are not presented. From Fig. 5, it can be seen that both e and U are
constant, while R decreases and Q increases with increasing CL.
Therefore, smaller R still leads to larger Q, while smaller U does not.
From above, it can be seen that neither U nor R can describe the
change of e when the heat capacity flow rates of the discussed heat
exchangers are not fixed. For this kind of cases, smaller R always
leads to larger Q, while smaller U does not always.
For three-stream heat exchangers, there are 12 flow arrangements
for the stream fluids if only the parallel flow and the counter
flow are considered [13]. Cheng et al. [13] analyzed the applicability
of Sg and R on the performance evaluation of the 12 arrangements.
The results show that R is more suitable. When only the
arrangements are discussed, the thermal conductance of the heat
exchanger is fixed. Considering Eq. (1), it can be seen that U has
the same variation tendency as that of Sg. Therefore, compared
with U, R is also more appropriate for describing the performance
of the discussed three-stream heat exchanger.