This paper reports the solution to the fundamental problem of how to m
aximize the mechanical power extracted from a hot single-phase stream
when the total heat transfer area bathed by the stream is constrained.
It is shown that the optimization has two degrees of freedom: the sha
pe of the stream temperature distribution as a function of the length
(x) traveled along the heat transfer surface, and the position of this
distribution on the absolute temperature scale. The optimal stream te
mperature distribution is exponential in x, and so is the temperature
distribution along the hot end of the system that converts the heat tr
ansfer into mechanical power. At any x, the temperature difference acr
oss the heat exchanger is proportional to the local absolute temperatu
re. Similar conclusions are reached for the cold end heat exchanger, w
hen the power system rejects heat to a cold single-phase stream. It is
shown that the optimal solution can be implemented in practice by usi
ng two counterflow heat exchangers. Each counterflow is imbalanced to
a degree recommended by thermodynamic optimization. The effect of the
sizes and capacity rates of the two heat exchangers is documented. (C)
1998 Elsevier Science Ltd. All rights reserved.