Heat Exchangers - Research Paper Example

College Heat exchangers A heat exchanger is an instrument which transfers heat energy from a hot fluid to a cold fluid,with rates being maximum while the investment and running cost being minimum. Sometimes the media is separated by a wall such that the fluids do not mix or get into direct contact. Heat exchangers have a broad area of use example being in ventilation, air conditioning and space heating. As the fluid passes through the exchanger it either lose or absorb heat due to latent heat. This makes the temperature of the fluid in the system to change, and so does the temperature of the dividing walls between the two fluids. Examples of heat exchangers are; intercoolers, preheaters, oil coolers of heat engine, condensers, and evaporators in refrigeration units.Various types of heat exchangers have been developed to meet the widely varying applications and are classified according to; nature of heat exchange process, physical state of fluid, constructional and design specs and relative direction of fluid flow. According to the nature of the heat exchange process, heat exchangers are classified as; direct and indirect contact heat exchangers. Plate and shell heat exchanger, combines both the plate heat exchangers and the shell exchangers for maximum efficiency. At the centre of the heat exchanger lies two plate that are pinned together by pressing them and welding them together. There is a nodle whose function is to carry fluids in and out of the whole system. This combination is very efficient in offering high operating temperatures, and high pressure a factor that is vital in the heat exchange process. The heat exchanger is able to function well without any gaskets. This means that it is able to work under these extreme conditions without any risk of leaking the fluid during the heat exchange process. Normally different types of heat exchangers are combined to come up with a more effective, cost friendly and efficient unit of heat exchanger and the plate and shell heat exchanger is just but one example. Plate and shell heat exchangers are not a counter-flow heat exchanger. This means that the fluids enter at the opposite ends of the unit enabling them to flow into the centre of the exchanger. As the fluids come close to each other, there is an exchange of heat among the fluids. As the fluid passes through the exchanger it either lose or absorb heat due to latent heat. According to the nature of the heat exchange process, plate and shell heat exchangers are classified as indirect contact heat exchangers. The exchange of heat takes place by direct mixing of hot and cold fluids allowing transfer of heat and mass to take place simultaneously in a direct contact heat exchanger and such a unit is applicable only where the two fluids are either harmless when in contact or are desirable. In an indirect contact heat exchanger there is a wall which prevents the two fluids from getting into direct contact. So heat exchange between the two fluids takes place by transmission through the wall. Plate and shell heat exchangers are classified as indirect contact heat exchangers. In a typical plate and shell heat exchanger, the fluids that are to be cooled (warm water or glycol solution) are flown through the plate side and a refrigerant circulating through the shell cools them. Plate and shell heat exchanger has a broad application in HVACR (heating, ventilation, air conditioning, and refrigeration.) in today life. In fuel oil heaters, large amount of heat are required to ensure that the process is a success, plate and shell heat exchanger can be used in such a situation to combat the high temperatures and pressure. With the thermal efficiency of plate type heat exchanger and the durability of shell, type heat exchanger makes it suitable for plate and shell heat exchanger to be used in ventilation system of a combustion room associated with high temperatures and high pressures like in the combustion room of a military submarine. Normally plate and shell heat exchanger are used in air condition system of an airplane propeller system as it functions in a wide range of temperature and pressure without malfunctioning (Thiele, 78). The relative cost of a plate and shell heat exchanger is low compared to the cost of other heat exchanger when viewed from the perspective of what it does because it only requires two plates as compared to the four plates that are required in a rectangular plate heat exchanger because one of the fluids enters through the shell opening and leaves through the same shell opening. In terms of serviceability the plate and shell heat exchanger needs sophisticated equipments and a lot of time to services since its tube diameter is small and are closely packed which requires a great attention although it requires a small space for installation because of its compactness as compared to other types which are usually easy to service because their tube diameter are large enough and greatly spaced which makes them to consume a big space during installation because the large space between the tube makes them bulky (Phillips, 56). For a plate and shell heat exchanger to function efficiently different types of valves, pumps are used to do different procedures and steps. Normally two-stage valves are used to hinder water hammer. Valves that are able to control the sequence are normally used to prevent thermal or mechanical stress from occurring during start up time. At the exchanger inlet, a pressure-reducing valve is normally installed in case the maximum discharge exceeds the maximum design pressure of the plate and shell heat exchanger. Due to harmonics and pulsation that accompanies this unit, vibration dampers usually equips the positive displacement pumps to minimize these effects. Normally globe or butterfly valves are included in plate and shell heat exchangers and are recommended that they be maintained in good working conditions to reduce their cases of malfunctioning. Just like all the other types of heat exchangers, plate and shell heat exchanger has its merits and demerits. Some of its merits are; it’s a compact unit and this means that it requires a small space for installation, it has no gaskets which implies that it provide a better and reliable working condition because it has no leakages that might cause accidents, it can be used in regions of high temperatures (up to 150bar) and in extreme temperatures (ranging from -100 degree Celsius to 540 degree Celsius). Its compatible with a large number of fluids and this widens its application in the engineering field. Its demerits are; it has critical operating principles which must be observed closely otherwise it might malfunction and this limits its operators to limited number of people and also in case it fails it difficult to troubleshoot the unit system because of its compactness (Aliabadi, 312). Some of the manufactures of plate and shell heat exchanger include EVAPCO (http://www.evapco.com/) and they use, evapco selection ice program, although it is not yet available for download. In addition, TRANTER (http://www.tranter.com) are manufactures of plate and shell heat exchanger although they do not have available downloadable software for its customers. Work cited Hebda, T., & Czar, P. (2009) Heat exchangers, (4th ed.). Upper Saddle River, N.J.: Practice Hall. Print Thiele, J. E. (2002). Heat Exchangers and their use (pp.347-358). St. Louis, MO: Mosby. Print Harriman, J. H. P. (2008). Heat Exchangers and their use. New York: Neal-Schuman Publishers. Print British Engineering Society. (2008). Heat Exchangers and their use. Chichester, Sussex, UK: Media Medica. Print Shah Khan, M. Z., Simpson, G., & Taylor, M. (1998). Heat Engineering. Melbourne, DSTO Aeronautical and Maritime Research Laboratory. Print Aliabadi, M. H., & Guagliano, M. (2006). Heat Excchangers theory. Southampton, WIT. Print Owen, M. J., Middleton, V., & Jones, I. A. (2000). Integrated design and manufacture of Heat exchangers. Boca Raton, FL, CRC Press. Print Phillips, K. A. (2004). Performance of Heat Exchangers. Blacksburg, Va, University Libraries, Virginia Polytechnic Institute and State University. Print Read More