You do get oil-free compressors, but the industrial heavy duty quality oil-free units are expensive. Airpower supplies non-lube piston compressors. These units do have oil in the sump. It has two pistons running together on top of one another. Between the two pistons running together, there is compressed air. This compressed air prevents any oil from passing the bottom piston rings. Compressors only pass oil past the rings, if the pressure under the piston is higher than on top of the piston. If your reciprocating piston compressor starts to pass oil, you must always check your air intake filter. If the compressor cannot suck air through the blocked filter, it must suck air past the rings from the oil sump, and then it will pass oil.
Purpose of oil in the compressors:
The oil in a reciprocating piston compressor lubricates all the moving parts but also cools the machine. The oil carries the heat away from the heated parts in the machine. The oil in a screw or vane compressor lubricates all the moving parts, seals the air between the screws and vanes but also cool the machine. The oil carries the heat away from the heated parts in the machine.
Ideal oil temperatures in compressors:
For every 10 °C the oil is increasing in temperature, you half the life expectancy of the oil. If the oil temperature is running for long periods of time at lower than 65 °C, moisture builds up in the oil and can cause rust and loses the lubrication function. The oil with too much moisture becomes milky in colour. The maximum temperature recommended is 105 °C on the oil. Above this temperature, your oil starts to break down and can also catch alight.
What oil must I use in my compressor:
Never use multi-grade oil in any compressor. It carbons up on the valves and outlet pipes. Use a straight monograde mineral oil only.
Why do some people use synthetic oils? Synthetic oils have 4 to 12 times the life expectancy of mineral oil. This reduces your service intervals, although you must still replace your oil filter and air filter on the recommended intervals. Your oil and separator filter will last much longer. With synthetic oils, you can get H1 and H2 approved food-grade oils. This is normally used in food processing factories. Never mix different synthetic oils by topping up with a different brand of oil. These oils can react with one another and become a thick syrup sludge and ruin the complete compressor. Synthetic oils run cleaner and cooler than mineral oils but are much more expensive.