Inherent operational characteristics and operational stability of a variable speed direct expansion air conditioning system

Previous studies on the inherent operational characteristics of a variable speed (VS) direct expansion (DX) air conditioning (A/C) system at a fixed degree of refrigerant superheat (DS) setting without considering its operational stability have been conducted. Therefore, a follow-up study on the inherent operational characteristics of a VS DX A/C system considering its operational stability has been carried out and the study results are reported. Using an experimental VS DX A/C system, the inherent correlations between its output total cooling capacity (TCC) and equipment sensible heat ratio (E SHR) at different combinations of compressor speed and supply fan speed were studied and the unstable operating points of speed combinations under different DS settings and inlet air states identified. The experimental results suggested while different DS settings may not significantly influence the inherent correlations between TCC and E SHR, but did impact the operational stability. A lower DS setting would result in a larger unstable operating region. Furthermore, VS operation and different inlet air states to the DX evaporator also influenced the operational stability. A higher compressor speed or a lower supply fan speed, and a lower inlet air temperature or relative humidity would lead to a higher chance to instability.