Co-flow Heat Exchangers
We consider co-flow heat exchangers (upper sign) first. The known inflow conditions are the temperatures TA (0) and TB (0), for which we find from (15.6)
According to these equations, the two temperatures approach a common value exponentially. The common value is obtained in the limit x →∞ as
In a heat exchanger of finite length L, the exit temperatures TA (L) and TB (L) differ from this value, see Fig. 15.3.
The entropy generation is a useful measure for the work loss associated with the heat exchanger. Since we consider the heat exchanger to be adiabatic to the outside, the entropy generation is just the difference between entropy flowing in and out; for simple incompressible fluids it is computed as
In a co-flow heat exchanger, all heat transfer takes places over finite temperature differences and the entropy generation is always finite. Therefore co-flow heat exchangers always have an associated work loss.