Shell-side flow patterns

Others have introduced hehcaUy wound or patterned fiber lay-up in the bundle geometries as a means for controlling and preserving the shell-side flow patterns within the module [27]. The use of hollow-fiber fabrics has also been proposed as a means for precision spacing of the hollow-fibers within the membrane module [28-31]. Many of these are assembled around a central mandrel that also serves as a fluid conduit for feed introduction or product or permeate withdrawal. Modules assembled in this fashion also accommodate distributors and baffles inserted within the fiber pack to aid in the direction and control of the flow sheU-side flow pattern. 

These methods can be used to make a crude estimate of the likely pressure drop. A reliable prediction can be obtained by treating the problem as one of two-phase flow. For tube-side condensation the general methods for two-phase flow in pipes can be used see Collier and Thome (1994) and Volume 1, Chapter 5. As the flow pattern will be changing throughout condensation, some form of step-wise procedure will need to be used. Two-phase flow on the shell-side is discussed by Grant (1973), who gives a method for predicting the pressure drop based on Tinker s shell-side flow model. 

There are many text books that describe the fundamental heat transfer relationships, but few discuss the complicated shell side characteristics. On the shell side of a shell and tube heat exchanger, the fluid flows across the outside of the tubes in complex patterns. Baffles are utilized to direct the fluid through the tube bundle and are designed and strategically placed to optimize heat transfer and minimize pressure drop. 

Tube holes cannot be drilled very close together, since this may struc-tually weaken the tube sheet. The shortest distance between two adjacent tube holes is called the clearance. Tubes are laid out in either square or triangular patterns as shown in Figure S-.i. The advantage of square pitch is that the tubes are accessible for external cleaning and cause a lower pressure drop when shell-side fluid flows perpendicularly to the tube axis. The tube pitch is the shortest center-to-center distance between adjacent tubes. The common pitches tor square putienis arc i-in. OD on... 

The baffle cut determines the fluid velocity between the baffle and the shell wall, and the baffle spacing determines the parallel and cross-flow velocities that affect heat transfer and pressure drop. Often the shell side of an exchanger is subject to low-pressure drop limitations, and the baffle patterns must be arranged to meet these specified conditions and at the same time provide maximum effectiveness for heat transfer. The plate material used for these supports and baffles should not be too thin and is usually minimum thick-... 

Figure 10-29. Three flow patterns for examining AT and LMTD. Note T = shell-side fluid inlet, and q = tube-side fluid inlet.

As discussed in Section 9.4.4, the complex flow pattern on the shell-side and the great number of variables involved make the prediction of coefficients and pressure drop very difficult, especially if leakage and bypass streams are taken into account. Until about 1960. empirical methods were used to account for the difference in the performance... 

It is shown in Section 9.9.5 that, with the existence of various bypass and leakage streams in practical heat exchangers, the flow patterns of the shell-side fluid, as shown in Figure 9.79, are complex in the extreme and far removed from the idealised cross-flow situation discussed in Section 9.4.4. One simple way of using the equations for cross-flow presented in Section 9.4.4, however, is to multiply the shell-side coefficient obtained from these equations by the factor 0.6 in order to obtain at least an estimate of the shell-side coefficient in a practical situation. The pioneering work of Kern(28) and DoNOHUE(lll who used correlations based on the total stream flow and empirical methods to allow for the performance of real exchangers compared with that for cross-flow over ideal tube banks, went much further and. 

The complex flow pattern on the shell-side, and the great number of variables involved, make it difficult to predict the shell-side coefficient and pressure drop with complete assurance. In methods used for the design of exchangers prior to about 1960 no attempt was made to account for the leakage and bypass streams. Correlations were based on the total stream flow, and empirical methods were used to account for the performance of real exchangers compared with that for cross flow over ideal tube banks. Typical of these bulk-flow methods are those of Kern (1950) and Donohue (1955). Reliable predictions can only be achieved by comprehensive analysis of the contribution to heat transfer and pressure drop made by the individual streams shown in Figure 12.26. Tinker (1951, 1958) published the first detailed stream-analysis method for predicting shell-side heat-transfer coefficients and pressure drop, and the methods subsequently developed... 

For reasons of compactness of equipment, the paths of both fluids may require several reversals of direction. Two of the simpler cases of Figure 8.3 are (b) one pass on the shell side and two passes on the tube side and (c) two passes on the shell side and four on the tube side. On a baffled shell side, as on Figure 8.4(c), the dominant flow is in the axial direction, so this pattern still is regarded as single pass on the shell side. In the cross flow pattern of Figure 8.5(c),... 

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

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