11
3.4 Function
3.4.3 Working Principle of Two-Stage Rotary vane pumps (ROTILABO
®
CR-DS 40)
Two pump stages (fore-stage and high-stage) are arranged in series in order to improve the end pressure
and the pumping speed at lower pressures. The intake takes place in the first stage (high-stage),
the compression and the outlet in the second stage (fore-stage).
When in operation, the intake stage is at lower pressure and the exhaust stage is at higher pressure.
Each stage contains a rotor assembly consisting of a rotor with two vanes and a stator. The two rotors
are combined on one shaft, and the two stators are combined in a common housing. The pump shaft
turns the rotors, causing the vanes in each chapter to sweep the surface of their stators. The vanes are
pressed against the stators by centrifugal force. Each stage has an exhaust valve. Gas expelled from
the first stage exhaust passes through an inter stage port to the intake of the second stage.
The second stage compresses the gas further, then expels it from the second stage exhaust valve to
the atmosphere. The oil carried along with the gas is separated out by a filter (available accessory).
An adjustable gas ballast valve is located in the pump’s exhaust stage. The purpose of the gas ballast
is to prevent vapours condensing in the vacuum pump. A dosed quantity of air (gas ballast) can be let
into the pump body during the compression by opening the gas ballast valve. The function of the
gas ballast valve is described more in detail in chapter 3.4.7 “Gas Ballast”.
Fig. 3 Pre-stage / high stage
Outlet opening with
exhaust valve
Outlet opening with
over pressure valve
Inlet opening
High stage
Pre-stage
3
DESCRIPTION
