22 • Refrigeration
REFRIgERATION
Defrost
Periodic defrosting to keep the evaporator coil free of frost is accomplished by a time clock used in conjunction with an electric or hot gas
defrost. Defrost is typically scheduled at night to avoid periods of heavy shopping or stocking. use a temperature-terminated defrost with a
defrost termination thermostat or probe for optimal performance.
The refrigeration technician should recheck coil conditions after one week of normal operation to ensure that the frequency and duration of
defrost is adequate.
ELECTRIC DEFROST - SETTINGS
for defrost settings.
The compressor stops when the defrost is initiated in a non-pump-down system. On pump-down systems, the liquid line solenoid will be de-
energized when the defrost is initiated. The clock will energize the defrost contactor, which will energize the defrost heater. This de-energizes
the fans and anti-sweat heaters. After the defrost period, the compressor will operate and fans will start on a delay.
Electric defrost thermostat are open-on-rise contacts that are wired in series with the defrost heaters. unless the installer rewires the defrost
thermostat, the defrost heater is de-energized when defrosting is complete.
SETTING
FROZEN FOOD
ICE CREAM
Defrost Type
Electric
Hot Gas
Electric
Hot Gas
Frequency
1 Per Day
1 Per Day
1 Per Day
1 Per Day
Duration
55 Minutes
30 Minutes
55 Minutes
30 Minutes
Temperature Termination Set Point
50°F
65°F
50°F
65°F
Note: Hot gas drip time is 5 minutes. A reduced temp option would take 40 minutes.
FIGURE 37: Defrost Settings
FIGURE 36: Case Temperature Internal Control Options
SETTING
FROZEN FOOD
ICE CREAM
Evaporator Temp
-7°F
-16°F
Return Air Temperature Set Point
0°F
-9°F
Return Air Temperature Differential
6°F
6°F
Discharge Air Temperature Set Point
-3°F
-12°F
Discharge Air Temperature Differential
6°F
6°F
SETTING
R-404A R-448A Sat. Temp R-404A R-448A Sat. Temp
Condensing Unit Cut-In
31 psig 23 psig
-2°F
23 psig 16 psig
-11°F
Condensing Unit Cut-Out
25 psig 18 psig
-8°F
18 psig 12 psig
-17°F
Note: Set points based on superheat of 6-8°F.
For high-glide refrigerants, use dew point for unit sizing.
Adjust evaporator pressure as needed to maintain discharge air temperature. To receive the full benefit of
high-glide refrigerant properties, the superheat may need to be lowered to 4-6°F. Contact Zero Zone with
questions.
