Chemglass AF-0350 Welch Vacuum Pump - Manual (Page 8)

2.5 the Principle of Gas Ballast
2.5a the Effects of Unwanted Vapor
systems which contain undesirable vapors cause difficulty both from the standpoint of attaining desirable ultimate
pressures as well as contamination of the lubricating medium. A vapor is defined as the gaseous form of any substance
which is usually a liquid or a solid. Water, oil and a mercury vapors are three of the more common vapors encountered in
typical vacuum systems. When such vapors exist in a system, the vapors or mixtures of gas and vapor are subject to
condensation within the pump; the precipitated liquid may thus ultimately dissolve or become emulsified with the
lubricating medium. This emulsion is recirculated to the chambers of the pump where it is again volatilized causing
increased pressure within the system.
2.5b the Presence and Removal of Condensate
Condensation takes place particularly in the compression stroke of the backing or second stage of a two-stage pump.
The compression stroke is that portion of the cycle during which the gas drawn from the intake port is compresses to the
pressure necessary to expel it past the exhaust valve. Condensation takes place when the ratio between the initial
pressure and the end pressure of the compression is high, that is, when the mixture of vapor and gas drawn from the
intake port is compressed from a low pressure to high pressure. By adding air though the gas ballast valve to the mixture
of vapor and gas being compressed, the pressure required for delivery past the exhaust valve is reached with a consider-
ably smaller reduction of the volume of the mixture; this, depending upon the amount of air added, condensation of the
vapor is wither entirely avoided or substantial reduced.
2.5c Pump function With Gas Ballast
In a pump functioning on a contaminated system and operating without the gas ballast, compression within the stage
takes place in the normal manner until the saturation pressure of the contaminating vapor contained within the mixture of
gas and vapor is reached. The saturation pressure of water vapor is that pressure and corresponding temperature at
which the dew point of the vapor is reached and condensation occurs. The saturation pressure of water vapor at an
ambient temperature of 20°C is 17.5 Torr, while at 60°C, the approximate operating temperature of a pump, the saturation
pressure is 149 Torr. The external side of the exhaust valve is subjected to atmospheric pressure. Consequently a
compressive force somewhat greater than atmospheric pressure is required to open the valve and permit expulsion of the
gas. Sometime during increased compression of the mixture of gas and vapors, the saturation pressure of 149 Torr for the
water vapor is reached and the vapor condenses. The condensate is then allowed to emulsify with the oil which is
recirculated within the pump stages this providing continued contamination of the system.
2.5d Pump function with Gas Ballast
On the other hand, when ballast air at atmospheric pressure is supplied to the compression stroke by means of the gas
ballast, the partial pressure of the unwanted vapor becomes a very small part of the total pressure of the mixture of gas,
vapor and newly supplied air. The vapor is thus prevented from reaching its saturation pressure corresponding to the
temperature of the pump and is finally expelled form the pump as a vapor.
2.5e Controlled Ballast Flow
Some degree of variation in ballast flow may be obtained by the amount of opening applied tot he gas ballast. Two or
more turns of the gas ballast are sufficient to open it wide. With the gas ballast open, the sound of the exhaust is similar
to that of a pump operating against a large leak. Because of the increased pressure introduced into the compression
stroke, the pump must work a little hard to function, thus resulting in an increased operating temperature of approximately
8°C over a prolonged period of time. Tests have shown that continuous and prolonged operation for several weeks under
these conditions is not injurious tot he pump.
2.5f Other Forms of contamination control
The application of the gas ballast is a moderate and very successful method for the removal of condensable vapors. For
very heavily laden systems, other means of removal such as oil separators may be required. For mild cases of contamina-
tion the simple expedient of a cold trap or a change of oil serve the purpose.
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