Air Handling Unit Supply Air Temperature Optimization During Economizer Cycles

Abstract

Most air handling units (AHUs) in commercial buildings have an air economizer cycle for free cooling under certain outside air conditions. During the economizer cycle, the outside air and return air dampers are modulated to seek mixing air temperature at supply air temperature setpoint. Mechanical cooling is always required when outside air temperature is higher than the supply air temperature setpoint. Generally the supply air temperature setpoint is set at 55°F for space humidity control. Actually the dehumidification is not necessary when outside air dew point is less than 55°F. Meanwhile the space may have less cooling load due to envelope heat loss and/or occupant schedule. These provide an opportunity to use higher supply air temperature to reduce or eliminate mechanical cooling and terminal box reheat. On the other hand the higher supply air temperature will require higher air flow as well as higher fan power. Therefore the supply air temperature has to be optimized to minimize the combined energy for fan, cooling and heating energy. In this paper a simple energy consumption model is established for AHU systems during the economizer and then a optimal supply air temperature control is developed to minimize the total cost of the mechanical cooling and the fan motor power. This paper presents AHU system energy modeling, supply air temperature optimization, and simulated energy savings.