanger will depend largely on the flow rates of the two fluid streams. If the flow rates are doubled, the capital charges will be cut almost in half, but the pumping power requirements will be increased by approximately a factor of eight.

Note that not only the costs of power for pumping ought to be included but also the capital charges dependent on the pumping equipment, since these charges may be a substantial fraction of the cost of the power required. Although individual cases differ widely, it has be en found that, for a pumping power requirement between 0.5 and 1.0% of the heat transmitted through the heat exchanger; the overall cost is usually fairly close to the minimum obtainable. Note that this is the case not only for the recuperator but also true of the steam generator

Stress ConsiderationsStress considerations are usually relatively unimportant in the design of heat exchangers

unless system pressures above 200 psi or metal temperatures above 300°F are employed. For pressures over 1000 psi or temperatures above 1000°F, stress considerations probably will be dominant. In these regions the tube headers are particularly likely to be the controlling factor in the selection of the heat exchanger geometry, and stress considerations are a major factor in determining the choice of the material to be employed.

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Differential thermal expansion is likely to pose important limitations on the design of the heat exchanger if temperature differences of 100°F or more are to be expected between the tubes and the shell; hence it is important to note this item in outlining the requirements to be met.

Material Requirements and Fabrication TechniquesCorrosion problems are almost always important in the selection of a material for a heat

exchanger application. if rather corrosive fluids are involved, it may be necessary to balance the cost of more corrosion-resistant materials against the higher maintenance and replace-ment costs of less expensive materials. The subject is far too complex to treat in a general fashion in the limited space available in this book. The special problems of widely used types of heat exchanger, such as steam boilers, are treated in later.

Fabrication problems may also be an important consideration in the choice of a material. If

the unit is to be soldered, brazed, or welded, the materials chose must be well suited to the fabrication operation, and a rice premium to minimize fabrication difficulties is of ten justified. if tubes are to be rolled into header sheets, the tube material should be carefully selected and specified to insure good ductility and suitable work-hardening characteristics.

Leaktightness.

The leaktightness required for any given application should be specified both for leakage from one fluid stream to the other and for leakage from either fluid stream to the surroundings. Methods for measurement of leaktightness are outlined in the chapter on heat exchanger testing. Heat exchanger specifications ordinarily should indude a statement of the method to be employed in checking the leaktightness.

Servicing, Repair, and Maintenance Maintenance requirements differ widely from one application to another, and often impose

important limitations on the design. Some of these limitations are discussed in the section on shell-and-tube heat exchangers, especially the influence of maintenance requirements on the design of the shell and the header sheet installation.

It is usually necessary to make some provisions for inspection and repair work. In some applications the design must be such that a leaking tube may be removed readily and replaced. In others, notably in boilers, the requirements may be relaxed, and a design considered acceptable if an inaccessible tube can be patched or readily blocked off with a plug at the header sheet in the event of a leak. If the incidence of tube leaks is small during the life of a unit, a few blocked off tubes will not result in an appreciable loss in heat exchanger capacity.

Heat exchangers in many applications tend to become fouled with deposits of one kind or another. Often these can be removed by cleaning with an oxalic, acetic or dilute hydrochloric acid solution, a detergent, or some solvent. In some applications these deposits are best removed by passing a special cleaning tool (of the type shown in Fig. 8.4) through the tubes. In some cases it may be better to remove the heat exchanger from the system rather than to try to dean it in situ. Where deposits present particularly severe problems, it may be best to employ a multiplicity of small heat exchangers in parallel so that units may be valved off and cleaned without interrupting plant operations.

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System Operating and Control Requirements

The response characteristics of a heat exchanger to changes in load of ten have important effects on the performance of a plant. The rate at which a plant may be started up or shut down, or the output level changed, may depend in large measure on the characteristics of the heat transfer equipment. All too often, in new types of plants, problems of this character are not recognized until after the plant is built and in operation. Wherever possible it is desirable to investigate the response characteristics desired not only at the design point but throughout the load range for which good control is required. This is particularly true where there may be difficulty in obtaining stable operation of a system. When this is the case, the performance characteristics of the major components and of the instrumentation and automatic control equipment must be considered.

Where system stability and control may be a problem, analysis of the dynamic characteristics of the system should be carried out so that the characteristics of the heat exchanger may be specified. Such an analysis usually most conveniently made with an analog computer-may lead to major changes in the choice of the operating conditions for the plant as a whole and the specification of unusual characteristics for the heat exchanger. For example, to obtain sufficiently rapid rates of response to changes in temperature conditions, it may be necessary to design the heat exchanger to give fairly high fluid velocities at low outputs and to accept a higher pumping power penalty at full-power conditions than would appear proper from a simple cost study that ignored control problems.

DESIGN APPROACHES TO THE SELECTION OF A HEAT TRANSFER MATRIX GEOMETRY

Once the design requirements are established by preparing a rough design specification as just outlined, the first step in the selection of a heat exchanger geometry is to examine commercially available units. if none is available in the size or operating temperature range desired, it is often possible to prepare charts to indicate the performance of special units scaled up or down from those commercially available.

Perturbations on Existing Designs When the procedure just described does not yield a unit that meets the desired

specifications, or when cost and environmental considerations justify an attempt to design an improved type of heat exchanger, it may be in order to explore the possibilities of solving the problem with an unconventional or unusual design. Of necessity the approach must be intuitive, but the point of departure will usually be some type of heat exchanger that has proved to be satisfactory for other applications and shows special promise of meeting the desired specifications.

Essential steps in the preparation of a new design are to compare the physical properties of the fluids employed in previous designs with the properties of the fluids to be used. It is usually more important that the density, viscosity, and thermal conductivity of the fluids be similar than that the heat exchanger be for a similar application. The physical properties of the two fluids in large measure establish the desirability of using finned surfaces, large- or small-diameter tubes, and the like. After thus narrowing the field of possible basic heat-transfer matrix geometries, a tentative selection of matrix geometry may be made on the basis of fabrication and maintenance considerations, particularly the extent to which heat-transfer surface fouling is to be expected and the requirements for its removal.

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Cut-and- Try DesignOnce basic heat-transfer matrix geometry has been selected, it is desirable to investigate the

effects of changing such parameters as the tube diameter and spacing. This can be done in several ways. It is possible to carry out a cut-and-try procedure by varying the major parameters in the region of interest, in each instance arriving by a series of successive approximations at a unit that meets the specifications. This procedure is clumsy, and is ordinarily not very effective except in the hands of an engineer experienced in the design of somewhat similar equipment.

Parametric Studies Because of the difficulty in proportioning the various dimensions of a heat exchanger to

give the desired heat-transfer performance together with the desired fluid system pressure drops, it is sometimes convenient to carry out a parametric study by varying the factors of greatest interest and evaluating the balance to meet the specifications. It is usually best to perform such calculations in a fashion that minimizes the number of calculational operations. The characteristics of the units meeting the desired specifications can then be plotted as functions of such factors as the tube diameter or tube length to yield curves showing the effects of greatest interest.

Optimization The large capital investment in same types of heat exchanger of ten justifies a systematic

selection of the key factors affecting the heat exchanger performance and an attempt to obtain an explicit solution for the design requirements at hand. It is usually possible to reduce most of the many factors just cited to a single common figure of merit such as east or weight. A computer code then can be set up to define the heat exchanger proportions for which the east, weight, or other parameters have the optimum value. it is usually not possible to obtain a heat exchanger that is at once both the least expensive and the lightest, for example; but the calculations can be set up to optimize first for one and then the other, and the results can then be compared.

In setting up a computer code for optimizing a heat exchanger, great care must be exercised to hold just enough parameters constant to give a solution without fixing so many conditions that a solution is impossible. Any approach is heavily dependent on the particular requirements for the application at hand, and hence no general procedure can be suggested. A specific example of such a treatment is given on steam boilers, where a technique is presented for obtaining an explicit solution to the tube length required to meet a given set of conditions.

While optimization studies can be carried out by hand, it is usually best to do them with a digital computer. Many codes for carrying out calculations of this sort are available, but these should be used with caution since the optimum set of proportions for one application is seldom the optimum for another.

EVALUATION OF PROPOSAL

An important set of problems are posed when making an engineering evaluation of several proposals involving widely different types of heat exchanger. Of ten such proposals may have been prepared on the basis of inadequate specifications. The first step in evaluating proposals of this type is an estimate of the heat-transfer performance to be expected. It is unfortunate but true that, under the pressure of competitive bidding conditions, proposals are often submitted offering equipment that is not adequate to meet the specifications, because the equipment designed was deliberately undersized in an effort to cut costs. Thus the lower

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initial costs of such units must be compared with possible increases in operating costs or losses in revenue that would be entailed by reductions in the heat exchanger capacity relative to the specifications. The proposals must also be evaluated for relative ease in servicing, probable relative leaktightness characteristics, corrosion problems, service life, ease of drainage, and stability and control.

Although not immediately obvious, flow distribution may pas e service problems in same types of heat exchanger and lead to serious losses in heat exchanger performance because of special problems such as hat spots, tube vibration, or flow instabilities. These problems are very difficult to evaluate analytically and ordinarily require testing. The probable extent of such problems should be considered in appraising competing proposals.

COST ESTIMATION

It is of ten important to make a rough estimate of the east of a he at exchanger when preparing a preliminary design for a plant. This section aids in formulating such preliminary rough estimates. The problems of making reliable estimates are complicated by the fact that not only do companies generally consider their basic east data proprietary-an understandable point of view-but east estimators as individuals usually guard their "secrets" jealously. Same data have been published, of which the most useful have appeared in a series of articles in Chemical Engineering. Information from these articles, through 1960, has been summarized and is available in book. These data, together with information from the authors' files, have served as the basis for this section on costs. It must be emphasized that costs are very sensitive to special requirements; hence the data presented here should be used only for very rough approximations. Seemingly small factors such as rigorous quality control can easily double the cost of a unit.

The values presented are for standard units of moderate size in low-volume production. It has been found that for virtually all types of material and equipment, the unit cost is inversely proportional to the square root of the annual production rate; that is, the cost per pound of equipment falls off by a factor of 10 for every factor of 100 increase in the annual production rate. This relation holds whether the increase is in the size of each item or in the average number of items produced per year. Even though the annual production rate is beyond the designer's control, he can sometimes choose a volume production unit in preference to a special design.

Escalation

Prices tend to increase from time to time. Adam Smith 10 showed that the inflation process had increased costs of basic commodities by roughly a factor of three per century from the twelfth to the eighteenth century. This rate of increase continued until World War I, but since 1914 it appears to have doubled so that prices roughly doubled from 1914 to 1957. While from 1957 to the time of writing (1965) prices seem to have increased at a lower rate, it is difficult to predict future trends. Cost data given here should be corrected by an escalation factor obtained from a major industrial price index, such as that published in Chemical Engineering or Engineering News Record. (The Chemical Engineering Cost Index for equipment rose from about 225 in 1957 to about 244 in 1965.)

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Selecting a Heat Transfer Fluid

Process engineers have a number of secondary coolants from which to choose. A nontoxic fluid with good thermophysical properties and a long service life usually is your best choice.

Figure 1. Secondary Refrigeration Process. A secondary refrigeration process uses smaller refrigerant amounts to cool the heat transfer fluid.

Low temperature heat transfer fluids are crucial components in many process applications. Current heat transfer fluid technologies include direct refrigerant use as well as liquefied gas and secondary refrigeration technology using various low temperature fluids. In a secondary refrigeration process (figure 1), a primary refrigerant or a liquefied gas is used to cool the heat transfer fluid. The cold heat transfer fluid then is circulated through the user's process to provide uniform temperature distribution. This secondary refrigeration process uses smaller refrigerant amounts to cool the heat transfer fluid. In general, the secondary refrigeration approach has several advantages:

Fewer refrigerant leaks due to substantially less refrigerant piping. As much as 75 to 80% reduction in the primary refrigerant charge. Fewer service calls. Stable process temperature.

But, if the heat transfer fluid is improperly selected or poorly maintained, it will reduce the system's efficiency, causing a loss in production and revenue. Fortunately, the process engineer has several options to choose from when selecting secondary coolants in operating temperature ranges from -170 to 32°F (-112 to 0°C). They include:

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An aqueous solution of organic compounds such as alcohols or glycols. These are water solutions of various concentrations including methanol/water, ethanol/water, ethylene glycol/water and propylene glycol/water.

An aqueous solution of inorganic salts such as sodium chloride and calcium chloride. Chlorinated or fluorinated hydrocarbons including methylene chloride, trichloro-

ethylene, R-11, hydrofluoroethers and perfluoropolyethers. Hydrocarbon-based fluids, including aliphatic, aromatic and terpene. Silicones.

Selecting a low temperature heat transfer fluid for a given application depends on a number of considerations. Choosing a particular fluid is invariably a compromise solution that best satisfies the specific application and economics. Strike a balance between several criteria:

Freezing point. Viscosity at low temperatures. Thermal properties (specific heat and thermal conductivity). Flammability (flash and fire points). Degree of corrosivness to materials of construction. Environmental concerns. Service life. Price.

The heat transfer fluid should posses a freezing point at least 20°C lower than the lowest operating temperature to avoid freezing on the wall. It also should have low vapor pressure or a high boiling point to avoid system pressurization at elevated temperatures. Additionally, a high flashpoint and autoignition temperature are desired so the fluid is less susceptible to ignition.

Good thermophysical properties are required to obtain high heat transfer coefficients and the low pumping power needed for the fluid to flow through a pipe with a particular flow rate. Therefore, high heat capacity and thermal conductivity is desirable in addition to low viscosity at low temperatures.

Apart from thermophysical properties, the fluid also should exhibit sufficient stability toward oxidative degradation. In the presence of air, most organic fluids oxidize at high temperatures and can form acidic and polymerization products in the system, which can initiate corrosion and fouling. This can severely affect the heat transfer system's efficiency. Corrosion also can be caused by a salt brine or glycol/water solution if proper inhibitors are not used.

The ideal heat transfer fluid would be nontoxic, environmentally friendly, classified as food grade and able to satisfy Food and Drug Administration (FDA) and United States Department of Agriculture (USDA) criteria for incidental food contact. Addition-ally, its vapor should neither contribute to the greenhouse effect nor ozone layer depletion. None of the currently used heat transfer fluids satisfy all of these requirements. A few satisfy most but are expensive.

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Comparing Aqueous Solutions

Commonly used as an antifreeze, ethylene glycol is used in refrigeration service and process cooling applications at lower temperatures. Ethylene glycol is colorless, practically odorless and miscible with water. When properly inhibited, it has relatively low corrosivity - an advantage when compared to salt-based brines. Ethylene glycol solutions can be used as a refrigeration system brine at temperatures as low as -40°F (-40°C). However, due to its high viscosity at low temperatures, ethylene glycol is most effective at 14°F (-10°C) or above. Ethylene glycol also is toxic, so it is not suitable for open baths or in food and pharmaceutical applications.

The water quality used to prepare a glycol solution is important. Typically, water with low chloride and sulfate ion concentration (<25 ppm) is recommended. Also, a schedule should be maintained to ensure that inhibitor depletion is avoided and solution pH is consistent. Once the inhibitor has been depleted, it is recommended that old glycol be removed from the system and a new charge be installed.

In its inhibited form, propylene glycol has the same low corrosivity advantages shown by ethylene glycol. In addition, propylene glycol has low toxicity, and certain grades can be used in food applications. Other than low toxicity, it has no advantages over ethylene glycol, being higher in cost and more viscous. Due to its high viscosity at low temperatures, propylene glycol is used at 14°F (-10°C) or above.

Methanol/water is an antifreeze solution used in refrigeration services and ground source heat pumps. Similar to glycols, this solution can be inhibited to stop corrosion. It can be used at temperatures as low as

-40°F (-40°C) owing to its relatively high heat transfer rate in this temperature range. Its main disadvantages as a heat transfer fluid are its toxicological considerations. It is considered more harmful than ethylene glycol and consequently is used only for process applications located outdoors. Methanol also is flammable and, as such, introduces a potential fire hazard where it is stored, handled or used.

Ethanol/water is an aqueous solution of denatured grain alcohol. Its main advantage is that it is nontoxic. Therefore, it is used in applications in breweries, chemical plants, food freezing plants and ground source heat pumps. As a flammable liquid, it requires certain handling and storage precautions.

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Selecting Salt Brines

A low temperature heat transfer fluid must posses a low freezing point to avoid freezing on the wall.

Sodium chloride water solutions are used for refrigeration services and other low temperature applications. Because salt brines have low toxicity and are nonflammable, they can be used in applications involving contact with foods and in open systems. A high heat transfer coefficient can be obtained with this type of fluid due to its good thermophysical properties. However, it has two main drawbacks:

A relatively high freezing point, which limits its use to approximately 14°F(-10°C). High corrosivity, requiring inhibitors that must be checked on a regular basis and

replenished to prevent an acid condition from occurring in the system.

A calcium chloride aqueous solution also can be used as a circulating brine. Similar to sodium chloride, this nonflammable, nontoxic solution has a lower freezing point and can be used at temperatures down to -35°F (-37°C). This solution's main disadvantages include high corrosivity, reduced heat transfer coefficient below -4°F (-20°C), and its incompatibility for use in direct contact with foods.

Considering Halocarbons

Below -40°F (-40°C), salt-based brines and glycol- or alcohol-based solutions do not perform well because of their high viscosity. Certain halocarbons such as methylene chloride, trichloroethylene and fluorocarbons can be used at these temperatures. Nonflammable and noncorrosive under normal operating conditions, chlorinated compounds such as methylene chloride or trichloroethylene are toxic and regulated by the Environmental Protection Agency (EPA). These fluids will be removed from existing systems in the next few years.

Fluorinated compounds such as hydrofluoroethers and perfluorocarbon ethers have unique properties that make them suitable for use in low temperature heat transfer fluid applications. Nonflammable and nontoxic, some fluorinated compounds have zero ozone-depleting potential and other environmental properties. Additionally, these fluids have low freezing point and low viscosity at low temperatures. However, they are expensive, and due to their extremely low surface tension, leaks can develop around fittings. Also, fluorocarbons posses a lower boiling point than many other heat transfer fluids. Therefore, they are not suitable for applications where both low and high temperatures are desired. Typical fluorocarbon-based fluid applications are in the pharmaceutical and semiconductor industries within a temperature range from -148 to -238°F (-100 to 150°C).

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Looking at Hydrocarbons

Good thermophysical properties are required to obtain high heat transfer coefficients and the low pumping power needed for the fluid to flow through a pipe with a particular flow rate.

Aromatic hydrocarbons such as diethyl benzene are common low temperature heat transfer fluids in the temperature range from -94 to 500°F (-70 to 260°C). The low temperature heat transfer characteristics and the thermal stability of aromatic compounds are good. However, these alkylated benzene compounds cannot be classified as nontoxic. They also have a strong odor that can be irritating to personnel. Few aromatic compounds have a freezing point lower than -112°F (-80°C), so most are used at temperatures above -94°F (-70°C) in closed, airtight systems typically found in chemical processing and industrial refrigeration.

Paraffinic and iso-paraffinic type aliphatic hydrocarbons also are used in some systems as a low temperature heat transfer fluid. Many petroleum-based aliphatic compounds meet FDA and USDA criteria for incidental food contact. In addition, these petroleum-based fluids do not form hazardous degradation byproducts. Most have an indiscernible odor and are nontoxic. Even with these advantages, though, they are not commonly used in low temperature applications because of their high viscosity at low temperatures. Also, thermal stability of aliphatic compounds is not as good as aromatic compounds. Some of the iso-paraffinic based fluids (with 12 to 14 carbons) can be used from -76 to 392°F (-60 to 200°C). They are preferred in food and pharmaceutical applications where toxicity is an issue.

Another class of low temperature heat transfer fluids is based on naturally derived terpenes such as d-limonene. D-limonene is the major component in citrus fruit oil and is present in trace quantities in orange juice. It is recovered in commercial quantities by distilling orange oil, which is obtained from citrus peels. Being derived from the citrus industry, d-limonene is considered a safe and environmentally friendly heat transfer fluid and is preferred in many food and pharmaceutical processes. The published melting point of d-limonene is approximately -140°F (-96°C). But, testing has shown that below -108°F (-78°C), it becomes a thick white, gel-like substance that is impossible to pump. In addition, at elevated temperatures, d-limonene oxidizes rapidly in the presence of air. This oxidation triggers the acidification and polymerization of the molecules.

Choosing Silicones

Another popular class of low temperature heat transfer fluids is dimethyl polysiloxane or silicone oil. Because this class is a synthetic polymeric compound, molecular weight and thermophysical properties can be adjusted by varying the chain length. Silicone fluids can be used at temperatures as low as -148°F (-100°C) and as high as 500°F (260°C). These fluids

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offer a long service life in closed systems in the absence of oxygen, and with low toxicity and essentially no odor, silicone fluids are known to be workplace friendly. However, with low surface tension, these fluids have the tendency to leak around pipe fittings - although this low surface tension improves the wetting property. Because silicone fluids have low toxicity, they are used most often in the pharmaceutical industry.

Several factors can affect the decision-making process when choosing a low temperature heat transfer fluid. However, proper selection can reduce the overall process cost in the long run. In addition to the fluids discussed, there are many more secondary coolants on the market that may be more efficient and environmentally friendly.

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