A modeling study on a direct expansion based air conditioner having a two-sectioned cooling coil

Abstract

In small- to medium- scale buildings such as residences and retails located in hot and humid regions, direct expansion based air conditioning systems are widely used for the control of indoor thermal environment. However, to simultaneously control indoor air temperature and humidity using direct expansion air conditioning systems, additional and costly provisions/installation spaces are usually required. Therefore, to address the inadequacies encountered in previous related studies, based on multi-evaporator technology, a novel direct expansion based air conditioning system having a two-sectioned evaporator or cooling coil (TS-DXAC) was proposed to provide variable dehumidification capacity and improved indoor air distribution. In this paper, the development of a steady-state physics-based mathematical model for the novel TS-DXAC system is presented. The model was developed by referring to the existing sub-models for the key system components, such as a compressor, a condenser and an evaporator, which were currently available in open literature. The developed model was experimentally validated using a purposely established prototype experimental TS-DXAC system, with the differences between experimental and predicted results of less than 6%. Using the validated TS-DXAC model, a follow-up modeling study was carried out on optimizing the sizes of the two sections. The simulation results suggested that a lower surface area ratio for the two sections (Rs) can lead to a larger variation ranges of both output total cooling load (TCC) and equipment sensible heat ratio (E SHR). For example, at the inlet state of 26 °C and 50% relative humidity, when Rs was altered from 1:1 to 1:3, the variation range for TCC was increased by 33%, and that for E SHR by 51%, which was beneficial to better dehumidification.