Cairo University Faculty of Engineering Chemical Engineering Department
Heat Exchangers Types and Applications
Submitted to:
Dr. Osama Abdel-Bary Chemical Engineering Department Faculty of Engineering, University of Cairo Prepared by: Sec.: 4
Moataz Said Eissa B.N.: 8
March, 2009
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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 solid particulates and a fluid, at differing temperatures and in thermal contact, usually without external 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 or condensation of a single or multicomponent fluid stream, and heat recovery or heat rejection from a system. In other applications, the objective may be to sterilize, pasteurize, fractionate, distill, concentrate, crystallize, or control process fluid. In some heat exchangers, the fluids exchanging heat are in direct contact. In other heat exchangers, heat transfer between fluids takes place through a separating wall or into and out of a wall in a transient manner. In most heat exchangers, the fluids are separated by a heat transfer surface, and ideally they do not mix. Such exchangers are referred to as the direct transfer type, or simply recuperators. In contrast, exchangers in which there is an intermittent heat exchange between the hot and cold fluids via thermal energy storage and rejection through the exchanger surface or matrix—are referred to as the indirect transfer type or storage type, or simply regenerators. Such exchangers usually have leakage and fluid carryover from one stream to the other. Heat exchangers may be classified according to transfer process, construction, flow arrangement, surface compactness, number of fluids and heat transfer mechanisms or according to process functions.
2. Shell and Tube Heat Exchangers 2.1. Overview 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 of this basic type of heat exchanger lie mainly in their construction features and the provisions made for handling differential thermal expansion between tubes and [1] shell . There are various design considerations to be taken into account such as routing of fluids (shell or tube), pressure drop especially in the case of increasing number of baffles and tube diameter and adjusting the area with the suitability of the exchanger to conduct the heat required to heat or cool a fluid with another one.
2.2. Illustration
Figure 1, shell-and-tube heat exchanger with baffles
1. Shell. 2. Floating Head Flange. 3. Shell Channel. 4. Shell Cover End Flange. 5. Shell Nozzle. 6. Floating Tube Sheet. 7. Floating Head.
8. Floating Head Flange. 9. Channel Partition 10. Stationary Tube Sheet. 11. Channel. 12. Channel Cover. 13. Channel Nozzles. 14. Tie Rods and Spacers
[2]
.
15. Transverse Baffles. 16. Impingement Baffle. 17. Vent Connection. 18. Drain Connection. 19. Test Connection. 20. Support Saddles. 21. Lifting Ring.
Figure 2, actual footage of a tube bundle.
Figure 3, actual footage of baffle arrangement.
2.3. Applications They are extensively used as process heat exchangers in the petroleum-refining and chemical industries; as steam generators, condensers, boiler feed water heaters and oil coolers in power plants; as condensers and evaporators in some air-conditioning and refrigeration applications; in waste heat recovery applications with heat recovery from liquids and condensing fluids; and in environmental control.
3. Double Pipe Heat Exchangers
3.1. Overview A typical double-pipe heat exchanger is shown in Figure 4. Essentially, it consists of one pipe placed concentrically inside another one of larger diameter, with appropriate end fittings on each pipe to guide the fluids from one section to the next. The inner pipe may have external longitudinal fins welded to it either internally or externally to increase the heat transfer area for the fluid with the lower heat transfer coefficient. The double-pipe sections can be connected in various series or parallel arrangements for either fluid to meet [3] pressure-drop limitations and LMTD requirements . 3.2.
Illustration
Figure 4, double pipe heat exchanger (one hair-pin)
[1]
.
Figure 5, actual footage of 7 hair-pins arrangement.
3.3. Applications The major use of double-pipe exchangers is for sensible heating or cooling of the process fluid where small heat transfer areas (typically up to 50 m.) are required. They may also be used for small amounts of boiling or condensation on the process fluid side. The advantages of the double-pipe exchanger are largely in the flexibility of application and piping arrangement, plus the fact that they can be erected quickly from standard [3] components by maintenance crews .
4. Compact Heat Exchangers 4.1. Overview One variation of the fundamental compact exchanger element, the core, is shown in Figure 5. The core consists of a pair of parallel 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 if a plane were drawn midway between the two plates, each half of the connecting metal members could [1] be considered as longitudinal fins . 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]
.
1. Plates. 2. Side Bars. 3. Corrugated fins stamped from a strip of metal.
Figure 7, Two-fluid compact heat exchanger [1] with headers removed .
Figure 8, actual footage of a cut-section in a compact heat exchanger.
4.3.
Applications [4]
Compact or plate-fin heat exchangers have a wide range of applications that include : • 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 are either smooth or have some form of corrugations, and they are either flat or wound in an exchanger. Generally, these exchangers cannot accommodate very high pressures, temperatures, and pressure and temperature differentials. These exchangers may be further classified as plate, spiral plate, lamella, and plate coil exchangers, as shown in Figure 9 the plate heat exchanger, being the most important of these, is described next.
5.2. Illustration
Figure 9, Plate and Frame heat exchangers
[2]
.
Figure 10, actual footage of a plate and frame heat exchanger.
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 board ships and oil production platforms. Pressures and temperatures are limited to comparatively low values because of the gasket materials and the construction. They are typically used for exchanging heat between two liquid streams in turbulent flow. They are occasionally used as condensers for fairly dense vapors (e.g., ammonia) or as vaporizers as for a reboiler. They are used in the food processing industry because they can be disassembled for cleaning and sterilization.
6. Spiral Heat Exchangers 6.1. Overview Several different versions of the spiral plate exchanger are available. This exchanger is formed by rolling two long, parallel plates into a spiral using a mandrel and then suitably welding the alternate edges of adjacent plates to form the channels. The plates are held apart by raised bosses on one of the plates. The open sides of the channels are sealed off against bypassing by cover plates (with gaskets) held in place by the bolted clamps around [3] the periphery . Connections are made at the center of the coil to each channel to act as inlet in one case and outlet in the other. Similar connections are made at the outer end of each channel. The spiral exchanger can be enclosed in a pressure vessel, or the outer panel can be incorporated to form the outside of the unit. The exchanger is closed top and bottom with 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 remova le top and bottom covers, this exch nger is easily cleaned and is therefore ideal for applications involving a high degree of fouling. Indeed, it is widely [3] used for the heating and coolin of slurries .
7. Re enerative Heat Exchangers 7.1. Overview The regenerator represents a class of heat exchangers in which heat is alternately stored and removed from a su face. This heat transfer surface is us ally referred to as the matrix of the regenerator. For continuous operation, the matrix m st be moved into and out of the fixed hot and cold flluid 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 of the matrix, the regenerator is referred to as a fixed matrix regene ator. In both cases the regenerator suffers from leakage and fluid entrainment probl ms, which must be [1] considered during the design process .
7.2. Illustration
Figure 11, Regenerators: (a) rotary, (b) fixed-matrix, and (c) rotat ing hoods.
Figure 1 , typical rotary regenerators or heat wheels.
7.3. Applications Rotary regenerators are used extensively in electrical power generating stations for air preheating. They are also used in vehicular gas turbine power plants, in cryogenic refrigeration 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 of fouling is very high or indeed the fluid is of very high viscosity, use is often made of scrapedsurface heat exchangers in which a rotating element has spring-loaded scraper blades which wipe the inside surface of a tube which may typically be 0.15 m in diameter. Doublepipe construction is often employed with a jacket; say 0.20 m in diameter, and one common arrangement is to connect several sections in series or to install several pipes within a common shell. Scraped- surface units of this type are used in paraffin- wax plants and for evaporating viscous or heat-sensitive materials under high vacuum.
8.2. Illustration
Figure 14, Scraper blade of scraped-surface exchanger
[6]
.
Figure 15, actual footage of a scraper-surface heat exchanger.
8.3. Applications The range of applications covers a number of industries, including food, chemical, petrochemical and pharmaceutical. The DSSHEs are appropriate whenever products are prone 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 been used extensively for heating, cooling, and dehumidifying air and other gases. The fins are preferably called transverse rather than radial because they need not be circular, as the latter term implies, and are often helical. The air-fin cooler is a device in which hot-process fluids, usually liquids, flow inside extended surface tubes and atmospheric air is circulated outside 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 case of integral low-fin tubing. However, it becomes increasingly difficult to extrude a high fin from ferrous alloys as hard as those required for high-temperature services, which are often amenable to work hardening while the fin is being formed. Whether fins are attached by arc welding or resistance welding, the fin-to-tube attachment for all practical design considerations introduces a neglible bond or contact resistance.
9.2. Illustration
[3]
Figure 16, typical high-finned tube used in air-cooled heat exchangers .
Figure 17, actual footage of various shapes of finned tubes.
9.3. Applications The large majority of applications are for transferring heat to atmospheric air. Finned tubes may be used in: water cooling of product, and air cooling of product, oil – air exchangers and oil, industrial and residential air heaters using burned gas heat, steam, hot water or resistance heating elements rolled inside finned tube, cooling and food processing industry and automotive industry.
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