When pumping atmospheric air (or gas) in a vacuum system, however “pure” it may appear to be, it will invariably contain some vapour.
During the compression process in the pump , this vapour will condense. Failure to remove it will form a contaminant which will prevent the pump from achieving its optimum vacuum pressure. Also, the condensate can enter the pump’s mechanism, for example the oil in oil-sealed rotary pumps, where, as a contaminant, it can have a detrimental effect.
In simple terms, a gas ballast valve incorporated into the system will allow a flow of air into the final part of the compression cycle and allow the vapour to be expelled without condensation or affecting the pump's overall performance.
What is the theory of gas ballast?
While the gas ballast theory is simple, the rationale behind its use in vacuum pumps is still sometimes shrouded in confusion and misinformation. As a result, the subject is frequently considered esoteric.
So, to give you a better understanding of gas ballast, let's talk a bit about its history and how it works
HISTORY AND OPERATION
Originally developed by Wolfgang Gaede in 1935, gas ballast was a significant addition to mechanical, oil-sealed vacuum pumps.
When used in mechanical, oil-sealed vacuum pumps, the gas ballast allows the pumping of vapours without them condensing and thus contaminating the pump’s sealing-oil. By reducing any potential condensation in the sealing oil, the pump can achieve its vapour duty at near full-specification; though the ballast flow does impact the ultimate pressure of the pump.
Edwards E2M28 speed curve: 1. Without ballast 2. With ballast
Gas ballast assembly. Source: Edwards
In vacuum practice, the gas stream evacuated from the vacuum chamber will often contain water vapour, solvent vapour and/or other unwelcome contaminants.
These contaminants occur because they have been “converted” (under vacuum pressure) from liquid to gas phase then (i) flowing back to the pump, where they are “converted" back from a gas to a liquid contaminant within the pump oil, or (ii) when using ballast exiting the pump itself, without contamination.
In simple terms, gas ballasting is when the introduction of the ballast gas flow allows the condensable elements of air or gas that is being pumped to be expelled from the pump before it condenses, contaminates the system and/or pollutes the sealing oil.
For instance: if the pump is “working” a gas that would naturally condense in the pump, the gas ballast mechanism opens the exhaust valve before the vapour has had a chance to condense. The result is that this condensable vapour is discharged (to the atmosphere).
Source: Edwards
A pump that has been subject to condensed vapour can be “cleaned-up” by using gas ballast.
To do so, the pump inlet port should be closed and the pump allowed to run with the gas ballast valve in the open position.. Continue purging for several hours or overnight (depending on the size of the system and the degree of contamination).
However, if the pump is “smoking” or emitting a visible mist in large quantities, you could return to a room full of “oil-fog”. Venting into an extraction hood or using exhaust mist filters will help to eliminate this problem. The use of a gas-ballast oil return kit resolves this issue.
It should be remembered that as pumped gases and vapours exit the system, they can enter the working environment. This means that depending on their content and/or concentration, they may pose a hazard.
Gas ballasting, which is associated with oil-sealed rotary vane and dry scroll pumps, is frequently employed in freeze-drying, rotary evaporation, distillation and gel drying.
The video below highlights the function of rotary vane vacuum pumps with gas ballast:
To find out more on the topic of vacuum measurement, download our eBook below.
Comments