In this post, I want to share a simple method to evaluate heat exchanger performance. One of the most helpful ways to evaluate the performance of heat exchanger is to determine its effectiveness by comparing the actual heat transfer rate to the maximum rate that is thermodynamically feasible. The simple formula is expressed below.
η = effectiveness
Q = actual heat transfer rate
Qmax = heat transfer rate which would be achieved if it were possible to bring the exit temperature of the stream with the lower heat capacity, to the inlet temperature of the other stream.
Using nomenclature in figure below, taking stream 1 as having the lower value of GCp, then:
The overall heat balance is:
Thus, based on stream 1:
and based on stream 2:
It is always recommended to use the positive value when determining temperature differences.
A flow of 1 kg/s of an organic liquid of heat capacity 2 kJ/kgK is cooled from 350 K to 330 K by a stream of water flowing countercurrently through a double-pipe heat exchanger. Estimate the effectiveness of the unit if the water enters the heat exchanger at 290 K and leaves at 320 K.
Calculate heat load,
Q = 1 × 2 × (350 – 330) = 40 kJ/s
Calculate flow of water (cool stream),
Gcool = 40 / [4.2 × (320 – 290)] = 0.32 kg/s
For organic liquid (hot stream),
(GCp)hot = (1 × 2) = 2 kW/K
For water (cool stream),
(GCp)cool = (0.32 × 4.2) = 1.34 kW/K
Water has lower GCp than organic liquid, then:
Qmax = 0.32 × 4.2 × (350 – 290) = 80.64 kW
η = 40/80.64 = 49.60%
That’s all a simple method to evaluate heat exchanger performance. I hope you find this post useful.
For more information, please read the following reference.