It depends. Basic thumb rule you need to follow is that, the temperature difference between hot fluid and cold fluid at any position needs to be >= minimum approach temperature. Let's keep that 10 degrees. For the case of co-current heat exchanger: hot outlet and cold outlet will be together, hence by the thumb rule, Thot-outlet needs to be greater than Tcold-outlet by atleast 10 degrees For the case of counter-current heat exchanger: hot outlet and cold inlet will be together, hence by the same rule, Hot outlet will always be 10 degrees higher than cold inlet. But that does not mean it will surely be higher than cold-outlet, which could be higher than hot outlet. hence, for co-current heat exchanger, hot outlet will always be greater than cold-outlet for counter-current heat exchanger, hot outlet may/may not be greater than cold-outlet.
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Do the hot stream outlet temperature need to be always higher than the cold stream outlet temperature ?
It depends. Basic thumb rule you need to follow is that, the temperature difference between hot fluid and cold fluid at any position needs to be >= minimum approach temperature. Let's keep that 10 degrees.
For the case of co-current heat exchanger:
hot outlet and cold outlet will be together, hence by the thumb rule, Thot-outlet needs to be greater than Tcold-outlet by atleast 10 degrees
For the case of counter-current heat exchanger:
hot outlet and cold inlet will be together, hence by the same rule, Hot outlet will always be 10 degrees higher than cold inlet. But that does not mean it will surely be higher than cold-outlet, which could be higher than hot outlet.
hence, for co-current heat exchanger, hot outlet will always be greater than cold-outlet
for counter-current heat exchanger, hot outlet may/may not be greater than cold-outlet.
It is nice,it helps
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