Development of Empirical Temperature and Humidity-Based Degraded-Condition Indicators for Low-Tonnage Air Conditioners

Abstract

A split-system direct-expansion air conditioner was used to empirically determine temperature and return-air humidity indicators that could detect performance degradation resulting from degraded conditions. The air conditioner test bench was equipped with the ability to use either a short-tube orifice (STO) or a thermal expansion valve (TXV). The degraded conditions studied include low evaporator airflow, high and low-charge, and a blocked condenser coil. The work presented in this thesis is experimentally based which could identify degraded effects that fall outside of a simulation-based approach. It sought to utilize only low-cost temperature sensing means, although return-air humidity was an important factor for obtaining early detection over a wide range of operating conditions. It was empirically based rather than model based to reduce computation time with a real-time processor. Further, the current work differentiated between a system that uses a shorttube orifice (STO), or fixed-orifice expansion, and a system that uses a thermal expansion valve (TXV), or variable-orifice expansion. This thesis shows that low-cost temperature sensors could be used to detect the degraded conditions studied. However, it is also shown that indicators of low evaporator airflow depend on three loading factors; outdoor-air temperature, return-air temperature, and return-air humidity. In the literature, the humidity sensing point has been neglected by automated detection systems for failures and degraded conditions of low-tonnage air conditioners. The effects of the loading factors on the indicators relative to improper charge and a blocked condenser coil were not studied. STO system and a TXV system relative to the degradations studied, a common set of indicators was identified that could detect degraded conditions without regard to the expansion device.