The difference between cooling-liquid return and supply temperatures for the same circuit under stated operating conditions.
Also called: coolant delta T; cooling water temperature rise; supply return temperature difference.
What does coolant delta T mean?
For a circuit absorbing heat from the process, coolant delta T is return temperature minus supply temperature. Identify both measurement locations. A temperature difference of 3 °C has the same magnitude as 3 K.
Do not confuse that difference with the gap between circulating fluid and facility cooling water, or between coolant and room air. These pairs describe different heat-transfer boundaries. Label both measurements and the intended subtraction before using delta T.
Does a small supply-return difference prove good cooling?
Not by itself. It can accompany low heat pickup or a different flow condition; it is not a direct seal or product-temperature measurement. Compare it with simultaneous flow, process state and actual supply temperature. A system can have a small difference while both fluid temperatures are unsuitable for the intended duty.
Also consider measurement capability when the difference is small. Two sensor readings contribute to the difference, so extra display digits do not guarantee a reliable small delta. Avoid diagnosing one fault from the difference alone; define the load state and measured boundary, then evaluate the process and utility evidence together.