Heat Exchangers Introduction, Types and Applications

1. Introduction

A heat exchanger is a device that is used for transfer of thermal energy (enthalpy)between two or more fluids, between a solid surface and a fluid, or between solidparticulates and a fluid, at differing temperatures and in thermal contact, usually withoutexternal heat and work interactions. The fluids may be single compounds or mixtures.Typical applications involve heating or cooling of a fluid stream of concern, evaporation orcondensation of a single or multicomponent fluid stream, and heat recovery or heatrejection from a system. In other applications, the objective may be to sterilize, pasteurize,fractionate, distill, concentrate, crystallize, or control process fluid. In some heatexchangers, the fluids exchanging heat are in direct contact. In other heat exchangers, heattransfer between fluids takes place through a separating wall or into and out of a wall in atransient manner.

In most heat exchangers, the fluids are separated by a heat transfer surface, andideally they do not mix. Such exchangers are referred to as the direct transfer type, orsimply recuperators. In contrast, exchangers in which there is an intermittent heatexchange between the hot and cold fluids via thermal energy storage and rejection throughthe exchanger surface or matrix—are referred to as the indirect transfer type or storagetype, or simply regenerators. Such exchangers usually have leakage and fluid carryover fromone stream to the other.

Heat exchangers may be classified according to transfer process, construction, flowarrangement, surface compactness, number of fluids and heat transfer mechanisms oraccording to process functions.

1.Shell. 8.FloatingHeadFlange. 15.TransverseBaffles.2.FloatingHeadFlange. 9.ChannelPartition 16.ImpingementBaffle.3.ShellChannel. 10.StationaryTubeSheet. 17.VentConnection.4.ShellCoverEndFlange. 11.Channel. 18.DrainConnection.5.ShellNozzle. 12.ChannelCover. 19.TestConnection.6.FloatingTubeSheet. 13.ChannelNozzles. 20.SupportSaddles.7.FloatingHead. 14.TieRodsandSpacers 21.LiftingRing.

2. Shell and Tube Heat Exchangers

2.1. Overview

Shell-and-tube heat exchangers are fabricated with round tubes mounted incylindrical shells with their axes coaxial with the shell axis. The differences between themany variations of this basic type of heat exchanger lie mainly in their construction featuresand the provisions made for handling differential thermal expansion between tubes andshell [1].

There are various design considerations to be taken into account such as routing offluids (shell or tube), pressure drop especially in the case of increasing number of bafflesand tube diameter and adjusting the area with the suitability of the exchanger to conductthe heat required to heat or cool a fluid with another one.

2.2. Illustration

Figure 1, shell-and-tube heat exchanger with baffles [2].

2.3. Applications

They are extensively used as process heat exchangers in the petroleum-refining andchemical industries; as steam generators, condensers, boiler feed water heaters and oilcoolers in power plants; as condensers and evaporators in some air-conditioning andrefrigeration applications; in waste heat recovery applications with heat recovery fromliquids and condensing fluids; and in environmental control.

Figure 2, actual footage of a tube bundle.

Figure 3, actual footage of baffle arrangement.

3. Double Pipe Heat Exchangers

3.1. Overview

A typical double-pipe heat exchanger is shown in Figure 4. Essentially, it consists ofone pipe placed concentrically inside another one of larger diameter, with appropriate endfittings on each pipe to guide the fluids from one section to the next. The inner pipe mayhave external longitudinal fins welded to it either internally or externally to increase theheat transfer area for the fluid with the lower heat transfer coefficient. The double-pipesections can be connected in various series or parallel arrangements for either fluid to meetpressure-drop limitations and LMTD requirements [3].

3.2. Illustration

Figure 4, double pipe heat exchanger (one hair-pin) [1].

Figure 5, actual footage of 7 hair-pins arrangement.

1.Plates. 2.SideBars.3.Corrugatedfinsstampedfromastripofmetal.

3.3. Applications

The major use of double-pipe exchangers is for sensible heating or cooling of theprocess fluid where small heat transfer areas (typically up to 50 m.) are required. They mayalso be used for small amounts of boiling or condensation on the process fluid side. Theadvantages of the double-pipe exchanger are largely in the flexibility of application andpiping arrangement, plus the fact that they can be erected quickly from standard

components by maintenance crews [3].

4. Compact Heat Exchangers

4.1. Overview

One variation of the fundamental compact exchanger element, the core, is shown inFigure 5. The core consists of a pair of parallel plates with connecting metal members thatare bonded to the plates. The arrangement of plates and bonded members provides both afluid-flow channel and prime and extended surface. It is observed that if a plane weredrawn midway between the two plates, each half of the connecting metal members could

be considered as longitudinal fins [1].Compact heat exchangers may be classified by the kinds of compact elements that

they employ. The compact elements usually fall into five classes:a. Circular and flattened circular tubes.b. Tubular surfaces.c. Surfaces with flow normal to banks of smooth tubes.d. Plate fin surfaces.e. Finned-tube surfaces.

4.2. Illustration

Figure 6, the core of a compact heat exchanger [1].

Figure 7, Two-fluid compact heat exchangerwith headers removed [1].

4.3. Applications

Compact or plate-fin heat exchangers have a wide range of applications that include [4]:• Natural gas liquefaction.• Cryogenic air separation.• Ammonia production.• Offshore processing.• Nuclear engineering.• Syngas production.

5. Plate and Frame Heat Exchanger

5.1. Overview

These exchangers are usually built of thin plates (all prime surfaces). The plates areeither smooth or have some form of corrugations, and they are either flat or wound in anexchanger. Generally, these exchangers cannot accommodate very high pressures,temperatures, and pressure and temperature differentials. These exchangers may befurther classified as plate, spiral plate, lamella, and plate coil exchangers, as shown in Figure9 the plate heat exchanger, being the most important of these, is described next.

Figure 8, actual footage of a cut-section in a compact heat exchanger.

5.2. Illustration

5.3. Applications

These exchangers are relatively compact and lightweight heat transfer surfaces,making them attractive for use in confined or weight-sensitive locations such as on boardships and oil production platforms. Pressures and temperatures are limited tocomparatively low values because of the gasket materials and the construction.

They are typically used for exchanging heat between two liquid streams in turbulentflow. They are occasionally used as condensers for fairly dense vapors (e.g., ammonia) or asvaporizers as for a reboiler. They are used in the food processing industry because they canbe disassembled for cleaning and sterilization.

Figure 9, Plate and Frame heat exchangers [2].

Figure 10, actual footage of a plate and frame heat exchanger.

6. Spiral Heat Exchangers

6.1. Overview

Several different versions of the spiral plate exchanger are available. This exchangeris formed by rolling two long, parallel plates into a spiral using a mandrel and then suitablywelding the alternate edges of adjacent plates to form the channels. The plates are heldapart by raised bosses on one of the plates. The open sides of the channels are sealed offagainst bypassing by cover plates (with gaskets) held in place by the bolted clamps aroundthe periphery [3].

Connections are made at the center of the coil to each channel to act as inlet in onecase and outlet in the other. Similar connections are made at the outer end of eachchannel. The spiral exchanger can be enclosed in a pressure vessel, or the outer panel canbe incorporated to form the outside of the unit. The exchanger is closed top and bottomwith covers bolted to the outer shell of the exchanger.

6.2. Illustration

Figure 10, top and side sections of a spiral heat exchanger.

Figure 12, actual footage of a spiral heat exchanger.

6.3. Applications

By virtue of the removable top and bottom covers, this exchanger is easily cleanedand is therefore ideal for applications involving a high degree of fouling. Indeed, it is widely

7.1. Overview

7.2. Illustration

used for the heating and cooling of slurries [3].

7. Regenerative Heat Exchangers

considered during the design process [1].

Figure 11, Regenerators: (a) rotary, (b) fixed-matrix, and (c) rotating hoods.

The regenerator represents a class of heat exchangers in which heat is alternatelystored and removed from a surface. This heat transfer surface is usually referred to as thematrix of the regenerator. For continuous operation, the matrix must be moved into andout of the fixed hot and cold fluid streams. In this case, the regenerator is called a rotaryregenerator. If, on the other hand, the hot and cold fluid streams are switched into and outof the matrix, the regenerator is referred to as a fixed matrix regenerator. In both cases theregenerator suffers from leakage and fluid entrainment problems, which must be

Figure 13, typical rotary regenerators or heat wheels. .

7.3. Applications

Rotary regenerators are used extensively in electrical power generating stations forair preheating. They are also used in vehicular gas turbine power plants, in cryogenicrefrigeration units, and in the food dehydration industry.

Fixed bed or fixed matrix regenerators are used extensively in the metallurgical,glassmaking, and chemical processing industries.

8. Scrapped Surface Heat Exchangers

8.1. Overview

In cases where a process fluid is likely to crystallize on cooling or the degree offouling is very high or indeed the fluid is of very high viscosity, use is often made of scraped-surface heat exchangers in which a rotating element has spring-loaded scraper bladeswhich wipe the inside surface of a tube which may typically be 0.15 m in diameter. Double-pipe construction is often employed with a jacket; say 0.20 m in diameter, and onecommon arrangement is to connect several sections in series or to install several pipeswithin a common shell. Scraped- surface units of this type are used in paraffin- wax plantsand for evaporating viscous or heat-sensitive materials under high vacuum.

8.2.Illustration

Figure 14, Scraper blade of scraped-surface exchanger [6].

8.3. Applications

The range of applications covers a number of industries, including food, chemical,petrochemical and pharmaceutical. The DSSHEs are appropriate whenever products areprone to fouling, very viscous, particulate, heat sensitive or crystallizing.

9. Transverse High-Finned Exchangers

9.1. Overview

Pipes, tubes, and cast tubular sections with external transverse high fins have beenused extensively for heating, cooling, and dehumidifying air and other gases. The fins arepreferably called transverse rather than radial because they need not be circular, as thelatter term implies, and are often helical. The air-fin cooler is a device in which hot-processfluids, usually liquids, flow inside extended surface tubes and atmospheric air is circulatedoutside the tubes by forced or induced draft over the extended surface.

High-fin tubes can also be extruded directly from the tube-wall metal, as in the caseof integral low-fin tubing. However, it becomes increasingly difficult to extrude a high finfrom ferrous alloys as hard as those required for high-temperature services, which areoften amenable to work hardening while the fin is being formed. Whether fins are attachedby arc welding or resistance welding, the fin-to-tube attachment for all practical designconsiderations introduces a neglible bond or contact resistance.

Figure 15, actual footage of a scraper-surface heat exchanger.

9.2.Illustration

9.3. Applications

The large majority of applications are for transferring heat to atmospheric air. Finnedtubes may be used in: water cooling of product, and air cooling of product, oil – airexchangers and oil, industrial and residential air heaters using burned gas heat, steam, hotwater or resistance heating elements rolled inside finned tube, cooling and food processingindustry and automotive industry.

Figure 16, typical high-finned tube used in air-cooled heat exchangers [3].

Figure 17, actual footage of various shapes of finned tubes.

10. Conclusion

Heat Exchangers have numerous different types and applications as discussed in thereport. Each type selection can only be determined by the application the device will beused for. The general design process can be summarized in the calculation of the requiredarea to transfer heat from one fluid to another by that the designer can determine theactual mechanical design parameters knowing the physical and chemical behavior of thefluids to be used.

The report discussed the most famous types of industrially used heat exchangerssuch as Shell-and-Tube heat exchangers, which are the most commonly used ones, that canwithstand high pressures with moderate area to volume ratio and Double-Pipe heatexchangers which are the simplest type in design and maintenance but they have relativelylow area of heat transfer. Compact heat exchangers which are famous of their capability touse different phases of fluids and Plate-and-Frame type that has very high area to volumeratio in addition to Spiral, Regenerative or Matrix, Scraped-Surface and High and Low-finned types.

General design considerations are routing of fluids and the suitability of thecalculated area of heat transfer according to fouling factor and other important parameterslike baffles arrangement to meet with the maximum pressure loss requirement in shell-an-tube heat exchanger.

References

[1] A. Bejan and A. D. Kraus, “Heat Transfer Handbook”, 2003.[2] Stanly M. Wallas, “Chemical Process Equipment, Selection and Design”, 1990.[3] Ernst U. Schltinder, “Heat Exchanger Design Handbook”, 1983.[4] Donald Q. Kern, “Heat Transfer”, 1982.[5] Warren M. Rohsenow, “Handbook of Heat Transfer”, 3rd Edition 1998.[6] Don W. Green and Robert H. Perry, “Perry’s Chemical Engineers’ Handbook”, 8th Edition2007.[7] Wikipedia.org, Cited in March 2009.