THERMODYNAMIC CRITERIA FOR HEAT-EXCHANGER NETWORKS DESIGN

The problem under consideration consists in selecting a heat exchanger network able to carry out a given request in heatings and coolings, i n steady-state behaviour with constant pressure, by using if necessary cold and hot utilities, and under the constraint DELTAT greater-than- or-equal-to e in order to restrict investment costs. The exchanged ene rgy and the produced entropy are compared in terms of operating costs. According to the request to be satisfied and the constraints of utili ty consumption, it is shown that the goal to minimize the produced ent ropy more or less agrees with the goal to minimize the exchanged energ y. In the last part, the case where the cost of utility use is assumed to be proportional to the flow rate, with a proportionality constant only depending on the input thermodynamic state, is studied thoroughly . Under this assumption, the minimization of operating costs is compat ible with the minimization of exchanged energy, and can be obtained vi a the maximization of the difficulty of the request part, made without using utilities. This point is based on the notion of a request easie r than another, which explicits the quite vague idea that a request is all the more easier because it involves less heatings at high tempera tures and less coolings at low temperatures.