Chiller models for plant design studies

Numerical models for air- and water-cooled centrifugal, reciprocating and screw chillers are presented. The models were established by curve-fitting manufacturers' chiller performance data. They relate the electricity demand of a chiller to its cooling output and to the temperature of the condenser inlet cooling medium. Including the cooling medium temperature as an independent variable in the model allows its effect on chiller performance to be accounted for explicitly. This contrasts with relying on an assumed relationship between this temperature and the chiller load (e.g. according to the ARI Standard rating conditions). Using chiller models based on such an assumption can lead to erroneous energy use predictions for chiller plants comprising multiple chillers. Predictions of the chiller models agreed reasonably well with chiller performance data extracted from reocrds of plant operation. The models have been incorporated into an air-conditioning system simulation program BECON as its built-in chiller model. A simulation study aimed at comparing the performance of alternative chiller number-size combinations for a building is described. This shows that such studies should be based on realistic rated COP values for chillers, as they will strongly influence the predicted electricity consumption and thus the conclusion on which combination is to be adopted.