The selection of pumping principle or vacuum pump is limited by specific parameters. These parameters are: minimum pressure, pressure range, pumping speed, exhaust pressure. In the ultra-high vacuum range, there are two other parameters: Pump selectivity and residual gas composition. The exhaust pressure is the pressure that the pump can exhaust under this pressure. According to this point of view, vacuum pumps can be roughly divided into three categories:
1) A pump that exhausts air into the atmosphere. This kind of pump is generally called rough pump or front stage pump. The air in the system is removed to a certain required working pressure, which is called rough pumping of the system. At the outlet of another pump to maintain a required low pressure is called the pre stage pressure. Mechanical rotary pump and jet pump are typical crude pump or pre stage pump.
2) A pump that exhausts only below atmospheric pressure. This kind of pump needs a front stage pump (Shenlian) to exhaust air into the atmosphere. Diffusion pump, roots pump and molecular pump all belong to this type. They all need a front stage pump.
3) A pump that binds the gas and steam in the system. This kind of pump does not need air outlet and is made according to the principle of ionization or absorption.
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