Countercurrent multipass heat exchanger

In the field of heat transfer, a good example of this category of shortcut design method is the famous F correction factor to correct the log mean temperature difference of shell and tube heat exchangers for deviations from true countercurrent flow. For multipass heat exchangers, the assumptions are ... 

Related Calculations. Mean temperature differences for multipass heat exchangers may also be calculated by using appropriate correction factors for the log mean temperature difference for countercurrent flow... 

The flow is either entirely cocurrent or countercurrent, as shown in Figures 6.42a, 6.42b, 6.43a, and 6.43b. This assumption is relaxed below for many multipass or cross-flow heat exchangers. 

The overall heat transfer coefficient is a composite number. It depends on the individual heat transfer coefficients on each side of the tube and the thermal conductivity of the tube material. The individual heat transfer coefficient in turn depends on the fluid flow rate, physical properties of the fluid, and dirt factor. The temperature along the tube is not uniform. The hot and the cold fluids may flow in the same (cocurrent) or in opposite (countercurrent) directions. Generally the hot and cold fluids come in contact only once, and such an exchanger is called single pass. In a multipass exchanger, the design of the... 

Parallel flow is rarely used in a single-pass exchanger such as that shown in Fig. 11.3 because, as inspection of Fig. 11,4a and b will show, it is not possible with this method of flow to bring the exit temperature of one fluid nearly to the entrance temperature of the other and the heat that can be transferred is less than that possible in countercurrent flow. In the multipass exchangers, described on pages 430 and 431, parallel flow is used in some passes, largely for mechanical reasons, and the capadty and approaches obtainable are thereby affected. Parallel flow is used in special situations where it is necessary to limit the maximum temperature of the cooler fluid or where it is important to change the temperature of at least one fluid rapidly. 

Flow arrangements in industrial exchangers are rarely of the simple 1 tube pass, 1 shell pass type shown in Figure 7.28. Fluids are usually passed through the shell and tubes more than once (in different directions) in order to increase the flow velocity and hence the heat transfer for a given area. In some sections of these multipass exchangers the flow is countercurrent and in others co-current. Correction factors to AT for multipass exchangers are available s. 

In a truly countercurrent system with no phase change and constant heat capacities in both fluids, the proper mean temperature differential becomes the logarithmic mean of the two terminal temperature differentials (LMTD). In Section 9.S.2.4 on caustic cooling, we rely on this approach. The literature contains standard corrections to the LMTD for configurations that do not allow true countercurrent flow, as for example in multipass shell-and-tube exchangers [9]. 

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