Brazed Aluminium Plate Fin Heat Exchangers-Construction,Uses Advantages in Cryogenic Refrigeration Systems

Paper 44a

BRAZED ALUMINUM PLATE FIN HEAT EXCHANGERS � CONSTRUCTION, USES, AND ADVANTAGES IN CRYOGENIC

REFRIGERATION SYSTEMS

Dan Markussen

Principal Sales Engineer

Chart Industries

Larry Lewis

CEO

SME Associates, Inc.

Prepared for Presentation at the 2005 AIChE Spring National Meeting

17th Annual Ethylene Producers� Conference

Atlanta, GA, April 10-14, 2005

Session TA011 Ethylene Plant Technology � Refrigeration Systems

April 13, 2005

AIChE shall not be responsible for statements or opinions contained in papers or printed in its publications.

BRAZED ALUMINUM PLATE FIN HEAT EXCHANGERS � CONSTRUCTION, USES, AND ADVANTAGES IN CRYOGENIC REFRIGERATION SYSTEMS

Dan Markussen, Chart Industries and Larry Lewis, SME Associates Abstract

This paper will provide an overview of the components of a brazed aluminum heat exchanger (BAHX) and how Chart Industries designs, manufacturers, and tests them. An overview is provided of the range of uses for BAHX in cryogenic process refrigeration systems with an emphasis on debottlenecking ethylene production facilities by optimizing their refrigeration systems.

Discussion is presented on the uses of BAHX and the advantages that they provide the designer of refrigeration systems and the owners of cryogenic refining facilities. Introduction

The continued emphasis on increasing plant efficiency driven by the increased costs of energy and feedstocks have led plant designers and owners to demand more from their heat exchangers. The required gains in efficiency and throughput targeted by a debottlenecking project can be achieved through the use of the highly efficient and flexible design of brazed aluminum heat exchangers, (BAHX.)

Over the past several decades, the use of BAHX has become quite prevalent in cryogenic refrigeration systems. Applications such as petrochemical processing, industrial gas processing, production of LNG and other hydrocarbon processing covers the general spectrum in which BAHX are commonly used. Heat Exchanger Design and Construction Today�s heat exchangers for typical industrial processing applications generally fall under two broad categories:

• Tubular type Heat Exchangers • Plate Type Heat Exchangers Brazed Aluminum Heat Exchangers (Compact Heat

Exchangers)

Shell-and-tube tubular type heat exchangers remain the most widely used units for heat transfer in process applications. For this type of heat exchanger, tubes are mechanically attached to tube sheets, which are enclosed inside a shell with ports for inlet and outlet fluid or gas. Shell-and-tube exchangers are versatile and robust, servicing applications requiring processing of dirty, corrosive fluids, where thermal performance and compactness are not important considerations.

Historically, shell-and-tube exchangers have proven to be cost effective when the application contains warm, corrosive or dirty process fluids. While this arrangement offered serviceable performance over time, issues of durability, limited application range and high horsepower requirements tended to increase operating costs and limit system reliability.

Brazed aluminum plate fin heat exchangers, a specialty subset of Compact Plate Heat

Exchangers, have become the most widely used heat exchanger in the cryogenic industry.

They offer the benefits of a compact design that is typically one-fifth the size of comparable carbon or stainless steel shell and tube heat exchanger and will weigh approximately 10 percent of the weight. BAHX smaller size is attributable to the high density of the heat transfer fins and the thermal conductivity of the aluminum material. Typical fin heights range from 0.200 to 0.380� and vary from 8 fins per inch to 25 fins per inch. Most of the surface is secondary surface with the parting sheet being the primary surface. The thermal conductivity of aluminum is very high resulting in fin efficiencies that are typically more than 80 percent. The heat transfer fin configuration is designed to create an optimum balance between pressure drop and heat transfer of the stream. Other benefits inherent in the aluminum plate fin design include close temperature approaches and a unique ability to configure multiple streams. The multiple streams can be different types of fluids, such as gas-to-gas, gas-to-liquid, two phase or any combination, with each stream balanced and optimized both thermally and hydraulically. Whereas a typical shell and tube heat exchanger is limited to two streams, brazed aluminum heat exchangers can be designed and configured in parallel or in series to create multi stream capabilities with up to 15 different streams.

Figure 1 shows the major components of a typical brazed aluminum fin heat exchanger.

Brazed aluminum heat exchangers are built by stacking layers of corrugated fins separated by parting sheets and sealed along the edges with side bars.

Figure 1

The matrix assembly is brazed in a large vacuum furnace to form an integral, rigid heat exchanger block. Exchanger block sizes can be in excess of 4�W x 5�D x 25�H. Headers and supports are welded onto the brazed matrix to complete the unit.

A wide range of fin patterns [plain, perforated, herringbone and serrated] accommodate

different thermal and hydraulic process requirements enabling the exchanger to be custom designed for an infinite number of processes. (See Figure 2)

Figure 2

Manufacturing

Brazed aluminum heat exchangers are manufactured as an all brazed and welded pressure vessel with no mechanical joints. The heat exchanger consists of a core block constructed of alternating layers of corrugated sheets, (fins), and flat parting sheets. Each layer is bound by bars and provided with inlet and outlet distributors. The core matrix is produced by vacuum brazing; that is joining, the fins, bars and parting sheets at high temperature in a clean, vacuum environment. Brazed aluminum exchangers, due to their inherently high surface area compactness, possess superior heat transfer capabilities and can be cost effective for non-corrosive gases and liquids as compared with traditional shell-and-tube exchangers. Historically, this type of heat exchanger has been used in cryogenic applications where small approach temperatures are important.

Codes

The design, construction, manufacturing and testing of brazed aluminum plate-fin heat exchangers are governed by recognized international codes that apply to pressure vessels. ASME Section VIII, Division I code stamp, PED, or other recognized international codes are normally applied. Associated piping is normally designed and manufactured in accordance with the ASME B31.3 Piping Code. Supplier Considerations

Consideration should be given only to exchangers that meet one of the recognized international design codes, and adhere to strict quality control procedures, including:

! Hydrostatic and pneumatic proof testing ! Performance flow testing ! External and vacuum helium leak detection ! Dye penetrant and radiographic testing

Ideally, a total commitment to quality and reliability should be manifested through

full accreditation to EN ISO9001.

Finally, customers and/or users of brazed aluminum exchangers should only

consider suppliers that conform to the standards of ALPEMA (Brazed Aluminum Plate-Fin Heat Exchanger Manufacturers� Association). ALPEMA standards provide consistent quality and conformance guidelines for the design and manufacture of brazed aluminum exchangers. More information regarding ALPEMA may be obtained from www.alpema.org Applications In addition to design, construction, quality and performance factors, application matching heat exchanger performance lies in the ability to match the application to the appropriate technology as specifically as possible. (see Figure 3) Correct decisions will help to ensure low-cost, effective operation over the long term.

As stated earlier, shell-and-tube heat exchangers are versatile, all-around products intended for warm, corrosive or dirty applications where thermal performance is not highly valued. Conversely, brazed aluminum heat exchangers are ideal for cold/cryogenic; clean process streams where thermal performance, close temperature approaches, compactness and low weight are important plant considerations. Key applications include industrial gas production, natural gas processing, refinery and petrochemical processing, hydrogen and helium liquefaction and recovery, and offshore platform or FPSO processing.

A variation of the brazed aluminum fin (BAHX) heat exchanger is the Core-In-Kettle®

Heat Exchanger. This approach consists of an aluminum plate fin heat exchanger assembled inside of a vessel. The aluminum plate fin heat exchanger assembly replaces the traditional tube bundle found in shell and tube kettle exchangers and other heat exchanger designs.

Core-in-Kettle designs are limited to clean fluids typical of hydrocarbon processing, natural gas processing and liquefaction. For petrochemical applications that produce ethylene, Core-in-Kettle, shell-and-tube and run back condensers are all suitable options.

Oftentimes, picking the right heat exchanger for the job is largely a process-driven task. In a refinement scenario, shell-and-tube exchangers are a natural fit for plant locations where warm conditions exist at the front end. As the product becomes more refined and the plant becomes colder, brazed aluminum exchangers then become an integral part of the heat transfer processing chain.

Figure 3 Cost is another consideration. While brazed aluminum offers many benefits over its

tubular counterparts, a smaller or more fundamental application may benefit from an equally utilitarian shell-and-tube heat exchanger or exchanger system. Moreover, as the availability and affordability of energy used to operate plants varies globally, the energy-efficient benefits of brazed aluminum heat exchangers may not dictate its use in certain applications.

Mercury Contamination Longtime fears about mercury�s corrosive effects on aluminum have fueled some reluctance to embrace brazed aluminum technology. In reality, mercury will only react with certain alloys of aluminum and then only when liquid mercury is allowed to exist in contact with the heat exchanger and there is water present.

Mercury removal systems upstream of the exchanger are commonly installed so that brazed aluminum plate-fin heat exchangers may be used with streams containing mercury. Operators may also use special shutdown procedures to restrict moisture and maintain temperatures below 100° C. In addition to mercury removal systems, equipment from Chart Industries can be designed to be mercury tolerant. Chart�s proprietary mercury-tolerant designs allow corrosive mercury to be present in the aluminum exchanger without causing the exchanger to fail. Benefits of The Core-In Kettle Heat Exchanger Technology Core-in-Kettle designs can possess up to ten times more heat transfer surface than traditional tube designs, providing numerous operational and performance advantages: Close temperature approaches can be accommodated generally to 1 degree C with resultant savings in power and operating costs. The Core-in-Kettle�s smaller vessel design creates cost savings though lower liquid inventory, reduction in insulation, smaller foot-print and associated space costs, And, savings in foundation and support costs as Core-in-Kettle designs are typically one-fifth the weight of tube and shell designs. Brazed aluminum construction eliminates mechanical joints and large diameter flanges that could leak. (See Figure 4)

Figure 4

Core-In Kettle Applications Core-in-Kettle heat exchangers are - ideal - for lowering initial and operating costs. In a

Core-in Kettle exchanger, a brazed aluminum core is immersed in a bath of refrigerant and extremely high UA/in3 are achieved due to the refrigerant path open flow area being quite large. The reduced approach temperatures lower refrigerant compression horsepower requirements and save money. The most common applications include C2 splitter condensers, C3 splitter heat pumps, C4 splitter heat pumps, and refrigeration condensers and evaporators. (See Figure 5)

Figure 5

Run Back Condensers

Run Back Condensers are high efficiency exchangers for the partial condensation of a range of fluids. Their sophisticated design accommodates heat and mass transfer effects while avoiding flooding. A special fin design is the heart of the refluxing stream delivering the optimum combination of free-flow area and hydraulic mean diameter. (See Figure 6)

Figure 6

Runback condensers are used extensively in ethylene recovery. Other applications include Helium recovery from natural gas, hydrogen purification, condensing argon, CO2 purification, and ammonia purge gas separation.

Benefits of Run Back Condensers include liquid runback to the tower which eliminates pumps, the elimination of several tower trays, and by shifting the refrigeration requirement for the tower to a warmer level � reducing tower cost and improving efficiency. Cold Boxes

A cold box - in terms of BAHXs - is a steel structure enclosed, (See Figure 7), with thin-gauged steel panel walls. A cold box can typically contain and support one or more BAHXs, process vessels, knock out drums, and associated interconnecting piping. Cold boxes can employ the use of specially designed control valves that allow access to the valve stems and the valve internals from the outside of the box. The piping penetrations through the box are typically free floating and insulated - pipe anchor points remain at the heat exchangers and vessels. Liquid level control lines, flow meters, and thermocouples with explosion proof junction boxes can also be included in the design and scope of supply in order to assist plant operators with monitoring plant performance. Cold boxes are typically filled with perlite insulation and provided with nitrogen purging to keep the interior components cold and safe. The compactness and ease of insulation of cryogenic process equipment makes cold boxes ideal for use in ethylene plants. Typically used in the coldest part of an ethylene plant, a cold box is usually supplied to perform the hydrogen purification requirements. Hydrogen purification typically is performed at temperatures well below -200°F and requires the use of two or more brazed aluminum exchangers, two process vessels, knock out drums, and cryogenic control valves. Although it is possible to mechanically insulate these components and its piping system, it is usually much more practical and economical to package the hydrogen purification system in a cold box.

Figure 7

Example of Heat Exchangers and Ethylene Extraction

In ethylene plants, brazed aluminum exchangers offer the advantage of pinch technology optimizing the refrigeration systems by tight approach temperatures and reducing refrigeration horsepower. Since brazed aluminum exchangers can accommodate as many as 15 streams, complex refrigeration schemes can be accomplished in services such as the ethylene recovery/purification train and the H2 purification unit.

As an example of the advantages of tighter approach temperature on refrigeration horsepower, we offer an example of a 12 MM BTU/hr Ethylene condenser (load) operating at �146.3 F @ 19.5 psia. Figure 8 shows a cascade refrigeration system utilizing tubular exchangers with 10 F approach temperatures. A simple cycle with 2 propylene refrigeration loops and one ethylene refrigeration loop was selected for the example. The tubular exchanger example requires 9,730 HP, excluding utility HP. This equates to a Coefficient of Performance (COP) of 0.484 BTU refrigeration/BTU power.

Figure 8

Figure 9

The same 12 MM BTU/hr Ethylene condenser load and cascade refrigeration system as

shown in Figure 9 was then simulated with BAHX exchangers achieving a 3 F approach temperature. The BAHX exchangers required only 8179 HP with a COP of 0.567. Reducing the approach temperatures from 10 F to 3 F resulted in a 16% compression HP Reduction. Composite cooling curves of both systems are shown in Figure 10 and Figure 11).

Figure 10

Figure 11

Further power reductions can be achieved by economizing the refrigeration loop and by increasing the number of loops. Conclusion

By reducing approach temperatures and by taking advantage of complex refrigeration circuitry available with brazed aluminum and other compact exchangers, plants such as ethylene purification units can reduce refrigeration horsepower or debottleneck existing systems without adding compression.

Sidebar FAQs

Although the use of brazed aluminum heat exchangers is fairly wide spread and well accepted as practical use in cryogenic processing industries, there remain many questions that are often unknown to the casual user of BAHX due to their special design and manufacturing process. The following issues are just a few that perhaps this paper has not covered or needs to be reiterated due to them often being addressed in industry.

1. How are BAHXs tested? a. A BAHX is first structurally tested. For hydrostatic test methods, each stream is

pressurized, with the other streams at zero pressure, to 1.3 times the design pressure per the requirements of ASME. The unit remains pressurized for 5-10 minutes. Pneumatic test methods can be used with the pressure level being adjusted to 1.1-time design pressure.

b. Leak testing is performed after the structural test has been successfully performed. Internal leak testing is performed by pressurizing each stream individually and monitoring small nozzle valves on the unpressurized streams using soap film solution. External leak testing is performed using soap solution on the exchanger external joints.

c. Helium vacuum leak testing may also be used to validate the leak tightness of an exchanger, usually at an extra expense.

2. Are the exchangers repairable if a leak is detected after the exchanger has been put in service?

a. Exchangers are repairable - to what extent depends on the type of leak and where it is located. Leaks develop from a number of sources - freeze failure, corrosion, and thermal fatigue are the most common. When a leak has been detected, a manufacturer of BAHX or qualified field repair company specializing in BAHX repairs should be called to consult and possible perform the repair. Repairs can include welding shut the leak or plugging of the passage that is leaking.

3. What are the temperature and pressure limits of BAHX? a. Temperature and pressure limits vary from supplier to supplier, however

generally most of the world leaders in the production of BAHX, members of ALPEMA (Aluminum Plate fin heat Exchanger Manufacturer�s Association) are capable of pressures from full vacuum to 1400 psi. Most manufacturers are capable of exceeding this pressure limit but the size of the exchanger and to what extent varies greatly between the different suppliers. Charts capability exceeds 2000 psig.

b. Temperature limits are dictated by ASME code and range from -450°F to 400°F. However, practically speaking, it is not desirable to exceed 150°F, especially at elevated pressure (in excess of 200 psi).

4. What aluminum alloys are used and what determines their selection? a. Economics, availability, and code limitations dictate the type of alloy used.

Generally speaking, 3003 alloy is used in the core block (consisting of parting

sheets, side/end bars, and fins) and 5083 alloy is used in the header and nozzle components. 6061 alloy is usually the alloy of choice for flanged connections.

5. Type of stainless steel to aluminum coupling preferred? a. For mechanical couplings, a RFWN flange is often used. For non-mechanical

connections there are a variety of suppliers of stainless to aluminum transition joints. It is best to consult with the BAHX supplier as to what supplier is recommended. Usually the BAHX supplier has experience with each of the suppliers and is capable of making a qualified recommendation as to which supplier(s) are adequate.

6. What process information is required for specification? a. Contact a BAHX supplier, they can provide a blank data sheet to be filled in.

Stream inlet and outlet temperatures and pressures are needed, as are either the physical properties at the inlet/outlet or the stream chemical composition. Chart prefers to receive a customer�s HYSYS file for the specified exchanger since Chart has a program which converts the HYSYS output to Chart�s design program input.

7. Installation concerns? Piping stress on nozzles? a. Consult with the supplier�s installation and operating manual. Usually the

supplier will have specified methods of lifting and installing the exchanger. The ALPEMA suppliers will perform thermal expansion/contraction mechanical calculations in order to provide adequate piping strength.

8. Commissioning requirements, start up and cool down rates

a. There are various considerations required for start-up and shutdown which are covered in the ALPEMA standards. Typically these considerations have to do with limiting thermal stresses imposed on the heat exchanger. Visit Chart�s web site for a PDF of the latest version of the ALPEMA standards (www.chart-ind.com).

9. What are the storage conditions? a. See the ALPEMA standards - www.chart-ind.com. General practice is to keep an

exchanger to be stored for any length of time in a clean, dry environment away from equipment that could potentially damage the exchanger. BAHX�s can be shipped with a nitrogen blanket and gages with valves to monitor the pressure. Damage in storage or shipping can be detected if pressure has been reduced or eliminated from the nitrogen blanket

10. When is upstream filtration required? Can blockages be removed? a. Due to the nature of the internal construction of BAHXs it is recommended that

filtration almost always be employed. Restrictions can be removed but can be difficult at times to eliminate. Consult with the ALPEMA specifications for the options of eliminating blockages (back puffing and solvent washing are most typical).

11. What fin types are available? What are their selection criteria? How are new fin designs developed and tested?

a. Fin types come in a wide variety from most of the ALPEMA suppliers. They range in height, thickness, configuration (straight, perforated, herringbone, and serrated), and fin count (fins per inch).

b. Fins are selected on the basis of required design pressure, thermal performance, and allowable pressure drop. The optimum fin is one that does all of these things at the lowest cost.

12. Corrosion and mercury features a. Chart is the developer and world leader in providing exchangers that are suitable

to perform in a mercury environment. Due to the variety of conditions for the exchanger to operate in with mercury present and the proprietary nature of Chart�s design, Chart recommends that a potential customer speak to us about their individual needs. Please note, Chart has/is presenting a paper about this subject at this conference.

About Chart Industries Chart Industries, Inc. is a leading global supplier of standard and custom engineered products and systems serving a wide variety of low temperature and cryogenic applications. Headquartered in Cleveland, Ohio, Chart has domestic operations located in seven states and an international presence in Australia, China, Czech Republic, Germany and the United Kingdom. For more information, contact Chart Industries Energy & Chemicals Group at 888-794-7608 or www.chart-ind.com

BRAZED ALUMINUM PLATE FIN HEAT EXCHANGERS –CONSTRUCTION, USES, AND ADVANTAGES IN CRYOGENIC REFRIGERATION SYSTEMS paper 44a

Presented by:Larry Lewis, SME AssociatesCo-authored by Dan Markussen, Chart Industries

2

BAHX’s are Compact, High Performance Designs

• Approx. 10% the weight of CS or SS tubular exchangers• Approx. 20% of the volume of CS or SS tubular exchangers• Single and multiphase, condensing or evaporating service• Complex designs with up to 15 streams are common• Single core modules can exceed 6 MM BTU/hr-F “UA”• Approach or pinch temperature of <1o F

3

BAHX’s offer a large degree of Design Freedom

• Design pressures above 2000 psig• Design Temperatures from –450F to +400F• Up to 15 streams in a single core• Single cores can exceed 4’ x 5’ x 25’• Double and Triple core Modules are common• Multi-core piped batteries are common • ASME Sec. VIII, Div 1, PED, and Other

Recognized International Codes apply• Can be designed as Mercury Toleranttm

4

Major Components of a BAHX

Brazed aluminum heat exchangers are built by stacking layers of corrugated fins separated by parting sheets and sealed along the edges with side bars. The matrix assembly is brazed in a large vacuum furnace to form an integral heat exchanger core. Exchanger core sizes can be in excess of 4’W x 5’D x 25’H, Headers and supports are welded onto the brazed matrix to complete the unit.

5

Wide Range of Fins Available

A wide range of fin patterns [plain, perforated, herringbone and serrated] accommodate different thermal and hydraulic process requirements enabling the exchanger to be custom designed for an infinite number of processes

6

Core-in-Kettle tm Designs

A three module core for a Core-in-Kettle tm

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Core-in-Kettle tm Designs

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Runback Condenser DesignsRun back condensers are high efficiency exchangers for partial condensation of tower overheads . Their design accommodates heat and mass transfer effects while avoiding flooding. A special fin design is the heart of the refluxing stream delivering the optimum combination of free-flow area and hydraulic mean diameter.

9

Cold Boxes

Typical H2 purification cold box for ethylene service

10

Typical Cold Box

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Cascade refrigeration system utilizing tubular exchangers w/ 10O F approach

12

BAHX cascade refrigeration system with 3O F approach

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BAHX cascade Refrig. System with 3 F approach

The BAHX exchangers

required only 8179 HP. Reducing the

approach temperatures from 10 F to 3 F resulted

in a 16% compression HP

Reduction.

14

By reducing approach temperatures and by taking advantage of complex refrigeration circuitry available with brazed aluminum and other compact exchangers, plants such as ethylene purification units can reduce refrigeration horsepower or debottleneckexisting systems without adding compression.

Conclusion

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Typical Designs

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Frequently Asked Questions1. When is upstream filtration required? 2. Can blockages be removed? 3. Are there commissioning requirements, start up and cool down

rates?4. What is the fin selection criteria? 5. What are Mercury Tolerant tm features?6. What are allowable piping stresses on nozzles?7. What process information is required for specification?8. How are aluminum exchangers coupled to CS and SS piping?9. Are the exchangers field repairable if a leak is detected?10.How are BAHXs tested?Please refer to written paper and Q&A session if your question is not specifically addressed