Effects of Water in Synthetic Lubricant Systems and Clathrate Formation: A Literature Search and Review

Rohatgi, Ngoc Dung T.

Air-Conditioning and Refrigeration Technology Institute, Arlington, VA (United States)

关键词: 36 Materials Science;    Clathrates;    Water;    Solid Solutions;    Water Vapor Refrigerants;   

DOI  :  10.2172/796197
RP-ID  :  DOEOR22674/ARTI-21CR/50035-01
RP-ID  :  FC05-99OR22674
RP-ID  :  796197

美国|英语

来源: UNT Digital Library

【 摘 要 】

An extensive literature search and a confidential survey were critically analyzed to determine the effects of water on the stability of hydrofluorocarbon/synthetic lubricant systems and to identify key areas requiring further investigation. Following are highlights from the analysis: Clathrate hydrates are solid solutions formed when water molecules are linked through hydrogen bonding creating cavities that can enclose various guest molecules from hydrate formers, such as hydrofluorocarbons R-32, R-125, R-134a, R-407C and R-410A. The four methods for preventing clathrate formation were drying the gas, heating it, reducing its pressure, or using inhibitors. The hydrolysis of polyolester lubricants was mostly acid-catalyzed and its reaction rate constant typically followed the Arrhenius equation of an activated process. Hydrolytic stability improved with hindered molecular structures, and with the presence of acid catcher additives and desiccants. Water vapor can effect the adsorption of long-chain fatty acids and the chemistry of formation of protective oxide film. However, these effects on lubrication can be either positive or negative. Fifty to sixty percent of the moisture injected into an air-conditioning system remained in the refrigerant and the rest mixed with the compressor oil. In an automotive air-conditioning system using R-134a, ice would form at 0 C evaporating temperature when the water content in the vapor refrigerant on the low-pressure side was more than 350 ppm. Moisture would cause the embrittlement of polyethylene terephthalate and the hydrolysis of polyesters, but would reduce the effect of amine additives on fluoroelastomer rubbers. The reactions of water with refrigerants and lubricants would cause formicary and large-pit corrosion in copper tubes, as well as copper plating and sludge formation. Moreover, blockage of capillary tubes increased rapidly in the presence of water. Twenty-four companies responded to the survey. From the responses, the water concentrations specified and expected for different refrigerant/lubricant systems varied depending on the products, their capacities and applications, and also on the companies. Among the problems associated with high moisture level, lubricant breakdown was of greatest concern, followed by acid formation, compressor failure and expansion valve sticking. The following research topics are suggested: 1. The air-conditioning and refrigeration industry needs to measure and record the water content and total acid number of the lubricant of newly installed systems as well as operating systems that are shutdown for service or repair. The reason for the shutdown needs to be documented. A database can then be established to correlate water content with type and cause of breakdown. 2. Detailed studies on the distribution of water in refrigeration and air-conditioning systems should be conducted to pinpoint problem areas associated with free water. 3. Research is needed to validate the current theories and mechanisms of formicary corrosion. Corrosion inhibitors need to be developed. 4. The conditions for clathrate formation and decomposition of other alternative refrigerants, such as R-23, R-41, R-116, R-125, R-143a, R-404A and R-507C, and water should be determined to avoid possible problems associated with tube plugging. The mechanism by which water facilitates or hinders lubrication needs to be studied.