Design of a Heat Exchanger Test Rig - Report Example

Design of a Heat Exchanger Test Rig Name Institutional Affiliation Date Table of Contents Literature Review 2 Introduction 2 Comparison of Various Types of Heat Exchangers 4 Working Principle of Plate Heat Exchangers 7 Heat Exchanger Modelling 14 Non-Dimensional Method Heat Exchangers’ Modelling 15 References 17 Literature Review Introduction Several scientific explanations have been done on the field of heat transfer and heat exchangers. Several laws of physics have been proved and accepted for conventional use in general application purposes. Such laws of physics suggest that heat has the ability to move from a body with higher temperatures into a body with lower temperatures. It, therefore, means that for heat transfer to take place there must be temperature difference between the two bodies (Hesselgreaves, 2001). However, heat transfer from one body to another takes place through various methods. Such methods include radiation, conduction and convection. Depending on the nature of matter involved, a specific method of heat transfer is always involved. Radiation normally involves energy transfer in form of electromagnetic radiations. The transfer of heat from sun to the earth is through the process of radiation. Heat transfer within solids takes place through conduction (Kakaç, 1999). Convection, however, is the transfer of heat by mixing of one part of a medium with another. Convection is a common means of heat transfer between fluids. Heat exchangers, therefore, perform work through such principles of heat. In the normal plate type heat exchangers, penetration of heat through the surface separating cold and hot medium occurs easily (Kakaç & Liu, 1998). Therefore, with the use of heat exchanger, cooling or heating of fluids with minimal levels of energy becomes possible. Heat exchangers are majorly a device that has the ability to transfer heat from one medium to another continuously. Two main types of exchangers have been developed over the last years. Such heat exchangers can be categorized as either a direct or an indirect heat exchanger. Thermal analysis, as well as proper design and use of heat exchangers, requires vast knowledge of fluid dynamics analysis for purposes of analyzing fluid flow, the mechanism for resistance and closure of fluids, as well as material knowledge in order to determine the most appropriate type of materials to be used (Kothandaraman, 2006). Kothandaraman (2006) asserts that heat exchangers are globally assumed to be operating under adiabatic conditions. It, therefore, means that, heat losses or gains between the heat exchangers and the environment are negligible. Further, thermal inertia of the heat exchanger becomes negligible. Therefore, the general balance equation of energy is reduced to: Where the total energy ht is a value that can be approximated by enthalpy and ∆ stands for the difference between the output and the input. Comparison of Various Types of Heat Exchangers Plate type heat exchangers consist of compacted heat exchanger where thin corrugated plates preferably 1 to 2 mm thick are arranged in a stacking manner and placed in such a way they remain in contact (Kothandaraman, 2006). The two fluids are allowed to flow along separate but adjacent channels in the corrugations. Appropriate channels , in most cases, are supported by the gaskets that control the flow of the two fluids, and can assist in allowing for parallel or cross flow in the required number of passes. However, in most cases, one pass is normally preferred The plate assembly in the plate type heat exchangers is as shown below. Figure 1: Plate Assembly for Plate Type Heat Exchangers Within the constraints of the equipment, widest pressure and temperature limits are offered by the plate type heat exchangers within the constraint of the equipment. Thus, plate type heat exchanger has been considered to have offered solutions to many of the thermal problems (Kuppan, 2000). There are several advantages that a plate heat exchanger offers to their user over the other types of heat exchangers. Dissimilar to the tradition of tube heat exchanger and shell, plate type exchangers occupy minimal space by far. The heat exchanger is also made of very thin materials on the heat transfer surface therefore giving it optimum heat transfer capabilities. Plate heat exchangers have a characteristic of higher turbulence. Such high turbulence results in a higher convection that leads to efficient heat transfer between the media (Kothandaraman, 2006). It therefore means that there will be a higher heat transfer coefficient per unit area making the whole operation efficient. Kreith & Bohn (2001) note that high turbulence has another advantage in the plate type heat exchanger. It results in self cleaning effect, therefore when compared with the other traditional types of heat exchangers like tube heat exchanger and shell, the fouling of the heat exchanger’s surface due to accumulation of debris is highly reduced. The construction and design of the plate type heat exchangers have put into consideration the need for expansion of its capacity. The framework consists of various exchanger plates than can easily be extended to accommodate or increase its capacity. The design of the plate type heat exchangers have also been done in such a way that, it is easy to open and carry out cleaning purposes when needed (Annaratone, 2010). However, this functionality is only limited to the casketed heat exchangers and not the brazed or fusion bonded units. Alifa Laval is among some of the greatest manufactures of plate type heat exchangers. Alifa Laval manufactures heat exchangers by employing the use of two different pressing patterns. Research has shown that, if the pattern of the plate is wider, the drop of pressure is smaller and, as a result, there is a smaller heat transfer coefficient (Kuppan, 2000). Therefore, this heat exchanger’s thermal channel will be shorter. However, if two plates that are subjected to a pressing pattern that is different are positioned next to each other, the output becomes a mixture of characteristics of short and long channels as well as mixed characteristics in pressure drop and effectiveness. There are some limitations in the plate type heat exchangers. Such limitation includes the maximum allowed pressure. Most plate type heat exchangers allow pressure up to a maximum of 1Mpa although there are emergence of other design that can allow up to a maximum pressure of 4Mpa (Rohsenow, Hartnett & Cho, 1998). Other limitations include the maximum allowed temperature range. For those plate type heat exchangers that use casketed material, a temperature of up to 150 ˚C. However, new designs in the market have been found to allow up to a maximum of 400 ˚C. In most cases, typical plate type heat exchangers are used for applications involving the heat transfer from one liquid to another. However, special plate design are in the market. Such designs have been developed to carry out process involving phase change. The developments of plate type heat exchangers are dated back to 1920s where it was developed mainly for use in food industries (Russell, Robinson & Wagner, 2008). It was purposely used for milk pasteurization. However, since then, the plate heat exchangers are taking over the market mainly because of its compactness and efficiency. Working Principle of Plate Heat Exchangers For a plate type heat exchanger, channels are formed between the plates and the inlet and outlet ports are arranged in such a way that, the two media with different temperatures pass through alternate channels (Serth, 2007). Heat transfer takes place through the plates between the channels thus creating a complete counter current flow that ensures that there is a higher efficiency. The corrugation of the plates acts as the channel of passage between the plates and supports each plate against the adjacent one thus resulting in turbulence. Turbulence in heat exchangers has been reported to be of a positive effect in the plate type heat exchangers. They ensure efficient heat transfer in the system. Figure 2: Arrangement of Heat Exchanger Plates and Type of Flow Currently, the shell & tube exchangers (STHE) are in use. STHE traces its origin from the jacketed coil distiller. However, it is mainly used in heavy industries as well as in residential hot water heating system. Steam condensers and boilers used in heavy commercial industries mostly employ the use of STHE (Wang, Sundén & Manglik, 2007). In the tube side, a pass normally stands for direct change in each main flow. Figure 3: Small Mounted STHE in Copper and a Cut out of the Similar Figure 4: The Schematic of an STHE with One Tube Pass & One Shell Pass Performing thermal analysis for any heat exchanger relies on its coaxial configurations that are normally simple (Kuppan, 2000). In most cases, a single flows in the pipe while the other in an annular section within a large cylindrical sheath. The sheath normally have opening at their ends. An example of a coaxial cross section is as shown below. Figure 5: Simple Annular Heat Exchangers In figure 5, (a) shows the temperature profile and sketch for a counter-flow configuration whereas (b) is for a co-flow. (c) is a representation of a cross-section sketch whereas (b) shows details across the separation surface. Normally, the transfer of heat from one fluid to another a combination of processes of conduction and convection (Wang, Chen, & Sunden, 2013). Therefore, at a steady condition, heat flux through the medium of separation between the two fluids from the hotter to the colder fluid is as shown below. For the case of overall heat transfer co-efficient, K is not effectively defined because it is only the product of co-efficient of heat transfer and area is considered in the equation (KA). Area between the two fluids has also posed a great challenge in its definition and measurement due to the corrugations. Boiler is another type of a heat exchanger that finds application in large scale and commercial purposes only (Kuppan, 2000). A boiler is a system of enclosed tubes where water is heated and in turn forms heat that is used for heating purposes. In cases where, an alternative fluid other than water is used, such a system is then referred to as a vaporizer (Serth, 2007). Most boilers are fired by fuel, therefore, can be viewed as shell & heat tube exchanger (Serth, 2007). In such cases, hot fluid is the burnt gas that produces heat due to its combustion. The cold fluid will thus be the water that is heated to form steam. Boilers, however experiences heat transfer through radiation due to the high temperaturesattained. Mostly the temperatures in the boiler go to as high as 2000 ˚K. Modern condensation boilers have been experimented and found to have an efficiency of close to 100%. This is as a result of the exhaust gases that are normally very hot have always been used in pre-heating of the gas and even fuel before the actual combustion. A boiler consumes a lot of energy, both at commercial or industrial and even at domestic levels. thus before a decision to use a boiler for a particular heating process, analysis of its viability is very important in order to determine the proper selection, operation and even maintenance. Boiler specifications are made depending on several variables. Such variables include mass flow rate capacity required, pressure level, fuel used as well as cost consideration both onthe requirements of the boiler as well as its productivity Hot gases out of the burner of a fire tube boiler pass through several tubes arranged in a particular manner and are surrounded by water that is to be heated. Fire tube boilers are both cheap and easy to maintain and their design are compact as compared to water tube boilers. In water tube boilers, water flows through the tubes that are fired by a furnace (Russell, Robinson & Wagner, 2008). The tubes are connected in such a way that there a mud drum bellow and a steam drum above. Water tube boilers are basically used for hot steam production for use in large scale applications. Figure 6: Fire-Tube Boiler Water tube boilers can produce steam of a higher pressure, normally up to 34MPa and temperatures of up to 900 K. Moreover, water tube boilers have been found to be more expensive than the fire tube types. According to Russell & Wagner (2008), boilers should be used under lot of precautions. The high pressure steam and temperatures generated by steam boilers are very dangers if not well controlled. Several safety measures are normally pre-installed in any boiler when in use. Suchsafety measures are normally meant to counteract any possible hazard that may result from any malfunctioning part of a boiler. Both reversible and irreversible safety valves are used that are normally spring loaded are used as a safety precaution. Reversible safety valve will close during low pressure recovery while the irreversible safety valve is meant to close during abnormal malfunctions. Apart from the safety devices, other One of the problems of heat transfer is majorly the impact of contamination on the surface (Kothandaraman, 2006). The contamination of the surface could be due to chemical reactions between foreign solids or elements in water with the steam or deposition in heat radiation surfaces. Fouling in heat exchangers is a term referring to the deposition of dirt and other deposits the surface of the heat exchanger plate (Kuppan, 2000). Fouling has a very bad effect on heat exchanger as it increases thermal resistance. Furthermore, fouling has proved to be one of the hardest deposits to clean there foe making the maintenance of such heat exchangers very expensive. Fouling furthermore reduces the life span of the heat exchanger especially if the heat exchanger plate is not subjected to regular cleaning practises. Fouling can be said to result from algae development on cold surfaces or from deposition of on hot surface. Clogging of unfiltered dirt on the heat exchanger plates is also one other major cause of fouling. All commercial, industrial or even domestic circuits cooled using the natural sea or fresh water is subjected to some biological foulings. This normally results from bio film settlements of living organisms (Kothandaraman, 2006). Several methods of getting rid of fouling in heat exchangers have been put in place. Although these methods do not guarantee a perfect functionality, they have helped to reduce the effect of fouling. Such methods include screening, physical screening and chemical dosing. Restriction from use of chemicals such has chlorine has impinged the use of chemicals in the process. The applications of heat plate exchangers are very diverse. Heat plate exchangers have been used at both industrial and domestic levels. Hot water heating system in most homes is based on heat plate exchanger technology (Kothandaraman, 2006). Hotels, hospitals, restaurants, military bases and many more institutions use plate heat exchanger for heating of water that is used for various purposes. In cases where instantaneous hot water is required for wash down purposes, heat exchanger is used. This therefore eliminates the need for large volumes of stored water. Some swimming pools that use hot water have been linked to the use of plate heat exchangers or boilers in maintaining of the swimming pool temperatures at the required levels. Highly sophisticated boilers are required in this case. The boilers should therefore have the ability to handle chlorinated water efficiently. Heat exchangers can be used in minimizing heat wastage by way of recovering the optimum amount of heat energy from waste products. Heat exchangers have also been used in chilled water systems. The heat exchangers are used in this case to maintain system temperatures close to those leaving the chillers. Heat Exchanger Modelling Considering a differential slice whose length dx along some counter-current heat-exchanger, the equation below can represent longitudinal energy balance and transversal energy balance. Whereas: T1 = the transversal flow of heat flow for hot fluid and T2 for cold dx = length dx dA = wetted annular It is assumed that the perfect-substance model eases the writing. Phase changes are treated with regard to enthalpies instead of temperature. Integrating equation four from x=0 to x=L, the heat exchanger will yield Equation 5. Where: ΔT12= mean K = overall heat-transfer coefficient \ A = choice of wet area Average temperature can be obtained through integration of equation 4 and 5. If equation 6 is integrated from x = 0 to x = L, the product is equation 7. Equation can be simplified to 8 as shown below: Testing problem is a process carried out in situation where the heat exchanger undergoes bench-testing by way of measuring all output and input parameters. Non-Dimensional Method Heat Exchangers’ Modelling The modelling methods illustrated above offer full dimensions where eight variables are linked by other equations. However, solving the performance problem stated above gives a system that is non-linear because of equation (8) LMTD. Further, this problem can be solved by employing the use of a different method that is based on a non-dimensional variable that was introduced by researcher Kay in London, back in 1955. This method mainly takes into consideration the several transfer units (N) which are defined by the lowest of the 2 thermal mass-flow-capacities. Therefore, the heat exchanger efficiency, η & heat capacity ratio, c, is defined by: The c, normally ranges from 0 in situations where fluid changes phases to1 in the cases where there is no phase change in both fluids (Annaratone, 2010). In a situation whereby one fluid is gaseous and the other liquid, mc-value for gas stream lowest as compared to its counterpart liquid and c=0. References Bottom of Form Top of Form Bottom of Form Top of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Top of Form Top of Form Bottom of Form Bottom of Form Bottom of Form ANNARATONE, D. (2010). Engineering heat transfer. Heidelberg, Springer. http://public.eblib.com/choice/publicfullrecord.aspx?p=571021. 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. ÇENGEL, Y. A. (1997). Introduction to thermodynamics and heat transfer. New York, McGraw-Hill. HESSELGREAVES, J. E. (2001). Compact heat exchangers selection, design, and operation. Amsterdam, Pergamon. http://site.ebrary.com/id/10206373. KAKAÇ, S. (1999). Heat transfer enhancement of heat exchangers. Dordrecht, Kluwer Academic Publishers. KAKAÇ, S., & LIU, H. (1998). Heat exchangers: selection, rating, and thermal design. Boca Raton, Fla, CRC Press. KOTHANDARAMAN, C. P. (2006). Fundamentals of heat and mass transfer. New Delhi, New Age International (P) Ltd., Publishers. http://site.ebrary.com/id/10318686. KREITH, F., & BOHN, M. (2001). Principles of heat transfer. Australia, Brooks/Cole Pub. 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. ROHSENOW, W. M., HARTNETT, J. P., & CHO, Y. I. (1998). Handbook of heat transfer. New York, McGraw-Hill. RUSSELL, T. W. F., ROBINSON, A. S., & WAGNER, N. J. (2008). Mass and heat transfer: analysis of mass contactors and heat exchangers. Cambridge, Cambridge University Press. SERTH, R. W. (2007). Process heat transfer principles and applications. Amsterdam, Elsevier Academic Press. http://search.ebscohost.com/login.aspx?direct=true&scope=site&db=nlebk&db=nlabk&AN=198948. WANG, L., SUNDÉN, B., & MANGLIK, R. M. (2007). Plate heat exchangers: design, applications and performance. Southampton, WIT Press. WANG, Q., CHEN, Y., & SUNDEN, B. (2013). Emerging Topics in Heat Transfer Enhancement and Heat Exchangers. SOUTHAMPTON, WIT Press. http://public.eblib.com/choice/publicfullrecord.aspx?p=1572957. Top of Form Bottom of Form Top of Form Bottom of Form Read More

Appropriate channels , in most cases, are supported by the gaskets that control the flow of the two fluids, and can assist in allowing for parallel or cross flow in the required number of passes. However, in most cases, one pass is normally preferred The plate assembly in the plate type heat exchangers is as shown below. Figure 1: Plate Assembly for Plate Type Heat Exchangers Within the constraints of the equipment, widest pressure and temperature limits are offered by the plate type heat exchangers within the constraint of the equipment.

Thus, plate type heat exchanger has been considered to have offered solutions to many of the thermal problems (Kuppan, 2000). There are several advantages that a plate heat exchanger offers to their user over the other types of heat exchangers. Dissimilar to the tradition of tube heat exchanger and shell, plate type exchangers occupy minimal space by far. The heat exchanger is also made of very thin materials on the heat transfer surface therefore giving it optimum heat transfer capabilities.

Plate heat exchangers have a characteristic of higher turbulence. Such high turbulence results in a higher convection that leads to efficient heat transfer between the media (Kothandaraman, 2006). It therefore means that there will be a higher heat transfer coefficient per unit area making the whole operation efficient. Kreith & Bohn (2001) note that high turbulence has another advantage in the plate type heat exchanger. It results in self cleaning effect, therefore when compared with the other traditional types of heat exchangers like tube heat exchanger and shell, the fouling of the heat exchanger’s surface due to accumulation of debris is highly reduced.

The construction and design of the plate type heat exchangers have put into consideration the need for expansion of its capacity. The framework consists of various exchanger plates than can easily be extended to accommodate or increase its capacity. The design of the plate type heat exchangers have also been done in such a way that, it is easy to open and carry out cleaning purposes when needed (Annaratone, 2010). However, this functionality is only limited to the casketed heat exchangers and not the brazed or fusion bonded units.

Alifa Laval is among some of the greatest manufactures of plate type heat exchangers. Alifa Laval manufactures heat exchangers by employing the use of two different pressing patterns. Research has shown that, if the pattern of the plate is wider, the drop of pressure is smaller and, as a result, there is a smaller heat transfer coefficient (Kuppan, 2000). Therefore, this heat exchanger’s thermal channel will be shorter. However, if two plates that are subjected to a pressing pattern that is different are positioned next to each other, the output becomes a mixture of characteristics of short and long channels as well as mixed characteristics in pressure drop and effectiveness.

There are some limitations in the plate type heat exchangers. Such limitation includes the maximum allowed pressure. Most plate type heat exchangers allow pressure up to a maximum of 1Mpa although there are emergence of other design that can allow up to a maximum pressure of 4Mpa (Rohsenow, Hartnett & Cho, 1998). Other limitations include the maximum allowed temperature range. For those plate type heat exchangers that use casketed material, a temperature of up to 150 ˚C. However, new designs in the market have been found to allow up to a maximum of 400 ˚C.

In most cases, typical plate type heat exchangers are used for applications involving the heat transfer from one liquid to another. However, special plate design are in the market. Such designs have been developed to carry out process involving phase change. The developments of plate type heat exchangers are dated back to 1920s where it was developed mainly for use in food industries (Russell, Robinson & Wagner, 2008). It was purposely used for milk pasteurization. However, since then, the plate heat exchangers are taking over the market mainly because of its compactness and efficiency.

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