Vickers Vane Pump description: The rotary vane pump […]
Vickers Vane Pump description: The rotary vane pump is designed so that the stator of the pump is immersed in oil and contains an eccentrically mounted rotor. The rotor contains two blades that slide in diametrically opposed slots. The blades may be spring loaded, but in addition to relying on centrifugal force to push them outward toward the stator wall. As the rotor rotates, the tip of the blade is always in contact with the stator wall.
The entire assembly is machined and assembled with tight tolerances such that the gap between the top of the rotor and the stator wall (commonly referred to as a "double seal") is approximately .025 mm (1.0 mil). The seal is filled with oil and provides a seal between the inlet side and the outlet side. The oil is circulated from the oil reservoir to the inside of the pump and discharged through the exhaust valve together with the pumped gas.
The ultimate pressure that can be reached by the pump is limited by the leakback of the Duo seal and the deflation of the lubricant. The outlet pressure can be as high as 1000 mbar (750 Torr) and the inlet pressure can be as high as 0.01 mbar (0.0075 Torr), which means that the pressure difference between the oil-filled seals is approximately 100,000:1 (1000:0.01). At pressure differences greater than this, back leakage will occur on the seal, which is one of the limiting factors for the ultimate vacuum obtained by the rotary vane pump.
A typical rotary vane pump has four stages of operation.
induction. The first 180° rotation of the rotor introduces gas into the pump chamber. Due to the crescent-shaped space formed by the offset-mounted rotor, the volume occupied by the gas is increased. Air pressure is proportional to its volume increase
Reduced (Powell's Law). This draws the gas into the pump and creates the required vacuum.
isolation. The uppermost blade passes through the air inlet and isolates it from the gas being pumped.
compression. Further rotation compresses and heats the gas in front of the lowermost blade, which reduces its volume due to the reduced space between the rotor and the stator.
exhaust. As the lowest blade continues to rotate, the pressure in front of the blade will increase enough to force the exhaust valve to open, expelling the gas at a pressure slightly above atmospheric pressure.
One of the key components in a rotary vane pump is an exhaust valve that is supplied with oil from multiple ports. A common valve design uses an elastomer (synthetic rubber) or a fluoroelastomer and a metal backing. The metal backing plate limits the movement of the elastomeric portion of the valve. Some valves are metallic and have no elastomer, but if the pump is stopped under vacuum, this design is susceptible to "sucking back". Since the valve does not use elastomer, oil may leak through it and then "suck back" through the pump and into the vacuum chamber or furnace. Since the valve opens and closes every time it is rotated, it is a source of noise and is prone to wear, whether or not an elastomer is used. For example, at a pump speed of 1750 RPM, the valve will open and close 2.5 million times every 24 hours at 29 Hz. The valve is mechanically operated and is forced to open by the pressure generated by the pump and then closed by atmospheric pressure.