How does the gas ballast work?
Gas ballast should generally be run all the time that vapour is being passed through the vacuum pump.
If vapour is being pumped, the vacuum pump’s ultimate pressure will be limited, not by ultimate pressure performance of the pump itself, but by the vapour pressure of the material being processed. Care should be taken to ensure that air/oxygen is not introduced if any of the process materials are flammable.
Gas ballast process
How can water vapour affect my vacuum pump?
A most common vapour for vacuum pumps to handle is water vapour.
Water has a saturated vapour pressure of ~24 mbar (18 Torr) at 20 oC (70 oF), this is the lowest pressure the pump can attain until all the water is pumped away.
Allowing water vapour to condense inside a pump will make the time to recover ultimate pressure much longer than if it remains in vapour phase.
This is because it has to be re-evaporated before it can be pumped out and that takes much more energy and time than if the water remains in vapour phase. Condensed water vapour can also degrade an oil sealed rotary vane pump oil quite quickly.
Mode switch: A unique feature to overcome condensables
The RV range of oil sealed rotary vane pumps provide a unique "mode switch" option allowing accelerated oil conditioning after exposure to condensables.
Vacuum pumps are quoted with a Maximum Water Vapour Handling Capacity, usually in grams/hour. This parameter is determined by the gas ballast flow rate, the running temperature of the pump and the pressure at which the exhaust valve of the pump opens.
An oil sealed vacuum pump running gas ballast for long periods of time should be fitted with a device designed to return oil back into the oil box while a large flow of gas goes through the pump. Gas ballast oil return accessories are available.
Oil free pumps, also called “dry pumps”, do not need these as they do not have oil within the vacuum envelope.
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Further reading
