Question

a)Draw the operational principle diagram for a roots-type vacuum pump.

b)Describe the advantages and disadvantages of using a roots pump over the more common

rotary vane pump.

Answer 1

A)

Roots vacuum pumps are rotary plunger type pumps where two symmetrically shaped impellors rotate in opposite directions inside the pump . The impellors are counter-rotating in such a way, that they are near to one another and to the housing without actual contacting . In rotor positions I and II ( figure) the volume of the intake is increased . As the rotors turn further to position III a part of the volume is cut off from the intake side . In position IV this volume is opened to the exhaust side and gas under forevacuum pressure (higher than the intake pressure) flows in . This gas compresses the gas coming from the intake . As the rotors turn further the compressed gas is ejected through the exhaust flange . This process repeats in the pump.

B) As the rotors do not come into contact with the pump’s housing Roots vacuum pumps may be operated at high speeds . Thus a high pumping speed is obtained from comparably small pumps.

start fast, can achieve in a short period of time limit of vacuum.Small power, low operation maintenance cost.

Roots vacuum pump have a wide pressure range (1 x 10 ^ 3 pa ~ 1 pa) and have a large pumping speed, can quickly get rid of all of a sudden release of gas , make up for the diffusion pump oil seal and mechanical pump in 1 x 10 ^ (3 pa – 1 pa) pumping speed resistance are small defects.Therefore, it is most suited as a booster pump.

It needs a secondary vacuum pump for work like rotary vane pump.

the ability to Suction high pumping hydrogen lower than the effect of rotary vane pump.