Designers of heat exchangers of all types normally have several degrees of freedom even while meeting the specified effectiveness exactly. One freedom is that of choosing the face-area ratios for the two (or more) fluids. A principal reason for choosing face-area ratio is to arrive at desired pressure drops for the fluids. The lowest pressure drop is not always beneficial: a low pressure drop can produce highly non-uniform flow that would degrade heat-exchanger performance. Obviously a high pressure drop penalizes system performance directly. In this paper it is shown that choosing face-area ratio is a good tool up to a point, one at which penalties in the form of increased size and cost of the overall heat exchanger begin to outweigh the benefits. This paper reports studies on the effects of choosing face-area ratios on rotary regenerative heat exchangers, but most results are applicable to fixed-surface recuperative heat exchangers also. However, one significant difference between the two types is that gas-turbine regenerators have short flow lengths, the thickness of the disk or drum. A short flow length is a virtue, because it reduces the regenerator disk volume and mass. But the disk thickness must not be allowed to be reduced to the point where there is substantial “short-circuit” thermal conduction between the hot and cold faces of a regenerator. These and other aspects of heat-exchanger design are explored in general and by means of examples, and design guidelines are suggested.

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