KRACK KR Series - SEQUENCE OF OPERATION

KR Electrical Defrost Unit Coolers (PN E108318_S) 26
FIGURE 10C: MODEL G - 3 PIPE HOT GAS WITH GAS PAN HEAT AND K - 2
PIPE KOOL GAS WITH GAS PAN HEAT – MOTOR TYPE D
9 SEQUENCE OF OPERATION
Step A – Normal Refrigeration Cycle
1. Power is supplied to terminals “N” and “4” on the defrost timer.
2. The heater safety thermostat is closed. The fan delay thermostat is closed and the defrost
termination thermostat is off.
3. The unit cooler fan motors are energized, and the fans operate continuously (the defrost
heaters are off).
4. The system compressor operates in accordance with the demand of the room thermostat.
5. Frost slowly builds up on the evaporator fins.
Step B – Defrost Cycle
1. Defrosting of the evaporator is started by the timer at predetermined intervals – typical settings
of the timer would be two defrost periods per 24-hour day. Systems using Carbon Dioxide
(R- 744) should defrost at least twice per day.
2. The timer mechanically disconnects power to terminal “4” thus closing the liquid line solenoid
valve and shutting off the evaporator fan motors. Simultaneously power is connected to timer
terminal “3,” which allows current to flow to the defrost heaters.
3. The heaters, embedded in slots in the coil face, give up heat directly to the evaporator fins.
This heat raises the coil temperature to 32°F causing the frost to melt.
4. As the frost melts it drips into the heated drain pan and flows down the drain.
5. When the frost has completely melted from the coil (typical cycle length is 20 to 25 minutes)
the coil continues to warm up above 32°F.
6. When the coil reaches the temperature setting of the defrost termination thermostat, it closes
which allows current to flow to terminal “X” on the timer which energizes the switching solenoid
in the timer. The timer disconnects power to terminal “3” thus turning off the defrost heaters.
At the same instant power is connected to terminal “4” of the timer. The heater safety thermo-
stat may open but under normal conditions it will remain closed. The heater safety thermostat
would open only if the defrost termination thermostat fails to close. The timer has a failsafe
(inner dial) which should be set for 30 minutes.
Step C – Return to Normal Refrigeration Cycle
1. Because there is power at terminal “4” the liquid line solenoid opens and the compressor
restarts.
2. The evaporator fan motor(s) remain off because the fan delay thermostat is open.
This prevents warm air from being blown into the refrigerated area.