Basic Concepts and Definitions Questions and Answers

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 \(10^{-5}\).

Answer: A

The keyed answer is chart (A), the only one in which the pumping speed falls to zero near a pressure of about 10^-3 torr.

A rotary (oil-sealed) pump cannot pump below its ultimate pressure, which is of the order of 10^-3 torr. Its speed therefore has to die away as the pressure approaches that value. Charts (C) and (D) carry the full speed all the way to 10^-6 torr, a range that a rotary pump cannot reach (it is the range of diffusion or turbomolecular pumps backed by a rotary pump), and chart (D) never falls at all. Chart (B) is a straight line that keeps rising past the rated 100 L/min, which no pump does.

What could not be confirmed is the shape of the rising part: in practice a rotary pump holds close to its rated speed over most of the range from atmosphere downward, and chart (A) shows it climbing from zero at one atmosphere. So (A) is the best of the four by elimination and by its fall to zero near the ultimate pressure, not a faithful picture of a measured curve.