Chemglass AF-0349 Welch Vacuum Pump - Manual (Page 9)

9
2.03 Effects of Continued Pressure Reduction
The quantity of gas in the vessel (6) is reduced with each evacuation cycle. The gas remaining in the vessel expands
to fill the vessel and consequently with each cycle the pressure in the vessel is reduced. This is a manifestation of
Boyle’s Law which states that, for a constant temperature, the volume of a body of gas is inversely proportional to its
pressure; i.e., if the volume is enlarged the pressure must be reduced.
As the amount of gas in the vessel is steadily diminished, its pressure is correspondingly reduced. The action of the
pump must therefore compress a successively smaller quantity of gas with each cycle to something greater than
atmospheric pressure in order to expel it from the pump.
At the beginning of an evacuation sequence, the compression ratio is very small. In the first cycle of operation the
pump draws in a volume of gas at atmospheric pressure and expels it at approximately atmospheric pressure. In
contrast, near its ultimate pressure, a pump draws in gas at (for example) 30 millitorr and must compress it to more
than 760,000 millitorr (atmospheric pressure) in order to expel it from the pump. Since the exhaust valve is generally
spring loaded to provide a good seal, the pressure required to open it is somewhat greater than atmospheric pressure.
Therefore, at an ultimate pressure of 1.3 x 10
-4 mbar 0.1 millitorr, (1 x 10 -4 Torr) the compression ratio performed by
the pump is greater than 1,000,000 to 1.
2.04 Ultimate Pressure
As described previously, a quantity of gas is removed from the system with each cycle of the pump. Therefore, the
pressure of the gas remaining in the system is reduced with each pump cycle. Since the pump can remove only a
small portion of the gas with each pump cycle, it is obvious that this method of evacuation can never completely
remove all the gas in the vessel. In addition to this, all the components of the system contain minute sources of gas
leakage which are impossible to seal completely against atmospheric pressure. Outgassing of materials within the
system provide additional sources of gas.
As a result, after prolonged pumping, a state of equilibrium is reached in which the gas introduced from all the leakage
sources is balanced by the ability of the pump to remove gas from the system. This state of equilibrium is referred to
as the ultimate pressure or blankoff pressure of the pump and its system. No matter how much additional pumping
time is provided, no further reduction in system pressure will be accomplished once ultimate pressure is attained.
2.05 Pump Mechanism Description
The Vacuum Pumps incorporate two in-line rotary-vane stages with interconnecting ports. When in operation, the
intake stage is at lower pressure and the exhaust stage is at higher pressure relative to each other. 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 section 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 interstage 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.
An adjustable gas ballast valve is located in the pump’s exhaust stage. The purpose of the gas ballast is to reduce or
eliminate vapor condensation in the pump. The function of the gas ballast valve is described later in this section.
The pump is mounted inside an oil case which is a reservoir for the oil that lubricates the pump. The electric motor
shaft drives the pump shaft via a coupling. There is a coupling body on the end of each shaft; a coupling spider
between the two coupling bodies transfers the power from the motor shaft to the pump shaft.
2.06 Intake Anti-Suckback Protection
When power to the pump is turned off, this valve closes automatically, maintaining vacuum in the system being
evacuated. When the pump is turned on, the plunger opens the valve to allow gas to flow into the pump intake.
Please Note: Pump model 8925 does not have the anti-suckback protection if the gas ballast is open.
The system vacuum will not be maintained.