Carrier 69NT40-561-201, Operation & Service Manual

Page: 39 / 117

3 − 10 T-364
2. The Defrost Interval Timer reaches or exceeds the
Defrost Interval.
3. During Pretrip, defrost may occur during tests P-8,
and P-10. Defrost is forced during Pretrip test P-9.
4. Temperature Probe Diagnostics logic determines
that a Probe Check is necessary based on temperat-
ure values currently reported by the supply and re-
turn probes.
5. An Initiate Defrost command is sent via communica-
tions.
6. The microprocessor can determine if defrost is re-
quired by calculating the difference between return
air temperature and supply air temperature (Delta
T). If the temperature difference between return air
and supply air is too great, it indicates reduced air-
flow over the evaporator coil, and a defrost cycle
may be required:
a. In Perishable Pull Down - Defrost will be initiated if
Delta T increases to greater than 12 C (53.6 F),
DTT is closed, and 90 minutes of compressor run
time have been recorded.
b. In Frozen Mode - Defrost will be initiated if Delta T
increases to greater than 16 C (60.8 F), DTT is
closed, and 90 minutes of compressor run time
have been recorded.
c. In Perishable Steady State – A baseline Delta T is
used to determine when defrost should be activ-
ated. The baseline is recorded after completion of
an initial defrost cycle. In order to record a baseline
Delta T, the unit must be cooling, and the evaporat-
or fans and heaters must remain in a stable state
for a period of five minutes. Defrost will then be initi-
ated if Delta T increases to greater than 4 C
(7.2 F) above the baseline, the DTT is closed, and
90 minutes of compressor run time have been re-
corded.
NOTE
During defrost, the Defrost Relay is energized,
the orange DEFROST light is illuminated, and
the orange HEAT light is illuminated.
Defrost may be initiated any time the DTS reading falls
below the set point. Defrost will terminate when the DTS
reading rises above the DTT set point. The DTT is not a
physical component. It is a controller setting that acts as
a thermostat, “closing” (allowing defrost) when the DTS
reading is below the set point and “opening” (terminating
or preventing defrost) when the DTS temperature read-
ing is above set point.
The DTT set point is set to 25.6
C (78 F).When a re-
quest for defrost is made through the manual defrost
switch, communications or probe check the unit will
enter defrost if the DTS reading is at or below the DTT
set point. Defrost will terminate when the DTS reading
rises above the DTT set point. When a request for de-
frost is made with the defrost interval timer or by de-
mand defrost, the DTS must be below 10 C (50 F).
When defrost is initiated, the controller closes the EEV,
opens contacts TC, TN and TE (or TV) to de-energize
the compressor, condenser fan and evaporator fans.
The blue COOL light is also de-energized. The control-
ler then closes contacts TH to supply power to the heat-
ers, and the orange DEFROST light is illuminated.
When the DTS reading rises to the DTT setting, the
de-icing operation is terminated.
24 VOLT POWER
ENERGIZED
DE-ENERGIZED
TN
ST
F
HPS
PB
IP-CM
TC
CF
TE
IP-EM1
EF
IP-EM2
HTT
TH
HR
ESV
TS
TV
EF ES
Figure 3 − 10 Defrost
NOTE
The EEV and DUV are independently operated
by the microprocessor. Complete schematics
and legends are located in Section 7.
3.3.20 Defrost Interval
There are two modes for defrost initiation, user-selected
timed intervals and automatic control.
The user-selected values are 3, 6, 9, 12, 24 hours,
AUTO, or PuLS; factory default is 3 hours.
Automatic defrost starts with an initial defrost at three
hours and then adjusts the interval to the next defrost
based on the accumulation of ice on the evaporator coil.
Following a start-up or after termination of defrost, the
time will not begin counting down until the DTS reading
falls below set point (DTT closed). If the reading of DTS
rises above set point (DTT open) any time during the
timer count down, the interval is reset and the count-
down starts over.