The below parameters for oil selection are required when selecting vacuum pump oils. However, the key parameters for oil selection are the ultimate pump pressure and the composition of the gas flow.
1. Ultimate pump pressure
As the lubricating oil is subject to vacuum, it may evaporate faster than when it’s under normal atmosphere. For vacuum applications, a lubricant should always be chosen whose vapour pressure at service temperature is at least two to three decimal powers of ten lower than the attainable pump pressure.
2. It is the gas flow that counts
The gas flow to be pumped may vary considerably depending on the process. Hence, it is essential to precisely know the composition of the gas.
3. Types of vacuum pumps
The following pump designs are common in vacuum technology:
Roots pumps 300 ... 1.0 mbar
Rotary vane pumps 1.0 ... 10-3 mbar
Turbopumps 10-3 ... 10-11 mbar
Diffusion pumps 10-7 ... 10-11 mbar
4. Oil temperature and oil used
- Temperature
Oil temperature in the pump is important because the vapour pressure of the oil decreases as the temperature rises
- Oil in use
Oil in use gives a rough indication of the oil's suitability in vacuum pumps in conjunction with the gas flow