Shell-side fluid

Selection of Flow Path In selecting the flow path for two fluids through an exchanger, several general approaches are used. The tube-side fluid is more corrosive or dirtier or at a higher pressure. The shell-side fluid is a liquid of high viscosity or a gas. 

When alloy construction for one of the two fJiiids is required, a carbon steel shell combined with alloy tube-side parts is less expensive than alloy in contact with the shell-side fluid combined with carbon steel headers. 

Acoustic Coupling When the shell-side fluid is a low-density gas, acoustic resonance or coupling develops when the standing waves in the shell are in phase with vortex shedding from the tubes. The standing waves are perpendicular to the axis of the tubes and to the direction of cross-flow. Damage to the tubes is rare. However, the noise can be extremely painful. 

The maximum baffle spacing for no tubes in the window of single segmental baffles is unhmited when intermediate supports are provided. These are cut on both sides of the baffle and therefore do not affect the flow of the shell-side fluid. Each support engages all the tubes the supports are spaced to provide adequate support for the tubes. 

Impingement Baffle The tube bundle is customarily protected against impingement by the incoming fluid at the shell inlet nozzle wmen the shell-side fluid is at a high velocity, is condensing, or is a two-phase fluid. Minimum entrance area about the nozzle is generally equal to the inlet nozzle area. Exit nozzles also require adequate area between the tubes and the nozzles. A full bundle without any provision for shell inlet nozzle area can increase the velocity of the inlet fluid by as much as 300 percent with a consequent loss in pressure. 

Dummy tubes or tie rods with spacers may be located within the pass partition lanes (and between the baffle cuts) in order to ensure maximum bundle penetration by the shell-side fluid. 

When tubes are omitted from the tube layout to provide entrance area about an impingement plate, the need for sealing strips or other devices to cause proper bundle penetration by the shell-side fluid is increased. 

In order to ehminate galvanic action the outer tube material may be stripped from the tube ends and replaced with ferrules of the inner tube material. When the end of a tube with a ferrule is expanded or welded to a tube sheet, the tube-side fluid can contact only the inner tube material, while the outer material is exposed to the shell-side fluid. 

Double Pipe Each tube has own shell forming annular space for shell side fluid. Usually use externally finned tube. Relatively small transfer area service, or in banks for larger applications. Especially suited for high pressures in tube above 400 psig. Services suitable for finned tube. Piping-up a large number often requires cost and space. 0.8-1.4... 

The temperature of the shell-side fluid in any shell-side pass is uniform over any cross section. 

These high velocities occur at the bundle entrance and exit areas, in the baffle windows, through pass lanes and in the vicinity of tie rods, which secure the baffles in their proper position. In conjunction with this, the shell side fluid generally will take the path of least resistance and will travel at a greater velocity in the free areas or by-pass lanes, than it will through the bundle proper, where the tubes are on a closely spaced pitch. All factors considered, it appears a formidable task to accurately predict heat transfer characteristics of a shell and tube exchanger. 

Some general considerations to bear in mind are (1) In all start-up and shutdown operations, fluid flows should be regulated so as to avoid thermal shocking the unit, regardless of whether the unit is of either a removable or non-removable type of construction (2) For fixed tubesheet (i.e., non-removable bundle) type units, where the tube side fluid cannot be shut down, it is recommended that both a bypass arrangement be incorporated in the system, and the tube side fluid be bypassed before the shell side fluid is shut down (3) Extreme caution should be taken on insulated units where fluid flows are terminated and then restarted. Since the metal parts eould remain at high temperatures for extended periods of time, severe thermal shock could occur. 

A hollow-fiber reverse-osmosis module consists of a shell which houses the hollow fibers (Fig. 11.3). The fibers are grouped together in a bundle with one end sealed and the other open to the atmosphere. The open ends of the fibers are potted into Ml epoxy sealing head plate after which the permeate is collected. The pressurized feed solution (denoted by the shell side fluid) flows radially from a central porous tubular distributor. As the feed solution flows around the outer side of the fibers toward the shell perimeter, the permeate solution penetrates through the fiber wall into the bore side by virtue of reverse osmosis. The permeate is collected at the open ends of the fibers. The reject solution is collected at the porous wall of the shell. 

For sheU-aiKl-l-ube heat exchangers with shell-side balile. die shell-side fluid flow is perpendicular to the tubes. In this arrangement, the outside film coefficient can be calculated from the following equation ... 

The approximate U values in the table do not differentiate between tube-side and shell-side fluids. Which fluid is on the inside of the tubes and which is on the outside does make a difference to the U value. This is beyond the accuracy of the table. 

Longitudinal baffles force the shell-side fluid to make more than one pass through an e.xchangci. With no longilurlinal baffle, such as in Figure... 

L the shell-side fluid makes one pass from inlet to outlet. With a longitudinal baffle, and with the nozzles placed 180° around the shell, the shell-side fluid would be forced to enter at the left, flow to the right to get around the baffle, and flow to the left to reach the exit nozzle. This would be required to approximate true counter-current flow, which was assumed in the heat transfer equations of Chapter 2. 

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... 

This tube is useful when the shell-side fluid is not compatible with the material needed for the tube-side fluid, or vice versa. The thicknesses of the two different wall materials do not have to be the same. As a general rule, 18 ga is about as thin as either tube should be, although thinner gages are available. In establishing the gage thickness for each component of the tube, the corrosion rate of the material should be about equal for the inside and outside, and the wall thickness should still withstand the pressure and temperature conditions after a reasonable service life. 

Figure 10-11. Duplex tube. Note inside liner is resistant to tube-side fluid and outer finned tube is resistant to shell-side fluid. (Used by permission Wolverine Tube, Inc.)...

Longitudinal baffles must also be compatible with the shell-side fluid, so they normally will be of the same material as tubes or baffles. This baffle never extends the full inside length of the shell, because fluid must flow by its fer end for the return pass in reaching the exchanger oudet. 

This is the net length of tube exposed inside the shell and available for contact by the shell-side fluid. This length accounts for the thickness of each tubesheet (and for the double tubesheets when used). For design purposes, it is usually estimated from experience, allowing about... 

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

Figure 10-31. Fluid flows through two passes in tubes part of flow is parallel to shell-side fluid, and part is counterflow.

R = Ratio of the heat capacities of tube-side to shell-side fluid, dimensionless... 

Figure 10-45. Chart for determining U-clean from tube-side and shell-side fluid film coefficients no fouling included. Note s = shell side, t = tube-side. (Used by permission (q...

Shell-side fluid properties. From previous process calculations, the following properties were determined for the dry gas stream ... 

JL = viscosity of shell-side fluid (at bulk temperature) lb/(ft) (hr)... 

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. 

With a dean shell-side fluid, 1.25 triangular pitch may be used and, from equation 9.211 ... 

Due to bypassing in the shell-side fluid, a high degree of extraction is often difficult to realize with solute-containing fluid on the shell side. It is desirable that the pores are filled with the fluid in which the. solute is most soluble. 

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