EXPLAIN THE PROPERTY OF LUBRICANT USED IN REFRIGERATION SYSTEM?
ANS) For satisfactory performance, all refrigeration lubricants – mineral oil or synthetic – must be compatible with the refrigerant in the system and have the following requirements:
1. Good miscibility and solubility to assist in good oil return to the compressor, where it belongs.
2. Chemical stability to resist chemical reaction with the refrigerant or other materials present in the system.
3. Thermal stability to eliminate excess deposits at compressor hot spots.
4. Low wax content to prevent separation of flocculent wax from the oil mixture at the low temperature points in the system.
5. Low pour point to prevent separated lubricant from congealing and restricting flow.
6. Proper viscosity, even when diluted with refrigerant, to ensure high film strength at elevated operating temperatures and still provide good fluidity under coldest operating conditions.
8. No contamination to prevent scarring of bearing surfaces, plugging of lines or oil ports and general deterioration.
Some major compressor manufacturers prefer alkyl benzene refrigeration oil for some applications with HCFC refrigerant blends such as R-22, R-123 and R-401A. However, alkyl benzene refrigeration oil with the proper viscosity can be used with most CFC and HCFC refrigerants as well as hydrocarbons and ammonia in most refrigeration and air-conditioning applications.
The benefits of high-quality alkyl benzene lubricants are high miscibility, low foaming, excellent thermal stability, very low flock points and good compatibility:
1. High miscibility: Miscibility is the ability of the refrigerant and lubricant to stay together as one homogeneous solution. Alkyl benzene has excellent miscibility with CFC and HCFC refrigerants, resulting in the oil and refrigerant remaining as one mixture at a wide range of temperatures and pressures.
2. Low foaming: The low foaming quality of alkyl benzene reduces carryover at compressor startup and subsequent oil loss from the crankcase.
3. Excellent thermal stability: Alkyl benzene can enhance the life of refrigeration systems by providing better thermal stability in the presence of CFC and HCFC refrigerants. It resists change under high temperatures, reducing problems with sludge, acids and copper plating.
4. Very low flock points: The flock point is the highest temperature at which wax-like materials precipitate from the oil in the refrigeration system. Because alkyl benzene is a synthetic lubricant, it contains little or no paraffin or wax, which can plug up parts of a system. This can be very desirable in low-temperature applications.
5. Good compatibility: Alkyl benzene can be blended with mineral oil of the same viscosity. It will not affect motor insulation and is compatible with most elastomers and additives often used to improve lubricity.
Preventing contamination problems is extremely critical in the refining and handling of all refrigeration oils. Great care must be used to assure that refrigeration oil is free of moisture and other contaminants. Service technicians must ensure that oil remains clean and dry.