A Sensor for Estimating the Liquid Mass Fraction of the Refrigerant Exiting an Evaporator

A traditional method of controlling evaporator superheat in a vapor compression air conditioning system is the thermostatic expansion valve (TXV). Such systems are often used in automotive applications. The TXV depends on superheat to adjust the valve opening. Unfortunately, any amount of superheat causes that evaporator to operate at reduced capacity due to dramatically lower heat transfer coefficients in the superheated region. In addition, oil circulation back to the compressor is impeded. The cold lubricant almost devoid of dissolved refrigerant is quite viscous and clings to the evaporator walls. A system that could control an air conditioner to operate with no superheat would either decrease the size of its existing evaporator while maintaining the same capacity, or potentially increase its capacity with its original evaporator. Also, oil circulation back to the compressor would be improved. To operate at this twophase evaporator exit condition a feedback sensor would have to quantify the quality of liquid mass fraction (when the exit stream is a mixture of droplets and superheated vapor) of the refrigerant exiting the evaporator.Air Conditioning and Refrigeration Project 7