Pressure Drop in the Shell Side

The pressure drop for the flow of liquid without phase change across the tubes in the shell side is given by following equations [6]  

The number of tube rows across which the shell fluid flows. Nr, which equals the total number of tubes at the center plane minus the number of tube rows that pass through the cut portions of the baffles. For 25% cut baffles, Nr may be taken as 50% of the number of the tubes at the center plane. 

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Pressure Drops. Shellside pressure drop has three components (1) pressure drop in the central (crossflow) section Apc, (2) pressure drop in the window area Apw, and (3) pressure drop in the shell side inlet and outlet sections, ApI 0. It is assumed that each of the three components is based on the total flow and that each component can be calculated by correcting the corresponding ideal pressure drops. 

Isothermal operation is maintained with n ligiUe pressure drop in the shell side. 

Estimating the pressure drop in the tube side is much easier than calculated pressure drop in the shell side. The pressure drop in the tube side is calculated using the following equations [6] ... 

A method for estimating the pressure drop on the shell-side of horizontal condensers is given in the Engineering Sciences Data Unit Design Guide, ESDU 84023 (1985). 

The total pressure drop for the shell-side is given by summing the pressure drops over all the zones in series from inlet to outlet ... 

Subcooling in a shell-and-tube condensers. Figure 13.3 is the same propane condenser shown in Fig. 13.2. Let s assume that the pressure drop through the shell side is zero. Again, we are dealing with a pure component propane. The inlet vapor is at its dew point. That means it is saturated vapor. Under these circumstances, the outlet liquid should be saturated liquid, or liquid at its bubble point. As the inlet dew-point temperature is 120°F, the outlet bubble-point temperature should be 120°F. But, as can be seen in Fig. 13.3, the outlet shell-side liquid temperature is 90°F, not 120°F. Why ... 

The baffle cut, shown in Fig. 19.3, is usually about 20 to 30 percent of the diameter of the baffle. The smaller the baffle cut, the more perpendicular the flow across the tubes. Perpendicular flow encourages desirable cross-flow velocity and vortex shedding. But a smaller baffle cut will also increase the pressure drop on the shell side. 

Ilie procedure to obtain the pressure drop on the shell side follows the steps outlined in Eqs. (14-46) through (14-51). The ideal cross-flow pressure drop is obtained from... 

Vladisavljevic and Mitrovic [50] developed a three-phase hollow hber membrane contactor with frame elements. The module consists of stacks of polygonal plates containing internal frames packed with hollow hbers and an external frame where headers for the inlet and outlet of the fluids flowing inside the hbers are provided. Plates can be monoaxial or biaxial, allowing two- and three-phase contact, respectively. Authors calculated, both at mbe and shell side, the pressure drops of this system. At the same huid how rate, the pressure drop at the shell side was lower than that of the mbe side and proportional to the gas how rate. The pressure drops at the mbe side were mainly related to the local obstacles in the module rather than the resistance in the hbers. 

Shellside Pressure Drop. Surprisingly little attention has been devoted in engineering literature to estimate two-phase pressure drop on the shell side of shell-and-tube heat exchangers [77, 78]. In engineering practice, the estimation of the two-phase flow pressure drop can be performed in some situations using modified single-phase flow correlations. This approach is, however, highly unreliable. 

Figures 2-35 and 2-36 represent the above equations in nomograph form. Apg is the pressure drop on the shell side with 16-foot-long tubes. If the tube length differs and is L ft. long instead, the permissible pressure drop Ap must be multiplied by a feictor 10/L Apg = Ap (16/L) in order to use the shell-side equation or Figure 2-35.

Based on the rating assessment in Example 6.2, it is observed that pressure drop on the shell side is too large to be allowed. Thus, a parallel arrangement is considered in this assessment as shown in Figure 6.4. The two exchangers are counted as one exchanger unit for the rating calculations below. 

An alternative method is presented in the following sections which can be used to determine pressure drop in the tube side and the shell side of the shell and tube heat exchanger. 

The shape of the cooling and warming curves in coiled-tube heat exchangers is affected by the pressure drop in both the tube and shell sides of the heat exchanger. This is particularly important for two-phase flows of multicomponent systems. For example, an increase in pressure drop on the shell side causes boiling to occur at a higher temperature, while an increase in pressure drop on the tube side will cause condensation to occur at a lower temperature. The net result being both a decrease in the effective temperature difference between the two streams and a requirement for additional heat transfer area to compensate for these losses. 

Coiled-tube heat exchangers frequently have flow distribution problems that include (1) tube distribution (2) two-phase tube distribution and (3) two-phase shell distribution. Good flow distribution within the tubes can be obtained by designing the headers in such a way that their pressure drop is considerably less than that for the frictional pressure drop in the tubes. To obtain good shell-side distribution one must use symmetric bundles and separately introduce the vapor and liquid phases to the bundles. It is also advisable to arrange for downflow of the shell-side fluid. For two-phase annular flow, the vapor will flow mostly in the space between the tube layers while the liquid needs to be carefully distributed in the radial direction for proportionate vapor-liquid flow normal to each tube layer. To avoid convection on the shell side due to density gradients, it is normal practice to use sufficiently large pressure drops on the shell side. 

Figure 16.3 is the same propane condenser shown in Fig. 16.2. Let s assume that the pressure drop through the shell side is zero. Again, we...

The second type of hoUow-fiber module is the bore-side feed type illustrated in Figure 23b. The fibers in this type of unit are open at both ends, and the feed fluid is usually circulated through the bore of the fibers. To minimize pressure drops inside the fibers, the fibers often have larger diameters than the very fine fibers used in the shell-side feed system and are generally made by solution spinning. These so-called capillary fibers are used in ultrafiltration, pervaporation, and in some low to medium pressure gas appHcations. Feed pressures are usually limited to less than 1 MPa (150 psig) in this type of module. 

Baffles in a horizontal in-shell condenser are oriented with the cuts vertical to facilitate drainage and eliminate the possibility of flooding in the upward cross-flow sections. Pressure drop on the vapor side can be estimated by the data and method of Diehl and Unruh [Pet. Refiner, 36(10), 147 (1957) 37(10), 124 (1958)]. 

Test, P. L., AStudy of Heat Transfer and Pressure Drop Under Conditions of Laminar Plow in the Shell Side of Cross Baffled Heat Exchangers, Paper No. 57-HT-3, ASME-AlChE Heat Transfer Conference, ASME, New York, NY (1957). 

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