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.