Shell-side bypassing

Elimination of one tubesheet reduces initial cost. Tube bundle is removable for inspection and cleaning. Full tube bundle minimizes shell-side bypassing. U-bends permit each tube to expand and contract individually. Tube bundle expansion is independent of shell no expansion diaphragm is required. 

Lower cost per fL of heat-transfer surface. Replaceable straight tubes allow for easy internal cleaning. Full tube bundle minimizes shell-side bypassing. No packedjoints or internal gaskets, so hot and cold fluids cannot mix due to gasket failure. 

SAmount of shell-side bypassing between the cross baffles and the shell, between tubes and tube holes in baffles, and between outermost tubes and shell depends on the manufacturing methods and tolerances for the exchanger. The amount of bypassing can have a large influence on the shell-side heat-transfer coefficient, the value of Fs is usually between 1.0 and 1.8, and a value of 1.6 is often recommended. 

The Liqui-Cel Extra Flow module of the CELGARD EEC (Charlotte, North California) is characterized by a higher mass-transport coefficient than parallel how conhgurations because of the presence of a central bufhe that forces the liquid stream (sent to the shell side) to how perpendicular to the hollow hbers. The central bufhe also minimizes the shell side bypassing the system has been designed for avoiding large pressure drops. 

I see this frequently when tube bundles are extracted from the shell during a unit turnaround. The distorted tubes interfere with the proper fluid flow through the shell side of the exchanger and likely promote both shell-side fouling and shell-side bypassing. Also, as the tubes plug off, tube-side AP increases. If half the tubes plug, then the differential pressure across the channel head pass partition baffle will increase by a factor of four and may result in the failure of the channel head pass partition baffle. 

Figure 28.1 The seal strips avoid shell-side bypassing.

Tube-Bundle Bypassing Shell-side heat-transfer rates are maximized when bypassing of the tube bundle is at a minimum. The most significant bypass stream is generally between the outer tube limit and the inside of the shell. The clearance between tubes and shell is at a minimum for fixed-tube-sheet construc tion and is greatest for straight-tube removable bundles. 

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. 

Higher overall heat transfer coefficients are obtained with the plate heat exchanger compared with a tubular for a similar loss of pressure because the shell side of a tubular exchanger is basically a poor design from a thermal point of view. Considerable pressure drop is used without much benefit in heat transfer efficiency. This is due to the turbulence in the separated region at the rear of the tube. Additionally, large areas of tubes even in a well-designed tubular unit are partially bypassed by liquid and low heat transfer areas are thus created. 

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. 

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. 

The shell-side leakage and bypass streams (see Section 12.9) will affect the mean temperature difference, but are not normally taken into account when estimating the correction factor Ft. Fisher and Parker (1969) give curves which show the effect of leakage on the correction factor for a 1 shell pass 2 tube pass exchanger. 

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... 

As the hot-vapor bypass valve opens, the condensate level in the shell side of the condenser increases to produce cooler, subcooled liquid. This reduces the surface area of the condenser exposed to the saturated vapor. To condense this vapor, with a smaller heat-transfer area, the pressure of condensation must increase. This, in turn, raises the tower pressure. This then is how opening the hot bypass pressure-control valve increases the tower pressure. 

Too much shell-side pressure drop can create a problem. The problem is flow through the bypass area shown in Fig. 19.3. This bypass area is caused by two factors ... 

The gap thus created between the shell ID and the outer row of tubes will permit the shell-side fluid to bypass around the tubes. This is obviously very bad for the heat transfer. And as the shell-side AP increases, the percent of fluid that is squeezed through the bypass area increases. 

The function of seal strips is to interfere with, and hence reduce, the fluid flow through the bypass area. Often, one pair of seal strips is used for every 18 in of shell ID (inner diameter). These seal strips encourage good shell-side cross-flow velocity and also help reduce localized fouling, caused by low velocity. 

The shell side with a number of segmental baffles presents more of a problem. It may be treated as a series of ideal tube banks connected by window zones, but also accompanied by some bypassing of the tube bundles and leakage through the baffles. A hand calculation based on this mechanism (ascribed to K.J. Bell) is illustrated by Ganapathy (1982, pp. 292-302), but the calculation usually is made with proprietary computer programs, that of HTRI for instance. 

The design calculations highlighted the shortcomings of the Kern method of exchanger design. The Kern method fails to account for shell-side inefficiencies such as bypassing, leakage, crossflow losses, and window losses. This leads to a marked overestimate of the shell-side heat-transfer coefficient and shell-side pressure drop. The Bell method is recommended to correct these deficiencies. 

On the contrary, no general expression is available for calculating the mass-transfer coefficient at the shell side. In the literature, in fact, different equations are proposed, depending on the type of module and on the type of flow (parallel or crossflow). Probably, this is due to the fact that the fluidodynamics of the stream sent outside the fibers is strongly affected by the phenomena of channeling or bypassing and it is not well defined as for the stream, which is sent into the fibers. Hereinafter some of the different expressions proposed are reported. 

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