2.4b 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
compressed 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 is compressed from a low pressure to a high pressure. By
adding air through 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 considerably smaller reduction of the volume of the mixture;
thus, depending upon the amount of air added, condensation of the vapor is either entirely avoided or substantially
reduced.
2.4c Pump Function Without 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 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 20C is 17.5 torr, while at 60C, 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 conden-
sate is then allowed to emulsify with the oil which is recirculated within the pump stages thus providing continued
contamination of the system.
2.4d 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 from the pump as a vapor.
2.4e Controlled Ballast Flow
Some degree of variation in ballast flow may be obtained by the amount of opening applied to the 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 approxi-
mately 8C over a prolonged period of time. Tests have shown that continuous and prolonged operation for several
weeks under these conditions is not injurious to the pump.
2.4f 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 contamination the simple expedient of a cold trap or a change of oil may serve the purpose.
