Design of a Heat Exchanger Test Rig - Coursework Example

Literature Review on Design of a Heat Exchanger Test Rig Name Institutional Affiliation Date Table of Contents Introduction 2 Working Principle of Plate Heat Exchanger 4 Heat Exchanger modeling 15 Non Dimensional Method of Heat Exchanger modeling 16 Factors that Affect the Performance of a Heat Exchanger 17 Conclusion 21 References 23 Introduction Heat exchangers are equipments in which thermal exchange occurs between fluids with or without respective fluids mixing or contaminating. These fluid streams are always at different temperatures and between them a metallic fixed intermediate wall is placed. Heat exchangers are used in refrigeration and air conditioning, automotives, chemical processing, power generation and electronics cooling. There is a high demand for the improvement of the efficiency and effectiveness of heat exchangers models entering the market (Sundén & Brebbia 2014). Heat transfer processes also need to be enhanced and this begins by making sure the heat exchanger is in good operating condition. Heat exchangers can be classified in terms of the type of fluid flow and geometrical build. For both momentum and heat transfers evaluation, the transfer coefficient has to be analyzed and the optimum value set to ensure there is good efficiency of the machine. Heat exchangers are used in all industrial processes that need thermal energy to be transferred from a hot to a cold medium like the beverage and food industries, and refrigeration applications (Thulukkanam 2014). This paper analyses the basic working principle of heat exchangers focusing more on the commonly used plate heat exchanger. It also seeks to address the means by which heat exchanger performances can be improved and enhanced. For this research, we will look proper modeling and design of heat exchangers and their thermal analysis, as well as neglecting heat gains and heat losses to the external environment as compared to the heat transfer occurring between the fluids and the thermal inertia if the medium used is not a mass of porous solid. Fluid is assumed to be at steady state. Working Principle of Plate Heat Exchanger Thulukkanam (2014) illustrates that plate heat exchangers transfer heat between media that run in co-joined corrugated plate layers. Tightening bolts are used to compress a mobile pressure plate and a fixed frame plate to make a plate pack. A gasket is fitted in the plates to direct media into channels by sealing interpolate channels. Plate heat exchangers are fitted with thin metal sheets arranged is a series with each pair forming a complex fluid surface for one stream to flow through. These plates are then sand witched leaving a space between each pair for the second fluid. The fluid’s physical properties, temperature range, drop in pressure and rate of flow determines the number of plates in each exchanger. Individual plates are corrugated to increase turbulence in the fluids and consequently raise the efficiency of the heat exchanger. Its application determines corrugation pattern used (Kuppan 2000). Surface area of heat transfer is a determinant factor of the amount of thermal energy transferred. When the media is distributed evenly over the transfer area, efficiency of the heat exchanger increases. Therefore, the heat exchangers are fitted with special area for distributing the fluids across the surface area near the port. This special area spreads the media on the plate face ensuring it is uniformly spread across the transfer surface as displayed in figure (1). Figure 1: A plate from a gasketed plate heat exchanger In one type of such an exchanger, the fluid that enters the exchanger from the bottom flows through the layers until it gets to the top. Likewise, for the fluid that enters through the top, it flows until it reaches the bottom layer then it flows out. These media flow counter to each other and therefore there is maximum heat transfer between them (Kuppan 2000). Most plate exchangers are single pass design. In this type, the two fluids flow once against each other. In such an exchanger, the fluids flow as shown in figure (2) below. Figure 2: fluid flow in a single pass plate heat exchanger Another plate exchanger design is the multi pass type with two or more passages. In this case, fluids counter flow more than once within the equipment. In figure (3) below is an example of a two pass design of the exchanger. Figure 3: Two pass plate exchanger Comparison of Various Types of Heat Exchangers The common types of heat exchangers are those of the plate and the shell and tube. However, other types include the open flow and the contact heat exchangers. Figure (4) below gives a summary diagram for most heat exchanger types. a)Shell and tube b)plates c) open flow d) rotating wheel Figure (4): Types of Heat Exchangers Heat exchangers are classified according to thermal transfer process as indirect or direct contact, geometrical construction as plate, tubes or extended surface, mechanism of heat transfer such as single or multi phase flow. They can also be classified depending on the arrangement of flow which include parallel, cross flow or counter flow. Finally, they can be simply classified into recuperators and generators. Heat exchangers can also be open flow or closed flow. In this paper, we will analyze some of these heat exchanger types (Kuppan 2000). Open flow contact heat exchangers do not have both streams of fluid piped. This enables the fluids to transfer energy by mixing and a consequent mass flow transfer is also achieved. An example is the car radiator where final heat is released to ambient air from a liquid and their origin is tube banks that are cooled by air. These heat exchangers are also used in manufacture of refrigeration and air conditioning condensers. For closed flow, the individual fluid streams are put in separated and enclosed pipes hence there is no mixing during heat transfer. Contact Heat Exchangers In contact heat exchangers, fluid streams are directly into contact and transfer of both mass and heat occurs simultaneously. When fluids separate using gravitational forces after being continuously in contact, it is referred to as a cooling tower. In regenerative exchangers, this contact involves a third external medium such as a solid rotating wheel in that the hot gas will lose its heat to the wheel as the cold gas retrieves this thermal energy. Intended contamination of these fluids may occur as the case with cooling towers or it may be inevitable as it is with heat recuperators for air conditioning. If mixing is such that there is no separation of the mixed fluids, the device ceases to act as heat exchangers and is now referred as a cooler or heater. Example of non-separation case is an evaporative cooler or water heaters where fluids are directly fed (Pavlovič & Venko 2006). In relation to the fluid type used, heat exchangers are classified into liquid to gas, liquid to liquid and gas to liquid heat exchangers. Vaporisers and condensers are heat exchangers that involve a phase change which can either be form liquid to gas, solid to liquid, liquid to solid or gaseous to liquid. Plate heat exchangers Stacked corrugated thin plates are in contact and a fluid flows along the channels, which are adjacent to the plates. The plates may be closed through the following methods; gaskets, which are clamped together, stainless steel, which is copper, brazed together, or the copper titanium welded together. Notably the most common type is the gasketed plate heat exchangers. The stacked plates are secured using a frame .The gaskets help to control how the fluid flows in the tubes. They have a conductance coefficient of K=6000 W/ (m2·K.it is possible to adjust the number of plates to fit into the intended needs and it uses fluids that are less viscous. Any time when the gaskets are dismounted, it is advisable to change them (Pavlovič & Venko 2006). Figure (5) above, is a display of the plate heat exchanger fitted with gaskets that is welded. Shell tube heat exchangers The shell and tube heat exchangers are known to be the most common and widely used heat exchangers. It is mostly used in boilers and steam condensers of heavy industries. The hot water heating systems in residential house also use this type of heat exchanger (Horvat & KončAr 2003). These exchangers and the tube design can be modeled as in the drawings of figure (6) below. Figure (6): Design and modeling of tube heat exchangers. Double piped exchangers In double piped heat exchangers, a pair of concentric pipes is used (Branson 2011). The pipe on the inside c can be either fitted with fins or maintained plain depending on the performance expectations of the exchanger. Two fluids counter flow one inside the pipe and the other through the space between the two pipes. This flow pattern ensures that high efficiency is achieved for a specific area of contact. Parallel flow can also be productive where streams require a wall temperature that is roughly constant disassembly is enough for cleaning to take place in this heat exchanger type and media flow is uniformly distributed within the flow channels. Double piped system is very effective in high pressure fluids due to the small diameters of its pipes. However, because of the high production cost these exchangers can be only used in heat exchangers where the surface area is small and usually less than (Branson 2011). Spiral tube exchangers In a spiral tubed heat exchanger, several coils are wound and fitted inside one shell. Because of these windings, surface area is extended and consequently more thermal energy is transferred compared to the longitudinally straight tubes. It is not possible to perform a thorough clean o n this exchanger type and there is no room for thermal expansion which increases the risk of bursting (Kakaç, Bergles, Mayinger & YüNcü 1999). Heat Exchanger modeling Consider a differential length dfor a counter flow heat exchanger. If the hot fluid is represented by subscript ‘h’ and the cold fluid by subscript ‘c’, for a separating wall, the energy balance for each fluid longitudinally is given by equation (1) below. Equation (1) Where and represent thermal energy flow between the hot and cold media across a differential length d, d is the heat exchanger surface area along d, dthe thermal energy and and represent the individual fluid heat capacities. All changes of phases are evaluated using temperatures instead of enthalpies in the model as opposed to the real exchanger. When equation (1) is integrated from length to with L as the full length of the heat exchanger, equation (2) is attained. Equation (2) Where the mean temperature for the cold and hot fluids, K is is the coefficient of heat transfer and A is the wetted area. When equations (1) and (2) are combined and compared, their integration within length leads to equation (3). Equation (3) Therefore, the mean temperature between the hot and cold fluids is given by equation (4) below. = Equation (4) Unlike in the multi pass and cross flow heat exchangers, when modeling co-flow exchangers, logarithmic mean temperature is used. Non Dimensional Method of Heat Exchanger modeling The measurement use when modeling heat exchangers using equation (5) creates a redundancy because it has two unknowns. For external measurements, the surface area and number of plates cannot be accurately measured. Thus, a non-dimensional figure is added to the equation in order to solve this issue (Kakaç, Bergles, Mayinger & YüNcü 1999). For the lowest rates of mass flow the number of units N is given by equation (5) below. The ratio of heat capacity and the efficiency of the heat exchanger can be given by expressions of equations (6) and (7) respectively. Equation (6) Equation (7) The ration of heat capacity is 0 for phase changing fluids and 1 when the fluid remains in one physical state during the transfer process. Similarly, mass transfer rate in gases is lower than in liquid streams. Factors that Affect the Performance of a Heat Exchanger Heat exchanger performance is increased when a close range of temperature is used in its operation. Even without an expansive operating surface area, optimum performance can be achieved provided the coefficient of heat transfer is increased. The surface area A, thermal energy transferred Q, driving force ∆T and the coefficient of heat transfer U are given by expression in equation (8) below. Equation (8) On considering phase change type and stream flow rate, estimates of the coefficient of heat transfer can easily be estimated. However, for its exact value to be found, heat transfer resistance has to be accounted for. These resistances may include the conductivity of the material, fouling effects and film coefficients. Performance of heat exchangers is affected by operation, available drop in pressure, fouling factors estimated and enhancements such as tubes, baffles, or fins. These enhancements lead to a decrease in fouling. We will discuss these modifications and their individual contribution to the heat exchanger performance (Hesselgreaves 2001). For fining, the external and internal surface areas are fitted with fins. It is a commonly used heat transfer enhancement method. It is commonly used where the transfer coefficient of the fluid is low as it is with gaseous streams. Fins lead to a larger surface area as well as the film coefficient. Consequently, the pressure drop increases. Fins are adjusted depending on the operating surface area involved in which case their number is raised when the area in increased. Fluid turbulence can also be achieved by putting inserts or turbulators inside the heat exchanger tubes. In laminar flow of viscous fluids, they can increase the film coefficient by up to five times. However, enhancement cannot be predicted following complexity of the relationship between the increases in thermal energy transfer, drop in pressure and insert geometry. Inserts also may lead to a high pressure drop where there is condensation occur within the tube Kakaç, Bergles & Mayinger 1995). Tubes or plates themselves can be skillfully deformed to form corrugated, fluted or twisted surfaces. In such surfaces, steam is condensed on the outer part as the water in side is heated. This increases the internal transfer coefficient by 400%. Deformation reduces vibration or the tubes and plates and eliminates baffles. They operate by enhancing fluid boiling and enhancing turbulence of the stream. Their limitation occurs when the pressure drops by up to twenty times then these deformed tubes are compared to the smooth ones. Baffles inserted into the tubes increase efficiency of fluid flow through the tube. When the fluid if bundled forcefully through the tube, there is a considerable loss of pressure and increases absorption of heat by introducing false surface area increase. In addition, dead areas and leakages are reduced when baffles are used. For highly viscous fluids, helical baffles shown in figure (7) below, are very effective tools which reduce shells used by 19% to 20% and are used in refineries Figure 7: Helical baffles as used in tube heat exchangers. The surface area can also be extended to increase the wetted area and hence the amount of thermal energy transferred. Methods used in creating these surfaces are injection, creation of electrostatic fields, suction and vibration of the surfaces (Roetzel & Xuan 1999). Figure (8) below represents the various modifications made on the geometry of heat exchangers to promote its optimum performance. Figure (8) Enhancements made on the heat exchanger geometry to increase its efficiency The chevrons angle is the angle of inclination of the plates in a plate heat exchanger. If the plate is inclines are too steep, fluid flow is turbulent and hence the transfer coefficient is increased leading to higher thermal energy transfer amounts. For laminar flows with lover plate elevation angle, the heat transfer rate is considerably low (Roetzel & Xuan 1999). Figure (9) below shows how corrugated chevrons are fixed on plates of heat exchangers. Figure (9): 3- dimensional diagram showing two plates of corrugated chevrons Conclusion Heat exchangers have are playing a critical role in our home appliances or in the heavy industries equipment such as the boilers. The fact that they help in the effective heat transfer for a hot to a cold fluid has improved the efficiency of the equipment. In thermal analysis, it is discovered that the heat flow in dependable. The heat lost to the environment is almost neglible. Therefore the role of heat exchangers cannot be underestimated. Looking at the various types of heat exchangers, it is recognized that each of the types have their unique characteristics which make them suitable for different uses. Contrary to the fact that heat exchangers conduct heat through conduction, they are known as heat convection tools. By involving the fluids in the tube necessitates the flow of heat through the fluids commonly referred to as convectional currents. As discussed, the shell tube heat flow has the two separate fluids flowing in tubes and transmitting the heat energy. In the case of plate heat exchangers, the stacked plates play a critical role in the creation of contact points. For single phase heat exchangers, the pressure drop and consequent increase in transfer of thermal energy are achieved through increasing the velocity of respective fluid streams and by regulating the transfer coefficient. From the discussion in this paper, it is clear that the more fouling factors an exchanger design has the bigger the component will be. Unfortunately, this leads to lower fluid flow rates and this makes fouling a problem. This can only be solved by balancing these fouling factors by periodic cleaning for smooth and fast flow of the fluid to increase the amount of thermal energy transferred. Also, various intensifications and modifications made on a heat exchanger helps raise the amount of heat transferred. Such modifications include introducing baffles, fins, and inserts in tubes, tube deformation and ensuring the chevrons angle is at optimum which in most cases is below 45°. Performance of a tube and shell heat exchanger can be increased by switching the tube and shell streams. However, this placement does not depend on which side the tube or the shell is placed but primarily on the pressure. In this case, the fluids at high pressures are put in the tubes while the stream at lower pressure is passed through the shell. References Branson, S. T. (2011). Heat exchangers types, design, and applications. Hauppauge, N.Y., Nova Science Publishers. http://search.ebscohost.com/login.aspx?direct=true&scope=site&db=nlebk&db=nlabk&AN=450551. Hesselgreaves, J. E. (2001). Compact heat exchangers selection, design, and operation. Amsterdam, Pergamon. http://site.ebrary.com/id/10206373. Horvat, A., & KončAr, B. (2003). Hierarchic modeling of heat exchanger thermal hydraulics. Proceedings. Kakaç, S., Bergles, A. E., & Mayinger, F. (1995). Heat exchangers: thermal-hydraulic fundamentals and design. Washington, Hemisphere Pub. Corp. Kakaç, S., Bergles, A. E., Mayinger, F., & YüNcü, H. (1999). Heat Transfer Enhancement of Heat Exchangers. Dordrecht, Springer Netherlands. http://dx.doi.org/10.1007/978-94-015-9159-1. Kuppan, T. (2000). Heat exchanger design handbook. New York, Marcel Dekker. http://search.ebscohost.com/login.aspx?direct=true&scope=site&db=nlebk&db=nlabk&AN=41510. Pavlovič, E., & Venko, E. (2006). Optimization of heat exchanger with natural convection in space heating devices. Congress Proceedings. Roetzel, W., & Xuan, Y. (1999). Dynamic behaviour of heat exchangers. Southampton, WIT Press/Computational Mechanics Publications. Sundén, B., & Brebbia, C. A. (2014). Heat Transfer Simulation and Experiments in Heat and Mass Transfer. SOUTHAMPTON, WIT Press. http://public.eblib.com/choice/publicfullrecord.aspx?p=1716089. Thulukkanam, Kuppan. (2014). Heat Exchangers Materials Selection, Fabrication, and Mechanical Design. CRC Pr I Llc. Read More

These plates are then sand witched leaving a space between each pair for the second fluid. The fluid’s physical properties, temperature range, drop in pressure and rate of flow determines the number of plates in each exchanger. Individual plates are corrugated to increase turbulence in the fluids and consequently raise the efficiency of the heat exchanger. Its application determines corrugation pattern used (Kuppan 2000). Surface area of heat transfer is a determinant factor of the amount of thermal energy transferred.

When the media is distributed evenly over the transfer area, efficiency of the heat exchanger increases. Therefore, the heat exchangers are fitted with special area for distributing the fluids across the surface area near the port. This special area spreads the media on the plate face ensuring it is uniformly spread across the transfer surface as displayed in figure (1). Figure 1: A plate from a gasketed plate heat exchanger In one type of such an exchanger, the fluid that enters the exchanger from the bottom flows through the layers until it gets to the top.

Likewise, for the fluid that enters through the top, it flows until it reaches the bottom layer then it flows out. These media flow counter to each other and therefore there is maximum heat transfer between them (Kuppan 2000). Most plate exchangers are single pass design. In this type, the two fluids flow once against each other. In such an exchanger, the fluids flow as shown in figure (2) below. Figure 2: fluid flow in a single pass plate heat exchanger Another plate exchanger design is the multi pass type with two or more passages.

In this case, fluids counter flow more than once within the equipment. In figure (3) below is an example of a two pass design of the exchanger. Figure 3: Two pass plate exchanger Comparison of Various Types of Heat Exchangers The common types of heat exchangers are those of the plate and the shell and tube. However, other types include the open flow and the contact heat exchangers. Figure (4) below gives a summary diagram for most heat exchanger types. a)Shell and tube b)plates c) open flow d) rotating wheel Figure (4): Types of Heat Exchangers Heat exchangers are classified according to thermal transfer process as indirect or direct contact, geometrical construction as plate, tubes or extended surface, mechanism of heat transfer such as single or multi phase flow.

They can also be classified depending on the arrangement of flow which include parallel, cross flow or counter flow. Finally, they can be simply classified into recuperators and generators. Heat exchangers can also be open flow or closed flow. In this paper, we will analyze some of these heat exchanger types (Kuppan 2000). Open flow contact heat exchangers do not have both streams of fluid piped. This enables the fluids to transfer energy by mixing and a consequent mass flow transfer is also achieved.

An example is the car radiator where final heat is released to ambient air from a liquid and their origin is tube banks that are cooled by air. These heat exchangers are also used in manufacture of refrigeration and air conditioning condensers. For closed flow, the individual fluid streams are put in separated and enclosed pipes hence there is no mixing during heat transfer. Contact Heat Exchangers In contact heat exchangers, fluid streams are directly into contact and transfer of both mass and heat occurs simultaneously.

When fluids separate using gravitational forces after being continuously in contact, it is referred to as a cooling tower. In regenerative exchangers, this contact involves a third external medium such as a solid rotating wheel in that the hot gas will lose its heat to the wheel as the cold gas retrieves this thermal energy. Intended contamination of these fluids may occur as the case with cooling towers or it may be inevitable as it is with heat recuperators for air conditioning.

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