Shell-side coefficient

Example If the shell-side coefficient of a unit is 25 Btu/hr (ft )(°F) and velocity in the shell is doubled, read the new shell-side coefficient, h as 36 (line a). If the tube-side coefficient is 25 and velocity is doubled, read the new tube coefficient, h, as 43.1 (line a). In other cases, pressure drop would increase by a factor of 4. Note This may be used in reverse for reduced flow. 

Shell-side coefficient vapor desuperheating or cooling. 

If the tube bundle is to be large in diameter, it is possible that the liquid head will suppress the boiling in the lower portion of the horizontal bundle thereby actually creating a liquid heating in this region, with boiling above this. Under such situations, the boiling in the unit cannot be considered for the full volume hence, there should be two shell-side coefficients calculated and the resultant areas added for the total. 

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... 

While it is possible to calculate the existing overall heat transfer coefficient from the operating data, it is not possible to calculate the individual film transfer coefficients. The individual film transfer coefficients can be combined in any number of ways to add up to an overall value of 285 W-m 2-K 1. However, the film transfer coefficients can be estimated from the correlations in Appendix C. Given that the tube-side correlations are much more reliable than the shell-side correlations, the best way to determine the individual coefficients is to calculate the coefficient for the tube-side and allocate the shell-side coefficient to add up to U = 285 W-m 2-K 1. Thus, to calculate the tube-side film transfer coefficient, KhT must first be determined. 

The correlation should be used with caution outside the range 0.6 < Tr < 0.8 and should not be used below a pressure of 0.3 bar. When dealing with a clean, nondegrading material, the process fouling coefficient should be increased to around 11,000 W m 2 K 1, but should be reduced to 1400 to 1900 W m 2 K 1 for material that has a tendency to polymerize17. If a shell-side coefficient of process fouling coefficient different from 5700 W m 2 K 1 is required, the corrected overall heat transfer coefficient can be calculated from17 ... 

For a preliminary design, except for very large or expensive heat exchangers, it is usually adequate to use approximate heat transfer coefficients. These can be found in references 22,23 and 24. When calculating individual heat transfer coefficients, it may simplify calculations to note that for streams that have a viscosity greater than 5 cp the tube-size coefficient is two or three times what the shell-side coefficient would be for the same material.2 This is often the deciding factor in determining which fluid should flow within the tubes. 

Case 2. Coolant flow rate is fixed. Here At2 is known, so the tube side and shell side coefficients and area are optimized. Use Equation (/) and (J) to find h0 and hv A0 is then found from Equation (b). 

When the shell-side coefficient is less than half that of the tube side the annular side coefficient can be made comparable to the tube side. 

In the common types of baffled shell-and-tube exchangers, the shell-side fluid flows across the tubes. The equations for predicting heat-transfer coefficients under these conditions are not the same as those for flow of fluids inside pipes and tubes. An approximate value for shell-side coefficients in a cross-flow exchanger with segmental baffles and reasonable clearance between baffles, between tubes, and between baffles and shell can be obtained by using the following correlation ... 

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