Welch Vacuum 1397 Duoseal Belt Drive - Manual (Page 7)

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2.20 Operation and System Optimization
2.21 High Pressure Operation
DUOSEAL two-stage pumps are designed to be most effi cient when operated at or near their ultimate blanked
off pressure. The pump is designed to remove air from a vacuum system at atmospheric pressure to a deep
vacuum. When the pump is operated at elevated pressures up to about 10 Torr for long periods of time, the
pump will run hotter. At elevated ambient temperatures under these conditions, the thermal protection switch
on single phase motors may cut out. Use of an exhaust fi lter with coalescing element is required.
CAUTION
Do not run this pump for prolonged periods of time at or above 10 Torr. The pump will overheat,
excessive oil mist will be emitted from the exhaust and the pump will eventually seize
Leaky vacuum systems, too large a vacuum chamber for pump size or high gas loads to the vacuum systems are
causes for extended runs of the pump at elevated pressure. If you need additional information, please contact Welch
at 847-676-8800 for further information and precautions.
2.22 Principle of Gas Ballast
2.221 Effects of Unwanted Vapor
Systems which contain undesirable vapors cause diffi culty both from the standpoint of attaining desirable
ultimate pressures as well as contamination of the lubricating medium. A vapor is defi ned 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 emulsifi ed 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.
2.222 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 considerably 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.
2.223 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.
2.224 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 fi nally expelled form the pump as a vapor.