CONTROL OF INDOOR HUMIDITY

Abstract

Recently, there has been a rising concern on the problem of high indoor humidity in hot and humid country like Singapore. When an air-conditioned space experiences only part of its design heat load, its humidity tends to rise as a result of the air-conditioning system trying to control the indoor temperature by reducing its cooling capacity. In an attempt to solve this problem, the part-load dehumidification performances of five temperature control strategies are compared. They are the Chilled Water Flow control, the Bypass Air control, the Variable Air Volume control, the Run-Around Coil control and the Low Face Velocity / High Coolant Velocity control strategies. Coil simulations are employed to study the part-load performances of the five control strategies. The coil model that is used is validated to be accurate in predicting the performance of a physical cooling coil. The relative humidity of a space depends on many factors such as the design sensible heat factor of the space, the temperature control strategy employed, the load condition experienced by the space and so on. If the part-load condition of a space is known, selection of control strategy can be based on the performance maps given in this thesis. If the load profile of the space is not known, the Bypass Air Control is recommended since it is able to provide an acceptable humidity over a wide range of load conditions, without having to affect the air movement in the space. Design ventilation rate affects the part-load performance of the control strategies. For the strategies employing air-side control, a higher ventilation rate causes the space humidity to rise during part-load. As for the strategies employing water-side control, increasing ventilation rate causes a slight drop in the part-load humidity.