Brochure

Parker Hannifin Corporation
Climate & Industrial Controls Group
Cleveland, OH
Catalog CIC-2003-1/US
Refrigeration Solenoid Valves
258
Typical Applications
This system is drawn with only two evaporators but it
is recommended that only twenty-five percent of any
multiplexed system be hot gas defrosted at any given
time.
An alternative to the hot gas defrost system is the cool
gas defrost which uses the gas from the top of the
receiver to defrost the evaporators. Because the cool
gas defrost operates at a lower temperature, the
thermal expansion of the refrigeration lines is reduced.
This often eliminates the need for special piping
techniques and leaks caused at line connections by
excessive thermal flexing.
Capacity Control System
A simple method of providing compressor unloading is
to use a solenoid valve connecting the discharge and
suction lines of the compressor. The solenoid valve is
controlled by a pressure control which responds to
suction pressure. When the switch closes, it opens
the normally closed solenoid valve and discharge
gas is short circuited back to the suction side of
the compressor.
In order to prevent overheating of the compressor,
a thermostatic expansion valve should be installed
to provide cooling to the compressor suction gas.
An alternative method consists of injecting hot gas
into the evaporator inlet. This prevents overheating
of the compressor and increases the velocity of the
gas through the evaporator.
This type of unloading should not be attempted
without thorough analysis of solenoid valve and
expansion valve sizing.
Liquid Line Shut-Off
In an effort to obtain a higher efficiency rating on resi-
dential and commercial air conditioning systems, a nor-
mally closed solenoid valve typically is installed in the
liquid line located near the air handler or furnace. In this
case, the solenoid valve is wired in parallel with the
contactor circuit (24 VAC) and may require a larger
transformer to accommodate the valve.
As an alternative method, a normally closed solenoid
valve may be located in the liquid line near the condens-
ing unit and wired directly to the compressor motor ter-
minal box. This solution improves system efficiency and
maintains the refrigerant charge in the condenser coil
during the off-cycle of the compressor which prevents
refrigerant migration when long piping runs are used.
If the application requires a fail-safe or open mode, a
normally open solenoid valve may be used. In this in-
stance, the valve may also be located in the condensing
unit and wired in series with the compressor crankcase
heater.
Hot Gas Defrost System
Capacity Control System
Liquid Line Shut-Off