The oil inside lubricated screw compressors not only protects the internal bearing surfaces from wear, but it also has other jobs. Compressor oil is much different than the lubricant inside other typical machines — therefore, it is best not to stretch the maintenance intervals of your compressor, and reduce the oil changes, especially in hot weather. For the purposes of this blog, we will use the word “oil” loosely … compressor lubricant is not necessarily oil in the traditional sense.
For other machines, various internal seals usually protect the lubrication from coming in contact with contaminants, such as water, dirt or gasses. In a lubricated screw compressor, the lubricant comes in direct in contact with the compressor inlet air and compressed air. When air is compressed the contained water vapor is squeezed out of the air, forming free water, which is picked up by the oil. The dust from the air also comes out of the air as the molecules are squeezed together. The compressor oil carries these contaminants to an oil sump, oil filter, and cooler. The heat from the compression process drives off the water vapor and the filtration system removes other contaminants. The oil contains anti-oxidants to try to reduce the breakdown of the lubrication properties due to internal conditions.
Inside the compressor, the lubricant acts to seal the gaps between the screw elements, helping with the compression process. The heat produced by the compression of the air is absorbed by the lubricant and carried away to the compressor lubricant cooler. In this process, the heat and oxygen contained in the compressed air break down the properties of the lubricant, causing excess aging if the level of heat is high. Compressor lubricant that has lost its properties allows excessive bearing wear and also forms a varnish that coats the internals of the compressors, reducing the efficiency of the heat exchangers and elevating the lubricant to undesirable levels — causing even more damage.
If you are using food grade lubricant, be aware that more frequent lubricant changes are required due to the different properties of the oil. Various other lubricant types have different recommended change intervals, depending on compressor operating hours and operating conditions.
If you are concerned frequent oil changes are costing you money, then test your oil, all compressor manufacturers offer lubricant analysis. It is not good practice to stretch your lubricant changes past the manufacturer’s recommendation, especially in hot humid weather lest compressor failure result, causing significant costs.