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Environmental concerns have spawned renewed interest in naturally occurring refrigerants such as carbon dioxide. CO2 has attractive features such as high enthalpy of evaporation and low cost compared to halocarbons. However, the vapor pressure of CO2 is high at temperatures normally encountered in refrigeration and air conditioning systems when compared to traditional and alternative refrigerants such as CFC-12 and HFC-134a. Major research efforts are underway to investigate the transcritical CO2 cycle, in which a gas cooler instead of a condenser accomplishes heat rejection to ambient, since carbon dioxide in this cycle is above the critical point. The vapor pressure in the gas cooler may exceed 120 bar (1,740 lb/in2). In this paper a reduced pressure carbon dioxide system is revisited1, 2. The working fluid is a mixture of CO2 and a non-volatile liquid, referred to as a co-fluid, in which CO2 is highly soluble and readily absorbed and desorbed.

Desiccant materials are commonly used in the automotive industry to reduce the level of moisture in vehicle air conditioning systems. The primary purpose for removing moisture from these systems is to avoid corrosion of metals, compatibility problems with polymeric materials, and possible freeze-up associated with free water. In nonpolar R-12/mineral oil systems with low solubility for water, moisture levels are usually controlled to 25 ppm or less. However, R-134a and PAG are highly polar and have good solubility for moisture, thus presenting reduced risk of free water in the air-conditioning systems. This paper addresses the questions of whether desiccants are required in air conditioning systems using R- 134a/PAG, and if required, what is the optimum quantity of desiccant for system stability and long-term system reliability Tests were conducted in the laboratory (accelerated sealed tube aging according to ASHRAE standard 97- 1989) as well as in the field (vehicle fleet tests).

Environmental concerns have spawned renewed interest in naturally occurring refrigerants such as carbon dioxide. CO2 has attractive features such as high enthalpy of evaporation and low cost compared to halocarbons. However, the vapor pressure of CO2 is high at temperatures normally encountered in refrigeration and air conditioning systems when compared to traditional and alternative refrigerants such as CFC-12 and HFC-134a. Major research efforts are underway to investigate the transcritical CO2 cycle, in which a gas cooler instead of a condenser accomplishes heat rejection to ambient, since carbon dioxide under these conditions is above the critical point. The vapor pressure in the gas cooler may exceed 120 bar (1,740 lb/in2). In this paper a reduced pressure carbon dioxide system is reported (Ref 1). Two companion papers will address properties of working fluids (Ref 2) and thermodynamic and cycle models (Ref 3) for the low pressure carbon dioxide cycle.