Discussion
Air Ballast valve in a rotary pump is used for :
Answer: B
The gas ballast valve is used for removing the condensable vapors.
In a rotary vane pump, vapour such as water drawn from the chamber is compressed during the exhaust stroke. If its partial pressure reaches the saturation pressure before the exhaust valve opens, it condenses into the oil, contaminates it and spoils the pump's performance. The gas ballast valve admits a controlled amount of air into the compression stage, so the exhaust valve opens before the vapour can condense and the vapour is expelled with the air.
It does not reduce noise or backstreaming, and it slightly worsens, rather than improves, the ultimate vacuum, which in any case for such pumps is of order \(10^{-3}\) Torr, not \(10^{-5}\).
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