Low delta-T syndrome in cooling systems: A systematic review of the signs, symptoms, and causes

Return water temperature and flow rate are indicators of the energy efficiency of chilled water systems. Since the late 1980s, the return water temperature has deviated from the designed value, resulting in an increased flow rate. Such deviations have been recognized as a persistent ‘disease’ named low delta-T syndrome. Based on a medical approach, this study aimed to categorise the key signs and symptoms, and causes to classify low delta-T syndrome into subclasses with individual properties; to connect individual causes to the subclasses; and to identify disagreements on individual causes. Through a systematic review of the literature, over 190 papers published since the late 1980s were identified and studied. By combining different return water temperature profiles and flow rates, low delta-T syndrome was classified into four subclasses with severities ranging from 1 (mild) to 4 (extreme). These subclasses were described with 12 signs and symptoms, each characterised by 19 (from a total of 52) individual or combined causes, to provide an improved overview and a fundamental basis for developing treatments. A fundamental analysis of low delta-T syndrome on a cooling coil revealed that cooling coils with a high chilled water temperature difference and a high chilled water supply temperature at design conditions have a higher risk of developing it. This literature review provides an improved understanding of as well as considerations regarding how to prevent, resolve, mitigate, and handle low delta-T syndrome during design and operation.