defrost. During electric defrost, the compressor relay is off for the defrost duration, the defrost
relay is on, and the evaporator fan relay or relays are typically off. See
for
more information on time-based and temperature-based defrost termination.
Hot gas defrost
Hot gas defrost systems use a bypass valve and extra refrigerant piping to temporarily reroute the
hot gas discharge from the compressor through to the evaporator coil. The defrost relay controls
the bypass valve. The rerouted hot gas melts the accumulated ice on the evaporator coil. Low
and very low temperature refrigeration applications use hot gas defrost. During defrost, both
the compressor and defrost relays are on and all of the configured evaporator fan relays are off.
Typically, hot gas defrost cycles use temperature-based defrost termination. You can also set up
time-based defrost termination.
Defrost termination types
The controller can terminate defrost durations based on time or temperature. Temperature
termination applications use the SN2 temperature sensor. On applications that use temperature-
based termination by sensor, you can define the maximum defrost duration value in minutes.
The defrost duration value terminates any defrost durations that do not reach the defined defrost
termination temperature.
Time-based defrost termination
Time-based defrost control terminates the defrost duration when the user-defined maximum
defrost duration elapses. You must set the defrost duration time at a sufficient length to remove
all of the ice that accumulates on the evaporator during the previous defrost interval. Time-based
defrost termination does not require a defrost termination sensor.
Temperature sensor defrost termination
Temperature sensor defrost termination uses the defrost termination sensor (SN2) mounted
on the evaporator coil to sense the coil temperature and terminate defrost at the user-selected
termination temperature. The defrost termination temperature value is the temperature at which
the evaporator coil is clear of ice. Set the termination temperature sufficiently high to ensure that
all of the accumulated ice has melted off the evaporator coil at the termination temperature.
Time-based termination overrides temperature termination
The maximum defrost duration time period overrides the temperature-sensor defrost termination
type, if the evaporator does not reach the termination temperature. The defrost duration
terminates when the evaporator reaches the selected defrost termination temperature or the
maximum defrost duration, whichever occurs first.
Defrost schedule types
Use the controller to schedule regular defrost interval times or use the adaptive defrost feature.
Adaptive defrost automatically determines the optimal time to initiate the defrost cycles for the
refrigeration system.
Scheduled defrost
You can set up a manual or automatic scheduled defrost. Schedule one to eight daily defrost cycles.
Use the manual setting to set the start times for each daily defrost cycle. Use the automatic setting
to set two parameters; the number of daily defrost cycles and the real time for the first scheduled
defrost. The system automatically calculates the daily schedule for the defrost intervals. The defrost
intervals are of equal length. After you create a schedule, you can use the manual option to change
one or more of the scheduled defrost duration start times. See
Setting up the defrost parameters
and
.
11
MR5 Series Panel Mount Case Controller Technical Bulletin
Discontinued
