Hussmann KRACK MK Series, Инструкция по установке и эксплуатации

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MK/MV – Medium Profile Series Unit Coolers (E206993_M) 24
7.1.2 STEP B: DEFROST CYCLE
The timer starts defrosting of the evaporator coil at predetermined intervals. A typical setting would be
two defrost periods per 24-hour day. Systems using Carbon Dioxide (R744) should defrost at least twice
per day.
1.
Upon initiation of the defrost cycle, the timer mechanically disconnects power to terminal “4” thus
closing the liquid line solenoid valve and shutting off the fan motors. Simultaneously power is
connected to terminal “3” which allows current to flow to the defrost heaters.
2. 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.
3. As the frost melts it drops into the heated drain pan and flows down the drain.
4. When the frost has completely melted from the coil the temperature of the coil will start to rise
above 32
°
F.
5. When the coil reaches the temperature setting of the defrost termination thermostat (75
°
F for fixed
Klixon), the thermostat 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.
6. Because there is power at terminal “4” the liquid line solenoid opens and the compressor restarts.
7. The evaporator fan motor(s) remain off because the fan delay thermostat is still open. This prevents
warm air from being blown into the refrigerated area.
8. The evaporator coil cools down approaching operating temperature.
9. When the coil temperature reaches 25
°
F (approximately 2 to 3 minutes after defrost termination)
the fan delay thermostat closes, thus allowing the fan motors to restart. The unit is now back in
operation.
10. The heater safety thermostat will only open if the defrost termination thermostat fails to close at its
set temperature. The heater safety thermostat is set to open at 80
°
F. The timer also has a fail-safe
(inner dial) timeout; the recommended setting is for 30 minutes.
NOTE: On systems where the room temperature is above +25
°
F the fan delay thermostat may not close
for an extended period. If the fan delay time is too long, it is permissible to install a jumper wire
between terminals “F” and “B” at the unit cooler. This allows the fans to turn on immediately after the
defrost period.