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36 TRG-TRC005-EN
notes
period five
Accessories
When the compressor is shut off, the evaporator contains a large quantity of
liquid refrigerant. This can present a problem if some of the refrigerant drains
into the suction line and slugs the compressor when it starts up again. To
prevent this from occurring, many systems pump the refrigerant out of the
evaporator and suction line before shutting the compressor off. This is called a
pump-down cycle. Instead of shutting the compressor off right away, the
solenoid valve is closed to stop the flow of liquid refrigerant into the
evaporator, and the compressor is allowed to run for a short period of time. The
compressor pumps the refrigerant from the low-pressure side of the system
(evaporator and suction line) to the high-pressure side of the system (discharge
line, condenser, and liquid line.)
As the low-pressure side of the system is pumped free of refrigerant, the
pressure in that part of the system drops. To end the pump-down cycle, a
pressure sensor is used to shut the compressor off when this pressure reaches
a predetermined set point. Prior to starting the compressor again, the solenoid
valve is opened, allowing the pressure on the low-pressure side of the system
to increase again.
The solenoid valve should be installed as close to the expansion valve as
possible. This will minimize the pump-down time and allow the liquid line to be
used for storing refrigerant when the system is off.
Liquid-Line Filter Drier
The next accessory to be discussed, the liquid-line filter drier, is installed
upstream of the solenoid valve and the expansion valve. It prevents moisture
(water) and foreign matter, introduced during the installation process, from
entering the expansion valve and the solenoid valve. Realize, however, that
there is no substitute for cleanliness during system installation.
Moisture and foreign matter can cause problems in any refrigeration system.
When water is mixed with refrigerant and oil, and heat is added by the
compressor, acids are formed that can damage the valves or compressor.
Additionally, certain foreign materials such as copper and brass particles can
Liquid-Line Filter Drier
solenoid
solenoid
valve
valve
expansion
expansion
valve
valve
liquid-line
liquid-line
filter drier
filter drier
evaporator
evaporator
liquid line
liquid line
Figure 53