Specifications

58
Oil
Its function
Choosing the right oil
Lubrication and anti-noise
device
Gas ballast
Oil has several important functions in the pump:
- It lubricates mechanical components (bearings, seals, rotor, vanes, etc.).
- It makes moving parts relatively tight by limiting internal leakage.
- It carries away the heat produced by the compressed gases.
Not all oils produce the same ultimate pressure in a given pump. Ultimate pressure
depends on the saturated vapour pressure of the oil, its viscosity and its ability to
dissolve gases.
Good pumping conditions are related to the type of oil used.
The choice depends on:
- Expected pump performance.
- Chemical aggression and corrosion of pumped gases.
- Accessories used.
- Desired maintenance intervals and total operating cost.
ALCATEL has selected various types of oil for its pumps (see page 67).
The pump is equipped with a lubrication system which regulates the oil flow rate
required in the vacuum pump. In addition this system also ensures the gassing of
the lubrication oil and therefore the low noise level of the pump.
When condensable vapours are being
pumped, gas is compressed beyond its
saturated vapour pressure in the
"compression" phase and can
condense, impairing pump
performance.
The gas ballast can be used to inject a
certain quantity of air (neutral or dry
gas) into the last stage of the pump
during the "compression" phase so that the partial pressure of the pumped gas is
less than its saturated vapour pressure at the temperature of the pump.
Condensation is therefore impossible if this limit is not reached. The maximum
admissible vapour pressure is obtained at pump inlet for this value.
At the end of "compression", the pressure in the exhaust chamber is greater than
atmospheric pressure. An anti-suckback device (valve + spring) prevents the gases
and oil from being drawn back into the inlet.
The saturated vapour pressure of a body is higher when the system is hot than
when it is cold; therefore, the pump must reach operating temperature before
pumping condensable vapours.
- Using the gas ballast increases the ultimate pressure of the pump as well
as the temperature.
- The gas ballast control, located at the front of the oil case cannot be used
to set the gas injection flow rate.
- When the gas ballast control is open, the pump is not tight when stopped. To
guarantee this tightness, install an automatic gas ballast.
In. Exh.
COMPRESSION
air