Question

List two methods that are used in heat exchanger design and analysis.

Answer 1

Following main two methods are used to design and anlyze the heat exchangers:

1) Log-mean temperature difference method also called as LMTD method:

In this method, inlet and outlet temperature of both hot ad cold stream are known. Based on those temperature, hot end and cold end temperature differences are determined. Then log mean temperature difference (LMTD) is calulated by using these hot and cold end aprroaches. Using LMTD, heat transfer rate can be determined by multiplying with heat transfer area and heat transfer coefficient

Thus, This method is practically used for the applications where both hot and cold stream inlet and outlet temperatures are known.

2) NTU(Number of transfer unit) Method:

When the exchanger size and its type are unknown and fluid outlet temperatures are not known and need to be deteremined, then this application method(NTU-effectiveness) is used to carry out the design and performance calculation.

Hot stream and cold stream heat capacity ratios are first calculated.Heat exchanger effectivenss is calulted as actual heat transfer rate to maximum heat transfer rate ratio. The number of transfer unit (NTU) is then calculated as a ratio of the overall thermal conductance to the smaller heat capacity rate.