THE LOG MEAN TEMPERATURE DIFFERENCE METHOD

1)

a) The expressions in parallel-flow and counter-flow heat exchangers.

b) For specified inlet and outlet temperatures, the log mean temperature difference for a counter-flow heat exchanger is always greater than that for a parallel-flow heat exchanger. That is,  and thus a smaller surface area is needed to achieve a specified heat transfer rate in a counter-flow heat exchanger. Therefore, it is common practice to use counter-flow arrangements in heat exchangers.

2) 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. A

3) A second kind of problem encountered in heat exchanger analysis is the determination of the heat transfer rate and the outlet temperatures of the hot and cold fluids for prescribed fluid mass flow rates and inlet temperatures when the type and size of the heat exchanger are specified.

a) The LMTD method could still be used for this alternative problem, but the procedure would require tedious iterations, and thus it is not practical.

b) The effectiveness–NTU method, which greatly simplified heat exchanger analysis.