Maximum benefits from the use of enhanced heat transfer surfaces

Abstract

The paper presents a critical review of the performance evaluation criteria (PEC) developed by Webb (Webb, 1981). These criteria evaluate the benefits that could be obtained in heat exchangers if their surfaces and inserts are designed to enhance the heat transfer, particularly in two-fluid heat exchangers in single-phase flow. This study revealed that the imposed constraint for fixed pumping power in the comparison of the augmented channel with the reference one is a major obstacle to achieving greater benefit. Accordingly, the elimination of the constraint of fixed pumping power and the objective of reduced pumping power decreases substantially the criteria of Webb. A new constraint related to the fixed augmentation entropy generation number has been involved replacing the constraint of fixed pumping power. The pumping power can be left to increase until the augmentation entropy generation number is less than unity. The maximum benefit can be obtained when the augmentation entropy generation number reaches the value of unity. The use of greater in tube heat transfer enhancement (with the use of compound heat transfer enhancement techniques instead of simple ones), and with an enhanced outer tube surface of the augmented exchanger is a useful instrument to increase the benefits, only if the augmentation entropy generation number does not cross the frontier of unity.