Heat exchanger

Suez University

Faculty Of Petroleum & Mining Engineering

Prepared by/

Student/ Mohamed salah abou El_hamed

Department/ petroleum refining

Year/ third

1Heat Exchanger Mohamed Salah /Student

A heat exchanger is a heat transfer device that exchanges heat between two or more process fluids. Heat exchangers have widespread industrial and domestic applications. Extensive technical literature is available on heat exchangers, but it is widely scattered throughout the technical journals, industrial bulletins, codes and standards.

• A heat exchanger is used to exchange heat between two fluids of different temperatures, which are separated by a solid wall.

• Heat exchangers are ubiquitous to energy conversion and utilization. They encompass a wide range of flow configurations.

• Applications in heating and air conditioning, power production, waste heat recovery, chemical processing, food processing, sterilization in bio-processes.

• Heat exchangers are classified according to flow arrangement and type of construction.

What is A heat exchanger?

They are devices

specifically designed for the efficient transfer of heat from one fluid to another fluid over a solid surface.


Types of heat exchangers 1. Double pipe heat exchanger

Double pipe heat exchangers are the simplest exchangers used in industries. On one hand, these heat exchangers are cheap for both design and maintenance, making them a good choice for small industries. But on the other hand, low efficiency of them beside high space occupied for such exchangers in large scales, has led modern industries to use more efficient heat exchanger like shell and tube or other ones.

But yet, since double pipe heat exchangers are simple, they are used to teach heat exchanger design basic to students and as the basic rules for modern and normal heat exchangers are the same, students can understand the design techniques much easier. To start the design of a double pipe heat exchanger, the first step is to calculate the heat duty of the heat exchanger.

2. Shell and tube heat exchanger

Shell and tube heat exchangers consist of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and tube heat exchangers are typically used for high-pressure applications (with pressures greater than 30 bar and temperatures greater than 260 °C. This is because the shell and tube heat exchangers

are robust due to their shape.


3. Plate heat exchanger

Another type of heat exchanger is the plate heat exchanger. One is composed of multiple, thin, slightly separated plates that have very large surface areas and fluid flow passages for heat transfer. This stacked-plate arrangement can be more effective, in a given space, than the shell and tube heat exchanger.

Advances in gasket and brazing technology have made the plate-type heat exchanger increasingly practical. In HVAC applications, large heat exchangers of this type are called plate-and-frame; when used in open loops, these heat exchangers are normally of the gasket type to allow periodic disassembly, cleaning, and inspection. There are many types of permanently bonded plate heat exchangers, such as dip-brazed, vacuum-brazed, and welded plate varieties, and they are often specified for closed-loop applications such as refrigeration. Plate heat exchangers also differ in the types of plates that are used, and in the configurations of those plates. Some plates may be stamped with "chevron", dimpled, or other patterns, where others may have machined fins and/or grooves.

CLASSIFICATIONOF HEAT EXCHANGERS

In general, industrial heat exchangers have been classified according to (1) construction, ( 2 )transfer processes, (3) degrees of surface compactness, (4) flow arrangements, ( 5 )pass arrangements,(6) phase of the process fluids, and (7) heat-transfer mechanisms. These classifications

1. Classification According to Transfer Process

These classifications are:1. Indirect contact type-direct transfer type, storage type, fluidized bed.2. Direct contact type-cooling towers.


1. Indirect Contact Heat Exchangers In an indirect contact type heat exchanger, the fluid streams remain separate, and the heat transfer takes place continuously through a dividing impervious wall. This type of heat exchanger can be further classified into the direct transfer type, storage type, and fluidized bed exchangers. Direct transfer type is dealt with next whereas the storage type and the fluidized.

2. Direct Transfer Type Exchangers In this type, there is a continuous flow of the heat from the hot fluid to the cold fluid through a separating wall. There is no direct mixing of the fluids because each fluid flows in separate fluid passages. There are no moving parts. This type of exchanger is designated as a recuperator.Some examples of direct transfer type heat exchangers are tubular exchangers, plate heat exchangers, and extended surface exchangers. Recuperators are further subclassified as prime surface exchangers, which do not employ fins or extended surfaces on the prime surface. Plain tubular exchangers, shell and tube exchangers with plain tubes, and plate heat exchangers are examples of prime surface exchangers.Direct Contact Type Heat Exchangers In direct contact type heat exchangers, the two fluids are not separated by a wall; owing to the absence of a wall, closer temperature approaches are attained. Very often, in the direct contact type, the process of heat transfer is also accompanied by mass transfer. The cooling towers and scrubbers are examples of a direct contact type heat exchanger.

2. Classification According to Flow Arrangement

The basic flow arrangements of the fluids in a heat exchanger are1. Parallel flow2. Counter flow3.Cross flow

The choice of a particular flow arrangement is dependent upon the required exchanger effectiveness,fluid flow paths, packaging envelope, allowable thermal stresses, temperature levels,and other design criteria. These basic flow arrangements are discussed next.

1.Parallel Flow Exchanger In this type, both the fluid streams enter at the same end, flow parallel to each other in the same direction, and leave at the other end.Fluid temperature variations, idealized as one-dimensional. This arrangement has the lowest exchanger


effectiveness among the single-pass exchangers for the same flow rates, capacity rate (mass x specific heat) ratio, and surface area. Moreover, the existence of large temperature differences at the inlet end may induce high thermal stresses in the exchanger wall at inlet. Although this flow arrangement is not used widely, it is preferred for the following reasons ;

1. Since this flow pattern produces a more uniform longitudinal tube wall temperature distribution and not as high or as low a tube wall temperature as in a counterflow arrangement, it is sometimes preferred with temperature in excess of 1100°C (2000°F).2. It is preferred when there is a possibility that the temperature of the warmer fluid may reach its freezing point.3. It provides early initiation of nucleate boiling for boiling applications.4. For a balanced exchanger (i.e., heat capacity rate ratio C* = l ) , the desired exchanger effectiveness is low and is to be maintained approximately constant over a range of NTU values.5 . The application allows piping only suited to parallel flow.

2.Countefflow Exchanger In this type, as shown in, the two fluids flow parallel to each other but in opposite directions, and its temperature distribution may be idealized as one-dimensional.Ideally, this is the most efficient of all flow arrangements for single-pass arrangements under the same parameters. Since the temperature difference across the exchanger wall at a given cross section is the lowest, it produces minimum thermal stresses in the wall for equivalent performance compared to other flow arrangements.In certain type of heat exchangers, counterflow arrangement cannot be achieved easily, due to manufacturing difficulties associated with the separation of


the fluids at each end, and the design of inlet and outlet header design is complex and difficult .

3.Crossflow Exchanger In this type, the two fluids flow normal to each other. Important types offlow arrangement combinations for a single-pass crossflow exchanger include:1. Both fluids unmixed2. One fluid unmixed and the other fluid mixed3. Both fluids mixedA fluid stream is considered “unmixed” when it passes through individual flow passage without any fluid mixing between adjacent flow passages. Mixing implies that a thermal averaging process takes place at each cross section across the full width of the flow passage.tube-fin exchanger with flat (continuous) fins and a plate-fin exchanger wherein the two fluids flow in separate passages (e.g., wavy fin, plain continuous rectangular or triangular flow passages) represent the unmixed-unmixed case. A crossflow tubular exchanger with bare tubes on the outside would be treated as the unmixed-mixed case, that is, unmixed on the tube sideand mixed on the outside. The both fluids mixed case is practically a less important case, and represents a limiting case of some multipass shell and tube exchangers (TEMA E and J shell).For the unmixed-unmixed case, fluid temperature variations are idealized as two-dimensional only for the inlet and outlet sections, and this is shown in Fig. 20. The thermal effectiveness for the crossflow exchanger falls in between those of the parallel flow and counterflow arrangements. This is the most common flow arrangement used for extended surface heatexchangers, because it greatly simplifies the header design. If the desired heat exchanger effectiveness is generally more than 80%, the size penalty for crossflow may become excessive. In such a case, a counterflow unit ispreferred. In shell and tube exchangers, crossflow arrangement is used in the TEMA X shell having a single tube pass.


components of heat exchanger

Several thermal design features must be considered when designing the tubes in the shell and tube heat exchangers:

Tube diameter : Using a small tube diameter makes the heat exchanger both economical and compact. However, it is more likely for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used. Thus to determine the tube diameter, the available space, cost and the fouling

nature of the fluids must be considered. Tube thickness : The thickness of the wall of the tubes is usually

determined to ensure:

1.1.There is enough room for corrosion1.2.That flow-induced vibration has resistance1.3.Axial strength1.4.Availability of spare parts1.5.Hoop strength (to withstand internal tube pressure)1.6.Buckling strength (to withstand overpressure in the shell)

Tube length : heat exchangers are usually cheaper when they have a smaller shell diameter and a long tube length. Thus, typically there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities. However, there are many limitations for this, including space available at the installation site and the need to ensure tubes are available in lengths that are twice the required length (so they can be withdrawn and replaced). Also, long, thin tubes are difficult to take out and replace.

Tube pitch : when designing the tubes, it is practical to ensure that the tube pitch (i.e., the centre-centre distance of adjoining tubes) is not less than 1.25 times the tubes' outside diameter. A larger tube pitch leads to a larger overall shell diameter, which leads to a more expensive heat exchanger.

Tube corrugation : this type of tubes, mainly used for the inner tubes, increases the turbulence of the fluids and the effect is very important in the heat transfer giving a better performance.

Tube Layout : refers to how tubes are positioned within the shell. There are four main types of tube layout, which are, triangular (30°), rotated triangular (60°), square (90°) and rotated square (45°). The triangular patterns are employed to give


greater heat transfer as they force the fluid to flow in a more turbulent fashion around the piping. Square patterns are employed where high fouling is experienced and cleaning is more regular

Baffle Design : baffles are used in shell and tube heat exchangers to direct fluid across the tube bundle. They run perpendicularly to the shell and hold the bundle, preventing the tubes from sagging over a long length. They can also prevent the tubes from vibrating. The most common type of baffle is the segmental baffle. The semicircular segmental baffles are oriented at 180 degrees to the adjacent baffles forcing the fluid to flow upward and downwards between the tube bundle. Baffle spacing is of large thermodynamic concern when designing shell and tube heat exchangers. Baffles must be spaced with consideration for the conversion of pressure drop and heat transfer. For thermo economic optimization it is suggested that the baffles be spaced no closer than 20% of the shell’s inner diameter.

Having baffles spaced too closely causes a greater pressure drop because of flow redirection. Consequently having the baffles spaced too far apart means that there may be cooler spots in the corners between baffles. It is also important to ensure the baffles are spaced close enough that the tubes do not sag. The other main type of baffle is the disc and donut baffle, which consists of two concentric baffles. An outer, wider baffle looks like a donut, whilst the inner baffle is shaped like a disk. This type of baffle forces the fluid to pass around each side of the disk then through the donut baffle generating a different type of fluid flow.

Fixed tube liquid-cooled heat exchangers especially suitable for marine and harsh applications can be assembled with brass shells, copper tubes, brass baffles, and forged brass integral end hubs.


Materials of heat exchanger 1. Tubes

Heat exchangers with shell diameters of 10 inches to more than 100 are typically manufactured to industry standards. Commonly, 0.625 to 1.5" tubing used in exchangers is made from low carbon steel, Admiralty, copper, copper-nickel, stainless steel, Hastelloy, Inconel, or titanium. Tubes can be drawn and thus seamless, or welded. High quality electro resistance welded tubes display good grain structure at the weld joints. Extruded tubes with fins and interior rifling are sometimes specified for certain heat transfer applications. Often, surface enhancements are added to increase the available surface or aid in fluid turbulence, thereby increasing the operative heat transfer rate. Finned tubes are recommended when the shell-side fluid have a considerably lower heat transfer coefficient than the tube-side fluid. Note, the diameter of the finned tube is slightly smaller than the un-finned areas thus allowing the tubes to be installed easily through the baffles and tube supports during assembly while minimizing fluid bypass. A U-tube design finds itself in applications when the thermal difference between the fluid flows would otherwise result in excessive thermal expansion of the tubes. Typical U-tube bundles contain less tube surface area as traditional straight tube bundles due to the bended end radius, on the curved ends and thus cannot be cleaned easily. Furthermore, the interior tubes on a U-tube design are difficult to replace and often requiring the removal of additional tubes on the outer layer; typical solutions to this are to simply plug the failed tubes.

2. Tube Sheets Tube sheets usually constructed from a round, flattened sheet of metal. Holes for the tube ends are teen drilled for the tube ends in a pattern relative to each other. Tube sheets are typically manufactured from the same material as tubes, and attached with a pneumatic or hydraulic pressure roller to the tube sheet. At this point, tube holes can both be drilled and reamed, or they are machined grooves (this significantly increases tube joint strength).


The tube sheet comes in contact with both fluids in the exchanger, therefore it must be constructed of corrosion resistant materials or allowances appropriate for the fluids and velocities. A layer of alloy metal bonded to the surface of a low carbon steel tube sheet would provide an effective corrosion resistance without the expense of manufacturing from a solid alloy.

The tube-hole pattern, often called ‘pitch’, varies the distance between tubes as well as the angle relative to each other allowing the pressure drop and fluid velocities to be manipulated in order to provide max turbulence and tube surface contact for effective heat transfer.

Tube and tube sheet materials are joined with weld-able metals, and often further strengthened by applying strength or seal weld to the joint. Typically in a strength weld, a tube is recessed slightly inside the tube hole or slightly beyond the tube sheet whereas the weld adds metal to the resulting edge. Seal welds are specified when intermixing of tube liquids is needed, this is accomplished whereas the tube is level with the tube sheet surface. The weld fuses the two materials together, adding no metal in the process. When it becomes critical to avoid the intermixing of fluid, a second tube sheet is designed in. In this case, the outer tube sheet becomes the outside the shell path, and the inner tube sheet is vented to atmosphere, so that a fluid leak can be detected easily effectively eliminating any chance of cross contamination.

3. Shell Assembly

The shell is constructed either from pipe or rolled plate metal. For economic reasons, steel is the most commonly used material, and when applications involving extreme temperatures and corrosion resistance, others metals or alloys are specified. Using off-the-shelf pope reduces manufacturing costs and lead time to deliver to the end customer. A consistent inner shell diameter or ‘roundness’ is need to minimize the baffle spacing on the outside edge, excessive space reduces performance as the fluid tends to channel and bypasses the core. Roundness is increased typically by using a mandrel and expanding the shell around it, or by double rolling the shell after welding the longitudinal seam. In some cases, although extreme, the shell is cast and then bored. When fluid velocity at the nozzle is high, an ‘impingement’ plate is specified to distribute fluid evenly in the tubes, thereby preventing fluid-induced erosion, vibration and cavitation.


Impingement plates effectively eliminate the need to configure a full tube bundle, which would otherwise provide less available surface. An impingement plate can also be installed above the shell thereby allowing a full tube count and therefore maximizing shell space .

4. Bo nnets and End Channels

Bonnets / end channels regulate the flow of fluid in the tube-side circuit, they are typically fabricated or cast. They are mounted against the tube sheet with a bolt and gasket assembly; many designs include a ‘machine grooved’ channel in the tube sheet sealing the joint.

If one or more passes are intended, the head may include pass ribs that direct flow through the tube bundle (figure C). Pass ribs are aligned on either end to provide effective fluid velocities through an equal number of tubes at a time ensuring a constant, even fluid velocity and pressure drop throughout the bundle.

Impingement plate distributing the fluid to the tubes preventing fluid-induced erosion, vibration and cavitation

Figure C. Heads

contain pass ribs that direct flow on

the tube-side fluid for one or more passes across the tube bundle.


Shell and tube configurations with up to (4) passes are the most common, however specialty designs do allow 20 or more crossings. The tube sheet configuration in a multi-pass shell and tube design must have provisions for the pass ribs, requiring either removal of tubes to allow a low cost straight pass rib or alternately a pass rib with curves around the tubes adding cost to the manufacture process. When a full bundle count is needed for the thermal requirement, machine pass ribs usually prevent the need to ‘upsize’ to the next larger shell diameter.

The material used in the cast bonnets / heads used in smaller diameters (ie 15” or less) are typically, poured from iron, steel, bronze, Hastelloy, nickel plated, or stainless steel. Pipe connections are normally NPT, others including SAE, tri-clamp, ASME flanged, BSPP, and others types are available.

5. Baffles

Baffles function in two ways, during assembly they function as tube guides, in operation they prevent vibration from flow induced eddies, last but most importantly they direct shell-side fluids across the bundle increasing velocity and turbulence effectively increasing the rate of heat transfer.

All baffles must have diameter slightly smaller than the shell in order to fit, however tolerances must be tight enough to avoid a performance loss as a result of fluid bypass around the baffles. This is where the concept of ‘shell roundness’ is of up most importance in sealing off the otherwise would be bypass around the baffle.

Baffles are usually stamped / punched, or machined drilled; such configurations vary based on size and application. Material selection must be compatible with the shell side fluid to avoid failure as a result of corrosion. It is not uncommon for some punched baffle designs to include a lip around the tube hole to provide more surfaces against the tube to reduce wear on the adjoining parts. Tube holes must be precisely manufactured to allow easy assembly and possible field tube replacement, all the while minimizing fluid flow through the hole and against the tube wall.

In typical liquid applications, baffles occupy between 20-30% of the shell diameter; whereas in a gas application with a necessary lower pressure drop, baffles with 40-


45% of shell diameter are used (figure D). Baffle placement requires an overlap at one or more tubes in a row to provide adequate tube support.

Additionally baffles are spaced evenly throughout the shell to aid in reducing pressure drop and even fluid velocity.

Impingement plate distributing the fluid to the tubes preventing fluid-induced erosion, vibration and cavitation

Figure C. Heads contain pass ribs that direct flow on the tube-side fluid for one or more passes across the tube bundle.

In a 'single-segmental’ configuration, baffles move fluid or gas across the full tube count. When high velocity gases are present, this configuration would result in excessive pressure loss thus calling fourth a ‘double-segmental’ layout. In a ‘double-segmental’ arrangement, structural effectiveness is retained, yet allowing gas to flow in a straighter overall direction. While this configuration takes full advantage of the full available tube surface, a reduction in heat transfer performance should be expected.


troubleshooting of heat exchanger

It is stressful when exchangers go online and don't perform as they should. But not all scary things go "bump in the night." Heat exchangers that go onstream and don't perform are also scary. This is especially true if there is a lack of thermal wells, pressure gages and flowmeters. In addition, you are told that many hundreds of thousands of dollars a day are being lost due to decreased production, so the problem has to be found immediately. In many cases, there has to be a shutdown. Here is helpful information on finding a cure, including:

• What information to collect and what to look for • The importance of calculated pressure drops and how they help analyze the

problem• Two-phase flow emphasizing low heat transfer due to stratified flow • Actual case histories of design and fabrication errors to help with the diagnosis.

The concentration is on thermal problems; problems due to vibration and exchanger leaks are not discussed.

INFORMATION COLLECTION Besides the obvious process information of flow, temperatures and pressure drops, you win probably need the manufacturer's heat exchanger drawings. Hopefuny, you will not have to run heat exchanger tests. But if you do, there are procedures in the literature.1, 2 Using the collected process information, make a full thermal design computer run. The printout will have much more information than a standard specification sheet. Check the printout with the following in mind: 1. Are there any error messages about the physical properties used? 2. Are there error messages for the input data? 3. Check the section that analyzes the design for comments. This is a section of the program that acts as expert system software. 4. Was the correct heat-transfer type specified on input?

5. Have any warnings been ignored in the heat exchanger's design? 6. Was the advice on bundle-sealing devices followed? 7. Ifthe problem is freezing or heat damage, could the temperatures in the clean condition be the problem?


In some cases, it is helpful to measure temperatures on the exchanger's exterior. This can identify unvented gas, stratified flow or fluid bypassing. Ifthese temperatures are not too hot or cold, you can check the shell by feeling with your hands.

1. Fouling. Ifthere is a gradual decline in heat transfer, fouling may be the culprit. Heat exchanger software can give the available fouling as compared to design fouling. Sometimes fouling is so severe that tubes can be plugged inside or the shellside ligaments between the tubes can be filled. This is sometimes seen when bundles from a refinery are sent to be repaired. Actual fouling can be much higher than the TEMA CThbular Exchanger Manufacturer's Association) specification. Ifyou suspect that fouling is a problem, check the exchanger's operating history. Are there deviations from design conditions? Are there periods of operation where flows are lower than design? Heat exchangers will foul faster at low velocities. Ifwater fouling is a problem, have the water flows been cut back in the winter? Ifyou determine that fouling is a problem, make a chemical analysis of the fouling material. Knudson4 discusses different fouling control methods and types of cleaning. Online and offline mechanical cleaning plus chemical cleaning is discussed. If the fouling cannot be controlled, a tube electropolishing process can slow scale and other buildup. It eliminates small ridges and pits that contribute to fouling.

2. Debris. Check to see if there is a strainer in the piping ahead of the inlet nozzles. Ifthere is no strainer, there may be debris in the exchanger. Itis amazing what types of debris can be found in heat exchangers after startupsuch things as rocks, trash, wrenches, gloves, weld rods, clothing, pencils, etc. Possibly during a work force shift change, the first shift left something that the second shift did not see before closing the piping.

3. Excess surface problems. In the design stage, the clean condition may not have been evaluated. Many exchangers are designed for fouled conditions only. Most of the time this is all that is necessary. However, in some situations the clean condition must be checked. More exchanger oversurface means more deviations from outlet design temperatures and a greater potential for problems. For high-temperature applications, the outlet temperature of the heated stream must be checked. It will be higher than the process design temperature. Ifthis temperature is higher than what was used to select the metallurgy, there may be a problem. Small5 relates a case where the effect of oversurface and the clean condition was not checked. It resulted in ruptured tubing and the loss of tube fins.


Another problem with higher-than-process-design outlet temperatures is that a liquid may degrade or lose its thermal stability. For cold applications, the outlet temperature ofthe stream to be cooled must be checked. It can cause stream freezing and tube plugging. It can also cause brittle tubing and tube failures. Another case where excess surface can be a problem is in vaporizer design. Ifvaporizing is done too well, there will be surging ofvapor leaving the exchanger. As a young engineer, I saw an ammonia vaporizer in a nitric acid plant creating surging vapor to the reactor. All the liquid would flash to vapor inside the kettle; liquid feed would then surge in and flash again. Our process group determined there was excess surface and plugged off some tubes. The kettle operation smoothed out and reactor efficiency improved. Excess surface problems are cured by plugging tubes in the inlet channel. There are many different types of plugs, but metal plugs with a slight taper are most common. Unless the temperatures are high, wooden plugs can be used in a pinch.

4. Venting.

Proper venting is a startup necessity. Improper venting usually occurs on startup and is recognized by poor heat transfer and a high pressure drop. Exchangers operating under a vacuum can be more ofa problem than those operating under pressure. The vacuum will suck air into the exchanger ifit isn't perfectly sealed. Vents should be located at the exchanger's highest points. The shellside is especially vulnerable to pockets of air or noncondensables. Gas can get trapped at the bundle~s top or by "ears" at the top of baffles. Ifa venting problem is suspected, talk to operations about their startup procedures. recommends startup procedures. YokelF has a more complete discussion ofvents, especially vertical fixed tube sheet exchangers.

5. Field mistakes. In one instance, a heat exchanger was piped up backwards. The fluid that should have been on the shells ide was piped to the channel side and vice versa. When both streams are in turbulent flow, this switch may go unnoticed. In this case, fluid that shouM have been in the shell was semiviscous. On the shellside, the fluid would have been turbulent and given better heat transfer. When on the tubeside, the fluid flowed in the transition region between turbulent and viscous. This gave a noticeably lower heat transfer, although better heat transfer than calculated.
