Heat exchangers design correction factors

Table 2.11 Cost Correction Factors for Shell-and-Tube Heat Exchangers —Design, Materials, and Pressure (Source Reference 13.)... 

If the heat exchanger is designed for true countercurrent flow (which usually is the most desirable design arrangement since less surface area is required for a given rate of heat flow), no correction factor need be applied to the calculated LMTD.t Also, a large temperature cross (i.e., where the outlet temperature of the cold side exceeds the exit temperature of the hot side) can be tolerated for such units. 

The corrected overall heat transfer coefficient is within the design range (140-260 Btu/ft h °F). The assumed value should match U-value estimated from the heat exchanger design specifications that depends on the film heat transfer coefficient of tube side and shell side, fouling factor, and metal resistance. 

In the field of heat transfer, a good example of this category of shortcut design method is the famous F correction factor to correct the log mean temperature difference of shell and tube heat exchangers for deviations from true countercurrent flow. For multipass heat exchangers, the assumptions are ... 

In Chapter 15, the Fr correction factor was correlated in terms of two dimensionless ratios, the ratio of the two heat capacity flowrates (R) and the thermal effectiveness of the exchanger (P). Practical designs were limited to some fraction of Pmax, that is7 ... 

Fhw = the window correction factor. This allows for flow through the baffle window and is a function of the heat transfer area in the window zones and the total heat transfer area. A typical value for a well-designed exchanger is near 1.0. 

Whenever a temperature cross exists, either (1) design the heat exchanger as a countercurrent unit, if thermally practical, or (2) increase the number of shells until the correction factor is satisfactory. (As a general rule, a minimum correction factor of 0.80 is considered satisfactory.) In the latter case, the correction factor should be based on intermediate temperatures (the temperatures between the shells) rather than temperatures fi om the plots for each zone. 

To this point, it has been assumed that the log-mean tenperature correction factor, F, for all exchangers is the same and equal to 0.8. The reason that F is not assumed to be equal to unity is that, for heat exchangers in most practical applications, the flows of the hot and cold streams are never purely countercurrent. The most common type of heat exchanger in use in the chemical process industries is the shell-and-tube (S T) type. These units are typically made as multiples of the basic 1-shell pass, 2-tube pass (1-2) design. When estimating the fixed capital investment associated with the purchase and installation of the heat-exchanger network, the number of 1-2 S T exchangers is needed in addition to the total surface area of the network. 

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