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Service Tools
Chapter 4 Page 4

Vacuum Pump

A good vacuum pump, properly maintained, is essential in the repair of a refrigeration system. There are three basic reasons for using a vacuum pump: 

1.  To remove air and non condensable gases from the system.
2.    To remove old refrigerant .
3.    To remove moisture.

Rotary-type compressors are used as vacuum pumps. They have the capability of pulling at least 29.8 inches of vacuum, which is a must in order to insure that all three of the above functions can be performed. 

NOTE: Reciprocating pumps do not have this capability and should therefore not be used as vacuum pumps. Two types of vacuum pumps in common use are shown.

vacuum punp.JPG

In use, water, acids, and dirt being removed from a system are drawn through the vacuum pump. These foreign materials contaminate the pump, in time impairing its efficiency. To be sure that the pump does not lose its efficiency, the oil in the pump should be changed at frequent inter­vals 3GS or 4GS refrigeration oil. Always change the oil after evacuating a burned-out or wet system.* In addi­tion, the pump should be checked periodically with a mercury manometer to be sure that its vacuum-pumping capability has not been reduced. 

* A burned-out system is one in which the insulation on the compressor motor has charred because of a short-cir­cuit or an overload. Since the motor windings are in the same housing as the refrigerant, the burned material will contaminate the refrigerant and oil in the system. 
A wet system is one in which moisture-laden air has en­tered, usually through the low side because it is under vacuum. The moisture contaminates the refrigerant and oil in the system. 

Vacuum Pump Maintenance

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