A rugged type of vacuum device is the rotary piston vacuum pump. Industries such as aerospace, vacuum degassing, evacuation, metal drying, and vacuum impregnation use the rotary piston vacuum pump to create a vacuum. This type of pump is attractive when handling dry, non-condensable loads.
How Rotary Piston Vacuum Pumps Create Vacuum
The piston is eccentric to the main bore of the pump cylinder. Gas is drawn into the chamber as the volume in the “bucket” expands as the piston moves around the chamber. When this volume is at its maximum, the bucket seals as the piston closes off the inlet port. As the sealed-off gas rotates around the chamber, the gas is compressed and discharged out of the outlet port at a pressure slightly above atmospheric.
A thin film of lubricating oil lines the inside of the vacuum chamber within the rotary piston pump to achieve a better vacuum seal. However, this oil can become entrained in the process vapors and is present in the discharge vapors of the vacuum pump. Contaminated process gas is unavoidable.
The rotary piston pump has two configurations, single-stage, and two-stage. Similar to liquid ring vacuum pumps, the two-stage configurations have higher capacities at a deeper vacuum than the single-stage designs. Additional components can be added to the rotary piston vacuum system to improve operability and convenience, including oil mist eliminators to collect and recycle entrained oil, after-condensers, and receivers to collect trace amounts of condensable on the discharge of the system, and many others.
Pros and Cons of Vacuum pump
The rotary piston vacuum pump has many advantages:
- Relatively cheap and high capacity at deep vacuum levels (<0.01 Torr)
- The rugged design allows for a long life
- Can allow trace amount of solvents
The limitations of this type of vacuum pump are as follows:
- Cannot handle liquids or any process carryover because these will damage the internals of the pump
- Cannot easily handle condensable vapors
- Solvent recovery is not possible due to oil contamination
- Designs can be noisy