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Calculate the log mean temperature difference ∆Tlm and the correction factor F, if necessary. 4.Obtain (select or calculate) the value of the ...
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Nimish Shah
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The LMTD method is very suitable for determining the
size of a heat exchanger to realize prescribed outlet temperatures when the mass flow rates and the inlet and outlet temperatures of the hot and cold fluids are specified.
With the LMTD method, the task is to select a heat
exchanger that will meet the prescribed heat transfer
requirements.
Tm an appropriate mean (average) temperature difference between the two fluids
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Multipass and Cross-Flow Heat Exchangers: Use of a Correction Factor
F correction factor depends on the geometry of the heat exchanger and the inlet and outlet temperatures of the hot and cold fluid streams. F for common cross-flow and shell-and-tube heat exchanger configurations is given in the figure versus two temperature ratios P and R defined as
1 and 2 inlet and outlet T and t shell- and tube-side temperatures
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Correction factor F charts for common shell- and-tube heat exchangers.
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Correction factor F charts for common cross-flow heat exchangers.
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is the rate of evaporation or condensation of the fluid
h (^) fg is the enthalpy of vaporization of the fluid at the
specified temperature or pressure.
The heat capacity rate of a fluid during a phase-change
process must approach infinity since the temperature
change is practically zero.
correction factor Ft, in addition to those made for LMTD:
across any cross section.
heat exchangers, the Ft values obtained from the curves will give an estimate of the “true mean temperature difference” that is sufficiently accurate for most designs.
Lets calculate one Example….!
Calculate for the following cases, the surface area required for a heat
exchanger which is required to cool 3200 kg/h of benzene (Cp=1.
kJ/kg 0 C) from 71 0 C to 42 0 C. The cooling water (Cp =4.18 kJ/kg 0 C) at
15 0 C has a flow rate of 2200 kg/h.
For each configuration, the overall heat transfer coefficient may be taken
as 0.28 kW/m 2 0C.
(i) Single pass counter‐flow
Use Chart for 1 ‐ 2 pass
Use Chart for 1 ‐ 2 pass F=0.
(ii) 1 ‐ 4 exchanger (one‐shell pass and four‐
tube passes)
(iii) Cross flow exchanger (one‐ fluid mixed
other unmixed)
cooling water…..
duty Q….
Use Chart for Cross flow
F = 0.
(iii) Cross flow exchanger (one‐ fluid mixed
other unmixed)
