Predicting the waterside temperature difference of a cooling coil in part load

Abstract

Chilled water plants must operate in the most energy efficient way possible. One of the threats to a good energy performance in part load is the ‘Low ΔT syndrome’, several causes of which are related to the cooling coils. In order to detect deviation behaviour of the waterside temperature difference (ΔT), one needs to know the normal return water temperature during part load operation of the cooling coils. However, until now it has not been possible to predict and quantify the waterside temperature in part load. This paper provides a mathematical derivation to predict the waterside temperature difference in part load. For a constant air and water flow, the performance is only determined by the nominal water leaving temperature and leaving air temperature. Both temperatures determine if the chilled water temperature decreases or increases in part load and the shape of the heat exchanger characteristic of the cooling coil. The result is tested for two known causes of the Low ΔT syndrome, namely 100% outdoor air handling units and economizers. The results show that placing the cooling coil behind (blow through), instead of in front of (draw through), the fan will result in an increased chilled water temperature difference in part load.