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Heat Exchangers CLASSIFICATION ACCORDING TO TRANSFER PROCESS DIRECT CONTACT TYPE INDIRECT CONTACT TYPE DIRECT TRANSFER TYPE STORAGE TYPE CLASSIFICATION ACCORDING TO SURFACE COMPACTNESS COMPACT (SURFACE AREA DENSITY >=700 m2 / m3) NON-COMPACT (SURFACE AREA DENSITY < 700 m2 /m3) HEAT EXCHANGER CLASSIFICATION ACCORDING TO CONSTRUCTION TUBULAR PLATE EXTENDED SURFACE REGENERATIVE GASKETED SPIRAL LAMELLA PLATE-FIN TUBE-FIN ROTORY FIXED-MATRIX DOUBLE-PIPE SHELL-AND-TUBE SPIRAL TUBE PLATE BAFFLE DISK-TYPE PLATE-FIN TUBE-FIN ROTORY DRUM TYPE HEAT EXCHANGER CLASSIFICATION ACCORDING TO FLOW ARRANGEMENTS SINGLE PASS MULTI PASS PARALLEL FLOW COUNTER FLOW CROSS FLOW EXTENDED SURFACE H.E. EXTENDED SURFACE H.E. CROSS CROSS COUNTER FLOW PARALLEL FLOW PARALLEL COUNTER FLOW SHELL AND MIXED SHELL-AND-TUBE IN TUBE PASSES SPLIT-FLOW DIVIDED-FLOW IN-PARALLEL PLATE MULTI-PASS CLASSIFICATION ACCORDING TO NUMBER OF FLUIDS TWO-FLUID THREE-FLUID N-FLUID (N>=3) CLASSIFICATION ACCORDING TO HEAT TRANSFER MECHANISM/FLOW ARRANGEMENTS SINGLE-PHASE CONNECTION ON BOTH SIDES SINGLE-PHASE CONNECTION ON ONE-SIDE, TWO-PHASE CONNECTION ON OTHER SIDE TWO-PHASE CONNECTION ON BOTH SIDES AND RADIATIVE HEAT OR COMBINED CONVECTION Figure 1 Classification of heat exchangers [1]. Chapter 1 ti T2 T t1 1, (0m) Figure 2 Double pipe heat exchanger. (a) Single pass with counterflow; and (b) multipass with counterflow [6]. The coiled tube heat exchanger offers unique advantages, especially when dealing with low-temperature applications for the following cases [9]: 1. Simultaneous heat transfer between more than two streams is desired. 2. A large number of heat-transfer units is required. 3. High operating pressures are involved. The coiled tube heat exchanger is not cheap because of the material costs, high labor input in winding the tubes, and the central mandrel, which is not useful for heat transfer but increases the shell diameter [5]. Glass Coil Heat Exchangers. Glass coil exchangers have a coil fused to the shell to make a one-piece unit. This prohibits leakage between the coil and shell-side fluids [10]. A glass coil heat exchanger is shown schematically in Fig. 3. More details on glass heat exchangers are furnished in Chapter 13. Plate Heat Exchangers Plate heat exchangers are less widely used than tubular heat exchangers but offer certain important advantages. Plate heat exchangers can be classified in three principal groups: 1. Plate and frame or gasketed plate heat exchangers used as an alternative to tube and shell exchangers for low- and medium-pressure liquid-liquid heat-transfer applications. 2. Spiral heat exchanger used as an alternative to shell and tube exchangers where low maintenance is required, particularly with fluids tending to sludge or containing slurries or solids in suspension. Chapter 1 2 and construction refer to Shah [1,2], Gupta [3], and Graham Walker [4]. For classification and systematic procedure for selection of heat exchangers, refer to Larowski et al. [5a,5b]. 3.1 Classification According to Construction According to constructional details, heat exchangers are classified as [1]: 1. Tubular heat exchangers—double pipe, shell and tube, coiled tube 2. Plate heat exchangers—gasketed, spiral, plate coil, lamella 3. Extended surface heat exchangers—tube-fin, plate-fin 4. Regenerators—fixed matrix, rotary Tubular Heat Exchanger Double Pipe Exchangers. A double pipe heat exchanger has two concentric pipes, usually in the form of a U-bend design as shown in Fig. 2. The flow arrangement is pure countercurrent. A number of double pipe heat exchangers can be connected in series or parallel as necessary. Their usual application is for small duties requiring, typically, less than 300 ft2 and they are suitable for high pressures and temperatures, and thermally long duties [5]. This has the advantages of flexibility since units can be added or removed as required, and the design is easy to service and requires low inventory of spares because of its standardization. Either longitudinal fins or circumferential fins within the annulus on the inner pipe wall are required to enhance the heat transfer from the inner pipe fluid to the annulus fluid. Design pressures and temperatures are broadly similar to shell and tube heat exchangers. The design is straightforward. and carried out using the method of Kern [6], or proprietary programs. Shell and Tube Heat Exchanger. In process industries, shell and tube exchangers are used in great numbers, far more than any other type of exchanger. More than 90% of heat exchangers used in industry are of the shell and tube type [7]. The shell and tube heat exchangers are the “work horses” of industrial process and heat transfer [8]. They are the first choice because of well-established procedures for design and manufacture from a wide variety of materials, many years of satisfactory service, and availability of codes and standards for design and fabrication. They are produced in the widest variety of sizes and styles. There is virtually no limit on the operating temperature and pressure. Coiled Tube Heat Exchanger Coiled Tube Heat Exchanger Used for Liquefaction Systems. One of the three classical heat exchangers used today for large-scale liquefaction systems is the coiled tube heat exchanger (CThe). The construction details are explained in Refs. 5 and 9. Construction of these heat exchangers involves winding a large number of small-bore ductile tubes in helix fashion around a central core tube, with each exchanger containing many layers of tubes along both the principal and radial axes. Tubes in individual layers or groups of layers may be brought together into one or more tube plates through which different fluids may be passed in counterflow to the single shellside fluid. The high-pressure stream flows through the small-diameter tubes, while the low-pressure return stream flows across the outside of the small-diameter tubes in the annular space between the inner central core tube and the outer shell. Pressure drops in the coiled tubes are equalyzed for each high-pressure stream by using tubes of equal length and varying the spacing of these in the different layers. Because of small-bore tubes on both sides, CTHES do not permit mechanical cleaning and therefore are used to handle clean, solid-free fluids or fluids whose fouling deposits can be cleaned by chemicals. Materials are usually aluminum alloys for cryogenics, and stainless steels for high-temperature applications. HEAT EXCHANGER DESIGN HANDBOOK T. KUPPAN Southern Railway Madras, India MARCEL DEKKER, INC. NEW YORK • BASEL ISBN: 0-8247-9787-6 This book is printed on acid-free paper. Headquarters Marcel Dekker, Inc. 270 Madison Avenue, New York, NY 10016 tel: 212-696-9000; fax: 212-685-4540 Eastern Hemisphere Distribution Marcel Dekker AG Hutgasse 4, Postfach 812, CH-4001 Basel, Switzerland tel: 41-61-261-8482; fax: 41-61-261-8896 World Wide Web http://www.dekker.com The publisher offers discounts on this book when ordered in bulk quantities. For more information, write to Special Sales/Professional Marketing at the headquarters address above. Copyright © 2000 by Marcel Dekker, Inc. All Rights Reserved. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. Current printing (last digit): 10 9 8 7 6 5 4 3 2 1 PRINTED IN THE UNITED STATES OF AMERICA To my parents Thulukkanam Senthamarai To my mentor Dr. Ramesh K. Shah Preface 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 tech- nical literature is available on heat exchangers, but it is widely scattered throughout the techni- cal journals, industrial bulletins, codes and standards, etc. This book is intended to consolidate into one volume the basic concepts of design and theoretical relationships useful in the design of heat exchangers, material selection, fabrication, and industrial practices. Thermal design information such as heat transfer and pressure drop data, thermal design methods, and flow-induced vibration for exchangers involving single-phase flow duties are discussed. Other books and handbooks are available that deal with the design of vaporizers and condensers in which there is two-phase flow. The book is an excellent resource for mechanical, chemical, and petrochemical engineers; process equipment and pressure vessel designers; and upper-level undergraduate and graduate students in these disciplines. The book is divided into 15 chapters covering most of the information required for selec- tion, design, material selection, fabrication, inspection, and operation of heat exchangers. Chapter 1 discusses the classification and selection of heat exchangers for the intended application. In addition to the principal types used in industry. such as compact, shell and tube exchangers, regenerators, and plate heat exchangers, other specialized types such as double pipe, heat pipe, spiral, lamella, jacketing, glass, graphite, and Teflon heat exchangers are also discussed. To successfully carry out the thermal design of heat exchangers, knowledge of thermo- hydraulic fundamentals is necessary. The discussion of thermal resistance variables, overall conductance equations, temperature distribution, mean temperature difference, temperature cor- rection factors, number of transfer units, and effectiveness formulas for various flow arrange- ments, pass arrangements, and compact and shell heat exchangers with thermal relation charts is presented in Chapter 2. Heat exchanger design methodology, heat exchanger design featuring rating and sizing vi Preface problems, computer-aided thermal design, pressure drop analysis, temperature-dependent fluid properties correction, performance failures, flow maldistribution, and uncertainties involved in thermal design are covered in Chapter 3. Compact heat exchangers are used in a wide variety of applications. The need for light- weight, space-saving, and economical heat exchangers has driven the development of compact surfaces. Basic construction types, surface geometrical parameters, j and f factors, fin effi- ciency, rating, and sizing are discussed in Chapter 4. Air coolers that use atmospheric air as the coolant are widely used in industry. They are also discussed in detail in Chapter 4. Shell and tube sheet exchangers are the workhorses of process industries. The major con- struction features, thermal design, sizing, and rating are shown in detail in Chapter 5. Thermal design procedures for disk and doughnut and rod baffle heat exchangers are also discussed. The drastic escalation of energy prices has made waste heat recovery more attractive over the past two decades. Recovery of waste heat from flue gas by means of heat exchangers can improve overall plant efficiency, serves to reduce national energy needs, and conserves fossil fuels. The objective of Chapter 6 is to acquaint the reader with various types of regenerators and with their construction details, their applications other than for heat recovery, and their thermal, and mechanical design. Additionally, some industrial regenerators for waste heat re- covery are discussed. In the 1930s, plate heat exchangers (PHEs) were introduced to meet the hygienic demands of the dairy industry. Today PHEs are universally used in many fields. They are used as an alternative to tube and shell exchangers for low- and medium-pressure liquid–liquid heat trans- fer applications. Design of PHEs, recent developments in their construction, and spiral plate heat exchangers are covered in Chapter 7. In recent years, increasing energy and material costs have provided significant incentives for the development of various augmented heat transfer surfaces and devices. Various forms of enhancement devices are discussed in Chapter 8. Most heat transfer processes result in the deposition of undesirable materials, commonly referred to as fouling. Fouling introduces perhaps the major uncertainty into the design and operation of heat exchangers, very often leading to extra capital and running costs, and reduces thermal performance. This necessitates a thorough understanding of the fouling phenomenon. Fouling mechanisms, prevention, and control are reviewed in Chapter 9. One of the major considerations in the design of shell and tube heat exchangers is that it is free from flow-induced vibration problems. Flow-induced vibration can cause potential tube failures. Chapter 10 presents a review of flow-induced vibration mechanisms, their evaluation, and vibration prevention guidelines. Chapter 11, on the mechanical design of shell and tube heat exchangers, deals with mini- mum thickness calculation procedures and stress analysis of various pressure parts such as tubesheets, heads, end closures, flanges, expansion joints, and nonpressure parts. Tubesheet design, as per ASME code, TEMA, BS 5500, and CODAP, is explained in detail. Important details of heat exchanger and pressure vessel construction codes and standards are also covered in detail. Metallic corrosion is a process that causes enormous material losses annually. Thus it is necessary to examine thoroughly the material and environmental interactions that adversely affect the performance and life of equipment. Chapter 12 discusses corrosion principles, vari- ous forms of corrosion and their evaluation, corrosion control and prevention, and monitoring. With few exceptions, water is the preferred industrial medium for removing heat from process fluids. An understanding of cooling-water corrosion is important for heat exchanger designers. Most problems associated with cooling water are identified, and their control and prevention are also discussed. vii Preface Proper material selection is important for desired thermal performance, strength considera- tions, safe operation, and achieving the expected life and economy. Thus it is necessary to have a thorough knowledge of various heat exchanger materials and their fabricability. Chapter 13 discusses the selection criteria for a wide spectrum of heat exchanger materials and their fabrication by welding. Quality of goods and equipment manufactured in the world market has become a matter of concern in recent years. For heat exchangers and pressure vessels, the overriding goal is to avoid the consequences of failure, which can be catastrophic in human, monetary, and environ- mental terms. Chapter 14 discusses various aspects of quality control and quality assurance, inspection, and nondestructive testing methods (NDT) and recent trends in NDT techniques. After thermal design, the heat exchanger unit is fabricated by shop floor operations. Be- yond the theoretical background, a knowledge of shop floor practices is required for a manufac- turer to efficiently achieve the desired quality and performance. Chapter 15 discusses various shop floor practices for shell and tube heat exchangers, and brazing and soldering of compact heat exchangers. The preparation of this book was facilitated by the great volume of existing literature contributed by many workers and scholars in this field. I have tried to acknowledge all the sources and have sought the necessary permissions. If omissions have been made, I offer my sincere apologies. Most materials manufacturers and research organizations responded to my inquiries and supplied substantial useful data and informative material. They are all acknowl- edged either directly or through references. Last, I make a special mention of my mentor and guide Dr. R. K. Shah, Vice President, ASME Board on Communications, Delphi Harrison Thermal Systems, Lockport, New York, who helped me throughout the preparation of the book and provided the basic literature required for the thermal design, flow-induced vibration, and thermal relations formulas for all types of arrangements. During the preparation of this book, my parents and family were deprived of much of my time and interest that they rightfully deserve. I apologize to them. T. Kuppan Contents Preface v 1. Heat Exchangers—Introduction, Classification, and Selection 1 2. Heat Exchanger Thermohydraulic Fundamentals 27 3. Heat Exchanger Thermal Design 129 4. Compact Heat Exchangers 159 5. Shell and Tube Heat Exchanger Design 229 6. Regenerators 303 7. Plate Heat Exchangers and Spiral Plate Heat Exchangers 347 8. Heat-Transfer Augmentation 373 9. Fouling 393 10. Flow-Induced Vibration of Shell and Tube Heat Exchangers 423 11. Mechanical Design of Shell and Tube Heat Exchangers 485 12. Corrosion 579 ix x Contents 13. Material Selection and Fabrication 667 14. Quality Control and Quality Assurance, Inspection, and Nondestructive Testing 863 15. Heat Exchanger Fabrication 955 References 1035 Index 1093 1 Heat Exchangers—Introduction, Classification, and Selection 1 INTRODUCTION A heat exchanger is a heat-transfer devise that is used for transfer of internal thermal energy between two or more fluids available at different temperatures. In most heat exchangers, the fluids are separated by a heat-transfer surface, and ideally they do not mix. Heat exchangers are used in the process, power, petroleum, transportation, air conditioning, refrigeration, cryogenic, heat recovery, alternate fuels, and other industries. Common examples of heat exchangers familiar to us in day-to-day use are automobile radiators, condensers, evaporators, air preheaters, and oil coolers. Heat exchangers could be classified in many different ways. 2 CONSTRUCTION OF HEAT EXCHANGERS A heat exchanger consists of heat-exchanging elements such as a core or matrix containing the heat-transfer surface, and fluid distribution elements such as headers or tanks, inlet and outlet nozzles or pipes, etc. Usually, there are no moving parts in the heat exchanger; however, there are exceptions, such as a rotary regenerator in which the matrix is driven to rotate at some design speed. The heat-transfer surface is in direct contact with fluids through which heat is transferred by conduction. The portion of the surface that separates the fluids is referred to as the primary or direct contact surface. To increase heat-transfer area, secondary surfaces known as fins may be attached to the primary surface. 3 CLASSIFICATION OF 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 (shown in Fig. 1) are briefly discussed here. For more details on heat exchanger classification 1 Heat Exchangers Figure 3 Glass coil heat exchanger [10]. 3. Plate coil or panel heat exchangers made from embossed plates to form a conduit or coil for liquids coupled with fins. Plate Heat Exchangers. A plate heat exchanger (PHE) essentially consists of a number of corrugated metal plates in mutual contact, each plate having four apertures serving as inlet and outlet ports, and seals designed to direct the fluids in alternate flow passages. The plates are clamped together in a frame that includes connections for the fluids. Since each plate is generally provided with peripheral gaskets to provide sealing arrangements, the plate heat exchangers are called gasketed plate heat exchangers. A plate heat exchanger is shown in Fig. 4. The PHE is covered in detail in Chapter 7, Plate Heat Exchangers. Spiral Plate Heat Exchanger. Spiral plate heat exchangers (SPHEs) have been used since the 1930s, when they were originally developed in Sweden for heat recovery in pulp mills. Spiral plate heat exchangers are classified as a type of welded plate heat exchanger. An SPHE is fabricated by rolling a pair of relatively long strips of plate around a split mandrel to form a pair of spiral passages. Channel spacing is maintained uniformly along the length of the spiral passages by means of spacer studs welded to the plate strips prior to rolling. An SPHE is shown in Fig. 5. For most applications, both flow channels are closed by alternate channels welded at both sides of the spiral plate. In some services, one of the channels is left open, whereas the other closed at both sides of the plate. These two types of construction prevent the fluids from mixing. The SPHE is intended especially for these applications [5]: 1. To handle slurries and liquids with suspended fibers, and mineral ore treatment where the solid content is up to 50%. 2. The SPHE is the first choice for extremely high viscosities, say up to 500,000 cp, especially in cooling duties, because of maldistribution, and hence partial blockage by local overcooling is less likely to occur in a single-channel exchanger. 3. SPHEs are finding applications in reboiling, condensing, heating or cooling of viscous fluids, slurries, and sludge [11]. Chapter 1 Figure 4 Plate heat exchanger. [From Hydrocarbon Processing, p. 123, (1996).] More details on the SPHE are furnished in Chapter 7, Plate Heat Exchangers. Panel Coil Heat Exchanger. These exchangers are called panel coils, plate coils, or embossed-panel or jacketing. The panel coil serves as a heat sink or a heat source, depending upon whether the fluid within the coil is being cooled or heated. Panel coil heat exchangers are relatively inexpensive and can be made into any desired shapes and thickness for heat sinks and heat sources under varied operating conditions. Hence, they have been used in many industrial applications such as cryogenics, chemicals, fibers, food, paints, pharmaceuticals, and solar absorbers. Construction Details of a Panel Coil. Two types of panel coil designs are shown in Fig. 6. The panel coil is used in such industries as plating, metal finishing, chemical, textile, brewery, pharmaceutical, dairy, pulp and paper, food, nuclear, beverage, waste treatment, and many others. Construction details of panel coils are discussed next. The text has been provided by M/s. Paul-Muller Company, Springfield, MO. 1. Single embossed surface. The single embossed heat-transfer surface (Fig. 7a) is an economical type to utilize for interior tank walls, conveyor beds, and when a flat side is required. The single embossed design uses two sheets of material of different thickness and is available in stainless steel, other alloys, and carbon steel, and in many material gauges and working pressures. 2. Double embossed surface. Inflated on both sides using two sheets of material and the same thickness, the double embossed construction (Fig. 7b) maximizes the heating and cooling process by utilizing both sides of the heat-transfer plate. The double embossed design is commonly used in immersion applications, and they are available in stainless steel, other alloys, and carbon steel, and in many material gauges and working pressures. 3. Dimpled surface. Dimpled on one side surface is shown in Fig. 7c. This surface is machine punched and swaged, prior to welding, to increase the flow area in the passages. It is available in stainless steel, other alloys, and carbon steel, in many material gauges and working pressures, and is available in both MIG plug welded and resistance spot welded forms. Heat Exchangers Figure 5 Alfa-Laval spiral plate heat exchanger. Figure 6 Panel coil heat exchanger. 8 Chapter 1 Double Embossed (a) Single Embossed (b) Dimpled (c) Figure 7 Embossing pattern of Paul-Muller panel coils. (a) double embossed surface; (b) single embossed surface; and (c) dimpled surface. Methods of Manufacture of Panel Coils. Basically, three different methods have been employed to manufacture the panel coils: (1) They are usually welded by the resistance spot welding process (RSW) and/or the resistance seam welding process (RSEW). An alternate method now available offers the ability to resistance spot weld the dimpled jacket style panel coil with a perimeter weldment made with the GMAW process (Fig. 8). Other methods are (2) the die-stamping process and (3) the roll-bond process. In the die-stamping process, flow channels are die stamped on either one or two metal sheets. When one sheet is embossed and joined to a flat (unembossed sheet), it forms a single-sided embossed panel coil. When both sheets are stamped, it forms a double-sided embossed panel coil. Types of Jackets. Jacketing of process vessels is usually accomplished by using one of lted jacket style template. the three main available types (Fig. 9): conventional jackets, dimple jackets, and half-pipe coil jackets [12]. Heat Exchangers 9 (a) (b) (c) Figure 9 Jacketing construction details. (a) Conventional; (b) dimple; and (c) half-pipe [12]. Advantages of Panel Coils. Panel coils provide the optimum method of heating and cooling process vessels in terms of control, efficiency, and product quality. Using a panel as a means of heat transfer offers many advantages [12]: All liquids can be handled, as well as steam and other high-temperature vapors. Circulation, temperature, and velocity of heat transfer media can be accurately controlled. Panels may often be fabricated from a much less expensive metal than the vessel itself. Contamination, cleaning, and maintenance problems are eliminated. Maximum efficiency, economy, and flexibility are achieved. In designing reactors for specific process, this variety gives the chemical engineer a great deal of flexibility in the choice of heat-transfer medium. Lamella Heat Exchanger. The lamella is a form of welded heat exchanger that combines the construction of a plate heat exchanger with that of a shell and tube exchanger. In this design, tubes are replaced by pairs of thin flat parallel metal plates, which are edge welded to provide long narrow channels, and banks of these elements of varying width are packed together to form a circular bundle and fitted within a shell. The cross section of a lamella heat exchanger is shown schematically in Fig. 10. With this design, the flow area on the shellside is a minimum Figure 10 Cross section of an Alfa-Laval lamella heat exchanger. 10 Chapter 1 and similar in magnitude to that of the inside of the bank of elements; due to this, the velocities of the two liquid media are comparable [13]. The flow is essentially longitudinal countercurrent "tubeside" flow of both tube and shell fluids [4]. Due to this, the velocities of the two liquid media are comparable. One end of the element pack is fixed and the other is floating to allow for thermal expansion and contraction. The connections fitted at either end of the shell, as in the normal shell and tube design, allow the bank of elements to be withdrawn, making the outside surface accessible. Lamella heat exchangers can be fabricated from carbon steel, stainless steel, titanium, Incoloy, and Hastelloy. They can handle most fluids, with large volume ratios between fluids. The floating nature of the bundle usually limits the working pressure to 300 psi. Lamella heat exchangers are generally less versatile than either PHFEs or shell and tube exchangers but are cheaper than the latter for a given duty [5]. Design is usually left to the vendors. Extended Surface Exchangers In a heat exchanger with gases or some liquids, if the heat-transfer coefficient is quite low, a large heat-transfer surface area is required to increase the heat-transfer rate. This requirement is served by fins attached to the primary surface. Tube-fin and plate-fin geometries (Fig. 11) are the most common examples for extended surface heat exchanger. Their design is covered in Chapter 4, in the section on compact heat exchanger design. Regenerative Heat Exchangers Regeneration is an old technology dating back to the first open hearths and blast furnace stoves. Manufacturing and process industries such as glass, cement, and primary and secondary metals account for a significant fraction of all energy consumed. Much of this energy is discarded in the form of high-temperature exhaust gas. Recovery of waste heat from the exhaust gas by means of heat exchangers known as regenerators can improve the overall plant efficiency. Types of Regenerators. Regenerators are generally classified as fixed-matrix and rotary regenerators. Further classifications of fixed and rotary regenerators are shown in Fig. 12. In the former the regeneration is achieved with periodic and alternate blowing of hot and the cold stream through a fixed matrix. During the hot flow period, the matrix receives thermal energy from the hot gas and transfers it to the cold stream during the cold stream flow. In the latter, the matrix revolves slowly with respect to two fluid streams. The rotary regenerator is commonly employed in gas turbine power plants where the waste heat in the hot exhaust gases is (a) (b) Figure 11 Extended surface heat exchanger. (a) Tube-fin; (b) plate-fin. Heat Exchangers Regenerator Fixed Matrix Rotary Regenerator Single bed Dual bed Disc type Drum valved (thermal type wheel) Figure 12 Classification of regenerators. utilized for raising the temperature of compressed air before it is supplied to the combustion chamber. A rotary regenerator is shown in Fig. 13 [14] and one type of rotary regenerator is shown in Fig. 14. Rotary regenerators fall in the category of compact heat exchangers since the heat-transfer surface area to regenerator volume ratio is very high. 2 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. 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 ex- changer 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 bed type are discussed in Chapter 6, Regenerators. 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 recupera- tor. 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. The discussion of cooling towers and scrubbers is not within the scope of this book. 2 Classification According to Surface Compactness Compact heat exchangers are important when there are restrictions on the size and weight of exchangers. A compact heat exchanger incorporates a heat-transfer surface having a high area density, P, somewhat arbitrarily 700 m'!m' (200 ft'/ft') and higher [1]. The area density, P, is the ratio of heat transfer area A to its volume V. A compact heat exchanger employs a compact surface on one or more sides of a two-fluid or a multifluid heat exchanger. They can often achieve higher thermal effectiveness than shell and tube exchangers (95% vs. the 60-80% typical for shell and tube heat exchangers), which makes them particularly useful in energy- intensive industries [15]. For least capital cost, the size of the unit should be minimal. There are additional advantages to small volume. Some of these are: 1. Small inventory, making them good for handling expensive or hazardous materials [15] 2. Low weight 3. Easier transport 4. Less foundation 5. Better temperature control Some barriers to the use of compact heat exchangers include [15]: 1. The lack of standards similar to pressure vessel codes and standards, although this is now being redressed in the areas of plate-fin exchangers [16] and air-cooled exchangers [17]. 2. Narrow passages in plate-fin exchangers make them susceptible for fouling and they can- not be cleaned by mechanical means. This limits their use to clean applications like han- dling air, light hydrocarbons, and refrigerants. Chapter 1 1 Rotating heat exchanger matrix Circumferential Radial seals Cool gas Rotary regenerator [14]. . . Rotating Figure 13 3 Classification According to Flow Arrangement The basic flow arrangements of the fluids in a heat exchanger are 1. Parallel flow 2. Counterflow 3. Crossflow The choice of a particular flow arrangement is dependent upon the required exchanger effec- tiveness, fluid flow paths, packaging envelope, allowable thermal stresses, temperature levels, and other design criteria. These basic flow arrangements are discussed next. 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 (Fig. 15). Fluid temperature variations, idealized as one-dimensional, are shown in Fig. 16. This arrangement has the lowest exchanger effective- ness among the single-pass exchangers for the same flow rates, capacity rate (mass x specific 1 Coursy " Figure 14 Rothemuhle regenerative air heater. Main parts: 1, stationary matrix; 2, revolving hoods. (Courtesy of Babcock and Wilcox Company.) 14 Chapter 1 Figure 15 Parallel flow arrangement. 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 [2]: Since this flow pattern produces a more uniform longitudinal tube wall temperature distri- bution 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). It is preferred when there is a possibility that the temperature of the warmer fluid may reach its freezing point. It provides early initiation of nucleate boiling for boiling applications. For a balanced exchanger (i.e., heat capacity rate ratio C* = 1), the desired exchanger effectiveness is low and is to be maintained approximately constant over a range of NTU values. The application allows piping only suited to parallel flow. Counterflow Exchanger In this type, as shown in Fig. 17, the two fluids flow parallel to each other but in opposite directions, and its temperature distribution may be idealized as one-dimensional (Fig. 18). 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, counter- flow 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 [2]. Crossflow Exchanger In this type, as shown in Fig. 19, the two fluids flow normal to each other. Important types of flow arrangement combinations for a single-pass crossflow exchanger include: Both fluids unmixed One fluid unmixed and the other fluid mixed Both fluids mixed A 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 aver- aging process takes place at each cross section across the full width of the flow passage. A 15 Heat Exchangers t th,i c th,i c tl t ci, ex co t c co 0 Cb < Cc Flow length Flow length Ch = Cc Flow length cj,i too th,o co th,o t ci,o ci tc,i Flow length Flow length ci th,o tc,o ci ti,o c th,i th,i th,i Figure 16 Temperature distribution for parallel flow arrangement. 16 Chapter 1 Figure 17 Counterflow arrangement. 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 pas- sages) 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 side and 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-dimen- sional only for the inlet and outlet sections, and this is shown in Fig. 20. The thermal effective- ness 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 heat exchangers, because it greatly simplifies the header design. If the desired heat exchanger effec- tiveness is generally more than 80%, the size penalty for crossflow may become excessive. In such a case, a counterflow unit is preferred [2]. In shell and tube exchangers, crossflow arrange- ment is used in the TEMA X shell having a single tube pass. 3.5 Classification According to Pass Arrangements These are either single pass or multipass. A fluid is considered to have made one pass if it flows through a section of the heat exchanger through its full length once. In a multipass arrangement, a fluid is reversed and flows through the flow length two or more times. Multipass Exchangers When the design of a heat exchanger results in either extreme length, significantly low veloci- ties, or low effectiveness, or due to other design criteria, a multipass heat exchanger or several single pass-exchangers in series or a combination of both is employed. Specifically, multipass- ing is resorted to increase the exchanger thermal effectiveness over the individual pass effec- tiveness. As the number of passes increases, the overall direction of the two fluids approaches that of a pure counterflow exchanger. The multipass arrangements are possible with compact, shell and tube, and plate exchangers. 3.6 Classification According to Phase of Fluids Gas–Liquid Gas–liquid heat exchangers are mostly tube-fin type compact heat exchangers with the liquid on the tubeside. The radiator is by far the major type of liquid–gas heat exchanger, typically cooling the engine jacket water by air. Similar units are necessary for all the other water-