Approach to Heat-Exchanger Design

The proper use of basic heat-transfer knowledge in the design of practical heat-transfer equipment is an art. Designers must be constantly aware of the differences between the idealized conditions for and under which the basic knowledge was obtained and the real conditions of the mechanical expression of their design and its environment. The result must satisfy process and operational requirements (such as availability, flexibility, and maintainability) and do so economically. An important part of any design process is to consider and offset the consequences of error in the basic knowledge, in its subsequent incorporation into a design method, in the translation of design into equipment, or in the operation of the equipment and the process. Heat-exchanger design is not a highly accurate art under the best of conditions.

Evaporator Types and Applications

Evaporators are often classified as follows:

1. heating medium separated from evaporating liquid by tubular heating surfaces,
2. heating medium confined by coils, jackets, double walls, flat plates, etc.,
3. heating medium brought into direct contact with evaporating liquid, and
4. heating with solar radiation.

Evaporators with tubular heating surfaces dominate the field. Circulation of the liquid past the surface may be induced by boiling (natural circulation) or by mechanical methods (forced circulation). In forced circulation, boiling may or may not occur on the heating surface

Solar evaporators require tremendous land areas and a relatively cheap raw material, since pond leakage may be appreciable. Solar evaporation generally is feasible only for the evaporation of natural brines, and then only when the water vapor is evaporated into the atmosphere and is not recovered.

Evaporators may be operated batchwise or continuously. Most evaporator systems are designed for continuous operation. Batch operation is sometimes employed when small amounts must be evaporated. Batch operation generally requires more energy than continuous operation.

Batch evaporators, strictly speaking, are operated such that filling, evaporating, and emptying are consecutive steps. This method of evaporation requires that the body be large enough to hold the entire charge of the feed and the heating element be placed low enough not to be uncovered when the volume is reduced to that of the product. Batch operation may be used for small systems, for products that require large residence times, or for products that are difficult to handle.

A more frequent method of operation is semibatch in which feed is continuously added to maintain a constant liquid level until the entire charge reaches the final concentration. Continuous-batch evaporators usually have a continuous feed, and over at least part of the cycle, a continuous discharge. One method of operation is to circulate from a storage tank to the evaporator and back until the entire tank is at a specified concentration and then finish the evaporation in batches.

Continuous evaporators have continuous feed and discharge. Concentrations of both feed and discharge remain constant during operation.

Evaporators may be operated either as once-through units or the liquid may be recirculated through the heating element. In once-through operation all the evaporation is accomplished in a single pass. The ratio of evaporation to feed is limited in single-pass operation; single-pass evaporators are well adapted to multiple-effect operation permitting the total concentration of the liquid to be achieved over several effects. Agitated-film evaporators are also frequently operated once through. Once-through evaporators are also frequently required when handling heat-sensitive materials.

Recirculated systems require that a pool of liquid be held within the equipment. Feed mixes with the pooled liquid and the mixture circulates across the heating element. Only part of the liquid is vaporized in each pass across the heating element; unevaporated liquid is returned to the pool. All the liquor in the pool is therefore at the maximum concentration. Recirculated systems are therefore not well suited for evaporating heat sensitive materials. Recirculated evaporators, however, can operate over a wide range of concentration and are well adapted to single-effect evaporation.

There is no single type of evaporator which is satisfactory for all conditions. It is for this reason that there are many varied types and designs. Several factors determine the application of a particular type for a specific evaporation result. The following sections will describe the various types of evaporators in use today and will discuss applications for which each design is best adapted.

Physical Properties in Evaporators

To calculate heat-transfer rates, physical-property data for the fluids being treated must be available. Physical property data should be as accurate as possible, especially as more accurate heat-transfer correlations become available. However, most physical properties of mixtures must be calculated or estimated; consequently there is little need to attempt to determine true film temperatures. Physical-property data at the average bulk fluid temperature are generally sufficient.

The following physical properties are usually required in order to obtain satisfactory calculated heat transfer rates:

1. viscosity
2. thermal conductivity
3. density
4. specific heat
5. latent heat
6. surface tension

When condensing or vaporizing over a temperature range, a curve representing heat load as a function of temperature should also be available. In addition, any concentration effects should be known.

Fouling Factors

When a heat exchanger is placed in service, the heat transfer surfaces are, presumably, clean. With time, in some services in the power and process industries, the apparatus may undergo a decline in its ability to transfer heat. This is due to the accumulation ofheat insulating substances on either or both ofthe heat transfer surfaces. The Tubular Exchanger Manufacturers’ Association (TEMA) undertook the establishment ofstandards defining design practices not covered by the ASME Code for Unfired Pressure Vessels. Because the ASME code is concerned primarily with safe pressure containment and the means for inspecting for it during construction, the contribution ofTEMA to sound mechanical construction has been substantial.

In addition, TEMA published a table of fouling factors to assist the designer in preventing the fouling ofa single item in a process, including several items ofheat transfer equipment. Resistances were tabulated which were to be added to the film resistances (1/Sihi and 1/hoSo) ofspecific process streams so that the operating period ofeach would be similar and assure some desired period ofcontinuous operation. The tables off ouling factors were intended as a crude guide toward the equalizations of cumulative fouling in all fouling streams in the assembly.

The fouling factors published by TEMA became entrenched in industrial heat exchanger design. Fouling factors, by the TEMA definition, are time dependent. They are not present when the apparatus is placed on stream; yet at some definite time in the future, when the apparatus has lost some of its heat transfer capabilities, the fouling factor is deemed to have arrived. TEMA does not delineate the in-between fouling process, and the fouling factor has shed little light on the nature of fouling. Significant is the fact that an item of equipment that failed to comply with the TEMA notion of a desired period ofcontinuous operation became a fouling problem. Within the scope of the definition of a fouling factor, the only means for ameliorating fouling was to employ larger fouling factors for repetitive services.

The entire concept ofthe fouling factor is somewhat indefinite. It is an unsteadystate effect that is added indiscriminately to steady-state heat transfer resistances. The difference between a clean and a fouled exchanger is that an intolerable portion of the available temperature difference between fluids must be used to overcome fouling. Thus, ifthe outside surface So ofa pipe or tube is the reference and rdo is the fouling or dirt factor.

Fouling Mechanisms

Somerscales and Knudsen (1981) have identified six categories off fouling:

1. Particulate fouling. The accumulation ofsolid particles is suspended in the process stream on the heat transfer surfaces. Typical examples include dust deposition, particles carried in condenser cooling water, and unburned fuel, or fly ash. If the solid deposition is due to gravity, the process is referred to as sedimentation fouling.
2. Precipitation fouling. Dissolved substances carried in the process stream are precipitated on the heat transfer surfaces. Examples include carbonates, sulfates, and carbonates. Scaling occurs when precipitation occurs on heated rather than cooled surfaces.
3. Chemical reaction fouling. In certain cases, deposits on the heat transfer surfaces which are not, in themselves, reactants are formed by chemical reactions. In this type off ouling, cracking and coking ofhydrocarbons and polymerization are typical examples.
4. Corrosion fouling. In this type of fouling, the heat transfer surface reacts, at certain pH levels, to produce products that adhere to the heat transfer surfaces, and in turn, this may promote the attachment of additional fouling materials. Sulfur in fuel oil and sulfur products in the flue gas, such as sulfur dioxide, can lead to sulfuric acid. This has caused, for example, significant damage to heat exchange surfaces in air heaters in the power industry.
5. Biological fouling. Materials such as algae, bacteria, molds, seaweed, and barnacles carried in the process stream cause biological fouling of the heat transfer surfaces. A prime example of biological fouling is in marine power plant condensers.
6. Freezing fouling. In this type off ouling, a liquid, or some ofits higher-eltingpoint components will deposit on a subcooled heat transfer surface.

Regenerator : Introduction

The regenerator represents a class ofheat exchangers in which heat is alternately stored and removed from a surface. This heat transfer surface is usually referred to as the matrix ofthe regenerator. For continuous operation, the matrix must be moved into and out ofthe fixed hot and cold fluid streams. In this case, the regenerator is called a rotary regenerator. If, on the other hand, the hot and cold fluid streams are switched into and out ofthe matrix, the regenerator is referred to as a fixed matrix regenerator. In both cases the regenerator suffers from leakage and fluid entrainment problems, which must be considered during the design process.

An example ofa rotary regenerator is shown in Fig. below. This is the Lungstrom air preheater used in power plants to warm the incoming combustion air using the exhaust or flue gases from the steam generator.

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A Lungstrom air preheater.

Classification of Compact Heat Exchangers

Compact heat exchangers may be classified by the kinds of compact elements that they employ. The compact elements usually fall into five classes:

1. Circular and flattened circular tubes. These are the simplest form of compact heat exchanger surface. The designation ST indicates flow inside straight tubes (example: ST-1), FT indicates flow inside straight flattened tubes (example: FT-1) and FTD indicates flow inside straight flattened dimpled tubes. Dimpling interrupts the boundary layer, which tends to increase the heat transfer coefficient without increasing the flow velocity.
2. Tubular surfaces. These are arrays oftubes ofsmall diameter, from 0.9535 cm down to 0.635 cm, used in service where the ruggedness and cleanability ofthe conventional shell-and-tube exchanger are not required. Usually, tubesheets are comparatively thin, and soldering or brazing a tube to a tubesheet provides an adequate seal against interleakage and differential thermal expansion.
3. Surfaces with flow normal to banks of smooth tubes. Unlike the radial low fin tubes, smooth round tubes are expanded into fins that can accept a number oftube rows, as shown in Fig. 11.16a. Holes may be stamped in the fin with a drawn hub or foot to improve contact resistance or as a spacer between successive fins, as shown, or brazed directly to the fin with or without a hub. Other types reduce the flow resistance outside the tubes by using flattened tubes and brazing, as indicated in Fig b and c below. Flat tubing is made from strips similar to the manufacture of welded circular tubing but is much thinner and is joined by soldering or brazing rather than welding. The designation considers staggered (S) and in-line (I) arrangements oftubes and identifies transverse and longitudinal pitch ratios. The suffix (s) indicates data correlation from steady-state tests. All other data were correlated from a transient technique. Examples include the surface S1.50-1.25(s), which is a staggered arrangement with data obtained via steady-state tests with transverse pitch-to-diameter ratio of1.50 and longitudinal pitch-to-diameter ratio of1.25. The surface I1.25-1.25 has an inline arrangement with data obtained from transient tests with both transverse and longitudinal pitch-to-diameter ratios of1.25.
4. Plate fin surfaces. These are shown in Figs. d through i below.
5. Finned-tube surfaces. Circular tubes with spiral radial fins are designated by the letters CF followed by one or two numerals. The first numeral designates the number offins per inch, and the second (ifone is used) refers to the nominal tube size. With circular tubes with continuous fins, no letter prefix is employed and the two numerals have the same meaning as those used for circular tubes with spiral radial ins. For finned flat tubes, no letter prefix is used; the first numeral indicates the fins per inch and the second numeral indicates the largest tube dimension. When CF does not appear in the designation ofthe circular tube with spiral radial fins, the surface may be presumed to have continuous fins.
6. Matrix surfaces. These are surfaces that are used in rotating, regenerative equipment such as combustion flue gas–air preheaters for conventional fossil furnaces. In this application, metal is deployed for its ability to absorb heat with minimal fluid friction while exposed to hot flue gas and to give up this heat to incoming cold combustion air when it is rotated into the incoming cold airstream. No designation is employed.

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Some compact heat exchanger elements

Compact Heat Exchanger

One variation ofthe fundamental compact exchanger element, the core, is shown in Fig below. The core consists ofa pair ofparallel plates with connecting metal members that are bonded to the plates. The arrangement of plates and bonded members provides both a fluid-flow channel and prime and extended surface. It is observed that ifa plane were drawn midway between the two plates, each halfofthe connecting metal members could be considered as longitudinal fins.

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Exploded view ofa compact heat exchanger core: 1, plates; 2, side bars; 3, corrugated fins stamped from a continuous strip of metal. By spraying braze powder on the plates, the entire assembly ofplates, fins, and bars can be thermally bonded in a single furnace operation.

Two or more identical cores can be connected by separation or splitter plates, and this arrangement is called a stack or sandwich. Heat can enter a stack through either or both end plates. However, the heat is removed from the successive separating plates and fins by a fluid flowing in parallel through the entire network with a single average convection heat transfer coefficient. For this reason, the stack may be treated as a finned passage rather than a fluid–fluid heat exchanger, and, ofcourse, due consideration must be given to the fact that as more and more fins are placed in a core, the equivalent or hydraulic diameter ofthe core is lowered while the pressure loss is increased significantly.

Next, consider a pair ofcores arranged as components ofa two-fluid exchanger in crossflow as shown in Fig. 11.15. Fluids enter alternate cores from separate headers at right angles to each other and leave through separate headers at opposite ends of the exchanger. The separation plate spacing need not be the same for both fluids, nor need the cores for both fluids contain the same numbers or kinds of fins. These are dictated by the allowable pressure drops for both fluids and the resulting heat transfer coefficients. When one coefficient is quite large compared with the other, it is entirely permissible to have no extended surface in the alternate cores through which the fluid with the higher coefficient travels. An exchanger built up with plates and fins as in Fig. below is a plate fin heat exchanger.

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Two-fluid compact heat exchanger with headers removed.

The discussion ofplate fin exchangers has concentrated thus far on geometries involving two or more fluids that enter the body ofthe compact heat exchanger by means ofheaders. In many instances, one ofthe fluids may be merely air, which is used as a cooling medium on a once-through basis. Typical examples include the air-fin cooler and the radiators associated with various types ofinternal combustion engines. Similarly, there are examples in which the compact heat exchanger is a coil that is inserted into a duct, as in air-conditioning applications.

Shell and Tube Heat Exchangers : Construction

Shell-and-tube heat exchangers are fabricated with round tubes mounted in cylindrical shells with their axes coaxial with the shell axis. The differences between the many variations ofthis basic type ofheat exchanger lie mainly in their construction features and the provisions made for handling differential thermal expansion between tubes and shell.

Awidely accepted standard is published by the Tubular Exchanger Manufacturers’ Association (TEMA). This standard is intended to supplement the ASME as well as other boiler and pressure vessel codes. The TEMA (1998) standard was prepared by a committee comprising representatives of27 U.S. manufacturing companies, and their combined expertise and experience provide exchangers ofhigh integrity at reasonable cost. TEMA provides a standard designation system that is summarized in and six examples ofthe shell-and-tube heat exchanger arrangements are shown in Fig. below.

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TEMA standard designation system for shell-and-tube heat exchangers. (From Saunders, 1988, with permission.)

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(a) Single-tube-pass baffled single-pass-shell shell-and-tube heat exchanger designed to give essentially counterflow conditions. The toroidal expansion joint in the center ofthe shell accommodates differential thermal expansion between the tubes and the shell. (b) U-tube single-pass-shell shell-and-tube heat exchanger. (c) Two-pass baffled single passshell shell-and-tube heat exchanger. (d) Heat exchanger similar to that of( c) except for the floating head used to accommodate differential thermal expansion between the tubes and the shell. (e) Heat exchanger that is similar to the heat exchanger in (d) but with a different type offloating head. (f) Single-tube-pass baffled single-pass-shell shell-and-tube heat exchanger with a packed joint floating head and double header sheets to assure that no fluid leaks from one fluid circuit into the other. (Courtesy ofthe Patterson-Kelley Co. and reproduced from Fraas, 1989, with permission.)

Heat Exchanger : Introduction

A heat exchanger can be defined as any device that transfers heat from one fluid toanother or from or to a fluid and the environment. Whereas in direct contact heat exchangers, there is no intervening surface between fluids, in indirect contact heat exchangers, the customary definition pertains to a device that is employed in the transfer of heat between two fluids or between a surface and a fluid. Heat exchangers may be classified (Shah, 1981, or Mayinger, 1988) according to (1) transfer processes,(2) number offluids, (3) construction, (4) heat transfer mechanisms, (5) surface compactness, (6) flow arrangement, (7) number offluid passes, and (8) type ofsurf ace.

Recuperators are direct-transfer heat exchangers in which heat transfer occurs between two fluid streams at different temperature levels in a space that is separated by a thin solid wall (a parting sheet or tube wall). Heat is transferred by convection from the hot (hotter) fluid to the wall surface and by convection from the wall surface to the cold (cooler) fluid. The recuperator is a surface heat exchanger.

Regenerators are heat exchangers in which a hot fluid and a cold fluid flow alternately through the same surface at prescribed time intervals. The surface of the regenerator receives heat by convection from the hot fluid and then releases it by convection to the cold fluid. The process is transient; that is, the temperature of the surface (and of the fluids themselves) varies with time during the heating and cooling of the common surface. The regenerator is a also surface heat exchanger.

Design Applications of Double Pipe Heat Exchangers

One benefit of the hairpin exchanger is its ability to handle high tubeside pressures at a lower cost than other removable-bundle exchangers. This is due in part to the lack of pass partitions at the tubesheets which complicate the gasketing design process. Present mechanical design technology has allowed the building of dependable, removable bundle, hairpin multitubes at tubeside pressures of 825 bar (12,000 psi).

The best known use of the hairpin is its operation in true countercurrent flow which yields the most efficient design for processes that have a close temperature approach or temperature cross. However, maintaining countercurrent flow in a tubular heat exchanger usually implies one tube pass for each shell pass. As recently as 30 years ago, the lack of inexpensive, multiple-tube pass capability often diluted the
advantages gained from countercurrent flow.

The early attempts to solve this problem led to investigations into the area of heat transfer augmentation. This familiarity with augmentation techniques inevitably led to improvements in the efficiency and capacity of the small heat exchangers. The result has been the application of the hairpin heat exchanger to the solution of unique process problems, such as dependable, once-through, convective boilers offering high-exit qualities, especially in cases of process-temperature crosses

Principles of Construction Double Pipe Heat Exchangers

Hairpin heat exchangers (often also referred to as “double pipes”) are characterized by a construction form which imparts a U-shaped appearance to the heat exchanger. In its classical sense, the term double pipe refers to a heat exchanger consisting of a pipe within a pipe, usually of a straight-leg construction with no bends. However, due to the need for removable bundle construction and the ability to handle differential thermal expansion while avoiding the use of expansion joints (often the weak point of the exchanger), the current U-shaped configuration has become the standard in the industry. A further departure from the classical definition comes when more than one pipe or tube is used to make a tube bundle, complete with tubesheets and tube supports similar to the TEMA style exchanger.

Hairpin heat exchangers consist of two shell assemblies housing a common set of tubes and interconnected by a return-bend cover referred to as the bonnet. The shell is supported by means of bracket assemblies designed to cradle both shells simultaneously. These brackets are configured to permit the modular assembly of many hairpin sections into an exchanger bank for inexpensive future-expansion capability and for providing the very long thermal lengths demanded by special process applications.

The bracket construction permits support of the exchanger without fixing the supports to the shell. This provides for thermal movement of the shells within the brackets and prevents the transfer of thermal stresses into the process piping. In special cases the brackets may be welded to the shell. However, this is usually avoided due to the resulting loss of flexibility in field installation and equipment reuse at other sites and an increase in piping stresses.

The hairpin heat exchanger, unlike the removable bundle TEMA styles, is designed for bundle insertion and removal from the return end rather than the tubesheet end. This is accomplished by means of removable split rings which slide into grooves machined around the outside of each tubesheet and lock the tubesheets to the external closure flanges. This provides a distinct advantage in maintenance since bundle removal takes place at the exchanger end furthest from the plant process piping without disturbing any gasketed joints of this piping.

Approach to Heat Exchanger Design

The proper use of basic heat-transfer knowledge in the design of practical heat-transfer equipment is an art. Designers must be constantly aware of the differences between the idealized conditions for and under which the basic knowledge was obtained and the real conditions of the mechanical expression of their design and its environment. The result must satisfy process and operational requirements (such as availability, flexibility, and maintainability) and do so economically. An important part of any design process is to consider and offset the consequences of error in the basic knowledge, in its subsequent incorporation into a design method, in the translation of design into equipment, or in the operation of the equipment and the process. Heat-exchanger design is not a highly accurate art under the best of conditions.

The design of a process heat exchanger usually proceeds through the following steps:

1. Process conditions (stream compositions, flow rates, temperatures, pressures) must be specified.
   
   
2. Required physical properties over the temperature and pressure ranges of interest must be obtained.
   
   
3. The type of heat exchanger to be employed is chosen.
   
   
4. A preliminary estimate of the size of the exchanger is made, using a heat-transfer coefficient appropriate to the fluids, the process, and the equipment.
   
   
5. A first design is chosen, complete in all details necessary to carry out the design calculations.
   
   
6. The design chosen in step 5 is evaluated, or rated, as to its ability to meet the process specifications with respect to both heat transfer and pressure drop.
   
   
7. On the basis of the result of step 6, a new configuration is chosen if necessary and step 6 is repeated. If the first design was inadequate to meet the required heat load, it is usually necessary to increase the size of the exchanger while still remaining within specified or feasible limits of pressure drop, tube length, shell diameter, etc. This will sometimes mean going to multiple-exchanger configurations. If the first design more than meets heat-load requirements or does not use all the allowable pressure drop, a less expensive exchanger can usually be designed to fulfill process requirements.
   
   
8. The final design should meet process requirements (within reasonable expectations of error) at lowest cost. The lowest cost should include operation and maintenance costs and credit for ability to meet long-term process changes, as well as installed (capital) cost. Exchangers should not be selected entirely on a lowest-first-cost basis, which frequently results in future penalties.