Log mean TD 1

[MrReds]

Hallo to everybody

Please, I would be grateful if someone of you could help me in following issue:
I am dealing with this issue, that it can be apparently simple, but I cannot find any reference about it.

In an heat exchanger, when defining logarithmic mean temperature,

Th1 = inlet temperature hot fluid
Th2 = outlet temperature hot fluid
Tc1 = inlet temperature cold fluid
Tc2 = outlet temperature hot fluid

In case of coutercurrent flow configuration

Lmtd = ((Th1 – Tc2) – (Th2 – Tc1))/log((Th1-Tc2)/ (Th2 – Tc1))

While in case of parallel flow configuration

Lmtd = ((Th1 – Tc1) – (Th2 – Tc2))/log((Th1-Tc1)/ (Th2 – Tc2))

Please, does anybody of you know which is the expression in case of cross-flow configuration?

Thanks in advance

 

[Christine74]

The usual way to do it is to find the LMTD correction factor and multiply it by the countercurrent LMTD.

The LMTD correction factor for cross flow can be obtained from charts similar to the one shown here:

http://www.wlv.com/products/databook/ch4_3.pdf

Make sure you use the natural log in your calculation, not log(10).

-Christine

 

[davefitz]

Unless one of the fluids is having a change of phase or there are more than 5 passes of the cross flow in the same direction, is better to use the e-NTU method instead of the LMTD method. The e-NTU method is the method of calculating the LMTD "correction factor".