4-16
Sequence of Operation
µ
P-D Controller
CRR-20 & CRR-40, September 1999
Null
• The controller calls for Null when the Return Air
Temperature decreases more than 1.0 C (1.8 F) below
setpoint (on temperature pull-down).
• The controller de-energizes the liquid line solenoid valve
to pump down the low side.
• The controller de-energizes the compressor contactor (to
terminate pump down) after 30 seconds.
• If the condenser fan was ON, it will operate for 30 sec-
onds and then stop. If the condenser fan was OFF
(based on condensing pressure), it will remain OFF.
• The evaporator fans continue to operate in low speed.
Defrost
During the Cool, Modulation Cool or Null modes, the con-
troller initiates the Defrost mode when the evaporator coil sen-
sor temperature is below 10 C (50 F) and:
• Demand Defrost algorithm determines when defrost is
required based on the return air temperature, evaporator coil
temperature, condenser coil temperature and the percent
modulation.
• Internal Defrost Timer calls for defrost based on a timed
defrost interval:
On Chill Loads
- Default: The initial time interval is 3 hours. One (1) hour is
added to the time interval each time two timed defrost
cycles occur without a demand defrost cycle between them.
Maximum accumulated time interval is 8 hours. Time inter-
val resets when the setpoint is adjusted to -10 C (14 F) or
below, when the unit is turned OFF for 48 hours, or when a
manual defrost occurs.
- Customer Configuration C5: Standard default times but
maximum time interval is 6 hours on chill loads.
- Customer Configuration C6: 2 hour initial defrost, then
maximum time interval is 6 hours on chill loads.
On Frozen Loads
- Default: The initial time interval is 8 hours. Two (2) hours
are added to the time interval each time two timed defrost
cycles occur without a demand defrost cycle between them.
Maximum accumulated time interval is 24 hours. Time
interval resets when the setpoint is adjusted to -9.9 C (14.1
F) or above, when the unit is turned OFF for 48 hours, or or
when a manual defrost occurs.
- Customer Configuration C5: Standard default times but
maximum time interval is 12 hours on frozen loads.
- Customer Configuration C6: 2 hour initial defrost, then
maximum time interval is 6 hours on frozen loads.
• A manual defrost is initiated (loaded) using the controller
keypad.
NOTE: If unit operating conditions do not allow
the unit to enter a defrost cycle, “DFRST INVAL”
(defrost invalid) appears on the controller display
when a manual defrost is initiated (loaded) via
the controller keypad.
NOTE: If the coil sensor fails, the controller sub-
stitutes the temperature of the controlling sensor
to determine when a defrost mode may initiated.
When the defrost mode is initiated:
• The controller de-energizes the compressor, evaporator
fan and condenser fan contactors, and liquid line sole-
noid valve.
• When the compressor stops, the controller turns ON the
Defrost LED and energizes the heater contactor, turning
on the electric heaters.
The controller terminates the defrost mode when:
• Chill mode: Evaporator coil sensor temperature reaches
30 C (86 F).
• Frozen mode: Evaporator coil sensor temperature
reaches 18 C (64.4 F).
• Time/temperature function: Controller terminates
defrost if the evaporator coil sensor rises and remains
above 8 C (46 F) for 10 minutes (Frozen mode only).
• Interval timer: Controller terminates defrost 90 minutes
after initiation if the coil sensor temperature has not ter-
minated defrost. An alarm code will be generated if this
occurs.
NOTE: If the coil sensor fails, the controller sub-
stitutes the return air (top) sensor. Defrost is ter-
minated when the top sensor reaches 18 C (64.4
F). If both the coil and top sensor fail, the con-
troller will terminate defrost 60 minutes after initi-
ation. An alarm code will be generated if this
occurs.
• When the controller terminates Defrost, the heater con-
tactor is de-energized. The controller starts the compres-
sor and condenser fan (if required) and energizes the liq-
uid line solenoid valve to pre-cool the evaporator coil.
The controller energizes the high or low speed evapora-
tor fan contactor (depending on return air temperature)
after the evaporator coil has been pre-cooled to minimize
heat energy release into the container.
Summary of Contents for CRR-20
Page 4: ......
