Some basic relationships to characterize tubular heat exchanger thermal performance are derived in terms of the well-known state variables. It is shown that the knowledge of η (NTU), R (heat capacity rate ratio), and partial derivatives of the temperature efficiency P with respect to η and R enables complete characterization of the exchanger performance around an operating point. Thus, the exchanger performance can be readily predicted for the so-called “subdesign” conditions. Likewise, additional criteria to compare various exchanger styles for a given range of operating conditions can be developed. Two such criteria are developed in this paper.

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