Welch Vacuum 8925 Direct Drive Rotary Vane Pump - Manual (Page 8)

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Section 2: PUMP FEATURES AND PRINCIPLES OF OPERATION
2.01 General Description
All of the Welch Vacuum Pumps are two-stage, rotary-vane, oil sealed vacuum pumps.
These Vacuum Pumps offer a number of features that improve performance, or protect the pump or
vacuum system under specific operating conditions.
2.02 Principles of Vacuum Pump Operation
The main purpose of a vacuum pump is to reduce the pressure in a vessel or a closed system. The
degree of pressure reduction is dependent upon the requirements of the application and the type of
vacuum pump employed. Rotary vane, oil-sealed vacuum pump operation is described in this section.
Pressure reduction in a closed system is accomplished by repeatedly removing a portion of the
original volume of gas contained in the system. Removal is performed by the action of the rotating
elements of the pump which cause a given space to be successfully enlarged and diminished. Figure 2.1
illustrates a section through a typical stage of rotary-vane pump. Note that this figure is not intended to
illustrate exactly the internal components of the pumps; its purpose is to illustrate the general operating
principles of vacuum pumps.
The rotary action of the pump creates a hollow space of chamber (1) which expands as the pump rotates.
As the chamber expands, the pressure in the chamber decreases. As a result, gas is drawn into the
chamber due to the difference in pressure between the chamber and the inlet (4) to the chamber. (The
inlet is the only place where gas can flow into the chamber.) Once the vane (3) moves past the inlet (4),
it seals the inlet against the chamber (1) and the gas becomes trapped between the vanes (2 and 3). The
chamber (1) formed by the enclosed space between the vanes then begins to decrease in volume as the
rotor revolves, compressing the gas. The pressure of the compressed gas becomes greater than
atmospheric pressure. When the vane (2) moves past the exhaust port (5) the compressed gas in the
chamber is forced out through the exhaust port.
Figure 2.1
Typical Rotary Vane
Pump, Schematic Diagram
This expansion/compression cycle constitutes one complete cycle of the pump operation. This cycle is
repeated as the vane (2) passes the intake port and seals it against the atmosphere. Therefore, two pump
cycles are performed during each revolution of the pump rotor.