Oil sealed or as popularly referred to as Rotary vane pumps are the main pumps in most heat treatment vacuum systems. They can also be called ‘backing’ pumps when incorporated with a booster pump, or with both a secondary (‘high vacuum’) pump and booster, turbo, cryo, and diffusion are all popular styles. In cases where a high vacuum is not needed, and a slower pump-down is sustainable, a rotary vane pump can be used independently.
They come in two-stage designs, which use two rotors sequentially to the pump. Single-stage designs offer a vacuum of 3 x 10-2 Torr (4 x 10-2 mbar), whereas two-stage designs can reach up to 3 x 10-3 Torr (4 x 10-3 mbar).
As a result of how rotary vane pumps have become prevalent, users and industrial vacuum equipment designers need to understand how these pumps work. This article series will encompass principles of operation, pump oils, pump designs, two-stage versus single-stage pump designs, common accessories, contamination and gas ballast (automatic and manual), applications, troubleshooting, and pump maintenance.
Principles of Operation
Out of all the vacuum pump innovations, rotary vane pumps are hugely deemed to be wet and positive displacement pumps. They are usually called ‘wet’ pumps because the gas that’s being pumped is brought into contact with oil used as a lubricant to provide the seal.
Because of this, oil is selected carefully and specially made for the application. Positive displacement means that the pump functions by trapping a volume of gas mechanically and moving it across the pump, which creates a low-pressure zone on the input side.
Vane Pump Design
Rotary vane pumps are built in such a way that the pump’s stator is doused in oil and has a rotor that is mounted eccentrically. The rotor comes with two vanes that slide in diametrically opposing slots. The vanes sometimes can be spring-loaded, but in most cases, they will depend on centrifugal force to assist in pushing outward against the stator wall. With the rotor continually turning, the blade tips will be exposed to the stator wall.
The whole assembly is assembled and machined with tight tolerances, such that the space between the stator wall and rotor’s top (commonly known as ‘Dou seal’) is roughly 0.025 mm (1.0 mils). Oil is filled in this seal to provide a seal between the outlet and inlet sides. Oil from the reservoir is circulated into the pump interior and is depleted through the exhaust valve alongside the pumped gas.
The optimal pressure that can be attained by the pump is restricted by back-leakage through the Duo seal and lubricating oil outgassing. The outlet pressure can reach up to 1000 mbar (750 Torr) while the inlet can be as low as .01 mbar (0.0075 Torr), meaning the pressure differentials across the oil-filled is approximately 100,000:1 (1000:0.01). When there are pressure differentials larger than this, back-leakage across the seal will happen, representing one of the restricting factors in the maximum vacuum attainable by rotary vane pumps.
The exhaust valve is among the most fundamental components in a rotary vane pump, given how several ports feed it. One frequently used valve design utilizes a fluoro-elastomer or elastomer (artificial rubber), coupled with a metal backing plate. The role of the metal backing plate is limiting the motion of the elastomer valve part. There are all metal valves and don’t have an elastomer, but this design is prone to experience a phenomenon referred to as ‘suck-back’ when the pump halts under vacuum. Given how the valve doesn’t have an elastomer, oil leaks past it and gets ‘sucked’ back through the pump and into the furnace or vacuum chamber. Considering that with every rotation, the valve opens up and closes, it offers a source of noise and is more prone to wear, regardless if an elastomer is used or not. Given a rotational speed of 1750 RPM, for instance, the valve will close and open 2.5 million times after every 24 hours and at a frequency of 29 Hz. The valve functions mechanically and is forced to open by the in-built pressure created inside the pump and closed with atmospheric pressure.
Rotary Pump Oils
Rotary pumps are lubricated using oil, which offers a seal between the low and high-pressure pump sides and lubricates the pump's bearings and other rotating parts. Traditional pump designs that use the circulation of lubricating oil depended on a vacuum feed distribution system where the vacuum created by the pump was also used to draw lubricating oil via rotor bearings. Some pumps utilize spring-loaded lip shaft seals on every side of the rotor shaft. This is a more dynamic seal style which will need lubrication.
Even though the feed oil system is still being utilized, more contemporary pumps utilize an independent oil pump to circulate the oil through passages machined in the stator to the seals and rotor bearings. When the vacuum pump is working, its rotation in the process rotates the oil pump that is mounted to the shaft and creates a positive oil feed pressure of 300 Torr (0.4 bar) above atmospheric pressure. This pressure, in turn, lists the spring-loaded elastomer disc, which enables oil to flow into a trough feeding the rotor bearings and pump interior as well as the vacuum pump vanes. As the vacuum pump comes to a stop, the oil pump pressure isn’t available to force open the elastomer disc and thus closes, avoiding oil suck back through the pump and directly in the vacuum chamber. Whether the oil pump has been used or not, the excess oil is depleted from the exhaust valve's pumping mechanism.
Vacuum pumps that utilize a separate pump can be integrated with an inlet isolation valve that is hydraulically operated. Here, the circulated oil is directed to a piston that is connected to an inlet valve and situated where the gas gains entry into the pump when coming from the vacuum chamber. The piston utilizes the hydraulic pressure created by the oil pump to open the inlet valve and allow gas to gain entry into the pump from the chamber. The valve spring loads and uses an elastomer seal to hinder the passage of gas within 0.5 seconds. This offers extra protection against suck back in the vacuum chamber.
When it comes to matters concerning oil-sealed rotary vane pumps, a lot more can be said, but the important thing is recognizing the vacuum furnace’s overall performance. It's integral that you are aware of how they function and how to use them correctly. Make sure you've changed the oil after every month and employ proper measures to maintain and take care of them, and you will be assured of numerous other years of trouble-free pump functioning.