Air conditioning and heat pump systems are designed to operate with only refrigerant and oil circulating within them. New field-piped systems and systems that have been opened for repair may be exposed to moisture and other contaminants. These systems must be evacuated and dehydrated before they are placed in operation (Figure 1-16). If dehydration is not performed properly, safe operation and the life expectancy of the system will be jeopardized.
The vacuum pump (Figure 1-17) is a motor-driven pump used to evacuate (remove) the air and other non-condensible gases from closed a ir conditioning and h eat pump systems and refrigerant storage cylinders. Vacuum pumps remove air by pumping the system down to a pressure below atmospheric pressure (14.7 psi). If the vacuum pump is operated so that deep levels of vacuum are obtained in the system, water will also be removed (dehydrated) as a result of the water boiling and evaporating. A deep vacuum satisfactory for dehydrating a system is considered to be any absolute pressure of 500 microns or below. 500 microns is about equal to an absolute pressure of 0.02 inches of mercury (in. Hg) absolute, which is a vacuum reading of 29.90 in. Hg vacuum.
A quality vacuum pump is a must for the service technician. A good pump is capable of evacuating a system down to 29.90 in. Hg vacuum (0.02 in. Hg absolute or 500 microns). Most use a direct drive, two stage, rotary-type vane pump driven by a motor with thermal overload protection. They usually have an oil level sight gauge, easy-to-fill oil port, and an oil drain. Pumps should have a solenoid-operated inlet line shutoff valve that prevents loss of vacuum pump oil if the pump input power is disconnected when the system is under vacuum. A gas ballast between the pump’s first and second stages is desirable to prevent some of the moisture from condensing in the vacuum pump crankcase.
Vacuum pumps need periodic maintenance. Since the oil in the vacuum pump becomes contaminated through normal use, it is a good practice to change the oil after every 10 hours of pump operation, and always immediately after pumping down a wet or contaminated refrigeration system.
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