Temperature correction factor, heat

The temperature correction factor, Ft, will normally be higher with plate heat exchangers, as the flow is closer to true counter-current flow. 

Figure 12.62. Log mean temperature correction factor for plate heat exchangers (adapted from Raju and Chand (1980))...

Correlations for friction factors and heat transfer coefficients are rated in HEDH. Some overall coefficients based on external bare tube surfaces are in Tables 8.11 and 8.12. For single passes in cross flow, temperature correction factors are represented by Figure 8.5(c) for example charts for multipass flow on the tube side are given in HEDH and by Kays and London (1984), for example. Preliminary estimates of air cooler surface requirements ram be made with the aid of Figures 8.9 and 8.10, which are applied in Example 8.9. 

Correlations for friction factors and heat transfer coefficients are cited in HEDH. Some overall coefficients based on external bare tube surfaces are in Tables 8.11 and 8.12. For single passes in cross flow, temperature correction factors are represented by... 

F logarithmic mean temperature correction factor or degrees of freedom h heat transfer coefficient or enthalpy... 

Fj temperature correction factor for shell tubes heat exchangers... 

EXAMPLE 4.9-1. Temperature Correction Factor for a Heat Exchanger A 1-2 heat exchanger containing one shell pass and two tube passes heats 2.52 kg/s of water from 21.1 to 54.4°C by using hot water under pressure entering at 115.6 and leaving at 48.9°C. The outside surface area of the tubes in the exchanger is A = 9.30 m. ... 

These refer to hot and cold fluid terminal temperatures, inlet of one fluid versus outlet of the other. For a cross exchanger with no phase change, the ATm gives exact results for true countercurrent flow. Most heat exehang-ers, how ever, deviate from true countercurrent so a correction factor, F, is needed. 

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 ... 

The ( ) factor enables temperature rises to be corrected for heat lost to the container or vessel. The ( ) factor approaches the value of one for large vessels and for extremely low-mass vessels. 

In the basic heat transfer equation it is necessary to use the log mean temperature difference. In Equation 2-4 it was assumed that the two fluids are flowing counter-current to each other. Depending upon the configuration of the exchanger, this may not be true. That is, the way in which the fluid flows through the exchanger affects LMTD. The correction factor is a function of the number of tube passes and the number of shell passes. 

Kj = valve coefficient of discharge = 0.92 Pi = flowing pressure, psia MW = molecular weight of gas = 17,4 Z = compres-sibility factor = 0.9561 C = gas constant based on ratio of specific heats Cp/C T = flowing temperature, R Kb = back-pressure correction factor... 

To determine the true overall temperature difference, the correction factors, F, shown in Figure 10-34 are used to correct for the deviations involved in the construction of multipasses on the shell and tube sides of the exchanger. Note that R of the charts represents the heat capacity rate ratio , and P is the temperature efficiency of the exchanger. 

A further advantage of the plate heat exchanger is that the effective mean temperature difference is usually higher than with the tubular unit. Since the tubular is always a mixture of cross and contra-flow in multi-pass arrangements, substantial correction factors have to be applied to the log mean temperature difference (LMTD). In the plate... 

The flow in a heat exchanger will clearly not be isothermal, and this is allowed for by including an empirical correction factor to account for the change in physical properties with temperature. Normally only the change in viscosity is considered ... 

The mean heat-transfer coefficient will depend on the number of tubes crossed. Figure 12.31 is based on data for ten rows of tubes. For turbulent flow the correction factor Fn is close to 1.0. In laminar flow the heat-transfer coefficient may decrease with increasing rows of tubes crossed, due to the build up of the temperature boundary layer. The factors given below can be used for the various flow regimes the factors for turbulent flow are based on those given by Bell (1963). 

U = overall heat transfer coefficient Aexisting = existing heat transfer area A A = additional area requirement ATim = logarithmic mean temperature difference Fj = logarithmic mean temperature difference correction factor... 

---