Heat exchanger specification: Coupling design and surface performance evaluation

Abstract

We develop an approach to obtaining the properties (flow area, path length and enclosed volume) of a heat exchanger surface that meets a design specification given simply in terms of the fluid properties, the flow rate, the allowable pressure drop, and the terminal fluid temperatures.With the thermal performance of the surface to be designed expressed in terms of NTU (=|ΔT|/ΔTm for the side in question), we obtain a dimensional composite design specification which can be calculated simply from the inputs to the design. This group (on the left side of the equation below) is shown to depend only on the thermohydraulic properties of a candidate surface (right side below), thereby linking the flow Reynolds number and hydraulic mean diameter of the surface that satisfy the design:ΔpNTU2ρμ2Pr2/3=fjHRe2dh−2Since the flow area and path length can each also be expressed in terms of Re and dh, the design is fixed once dh is specified. The solutions are conveniently shown graphically such that the effects of using different passages sizes and different enhanced surfaces are revealed explicitly, without undertaking detailed design work. The method is ideal for early size and shape assessments, surface selection and optimisation of the design specification.