Producing low-cost, oil-free, maintenance-free mechanical pumps for roughing/backing applications has been the goal of vacuum pump manufacturers for the past 20 years. One impediment to attaining this objective has revolved around eliminating oil, which plays many roles in the operation of mechanical pumps (and we don’t mean Palm oil). Not only does oil reduce friction between the sliding and rolling elements inside the pump, it also seals vacuum compartments and conducts heat away from the pump’s interior regions.
One way to eliminate oil or other forms of lubrication, at least in the spaces where the pumping action takes place, is to adopt a “contactless” design. Contactless pumps have been used for many years to pump the often corrosive gases and abrasive by-products produced in the manufacture of semiconductors. Unfortunately pumps based on these designs are too large, too complicated, and too expensive for applications where generally clean gases are pumped. These applications range from process chamber loadlocks in semiconductor manufacturing to backing turbo pumps used in medical and scientific instruments and in many industrial processes such as chemical and food processing and the manufacture of TV tubes and light bulbs.
One difficulty in arriving at an appropriate contactless pump design for these applications concerns the very narrow spaces needed between the stationary and moving surfaces inside the pump to produce effective vacuum sealing and efficient pumping. If the pump is a high-speed device, additional potential problems arise with the precision the rolling element bearings and associated seals must maintain over long periods of time.
Heat removal is also a problem that must be solved with a contactless pump that contains no oil in its working chambers. Heat generated by the compression of the pumped gases must be removed either via the support/drive shaft or by means of complicated streams of supplementary inert purge gas near the bearings and water cooling. While it is desirable to operate pumps at elevated temperatures, to avoid the problem of condensation (often water) inside the pump, there is a practical upper limit to the working temperature. If bearings and seals are operated at much more than 100 [degrees] C, the very long lifetimes demanded by modern manufacturing will be difficult to meet.
Leybold Vacuum GmbH, Cologne, Germany, approached these challenges by designing an oil-free roughing vacuum pump based on the proven, highly reliable, low-cost, piston principle. The EcoDry L pump, which is mechanically very simple, needs only ambient air for cooling; no supplementary purge gases or water are required. These savings, coupled with very low electrical power consumption at idle, combine to reduce operating expenses to a minimum for end users.
To avoid many of the difficulties and costs inherent in contactless pumps, the new piston design incorporates a low coefficient of friction material between the stationary and moving parts, so that controlled contact can be achieved without using any liquid lubricants. The relatively thick PTFE-like material that coats the outside of each cylindrical piston also makes it possible for the contact surfaces to dissipate the heat generated during gas compression.
By making the piston long in relation to the slender gap between the piston and cylinder wall, the compression space can be sealed effectively without generating any high contact forces. This assures relatively cool operation and extended maintenance-free intervals. The support bearings, sealed and lubricated for life, are situated outside the vacuum compression spaces and never need maintenance.
Piston pumps generally use an inlet valve that is opened and closed automatically when the piston, during its motion, uncovers and covers a narrow slot in the wall of the cylinder at one extreme of the piston’s travel. This makes the inlet valve independent of the inlet pressure, a feature that has been retained in the Leybold design.
The usual solution for the exhaust valve is to provide a rigid valve plate, containing the movable valve, at the other extreme of the piston’s travel. The problem with this approach is that in order to increase the compression ratio, and therefore the ultimate pressure, only a very small gap must remain between the top of the piston and the rigid exhaust valve plate. Any mechanical inaccuracy in the position of this solidly fixed plate and the top of the piston will cause conditions that negatively affect the performance of the pump. Also, any contact between valve plate and piston, whether from mechanical inaccuracies or from ingested particles that become lodged between the two during operation, will cause mechanical damage and noise.
On the other hand, should the gap between piston and valve plate get too large, the pump would be prevented from reaching the desired low ultimate pressure. It is clear that to be able to operate a piston pump for extended periods, a simple yet effective alternative solution to the exhaust valve problem must be found that would additionally provide tolerance for foreign particles and liquids.
In the EcoDry L pump, the rigid valve plate has been replaced by a lightweight stressed metallic membrane that acts both as the valve plate and closing spring. The membrane, which is constructed from corrosion-resistant spring material, is larger than the piston and is fixed in such a way that it can be bumped by the piston at the extreme of the exhaust stroke and flexed open while producing little or no noise and incurring no mechanical damage. Only a very small valve opening is needed for good vacuum performance, due to the large valve area. The flexible nature of the exhaust valve also largely eliminates any performance limitations due to mechanical inaccuracies.
Because of the small flex distance, extremely long valve life can be expected. Additionally, accidental piston-valve contact can now take place without concern for mechanical damage.
The EcoDry L piston pump has four pistons, arranged in opposition, with two stages of compression per piston, to produce a sufficiently low pressure for most purposes. The pistons are arranged so that both compression stages on two of the pistons are used in parallel to produce a combined pumping speed of 48 [m.sup.8]/hr. The next piston has both compression spaces in series with the first pair of pistons. The last piston is arranged so that both compression spaces are independent of each other, providing the final stages of compression.
During high-pressure operation (near atmospheric pressure), the pump is provided with a pressure-sensitive bypass valve to automatically remove three stages of compression, as they are unnecessary during this phase of operation. This is done to avoid the needlessly high power consumption and heating associated with the extra compression. Providing this bypass also allows pump operation for long periods at atmospheric pressure without causing overheating to take place.
Because of its flexible exhaust valves, the EcoDry L piston pump can tolerate foreign particles, such as broken semiconductor wafers, and liquids without affecting its long-term performance or lifetime. The exhaust valves, which do not wear in any way, guarantee trouble-free operation for more than 20,000 hr, while providing the level of pumping performance normally associated with conventional oil-sealed pumps.
In summary, the EcoDry L roughing pump’s new simple design provides approximately two years of maintenance-free, contamination-free operation without the use of a blower or water cooling, and of course with no oil or grease-lubricated bearings on the vacuum side. Additionally, the pump has been designed so that easily measured performance changes give ample warning of upcoming maintenance requirements.