Multipass

Ahmad, S., LinnhoflF, B., and Smith, R., Design of Multipass Heat Exchangers An Alternative Approach, Trans. ASME, J. Heat Transfer, 110 304, 1988. 

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. 

Experiments were carried out on samples, made of austenitic steel with thickness from 10 to 50 mm using equipment, described above. The samples were a) multipass austenitic weld and b) base metal. 

Figure 2 a) multipass austenitic weld and b) base metal. 

Experiment was carried on samples, made of austenitic steel type 316 (18-12), with the thickness of 15 mm. The samples were in the form of a segment of a pipe with the diameter 300 mm having multipass welding. One of the variants of the sample is shown in Figure3. 

Le Blanc C, Grillon G, Chambaret J P, Migus A and Antonetti A 1993 Compact and efficient multipass Ti sapphire system for femtosecond chirped-pulse amplification at the terawatt level Opt. Lett. 18 140-... 

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 ... 

Use the average of the two values for F, then increase the required multipass UA as follows ... 

Condensers The vapor from the last effect of an evaporator is usually removed by a condenser. Surface condensers are employed when mixing of condensate with condenser coohng water is not desired. They are for the most part shell-and-tube condensers with vapor on the shell side and a multipass flow of cooling water on the... 

Multipass clarifier Continuousf Batch manual Zero To 3,000 gal/h... 

The annular gap mill shown in Fig. 20-36 is avariation of the bead mill. It has a high-energy input as shown in Fig. 20-37. It may be lined with polyurethane and operated in multipass mode to narrow the residence-time distribution and to aid cooling. 

Among new mill developments, annular-gap bead mills and stirred bead mills are being used. These have a high cost, but result in a steep particle-size distribution when used in multipass mode [Kolb, Ceramic Forum International, 70(5), 212-216 (1993)]. Costs for fine grinding typically exceed the cost of raw materials. Produces are used for high-performance ceramics. 

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. 

Elimination. In multipass welding, care must be exercised to remove cracks in the initial pass, before subsequent passes are applied. Pre-... 

Minimum diameter for multipass trays is given in Table 2. 

Minimum Practical Diameter for Multipass Baiiast Trays... 

Multipass Tray Number of Passes Minimum Diameter (ft) Preferred Diameter (ft)... 

Here is a tip for possible capacity increase for towers with sloped downcomers. Usually, the tray vendor doesn t use the dead area next to the bottom part of the sloped downcomer as active area if the trays are multipass, since he would require a different design for alternate trays. This area could be used for additional vapor capacity in an existing column. 

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 ... 

The direction of flow is important, as it has a pronounced effect on the efficiency of a heat exchanger. The flows may be in the same direction (parallel flow, cocurrent), in the opposite direction (counterflow), or at right angles to each other (cross-flow). The flow may be either single-pass or multipass the latter method reduces the length of the pass. 

Eor one shell and multipass on the tube side, it is obvious that the fluids are not in true counter-current flow (nor co-current). Most exchangers have the shell side flowing through the unit as in Eigure 10-29C (although some designs have no more than two shell-side passes as in Eig-ures 10-IJ and 10-22, and the tube side fluid may make two or more passes as in Eigure 10-IJ) however, more than two passes complicates the mechanical construction. 

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

Correction for Multipass Flow through Heat Exchangers... 

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. 

To determine the true overall temperature difference, the correction factors, F, shown in Figure 10-34 are used to correct for the deviations involved in the construction of multipasses on the shell and tube sides of the exchanger. Note that R of the charts represents the heat capacity rate ratio , and P is the temperature efficiency of the exchanger. 

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). 

Care should be used in determining the temperatures that prevail at tube inlet and oudet, as well as the shell side in and out for the subcooling portion. This becomes particularly tedious for multipass units. 

---