Effect of variable flow on centrifugal chiller performance

This research was conducted to determine the effect of varying chilled and condenser water flows on centrifugal chiller efficiency. Three methods were used: (1) theoretical heat transfer equations were examined to assess the effect of variable flow on vessel heat transfer, (2) extensive packaged chiller computer modeling was done to examine a variety of flow and load conditions, and (3) a chiller was run on an ARI-certified test stand under varying conditions to record data, correlate results with the computer studies, and determine if any unstable operating or control situations would occur. Computer modeling indicated that there was no significant difference in a chiller's coefficient of performance (COP) when operating at full evaporator flow or at a flow reduced equal to the percent reduction in load. Calculated pressure drops were proportional to flow to the 1.8 power. Test stand operation correlated with the modeling results within ARI tolerances. In stability in evaporator performance was observed at a tube velocity of less than 2 ft/s (0.61 m/s). Condenser performance was normal at tube velocities as low as 1 ft/s (0.305 m/s). The testing was done on internally rifled tubes and the results may differ with smooth-bore tubes.