Properties of Working Fluids for Reduced Pressure Carbon Dioxide Systems

In a companion paper, a reduced pressure carbon dioxide automotive air-conditioning system is reported [1], in which the working fluid is a mixture of CO₂ and another, less volatile, liquid referred to as the co-fluid. Data essential for the study of this system includes the vapor pressure, density, viscosity and heat capacity of the pure co-fluid as well as mixtures of the co-fluid and carbon dioxide at concentrations encountered in the system. Utilization of the pressure-volume-temperature and heat capacity data is discussed in another companion paper [2]. Viscosity of the mixtures is important in studies related to compressor lubrication and heat transfer/pressure drop in the system.

This paper describes the equipment and methodology used to obtain this data, and results are given for several different co-fluids, at concentrations of 0, 10 and 20 weight percent carbon dioxide over the temperature range -20 to +100 degrees Celsius. Viscosity and pressure data are presented in the form of a Daniel Chart; accompanying plots give the density and heat capacity. Best-fit regression curves have been generated, and the properties are given in equation form as functions of the variables temperature and mass fraction CO₂.