Multipass exchangers

One particularly important property of the relationships for multipass exchangers is illustrated by the two streams shown in Fig. E.l. The problem overall is predicted to require 3.889 shells (4 shells in practice). If the problem is divided arbitrarily into two parts S and T as shown in Fig. El, then part S requires 2.899 and Part T requires 0.990, giving a total of precisely 3.889. It does not matter how many vertical sections the problem is divided into or how big the sections are, the same identical result is obtained, provided fractional (noninteger) numbers of shells are used. When the problem is divided into four arbitrary parts A, B, C, and D (Fig. E.l), adding up the individual shell requirements gives precisely 3.889 again. 

The cases of multipass exchangers with liquid contimionsly added to the tank are covered by Kern, as cited earlier. An alternative method for all mnltipass-exchanger gases, including those presented as well as cases with two or more shells in series, is as follows ... 

With respect to selecting measurements, emphasis should include measurements within the equipment such as tower internal temperatures and compositions, internal reac tor conditions, and intermediate exchanger temperatures in multipass exchangers. Trace component compositions provide particular insight into distillation-column performance. Those components that fall between the heavy and light keys and distribute in the products can usually be described by a variety of models and parameter estimates They provide little insight into the column performance. 

Correction factors are given in Figures 10-34A-F to modify the true counter-current LMTD for the multipass exchanger... 

In most multipass exchangers, a combination of counter-current and co-current flow exists as the fluid flows through alternate passes (see Figure 10-29). The mean temperature is less than the logarithmic mean calculated for counter-cur-rent flow and greater than that based on co-current flow. 

Ratnam and Patwardhan present graphs to aid in analyzing multipass exchangers, based on equations developed. Turton, et al. also presents performance and design charts based on TEMA charts (Figure 10-34J) and combining these with Temperature Efficiency Charts from TEMA (Figures 10-35A-C). 

Bowman RA (1936) Mean Temperature Difference Correction in Multipass Exchangers, Ind Eng Chem, 28 541. 

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... 

Determine the thermal effectiveness. The thermal effectiveness of a 1-2 multipass exchanger can be determined from Fig. 7.15. Now,... 

Temperature control, 42,45-47 Temperature difference, 172 logarithmic mean, 172 multipass exchangers, 173, 175-177 Temperature profiles, heat exchangers,... 

K. A. Gardner. "Variable Heat Transfer Rate Correction in Multipass Exchangers, Shell Side Filin Controlling. Transactions of the ASME 67 (1945), pp, 31-38. 

Mean Temperature Difference 172 Single Pass Exchanger 172 Multipass Exchangers 173 F-Method 173... 

Figure 8-1. Flow patterns in a multipass exchanger (1 2 floating head as shown).

Because S cannot be expressed explicitly. Equation 8-179 can be solved only by trial and error, assuming different values of S until an equality is attained. The equations for batch heating and cooling are the same as those developed for the 1-2 multipass exchangers, except that the value of S from Equation 8-179 replaces the value of S from Equation 8-178. 

Such a long tube needs many 1-1 shell-and-tube exchangers in series or a multipass exchanger with many passes on the tube side. A 1-1 type heat exchanger is considered here for simplicity. 

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. 

MULTIPASS EX, a] ERS. In multipass shell-and-tube exchangers the flow pattern is complex itlrparallel, countercurrent, and crossflow all present. Under these conditions, even when the overall coefficient t/is constant, the LMTD cannot be used, Calculation procedures for multipass exchangers are given in Chap. 15. 

An even number of tube-side passes is used in multipass exchangers. The shell side may be either single-pass or multipass. A common construction is the... 

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