Shell-side exchanger pressure

The shell-side exchanger pressure drop, which includes the effect of the baffle height. 

For short exchangers with no shell-side haffles, pressure drop is usually negligible. Allowances should he made for nozzle entrances and exits if the pressure level of the system warrants this detail. 

APm.AP,., Pressure drop for ideal-tube-bank cross-flow and ideal window respectively AP for shell side of baffled exchanger kPa Itf ft ... 

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. 

For an all-steel heat exchanger AAith mixed design pressures the total extra for pressure is 0.7 X pressure extra on shell side plus 0.3 X pressure extra tube side. 

For an exchanger AAith alloy parts and a design pressure of 150 Ibf/im, the alloy extras are added. For shell and shell cover the combined alloy-pressure extra is the alloy extra times the shell-side pressure extra/100. For channel and floating-head cover the combined alloy-pressure extra is the alloy extra times the tube-side pressure extra/100. For tube sheets and baffles the combined alloy-pressure extra is the alloy extra times the higher-pressure extra times 0.9/100. (The 0.9 factor is included since baffle thickness does not increase because of pressure.)... 

The shape of the coohng 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 shellside causes boiling to occur at a higher temperature, while an increase in pressure drop on the tubeside will cause condensation to occur at a lower temperature. The net result is 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. 

The price of a shell and tube exchanger depends on the type of exchanger, i.e., fixed tube, U-tube, double tube sheets, and removable bundles. The tube side pressure, shell side pressure, and materials of construction also affect the price. If prices cannot be obtained from endors, correlating in-house data by plotting /fr vs. number of ft with correction factors for the variables that affect price will allow estimating with fair accuracy. If not enough in-house data is available to establish good correlations. it will be necessary to use the literature, such as References 16. 17. and 18. 

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. 

Figure 10-8. Single-pass shell and tube Teflon tube heat exchanger, countercurrent flow. Tube bundles are flexible tube Teflon joined in integral honeycomb tubesheets. Shell-side baffles are provided for cross-flow. Standard shell construction is carbon steel shell plain or Teflon (LT) lined. Heads are lined with Teflon . Tube diameters range from 0.125-0.375 in. O.D. the temperature range is 80-400°F pressures range from 40-150 psig. (Used by permission AMETEK, Inc., Chemical Products Div., Product Bulletin Heat Exchangers of Teflon . )...

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-... 

Pressure losses through the shell side of exchangers are subject to much more uncertainty in evaluation than for tube side. In many instances, they should be considered as approximations or orders of magnitude. This is especially true for units operating under vacuum less than 7 psia. Very little data has been published to test the above-atmospheric pressure correlations at below-atmospheric pressures. The losses due to differences in construction, baffle clearances, tube clearances, etc., create indeterminate values for exact correlation. Also see the short-cut method of reference 279. 

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). 

Whitley, D. L., Galculating Heat Exchanger Shell-Side Pressure Drop, Chem. Eng. Prog, V. 57, No. 9, p. 59 (1961). 

Starczewski, J., Short Cut Method to Exchanger Shell-Side Pressure Drop, Hydro. Proc., V. 50, No. 6, p. 147 (1971). 

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. 

ESDU Engineering Sciences Data Unit Report 83038 Baffled shell and tube heat exchangers flow distribution, pressure drop and heat transfer on the shell side. (ESDU International, London 1983)... 

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... 

The procedure for calculating the shell-side heat-transfer coefficient and pressure drop for a single shell pass exchanger is given below ... 

Though Bell s method will give a better estimate of the shell-side pressure drop than Kern s, it is not sufficiently accurate for the design of exchangers where the allowable pressure drop is the overriding consideration. For such designs, a divided-flow model based on Tinker s work should be used. If a proprietary computer program is not available,... 

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