Online Measurement Techniques for Determining Oil Circulation Rate

Abstract

Refrigeration and air-conditioning (AC) systems employ refrigerant as the working fluid; however, a portion of oil is discharged from the compressor as part of the compression process and also circulates through the system. This small amount of parasitic fluid causes heat transfer and pressure drop correlations that were developed for pure refrigerant flow to fail and needs to be determined for proper design of heat exchange equipment and connection piping. It is desired to be able to measure the small concentrations of oil circulating as a component of the working fluid online in real time. The oil in circulation as a fraction of the total fluid flow rate is termed the oil circulation rate or oil circulation ratio (OCR). The goal of this study was to determine which combination of fluid property measurements could be used to accurately and precisely quantify OCR. Oil, which is needed to lubricate the compressor, is carried with the refrigerant throughout the system. Oil affects fluid properties such as enthalpy, thermal conductivity, and viscosity and can impact the ability to accurately measure heat exchanger and system performance. Fluid property and flow maps have been developed for various refrigerant-oil mixtures; in combination with these maps the ability to accurately measure OCR online may prove to be a powerful tool in quickly measuring, analyzing, and improving system performance. Without this ability to accurately measure the oil circulation rate over the range of operating conditions, it is impossible to create accurate thermodynamic balances based entirely on the properties of the refrigerant portion of the working fluid. The refrigerant-lubricant mixture selected for this study is a commonly used mixture for automotive AC systems: R134a with ND-8 oil. In a typical air conditioner, utilizing R134a with ND-8, a single phase exists only in subcooled portions of the condenser and the liquid line. Therefore, the experiments were conducted at typical automotive AC conditions between 20 °C and 45 °C, pressures ranging from the saturation pressure up to 1900 kPa, and an OCR between 0% and 12%, and a fixed mass flux of nominally 300 kg/m2s. For a single phase fluid comprised of two components, it is necessary to measure three independent fluid properties to completely describe its state. Since the temperature and pressure are easily obtainable, additional readily available properties to determine the liquid composition were selected: density, ultra-violet light absorptivity, and refractive index. The accuracy and precision of calculating the OCR with these measurements are compared analytically and experimentally. The experimental apparatus was located within an environmental chamber which was capable of controlling the temperature over the range of test conditions. The working fluid was circulated using an oil free gear pump and the pressure of the mixture was controlled via a hydraulic cylinder which was attached to a variable pressure source. Precise quantities of oil were incorporated into the working fluid with a high pressure liquid chromatography pump. A length of clear nylon tubing permitted flow visualization.