Oil-free two stage scroll vacuum pump and method for controlling the

Abstract

An oil-free two-stage vacuum pump includes first and second pump stages coupled in series. The first and second pump stages have discharge spaces capable of communicating with each other via a bypass passage. The bypass passage is provided with a pressure control valve which is closed when pressure in the bypass passage is lower than a predetermined pressure. This permits a reduction in scroll size. In addition, the pump is free from problems present in pumps with large scroll sizes such as drive shaft vibrations due to warping thereof at high speed rotation, noise and heat generation, and durability reduction due to causes such as non-uniform contact between stationary and revolving scrolls. Moreover, in a compression step in the first pump stage, withdrawn gas is under high pressure; the gas pressure in a sealed vessel is close to the atmospheric pressure in an initial stage of driving. With a pressure increase beyond a predetermined pressure, a pressure control valve is opened so that compressed gas under high pressure is no longer supplied to the second pump stage; the compressed gas is instead exhausted to the outside environment. The second pump stage thus does not withdraw compressed gas under a pressure higher than atmospheric pressure, and is free from heat generation that might result from excessive compression.