Designs of Heat Exchangers Essay
Designs of Heat Exchangers
Heat transfer from one substance to another plays a major role in the process of some commonly use technology today. The heat transfer is done to control temperature, to heat up a cooler substance, or to cool down a hotter substance. This process is done by a machine called the heat exchanger. The heat exchanger is commonly used in the space heating, in cooling of engines, refrigeration and other processes which need temperature control. They are classified according to their flow arrangement design. One is the parallel-flow design which the two substance flow from the same direction and exit at the same direction also.
The other one is the counter-flow design on which the fluids flow on opposite direction. The last one is the cross-flow heat exchanger where the fluids flow perpendicular to each other (Heat Exchanger Design, Inc. 2009). Figure 1. Parallel-flow and Counter-flow Design (Engineering Edge 2009) There are also several types of heat exchangers depending on its design. Some are shell and tube heat exchangers, plate heat exchangers, Regenerative heat exchangers and many others. The shell and tube design of the heat-exchanger consists of two sets of tubes.
The first set of tubes is where the cooled or heated fluid flows and on the other set of tubes is the fluid that needed to be heated or cooled. The fluids are separated by the tube’s wall. This design ensures a better protection from leakage because of the tube design. So this design is ideal for the treatment of highly pressurize fluids (Kakac & Liu 2002:8). The plate heat exchanger is consisting of multiple, thin and slightly separated plates which allow a larger fluid flow area fro heat transfer. It is said to be more effective than the shell and tube design because it provides more surface area for heat transfer.
Additionally, the plate heat exchangers have recent developments on its brazing and gasket that makes it practical to use than other heat exchangers. However, the plate heat exchanger is needs a lot of space and it is more susceptible to leakage (Wang, Sunden & Manglik 2007). The regenerative heat exchangers are heat exchangers that make use of the heated fluid after a process to be the heating fluid for the next process. This design can be on a plate or shell and tube structure. The advantages of this type are adapted from the design it is structured.
Furthermore, this design brought an economical perspective to the first two designs. On the other hand, this design is only intended for the use of gas substances as fluids (Saunders 1988). Figure 4. Regenerative Heat Exchanger (Defense Research & Development Organization 2004) The adiabatic wheel heat exchanger is a machine which uses a third substance, fluid or solid, to transfer heat between the streams. The disadvantage of this design is that there will be a small mixing of the two streams in the process of heat transfer (Saunders 1988). The plate fin heat exchanger is highly used in different industries.
The plate-fin heat exchangers utilize plates and finned chambers for relatively high heat transfer. It is preferred by other industries because of its ability to transfer heat with to small difference between the streams. Additionally, it has a compact design and is very lightweight (Kakac & Liu 2002:17). Another type is the fluid heat exchangers. The process being done is by showering fluids to a gas stream in an upward direction. The process is used in machines that needed gas cooling and purification at the same time (Saunders 1988). Figure 6. Thermal Fluid Heat Exchanger (Thermotech Industries n.
d. ) A waste heat recovery unit is a heat exchanger that recovers waste heat from other mechanism and uses it to transfer heat the fluid being heating. This design is very desirable for refineries and plants since they have an abundant amount of waste heat from their machineries. This design is very economical since waste recycled to something useful (Saunders 1988). Figure 7. Waste Heat Recovery Unit (Turner 2005) Dynamic or scraped surface heat exchangers are heat exchangers that have high maintenance cost because of the constant surface scraping.
The scraping is due to the process these machines are used. Such processes are heating and cooling of high-viscosity fluids and other highly fouling substance. However, this design has a sustainable heat transfer and it avoids fouling of the surface of transfer (Saunders 1988). Figure 8. Scraped Surface Heat Exchanger (GEA Niro 2008) The phase-change heat exchangers are machines that utilizes liquid and gas phase in heat transfer. These heat exchangers either heat a liquid to boil and evaporate or cool hot gases to condense. This process is common in distillation set ups.
The distillation is a process of which a liquid is heated up and cooled down to remove impurities. It is also used in power plants such as nuclear power plants and fossil fuel plants. These power plants usually use steam driven turbines to generate electricity. In which the steam came from the heat exchange between the heater and the liquid (Saunders 1988). The heat exchangers above are heat exchangers which utilizes a wall to separate the two streams of liquids. The classifications above are based on the design of which how the two streams are separated.
However, there are other heat exchangers that do not use walls or separators. These are called direct contact heat exchangers. These heat exchangers use two different phases in heat transfer (Saunders 1988). Figure 9. Phase-changed Heat Exchanger (NationMaster. com 2003) There are other heat exchangers in the market today. Some of these are the multi-phase heat exchangers and spiral heat exchangers. Selecting a proper heat exchanger can be a challenging task. It requires technical knowledge about the designs since there are many factors to be taken into consideration.
Some are temperature ranges, the composition of the substance to be heated and pressure limits. That is why the decision about the design is made by a computer program or an engineer. Additionally, large companies and corporations tend to design their own heat exchanger to tend to their needs (Wang, Sunden & Manglik 2007). Heat exchangers are seen everywhere without you noticing that they are there. The advances in the technology of heat exchangers can produce better results for engineers, researcher and many industrialists since it will increase the efficiency of the process which common machines have today.
List of References Defense Research & Development Organization (2004). Mechanical & Propulsion System. Technology Focus [online] available from http://www. drdo. org/pub/techfocus/oct04/welcomeoct04. html [17 April 2009] Engineering Edge. (2009). Heat Transfer. [online] available from http://www. engineersedge. com/heat_transfer/parallel_counter_flow_designs. htm [17 April 2009] GEA Niro. (2008). Preheating System. [online] available from http://www. niro. com/niro/CMSDoc. nsf/WebDoc/ndkw5y7gby [17 April 2009] Heat Exchanger Design, Inc. (2009).
Shell and Tube Heat Exchanger. [online] available from http://www. hed-inc. com/shell-tube. html [17 April 2009] Kakac, S. & Liu, H. (2002). Heat Exchangers: Selection, Rating and Thermal Design (2nd Edition). CRC Press Linde Engineering Division (2005). Plate Fin Heat Exchangers. [online] available from http://www. linde-engineering. com/plant_components/plateheatexchanger_aluminium. php [17 April 2009] NationMaster. com (2003). Heat Exchanger. [online] available from http://www. nationmaster. com/encyclopedia/Heat-exchanger [17 April 2009] Saunders, E. A. (1988).
Heat Exchanges: Selection, Design and Construction. New York: Longman Scientific and Technical Thermotech Industries (n. d. ) Products. [online] available from http://www. thermotech-finnedtubes. com/products. htm [17 April 2009] Turner, J. (2005). Making the Most of Waste Energy. Science and Technical Information- National Aeronautics Space Administration [online] available from http://www. sti. nasa. gov/tto/Spinoff2005/er_7. html [17 April 2009] Wang, L. , Sunden, B. , & Manglik R. M. (2007). Plate Heat Exchangers: Design, Applications and Performance. Southamton, Boston: WIT Press