Welch Vacuum 1376N Chemstar - Manual (Page 7)

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2.20 Operation and System Optimization
2.21 High Pressure Operation
CHEMSTAR 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 Vacuum at 847-676-8800, extension 1, 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.
2.225 Controlled Ballast Flow
Some degree of variation in ballast fl ow may be obtained by the amount of opening applied tot he gas ballast.
Two or more turns of the gas ballast are suffi cient 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 to the pump.