What is the mode switch feature?

In a two stage oil sealed rotary vane pump, two pumping stages are used in series:

• first, the high vacuum stage which exhausts into 
• the second low vacuum stage and ultimately exhausts to atmospheric pressure.

Vacuum pump oil is injected into the stages via an oil pump and an oil pressure regulator that is part of the pumping mechanism. The oil performs a number of duties, including lubricating, sealing and transferring heat away.

With the mode switch:

• in high vacuum mode the oil is degassed in the interstage area before being allowed into the high vacuum stage at a controlled rate. This degassing allows best ultimate to be attained.
• in high throughput mode, high oil flow is directed into the high vacuum stage to ensure good lubrication.

In high throughput mode we are not concerned with the ultimate performance of the pump, just its ability to cope with large gas loads without the high vacuum stage being starved of lubrication.

For applications where the pump spends a considerable amount of time at relatively high inlet pressures (high pressure in this case meaning greater than 75Torr/100mbar or about 1/10th of atmospheric pressure), the pump should be operated in high throughput mode.

With a conventional two stage pump operating during high pressure/high throughput operation like this, the amount of gas passing through the high vacuum stage can get so high that it carries out the oil normally inside it causing it to “run dry”, meaning direct metal-to-metal contact between the moving and non-moving parts of the pump. This results in very rapid wear and damage.

The mode selector switch changes the oil distribution path inside the pump body to increase the flow of oil into the high vacuum stage.  This keeps it lubricated and also helps the pump handle large amounts of condensable vapor which may otherwise condense inside the pump.

An alternative to a mode control switch is to have two dedicated pumps:

• a single stage pump for high throughput/pressure operation
• and a two stage pump for high vacuum use.

The mode switch allows a single pump to do double duty in both high throughput and high vacuum operation without compromising performance in either mode.