SM/SV-Space Master Unit Coolers (E316008_B)
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7.4
ELECTRIC DEFROST SEQUENCE OF OPERATION
The electric defrost cycle is time clock initiated and temperature terminated with a timer and or high temperature
over-ride. For systems with multiple unit coolers and a single defrost time clock the defrost termination
thermostat must be wired in series. Reference figures 4 and 5 for electric defrost wiring diagrams.
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 and fan delay thermostat are closed, the defrost termination thermostat is off and the
defrost heaters are off.
3.
The unit cooler fan motors are energized and the fans operate continually.
4.
The systems compressor operates in accordance with the demand of the room thermostat.
5.
Frost slowly builds up on the evaporator fins.
STEP B: Defrost Cycle
The timer starts defrosting of the evaporator coil at a predetermined interval. A typical setting would be two
defrost periods per 24-hour 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 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 it’s 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 of time. 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.
