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Gas ballast is a constructional feature widely applied in many primary vacuum pumps for the pumping of vapours. Wolfgang Gaede invented the gas-ballast principle in 1935 and was originally focussed on primary oil sealed pumps. Gas ballast is useful in reducing the extent of vapour contamination in the oil (and/or other parts) thus extending oil life and at a more fundamental level permitting the pump to operate on vapour duty at nearly full specifications. Put simply, depending on the percentage make-up of vapour in the pumped load, then without gas ballast the pump can ‘stagnate’.
A typical physical representation of the gas ballast configuration is shown below
The basic principle of ballast is that atmospheric air (or CDA or inert gas) is admitted into a pump during the (late) compression stages (as above). This increases the percentage of non-condensable gas such that the partial pressure of the vapour being pumped is below its saturated vapour pressure when the exhaust valve opens (typically at a pressure of 1.05 to 1.2 bara). This means that the vapour is discharged from the pump without liquefaction.
The work performed from ballasting tends to increase the operating temperature which again helps with vapour handling.
N.B. when we talk about vapour pressure we usually mean the saturated vapour pressure.
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