Basic Design Methods of Heat Exchange.pptx

Heat Exchanger

1. ADVANCED THERMODYNAMICS & HEAT ENGINES Presented by Saurab Kumar Yadav M.Sc. in Mechanical System Design & Engineering I/I 1 HEAT EXCHANGER

2. INTRODUCTION Heat exchangers are mechanical devices that help two fluids with differing temperatures exchange heat. Heat exchangers find widespread usage in various applications, ranging from home air conditioning and heating to large-scale chemical processing and power generation. Heat can be transferred between two fluids in an exchanger through direct contact or by transmission through a wall that separates the fluids. While the latter are known as regenerators, recuperators , or surface exchangers, the former are referred to as direct contact heat exchangers. 2

3. TYPES OF HEAT EXCHANGER Heat exchangers are typically categorized based on the orientations in which the hot and cold fluids flow relative to one another, or based on the distribution of temperatures between the two fluids along the exchanger's length. As a result, the following kinds of heat exchangers could exist: Parallel-flow heat exchanger Counter-flow exchanger Cross-flow exchanger 3

4. PARALLEL-FLOW HEAT EXCHANGER The term "parallel-flow" refers to the direction in which the hot and cold fluids flow in this kind of heat exchanger. The temperature differential between the hot and cold fluids in a parallel-flow exchanger continuously decreases from the inlet to the exit. This class includes a wide range of devices, including water heaters, oil warmers, oil coolers, etc. 4

5. COUNTER-FLOW EXCHANGER The term "counter flow" refers to the situation in which the two fluids flow through the exchangers in opposing directions. Compared to the parallel-flow type, it is evident that the temperature differential between the two fluids stays more or less constant. Because they provide the highest heat transfer rate for a given surface area, counter-flow heat exchangers are the best equipment for heating and cooling fluids. 5

6. CROSS-FLOW EXCHANGER The two fluids of a cross flow heat exchanger flow at right angles to one another. 6

7. ACCORDING TO CONSTRUCTION 7 Double-pipe heat exchanger Shell-and-tube heat exchanger

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9. FOULING FACTOR Heat-transfer surfaces of an exchanger may corrode due to the interaction of the fluids with the material used in the heat exchanger's construction, or they may get coated with different deposits found in the flow systems after a period of operation. In either scenario, the performance is reduced since this coating adds to the heat flow resistance. Usually, a fouling factor or fouling resistance is used to indicate the total effect. This resistance must be considered in addition to the other thermal resistances that make up the overall heat transfer coefficient. 9

10. FOULING FACTOR By calculating the values of U in the heat exchanger under both clean and dirty situations, fouling factors must be acquired experimentally. Thus, the definition of the fouling factor is 10

11. ANALYSIS OF HEAT EXCHANGERS Heat exchangers are analyzed to determine the heat transfer rate for a given temperature change in a fluid stream of known mass flow rate, or to predict the outlet temperatures of the hot and cold fluid streams in a specified heat exchanger. For the first task, the effectiveness–NTU approach works well, and for the second, the log mean temperature difference, or LMTD method. 11

12. The temperature difference between the hot and cold fluids varies along the heat exchanger, and therefore, it is convenient to have a mean temperature difference for use in the relation   12 LMTD for PARALLEL FLOW HEAT EXCHANGER

13. 13 An energy balance on each fluid in a differential section of the heat exchanger can be expressed as …(3) …(2) Rearranging the above equations for d and d gives   …(1) …(4) Taking their difference , …(5)

14. 14 The rate of heat transfer in the differential section of the heat exchanger can also be expressed as ...(7) Substituting this equation into Equation (5) ...(6) Integrating from the inlet of the heat exchanger to its outlet ...(8) ...(9) ...(10)

15. 15 Rearranging for heat transfer w hich can be expressed as w here , is the log mean temperature difference, which is the suitable form of the average temperature difference for use in the analysis of heat exchangers.

16. 16 Here and represent the temperature difference between the two fluids at the two ends (inlet and outlet) of the heat exchanger. It makes no difference which end of the heat exchanger is designated as the inlet or the outlet.   The temperature difference between the two fluids decreases from at the inlet to at the outlet.  

17. 17 The logarithmic mean temperature difference is obtained by tracing the actual temperature profile of the fluids along the heat exchanger and is an exact representation of the average temperature difference between the hot and cold fluids. It truly reflects the exponential decay of the local temperature difference.  

18. 18 LMTD for COUNTER FLOW HEAT EXCHANGER In this scenario, the cold fluid's exit temperature may be higher than the hot fluid's because the hot and cold fluids enter the heat exchanger from different ends . The cold fluid will be heated to the hot fluid's inlet temperature in the limiting scenario. .

19. 19 Heat exchangers with counter-flows can also use the log mean temperature difference that was created for parallel-flow heat exchangers. But this time, and are expressed as shown in figure.  

20. 20 A counter-flow heat exchanger's log mean temperature differential is always larger than a parallel-flow heat exchanger's for given intake and exit temperatures. In a counter-flow heat exchanger, a lower surface area—and consequently, a smaller heat exchanger—is therefore required to obtain a given heat transfer rate. For this reason, using counter-flow arrangements in heat exchangers is standard procedure.

21. 21 MULTI-PASS AND CROSS-FLOW HEAT EXCHANGERS The previously proposed log mean temperature difference ( ) relation is exclusive to heat exchangers that operate in parallel or counterflow . Similar relations are also derived for multi-pass shell-and-tube heat exchangers and cross-flow heat exchangers, but the complex flow conditions in the resulting expressions make them too complicated.  

22. In these situations, it is practical to link the counter-flow case's equivalent temperature difference to the log mean temperature difference ratio as where F is the correction factor, which is determined by the heat exchanger's design as well as the temperatures of the hot and cold fluid streams at the intake and outflow . Figure presents the correction factor F for typical cross-flow and shell-and-tube heat exchanger systems versus two temperature ratios, P and R, which are defined as 22

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25. NTU METHOD 25 When the type and size of the heat exchanger are defined, the NTU Method is used to determine the heat transfer rate, as well as the outlet temperatures of the hot and cold fluids, for the prescribed fluid mass flow rates and inlet temperatures. This method is based on a dimensionless parameter called the heat transfer effectiveness , defined as  

26. NTU METHOD An energy balance on the hot or cold fluids can be used to calculate the real heat transfer rate in a heat exchanger, which can be stated as where and are the capacity rates of the cold and hot fluids, respectively. The greatest temperature differential in a heat exchanger is the difference between the hot and cold fluids' entrance temperatures. This knowledge is necessary to calculate the highest feasible heat transfer rate in a heat exchanger.   26

27. 27 The maximum heat transfer rate in a heat exchanger is thus the temperature change that the fluid with the lower heat capacity rate will undergo. The actual heat transfer rate can be determined from In addition to flow arrangement, heat exchanger geometry determines heat exchanger efficacy. Hence, the effectiveness relations of various heat exchanger types vary. Equation for a parallel-flow heat exchanger can be rearranged as

28. 28 Solving for   Substituting this relation into above equation

29. 29 We now manipulate the definition of effectiveness to obtain Substituting this result into the above Solving for gives the following relation for the effectiveness of a parallel-flow heat exchanger  

30. 30 Taking either or to be , the relation above can be expressed more conveniently as   is the smaller heat capacity ratio and is the larger one, and it makes no difference whether belongs to the hot or cold fluid.   Effectiveness relations of the heat exchangers typically involve the dimensionless group U / . This quantity is called the number of transfer units NTU and is expressed as   NTU is proportional to . For specified values of U and , the value of NTU is a measure of the heat transfer surface area . The larger the NTU, the larger the heat exchanger.  

31. 31 In heat exchanger analysis, it is also convenient to define another dimensionless quantity called the capacity ratio c as The capacity ratio (c) and the number of transfer units (NTU) determine a heat exchanger's performance . = function (U / , / ) = function (NTU, c)  

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35. 35 The efficacy has a value between 0 and 1. For small values (up to around NTU 1.5), it increases quickly with NTU; however, for larger values, it increases rather slowly. As a result, it is not economically justified to employ a heat exchanger with a big NTU (often greater than 3) and thus a large size, as a large increase in NTU in this instance only results in a slight increase in effectiveness. Therefore, from the perspective of heat transmission, a heat exchanger with a very high efficacy may be quite desired, but from an economical perspective, it may be relatively unattractive.

36. 36 The cross-flow heat exchangers with both fluids unmixed are almost as effective as the counter-flow heat exchangers for a given NTU and capacity ratio c. For NTU values less than around 0.3, a heat exchanger's effectiveness is independent of the capacity ratio c.

37. SELECTION OF HEAT EXCHANGERS 37 Heat transfer requirements Temperature and pressure conditions Flow rates Pressure drop limitations Space and weight constraints Material compatibility Fouling tendencies Maintenance and cleaning needs Cost considerations