Monitoring the transient concentration of refrigerant in oil using ultrasonic TOF method.

The viscosity of the oil sump is crucial for the reliability of refrigerant compressors. Due to the high solubility of refrigerant in lubricating oil, the viscosity of the oil/refrigerant mixture in high-pressure shell compressors is significantly affected. Therefore, monitoring the refrigerant concentration in the oil sump is necessary, especially during startup and defrosting processes. In this paper, the time of flight (TOF) of ultrasonic waves is developed to obtain the transient concentration of refrigerant in oil. An apparatus was constructed to observe and measure the sent and received signals of ultrasonic pulses in a vapor-liquid equilibrium (VLE) chamber. Temperature and pressure were monitored to calibrate the relationship between sound speed and concentration. The results show that sound speed decreases with the increase in refrigerant concentration. The calculated transient concentration during the pressurization process illustrates that TOF can be an efficient, non-invasive method to monitor the status of the refrigerant/oil mixture.