www . refrigerationandaircon . co . za RACA Journal I October 2024 41
Support container to another . Try to use all the oil once the container has been opened .
Polyolester oils , unlike mineral or alkylbenzene oils , are highly hygroscopic . This means that they absorb moisture quickly . These oils come in metal containers to keep them as dry as possible . Do not store polyolesters in plastic containers . Polyolesters can absorb moisture through the walls of a plastic container .
Exposure to the atmosphere must be kept to a minimum . Polyolester oils will typically saturate at 1 000 ppm moisture when exposed to the atmosphere . Compare this with about 100 ppm for mineral oils . Oils with a high moisture level can create problems in a refrigeration system . Always follow the manufacturer ' s instructions .
REMOVING OIL FROM A SYSTEM Normally , you remove oil from a system by draining it from the compressor crankcase . A small amount of residual oil remains in the system . Consult the compressor manufacturer for information and specifications . Get into the habit of changing the filter-drier with each oil change . It is a good idea to change the filter-drier anytime the system has been opened .
You must drain the oil from systems that use oil separators , oil reservoirs , oil level controls , and accumulators .
Removing oil from systems with hermetic compressors can be difficult . There is usually no simple means for draining the system oil . For these systems , oil change is often done only when the compressor must be replaced .
Remember that refrigerant oil may contain acids and personal protective equipment should be worn when replacing refrigerant oil .
EXCESSIVE OIL TEMPERATURES It is desirable to limit discharge temperatures to below 135 ° C to assure adequate packing life and to avoid oil degradation . At temperatures above 150 ° C , eventual lube oil degradation is likely . Under no circumstances should the discharge temperature be allowed to exceed 180 ° C .
Why does overheating have such a devastating effect on the compressor and the oil ?
• Loss of lubrication film : Refrigeration oils have been highly refined to elevate the temperature at which chemical decomposition will occur . As such , they are vulnerable to losing the lubrication film necessary to prevent metal-to-metal contact between bearings and journals , or piston rings and cylinders , prior to the temperature at which decomposition begins . When these temperatures are achieved , the probability of extreme piston and ring wear is likely .
• Chemical decomposition : This happens at elevated temperatures and is accelerated in the presence of other contaminants such as air or water . The process by which the refrigerant and / or oil chemically breaks down can occur in a matter of seconds under excessively high temperature conditions .
Oil will start to decompose and as temperatures increase above this threshold , the oil starts to polymerise . First the oil is transformed into a dark thick oil , then a sludge .
The presence of oil breakdown in the refrigeration system has many negative side effects . Sludge and solid particulate can plug up the oil inlet screen in the compressor sump or the lubrication passages in the crankshaft . Each of these will have the same devastating consequence : loss of lubrication , and ultimately failed compressors .
DIELECTRIC PROPERTIES The amount of moisture in a refrigeration lubricant can be measured by its resistance to the flow of a current of electricity through it without breaking down . This is known as its dielectric strength . A refrigeration lubricant with acceptable moisture content should have a minimum dielectric value of 25 000 volts .
HANDLING REFRIGERATION LUBRICANTS Refrigerant lubricant must be kept in sealed containers and not exposed to air where it will absorb moisture . Refrigeration lubricant comes in one and five litre containers . It is advisable to purchase lubricant in small , sealed containers , holding enough for each separate service operation . Excess lubricant should be allowed to remain in its sealed container and not transferred from one container to another , as it may pick up some moisture and dirt . Always seal lubricant containers after using them . Contaminated refrigerant my contain acids . Correct use of personal protection equipment is an important factor when handling refrigerant oils .
CONTAMINATED LUBRICANT Lubricant that has been removed from a system should be clear . Any discoloration means that the lubricant is contaminated . When this has happened , new driers and filters should be placed in the system . These will keep the new lubricant clean . Another indicator of contaminated lubricant is odour . Dark-black , pungent oil is an indicator of compressor failure . Metal shavings and chips are another sure sign that the compressor needs replacement .
Ryan , I hope that this helps with your understanding of refrigerant oils . Incorrect oil will almost certainly cost you a compressor . Always dispose of contaminated oil in the correct manner and remember that used refrigerant oil may be acidic , always use the appropriate personal protective equipment . RACA
Grant Laidlaw
REFERENCES : 1 . ACRA 2 . Danfoss
www . refrigerationandaircon . co . za RACA Journal I October 2024 41