Heatcraft Mohave, Installation & Operation Manual, Page: 44 / 76

44
Defrost Operation
Defrost Operation
Defrost Timing/Schedule Programming
The hot gas controller can be programmed with up to 12 defrost start times.
The Program Review menu section describes the process to program or
delete a valid start time. Note that clearing a start time by pressing
CLEAR
and the ENTER will disable all start times following the one being cleared.
There must be 30 minutes of elapsed time following a start of defrost before
another defrost can be scheduled. A defrost cycle can be initiated manually
at any time.
Force Defrost Manually
To manually force the start of a defrost-cycle, press the FORCE DEFROST
button. If the system is in
COOL
mode, the system will pump down and go
to the off mode before the defrost process is started.
Defrost Process
The defrost process has four steps: Pre-defrost (
DEF1
) pressure
equalization, Defrost operation (
DEF2
), Post-defrost (
DEF3
) pressure
equalization and drain down, and Refreeze (
FREZ
).
DEF1 :
Pre-defrost, or defrost step 1, always follows activation of Off mode. If the
system is in Cool mode when the defrost cycle is activated, the control will
pump down and go to Off mode before activating pre-defrost.
The purpose of
DEF1
is to equalize the refrigerant pressures between
the condensing unit receiver and the evaporator(s). This is accomplished
by first deactivating the evaporator fans, and the suction solenoid. Next the
Bypass Solenoid and the evaporator Pan Heater contactor are activated.The
time duration of the pressure equalization is programmed as
EQUT
in the
PROGRAM REVIEW menu.
DEF2 :
After the equalization time has elapsed, the Bypass Solenoid is deactivated.
The Defrost Solenoid is activated 1 second later, followed by the 3-Way valve
solenoid 1 second after that. Next, the pressure regulator control algorithm
is enabled. The initial position is full open, but it quickly makes adjustments
in order to stabilize the pressures and temperatures seen at the inlet of the
suction accumulator.
The compressor contactor turns on at the same time that the pressure
regulator is activated. The ambient temperature is measured, and a
determination is made of how many condenser fans should be operating.
The correct number of condenser fans is activated at the same time as the
compressor contactor.
During the
DEF2
operation, the pressure regulator continues to maintain
the appropriate volume of refrigerant flow through the system based upon
the current ambient conditions, the refrigerant type, and the type of cooling
application.
Termination of defrost is accomplished by either both evaporators reaching
their target termination temps, or the liquid pressure measured between the
receiver and the condenser coil reaching its target pressure, or the pre-
programmed fail safe time.
When one of the termination factors is realized, the compressor, condenser
fan(s), and the defrost solenoid are turned off. The 3-Way valve and the Pan
heaters are left on. The pressure regulator is activated to 100% open, and
DEF3
begins.
DEF3
:
Post-defrost has two purposes. The first is the transfer of high pressure
refrigerant at the evaporator back to the condenser receiver by way of the
pressure regulator. The pressure regulator is open 100% during this step.
The second purpose is drain down time for the warm evaporators. This
allows the water that was melted off of the coil to drain out of the evaporator
drain pan. The time duration for this step is identical to the
DEF1
equalization time.
After the completion of the delay time period, the 3-Way valve and the
pan heater contactor are turned off. One second later the Suction Solenoid
is turned on, and then one second after that the compressor turns on.
The condenser fan control algorithm is also enabled. When the pressure
measured at the suction sensor falls below -10°F SST, the liquid line
solenoid is activated and the process step changes to Refreeze.
FREZ
:
The Refreeze step is identical to Cooling mode except that the evaporator
fans are turned off. This is to allow the evaporator coils to freeze any
remaining water that might be left over from the drain down step so that
when the fans turn on, the water will not be sprayed into the refrigerated
space. When the evaporator reaches the refreeze set-point, or the refreeze
time limit elapses, the fans turn on and the system begins a cooling cycle. If
the thermostat is satisfied, or deactivated, the system will run a cooling cycle
for two minutes and then pump down and shut off.
After the compressor is energized, the suction stop valve may be pulsed to
limit the suction pressure at the compressor.
Through the refreeze period and into cooling mode,the liquid solenoid may
energize and de-energize to hold the evaporator below 10 °F SST and allow
the expansion valve to better regulate as the system enters cooling mode.
Parameter LLSP controls the duration of this pulsing event.