How to Select a High Vacuum Pump, Part 2: Positive Displacement Pumps
The Positive Displacement Pump category is comprised of rotary vane, piston, roots, scroll and diaphragm pumps.
The rotary vane vacuum pump is the most widely used type in both industry and research. These pumps are relatively quiet, have no external moving parts, and are sold in a variety of sizes from a few cubic feet per minute (CFM) to many hundreds of CFM. The pump designs are all similar in that an electric motor is connected in line to the pump which basically consists of a stator and an offset rotor mounted very tightly to the stator wall.
As the rotor spins, the spaces between the blades are exposed to the chamber being evacuated. A small number of gas molecules expand into this space and are then swept around and then exhausted through the one way valves to atmosphere. These pumps are filled with oil, including the area inside the stator walls, which fills the small empty spaces between the rotor and stator, preventing the gas from returning to the evacuated chamber.
Piston pumps are similar to the design in a car engine in that when the piston is at bottom dead center, the piston cavity fills with expanded gas from the chamber being evacuated. As the piston reverses direction and heads for top dead center, the gases are expelled across one way valves that prevent the reentry of exhausted gas. Due to the off center pumping mechanism, these pumps are inherently noisier and create more vibration than rotary vane pumps.
Roots blowers usually have dual rotating shafts inside a common housing, and mounted on these two shafts are impellers which rotate close to each other and close to the stator walls. The spaces between the impellers are filled with expanding gas from the chamber. The gas is then swept into a contracted space, thereby increasing the pressure and exhausting it to atmosphere.
Scroll pumps use a similar principal; but instead of using impellers that rotate close to each other, the scroll pump uses a spiral design that takes the expanded gas into large circular spaces which narrow as they reach the top of the spiral spinning rotor.
Diaphram pumps use a flexible diaphragm which is alternately moved in opposing directions so that gas is allowed to enter into the space above the diaphragm as it maximizes the space above it. Then the inlet valve closes and the exhaust valve opens to push the gas to atmosphere.
As you might imagine each of these positive displacement pumps have pros and cons including price, performance, reliability and ability to handle various gases and chemicals. As a very rough guideline, here are the base pressure ranges that these pumps can achieve (note: 760 torr equals one atmosphere (14.7psi):
| Pump Type |
Base Pressure Range (mTorr) |
| |
|
| Piston |
20-50 |
| Rotary Vane |
1-20 |
| Scroll |
25-250 |
| Diaphragm |
10-100 |
Another reason these pumps are used so extensively is that many other vacuum pumps which achieve considerably lower base pressures need to be “backed” by some type of vacuum pump which is capable of exhausting directly to atmosphere.
Next related blog:
How to Select a Vacuum Pump: Part 3, Momentum Transfer Pumps
This post was authored by Michael A. Grandinetti. You can learn more about Mike at http://LinkedIn.MichaelGrandinetti1
IES Technical Sales is a value added technical sales, distribution, and solutions provider serving the high technology vacuum, plasma/thin film, temperature, fluid handling and metrology markets.