|Figure 1 | Cross-sectional view of a rotary vane pump (courtesy of Edwards Vacuum)|
Oil sealed rotary vane pumps (aka rotary vane pumps) are the primary pumps on most vacuum systems used in the heat treatment industry. They are also referred to as a “backing” pump when used in combination with a booster pump, or with both a booster and secondary (“high vacuum”) pump, typically a diffusion style. A rotary vane pump can also be used alone when high vacuum is not required and slower pumpdown is acceptable.
Two-stage designs are available, which utilize two rotors in series internal to the pump. Single-stage designs can provide a vacuum of 3 x 10-2 Torr (4 x 10-2 mbar), while two-stage designs can achieve 3 x 10-3 Torr (4 x 10-3 mbar).
Due to the prevalence of rotary vane pumps, it is important for designers and users of industrial vacuum equipment to have a good understanding of how these pumps function. This series of articles will cover pump principles of operation, pump designs, pump oils, single-stage versus two-stage pump designs, contamination and gas ballast (manual and automatic), common accessories, applications, troubleshooting and pump maintenance.
Principles of Operation
Of the various vacuum pump technologies, rotary vane pumps are considered wet, positive displacement pumps. They are often called “wet” pumps because the gas being pumped is exposed to oil used as a lubricant to help provide the seal.
For this reason, the oil is carefully selected and specially designed for the application. Positive displacement indicates that the pump works by mechanically trapping a volume of gas and moving it through the pump, creating a low
pressure on the inlet side.
|Figure 2 | Internal view of the upper portion of a rotary vane pump (courtesy of Edwards Vacuum)
Rotary vane pumps (Fig. 1) are designed so that the stator of the pump is immersed in oil and contains a rotor which is eccentrically mounted. The rotor contains two vanes which slide in diametrically opposed slots. The vanes can be spring loaded, but otherwise rely on centrifugal force to push outward against the stator wall. As the rotor turns, the tips of the blades are in contact with the stator wall at all times.
The entire assembly (Fig. 2) is machined and assembled with tight tolerances so that the gap between the top of the rotor and the stator wall (often referred to as the “Dou seal”) is approximately .025 mm (1.0 mils). This seal is filled
with oil, providing a seal between the inlet and outlet sides. The oil is circulated from the oil reservoir into the pump interior and is exhausted through the exhaust valve with the pumped gas.
The ultimate pressure achievable by the pump is limited by back-leakage through the Duo seal and by the outgassing of the lubricating oil. The outlet pressure can be as high as 1000 mbar (750 Torr) and the inlet as low as .01 mbar
(0.0075 Torr), which means the pressure differential across the oil-filled seal is roughly 100,000:1 (1000:0.01). At pressure differentials greater than this, back-leakage across the seal will occur, which represents one of the limiting
factors in the ultimate vacuum achieved by rotary vane pumps.
|Figure 3 | Four stages of a rotary vane pump (courtesy of Edwards Vacuum)
There are four stages of operation in a typical rotary vane pump (Fig. 3)
- Induction. The first 180° rotation of the rotor induces the gas into the pumping chamber. The volume occupied by the gas increases due to the crescent-shaped space created by the offset-mounted rotor. The gas pressure decreases in proportion to the increase in its volume (Boyle’s law). This draws the gas into the pump and generates the required vacuum.
- Isolation. The upper most vane passes the inlet port, sealing it off from the gas being pumped.
- Compression. Further rotation compresses and heats the gas ahead of the lowermost vane, reducing its volume due to the decreasing space between the rotor and stator.
- Exhaust. As the lowermost vane continues its rotation, the pressure in front of it increases sufficiently to force the exhaust valve open, discharging the gas at a pressure slightly above atmospheric.
|Figure 4 | Exhaust valve on a rotary vane pump (courtesy of Edwards Vacuum)
One of the critical components in a rotary vane pump is the exhaust valve (Fig. 4), which is fed by multiple ports. One common valve design uses an elastomer (artificial rubber) or flouro-elastomer, with a metal backing plate. The metal backing plate limits the movement of the elastomer part of the valve. Some valves are all metal with no elastomer but this design is susceptible to an effect known as “suck-back” if the pump stops under vacuum. Since the valve does not use elastomer, oil can leak past it and be “sucked” back through the pump and into the vacuum chamber or furnace. Since the valve opens and closes with every rotation it is a source of noise and is susceptible to wear, whether elastomer is used or not. With a pump rotational speed of 1750 RPM, for example, the valve will open and close 2.5 million times every 24 hours at a frequency of 29 Hz. The valve operates mechanically and is forced open by the pressure created by the pump, then closed by atmospheric pressure.
Rotary Pump Oils
Rotary pumps are lubricated with oil, which not only provides a seal between the high and low-pressure sides of the pump, but also lubricates the pump bearings and other rotating components. Some pump designs, especially older ones using circulation of the lubricating oil relied on a vacuum feed system whereby the vacuum generated by the pump itself was also employed to draw the lubricating oil through the rotor bearings. Other pumps use spring loaded lip type shaft seals around the rotor shaft. This is a dynamic style seal, which also requires lubrication.
Although vacuum feed oil distribution is still used, more modern pumps use a separate oil pump, to circulate the oil via passages machined into the stator to the rotor bearings and seals (Fig. 5). When the vacuum pump is operating, its rotation also rotates the oil pump, which is mounted to the same shaft, and develops a positive oil feed pressure of 0.4 bar (300 Torr) above atmospheric pressure. This pressure lifts a spring-loaded elastomer disc which allows oil to flow into a trough feeding the pump interior and rotor bearings as well as the vanes of the vacuum pump. When the vacuum pump stops, the oil pump pressure is no longer present to force the elastomer disc open, and therefore it closes, preventing the suck back of oil through the pump and into the vacuum chamber. Whether or not an oil pump is used, the excess oil is exhausted from the pumping mechanism via the exhaust valve.
|Figure 5 | Oil pump and distribution system (courtesy of Edwards Vacuum)|
On vacuum pumps that use a separate oil pump, a hydraulically operated inlet isolation valve can also be incorporated (Fig. 6). In this design, some of the circulated oil is directed to a piston, which is connected to an inlet valve located where the gas enters the pump from the vacuum chamber. The piston uses the hydraulic pressure generated by the oil pump to open the inlet valve, permitting the gas to enter the pump from the chamber. The valve is spring loaded and uses an elastomer seal to stop the flow of gas within 0.5 second of pump stoppage. This provides additional protection against suck back into the vacuum chamber.
|Figure 6 | Hydraulically operated inlet isolation valve (courtesy of Edwards Vacuum)
Types of Oil
The oil used in rotary vane pumps is carefully selected. In addition to providing lubrication of the rotor bearings, it must:
- Provide a seal between the vanes and the rotor.
- Generate the Duo seal between the tips of the vanes and the stator.
- Provide cooling of the stator by transferring heat to the outer casing.
- Offer corrosion protection of the metal parts from the gas being pumped.
|Figure 7 | Oil mist filter (courtesy of Edwards Vacuum)
In addition, the vapor pressure of the oil is critical because the oil is exposed to the gas being pumped from the chamber. If the oil pressure is too high, it will vaporize when exposed to vacuum, allowing oil vapor to contaminate the vacuum chamber (referred to as backstreaming). The oil vapor pressure is generally one of the factors limiting the ultimate achievable. For the reasons above, careful consideration must be given to the selection of the oil. Typical motor oil, for example, is not sufficiently refined for use in a vacuum pump, has insufficient resistance to chemical attack, and contains additives that may be detrimental to the process being performed in the vacuum chamber. In addition, the viscosity must be considered. Lower viscosity oils are used for lower operating temperatures, and smaller pumps, while medium viscosity oils are used for medium to large pumps.
Oils designed specifically for rotary pumps are distilled mineral oils to which hydrogen atoms have been attached to any loose molecules in the chain. This process, referred to as hydro-treating, provides a strong, stable formulation
with a low vapor pressure. For applications where the vacuum pump may be exposed to reactive or corrosive gases carried in the pumped gas, specially engineered oil is used which has been further processed to remove impurities. Where a high concentration of oxygen or other chemically reactive gases are present, highly inert, man-made lubricants are recommended. These perflouropolyether (PFPE) fluids have good temperature resistance but must not be exposed to temperatures above 280°C (535ºF) at which point they release toxic vapors. PFPE fluids are available under the trades names (e.g., Fomblin (Solvay Solexis) and Dupont’s Krytox). If the incorrect oil is used in a chemically aggressive environment, it will break down and leave a tar-like residue, which will block the internal passageways and cause pump overheating and failure resulting from insufficient lubrication.
Due to the inherent design of the rotary pump as a “wet” pump, some oil is expelled out of the pump as a mist, along with the gas being transferred. For this reason, an oil mist filter (Fig. 7) is employed to capture the expelled oil.
After leaving the pump, the pumped gas is passed through the mist filter, which contains a filter element that reduces the oil mist into droplets and collects it. The captured oil can be drained manually, or through other accessories
returned to the pump in a closed loop. It can return either by gravity to the oil box, or by suction through the gas ballast (to be discussed later). The filter element is a consumable product and must be periodically replaced.
Next Time: We continue our discussion on rotary vane pumps.
Daniel H. Herring / Tel: (630) 834-3017) /E-mail: [email protected]
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.