Logarithmic-mean temperature correction factor

Figure 4.7 Definition of parameters for the logarithmic-mean-temperature correction factor.

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

The value of T is calculated from the logarithmic mean temperature difference multiplied by a correction factor. With single-pass operation, this factor is about 1 except for plate packs of less than 20, when the end effect has a... 

F Geometric factor for radiation or correction factor for logarithmic mean temperature difference ... 

The usual practice in the design of shell and tube exchangers is to estimate the true temperature difference from the logarithmic mean temperature by applying a correction factor to allow for the departure from true counter-current flow ... 

A pure, saturated, vapour will condense at a fixed temperature, at constant pressure. For an isothermal process such as this, the simple logarithmic mean temperature difference can be used in the equation 12.1 no correction factor for multiple passes is needed. The logarithmic mean temperature difference will be given by ... 

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... 

Next, calculate the logarithmic-mean temperature difference correction factor, F, from Equation 4.5.4. Calculate F either from Equation 4.10 or use plots of Equation 4.10 given in the chemical engineering handbook [1]. In either case, first calculate the parameters R and S. R and S are defined in Figme 4.7. 

Calculate the logarithmic-mean temperature-difference correction factor, F, from Equation 4.7.4. 

Obtain a relation for the logarithmic mean temperature difference for use in the LMTD melhfld, and modify it (or different types of heat exchangers using the correction factor,... 

It was seen from the discussion of heat exchangers that the fluid streams are not strictly countercurrent. Baffles on the shell side induce crossflow, and in a two-tube-pass heat exchanger both countercurrent and cocurrent flow occur. To account for deviations from countercurrent flow, the logarithmic-mean temperature difference is multiplied by a correction factor, F. Thus,... 

Because the cost of a heat exchanger depends on its size, and because its size will depend on the heat-transfer rate, a rate equation must be introduced. The rate equation is given by Equation 4.4.3. The logarithmic-mean temperature difference in Equation 4.4.3 is given by Equation 4.4.4. Because perfect countercurrent flow can never be achieved in an actual heat exchanger, the logarithmic-mean temperature difference correction factor, F, is needed. For simplicity, Equation 4.10, discussed earlier, is expressed as Equation 4.4.5, which states that F depends only on the terminal temperatures, once a particular heat exchanger is selected. 

Fi correction factor to ATim, fraction h enthalpy HP horse power 7 electrical current k ratio of specific heat = Cp/Cy m, M mass flow AP pressure drop P pressure PE power factor Q heat content r compression ratio = P2IP1 T-[, ti supply temperature of hot (cold) stream T2, ti target temperature of hot (cold) stream ATi hot end temperature approach AT2 cold end temperature approach ATim logarithmic mean temperature differenee (LMTD)... 

Heat transfer of a heal eKchanger, heat exchan ger duty, is proportional to its oveiall heat transfer coeflident, heat transfer area, logarithm mean temperature diiTerence, and a correction factor. 

Q is heat exchanger duty, in htu/hr U is overall heat transfer coefficient, in btu/hr-ft2-"F A is total heat transfer area, in fl2 LMTD is logarithm mean temperature difference, in F F is LMTD correction factor, in fraction. 

Logarithmic mean tcmperaruie diffnence is calculated by Eq. (5). Logarithirtic mean tiemperaturc difTerence multiplied by correction factor (F> is the effective mean temperature difference. It is a measure of effective heat transfer driving force in a heat exchanger. 

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