The thermal design and rating of multistream plate-fin heat exchangers

Multistream plate-fin heat exchangers are introduced and their applications in various industries are briefly outlined. The difficulties in extending traditional two-stream design theory to deal with multistream heat exchangers are detailed. Key aspects of the design and performance, and some peculiarities in the behaviour of multistream heat exchangers are discussed. Existing literature on the development of design theory for multistream plate-fin heat exchangers is reviewed. The implications of using certain simplifying assumptions to derive design theory for multistream plate-fin heat exchangers are noted. The basic equations for heat transfer along a fin and across a separating surface in a multistream plate-fin heat exchanger are derived. Three distinct mechanisms that operate in heat exchange between neighbouring passages are noted and their significance is discussed. Equations for heat transfer in a discrete section of a multistream plate-fin heat exchanger with an arbitrary arrangement of passages are written down. The development of a general rating method based on a matrix solution of these equations is outlined. The results of a number of realistic simulations done,vith the computer program STACK are used to illustrate the behaviour of multistream plate-fin heat exchangers in practical situations. The effects of stacking pattern, intermediate entry/exit of stream(s), multiple intermediate withdrawal of stream(s) etc. are illustrated. Based on conclusions drawn from the examples, criteria for achieving effective thermal design are enumerated.