Constant-volume air-conditioning systems provide space conditioning in many residential and commercial buildings. In these applications, the compressor typically cycles on and off to meet the cooling load, while the supply fan operates constantly to ensure the zone is well mixed. Under this scenario, it has been demonstrated that the dehumidification capacity of an air-conditioning system degrades at part-load conditions because moisture on the cooling coil surface evaporates back into the airstream when cooling is deactivated. Henderson and Rengarajan (1996) developed an engineering model of this phenomenon suitable for use in hourly building simulations as well as other types of analysis. The model considers conditions entering the coil, thermostat cycling rates, air-conditioner transient performance, and the moisture- retaining characteristics of the cooling coil.

Previous laboratory measurements have shown that during the OFF cycle, the cooling coil essentially acts as an evaporative cooler, adiabatically providing both sensible cooling and moisture addition. This paper uses that knowledge to determine the moisture evaporation characteristics of a cooling coil installed in a residence with relatively simple temperature measurements. Field-monitored data for a residential water-to-air heat pump are used to determine required parameters for the latent degradation model. The model was able to properly predict the measured latent degradation effects measured over the cooling season. The paper also discusses the other types of experimental data that would be required to further validate the model for various classes of cooling coils and operating scenarios.

Authors: Hugh I. Henderson, Jr., P.E.
Citation: IAQ and Energy 98 Using ASHRAE Standards 62 and 90.1 Conference Papers
Keywords: October, Louisiana, 1998