The Defrost Cycle
The
refrigeration cycle results in frost formation on the surface
of the evaporator. This frost will eventually build up to
the point where it will restrict the air flow causing a
loss of refrigeration capacity. To prevent this, a timeclock
(I), usually set to repeat every 6 or 8 hours, initiates
a defrost cycle which melts the frost.
The
clock de-energizes (closes) the liquid line solenoid valve
which causes the compressor to pumpdown and shut off from
the low pressure control. The clock also energizes the drain
pan heater (J) in the evaporator and timer relay which after
a two minute delay energizes (opens) the 3-way valve and
hot gas solenoid valve (K) which then builds up pressure
in the evaporator causing the low pressure control to close
and start the compressor.
The
hot discharge gas from the compressor flows through the
3-way valve, Hot gas valve and check valve forcing all the
liquid left in the liquid line into the evaporator. If pressure
builds up too high the 3-way valve pressure control (L)
will de-energize the solenoid valve and allow pressure to
relieve through the condenser. Pressures within the evaporator
will remain steady and once all the frost has melted the
pressure will rise until the defrost termination pressure
control (M) energizes the timeclock's internal solenoid
terminating the defrost cycle. The 3-way valve, hot gas
valve solenoids are then de-energized. The liquid line solenoid
valve opens and the compressor continues to run. The evaporator
fans do not start up until the pressure in the evaporator
is low enough to close the Fan delay control (N). By delaying
the fans this allows any moisture left on the coil to drain
away or freeze. As soon as the evaporator fans are energized
the system will then resume back to the refrigeration cycle.
The
cycle continues until the room temperature is satisfied.
This
de-energizes the liquid line solenoid, initiating a pumpdown
cycle that reduces the suction pressure to the cut-out setting
on the low pressure control which de-energizes the compressor.
To
view the Refrigeration Cycle, Click on the button below
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