TechArchive

This article was originally written in the period 1995-2000

Bob Brooks and Noel West from Rietschle UK compare vacuum pump technologies, explaining how the Inovac Roots-type design has evolved.

Traditionally, vacuum for the chemical and pharmaceutical processing industries has been generated by steam ejectors, steam ejector liquid ring hybrids and fresh oil pumps. Each has the disadvantage that the process gas passing through the vacuum generating source comes into contact with another medium.

Single stage and multi-stage radial blowers, as well as those with direct or gearbox drive, produce vacuum up to 230mbar or pressure up to 300mbar. For vacuum operation, a pipeline is connected to the suction port of the blower. Capacities range from 100-3000 m<3>/hr. Radial blowers are air-cooled and non-contact operation means there is no wear, making the design virtually maintenance-free.

At a higher motor speed, for example 60Hz operation instead of 50Hz, both capacity and pressure difference are increased. The performance of the blower – and subsequently the required motor power – are also increased considerably.

The side channel principle is also based on the method of dynamic compression – transforming flow energy into pressure energy. The side channel works as a vacuum pump when throttled on the suction side and as a compressor when throttled on the pressure side. It is suited for applications requiring both pressure and vacuum. The air is accelerated and compressed in the impeller several times. The more the blower is throttled, either at the inlet or discharge, the greater the number of impeller re-entries and hence increased compression.

Depending upon model and size – single-stage or two-stage – vacuum up to 500mbar in suction operation and pressure up to 650mbar in pressure mode can be obtained. The capabilities range from 18-1100m<3>/hr. Non-contact operation means side channel blowers are practically free of wear and maintenance. The pumping medium is not contaminated from carbon dust or oil, as occurs with dry-running or oil-lubricated rotary vane pumps.

Pressure increase by volume reduction is the principle behind rotary vane operation. This static design offers excellent service in pressure, vacuum or a combination of both. Depending upon size or design – oil lubricated or dry running – vacuum up to 0.5mbar can be reached with capacities ranging from 3-100m<3>/hr and pressures up to 2.5bar is possible with capacities ranging from 3 to 500 m<3>/hr. When used as a combined unit, 0.8 bar vacuum and 0.8 bar pressure can be achieved simultaneously. The ratio between vacuum and pressure capabilities is influenced by the arrangement of the inlet ports.

Roots vacuum pumps are similar to rotary vane pumps, in that they are also static compression systems, although the compression dies not result from an “internal” volume reduction. Because of the non-contact operation of the Roots pump, there is no possibility of contamination of the pumped gas.

Single-stage designs are designed for use in pump sets in combination with a rotary vane backing vacuum pump. The conveyed air is not discharged to atmosphere, but piped into the inlet port of the connected high pressure stage rotary vane pump. The achievable capacities range from 500 to 4000 m<3>/hr at an ultimate vacuum of 0.01mbar.

The limitation, however, of a Roots-type blower has been its inability to achieve a respectable vacuum without using a backing pump. This is because the gas entering the pump is not compressed in any way and the compression is achieved on the discharge, resulting in recompression of the gas – for each unit of gas throughput, the gas is compressed twice.

Adapting the Roots design principle, Rietschle fitted a non-return valve on the discharge to stop recompression and limit the space in which any gas can be caught, thus not allowing it to expand. This has transformed a three-stage Roots pump from a middle of the road vacuum-generating source capable of only 350mbar abs into a multi-purpose, high vacuum pump capable of 0.5mbar abs, available on a single base plate with one drive motor.

The seal is achieved using an O-ring in compression. At each end of the stage, the process gas is isolated from the outside world by two PTFE piston rings and can be supported additionally with an inert gas purge if the process gas concerned is deemed hazardous. The inert gas purge connections are fitted on every pump whether used or not. Outside the PTFE piston rings is a lip seal to ensure that the oil bath used to lubricate the bearings does not leak into the process.

At the non-drive end is a set of helically-cut spur gears, which provide the timing of each lobe of the pump. These gears are fitted using taper lock bushes, which eliminate the need for keying the shaft or hot fitting the gears.

Historically, a typical multi-stage system requires liquid cooling together with sound attenuation. This problem has been solved by fitting all three stages inside a cooled water bath. This has resulted in near-perfect temperature at the same time as reducing the overall noise level to acceptable limits. A typical three-stage Roots pump creates 110dBa, but the Inovac 160 only creates 76dBa.

• Werner Rietschle
• Tel: 01892 835237
• Fax: 01892 834643