The oil inside lubricated screw air compressors does more than just prevent deterioration of the internal bearing surfaces. Compressor oil differs significantly from lubricant typically used in other machines, so it is best not to extend your compressors maintenance intervals and reduce oil changes, especially in hot weather. We use the word “oil” lightly in this article because compressor lubricant is not exactly oil in the traditional sense.
In other machines, there are various internal seals that typically prevent the lubrication from coming in contact with contaminants like dirt, water, or gasses. Lubricated screw compressors differ in that the lubricant makes direct contact with the compressor inlet air and compressed air. When air is compressed, the water vapor in the air is squeezed out, making free water, which is then picked up by the oil. The dust in the air also comes out as the particles are squeezed together. The compressor oil takes these contaminants to an oil sump, oil filter, and cooler. The heat from the compression process dispels the water vapor and the filtration system eliminates any other contaminants. Compressor oil contains antioxidants, in attempt to reduce the breakdown of the lubrication properties due to internal conditions.
Within the compressor, the lubricant works to seal the gaps between the screw components, improving the compression process. During compression, heat is generated and then absorbed by the lubricant and taken to the compressor lubricant cooler. During this process, the heat and oxygen in the air break down the properties of the lubricant, which causes additional aging if the heat level is high. Once it has lost its properties, compressor lubricant allows excess deterioration of bearings and forms a varnish that covers the compressors internals. This reduces the efficiency of the heat exchangers and raises the lubricant to undesirable levels, causing even more damage.
Keep in mind that if you are using food grade lubricant, more frequent lubricant changes are needed due to the different properties of the oil. Various types of lubricant have different recommended change intervals, depending on compressor running hours and conditions.
If the cost of frequent oil changes is a concern, you should test your oil. All compressor manufacturers offer lubricant analysis. Extending lubricant changes past the manufacturers recommendation is not a good practice, especially in hot humid weather in the event that your compressor fails, creating an even bigger cost.