Carrier 69NT40-541-505, Operation And Service Manual

Page: 35 / 143

4–5 T-368
Figure 4.5 Controller Operation - Perishable Mode
4.3.6 Perishable Mode Dehumidification
Dehumidification is provided to reduce the humidity
levels inside the container. Dehumidification is acti-
vated when a humidity value is set at Cd33. The yellow
SUPPLY LED will flash ON and OFF every second to
indicate that dehumidification is active. Once dehumidi-
fication is active and the following conditions are satis-
fied, the controller will activate the heat relay to begin
dehumidification.
1. The humidity sensor reading is above the
humidity set point (Cd33).
2. The unit is in the perishable steady state mode,
and the supply air temperature is less than
0.25°C (0.45°F) above set point.
3. The heater debounce timer (three minutes) has
timed out.
4. Heater termination thermostat (HTT) is closed.
If the above conditions remain true for at least one hour,
the evaporator fans will switch from high speed to low
speed. Evaporator fan speed will then switch every hour,
as long as the 4 conditions are met (see Bulb Mode,
Section 4.3.7 for different evaporator fan speed options).
If any condition except for item (1) becomes false OR if
the relative humidity sensed is 2% below the dehumidi-
fication set point, the high speed evaporator fans will
be energized.
During dehumidification, power is applied to the defrost
heaters. This added heat load causes the controller to
open the SMV to match the increased heat load while
still holding the supply air temperature very close to the
set point.
Opening the SMV reduces the temperature of the
evaporator coil surface, which increases the rate at
which water is condensed from the passing air. Remov-
ing water from the air reduces the relative humidity.
When the relative humidity sensed is 2% below set
point, the controller de-energizes the heat relay. The
controller will continue to cycle heating to maintain rela-
tive humidity below the selected set point. If the dehu-
midification mode is terminated by a condition other
than the humidity sensor, e.g., an out-of-range or com-
pressor shutdown condition, the heat relay is de-ener-
gized immediately.
Controller Set Point ABOVE ï 10 q C (+14 q F),
or
ï
5
q
C (+23
q
F) optionally
ï
.20
q
C
ï
.50
q
C
+.20 q C
ï
.20
q
C
Air Circulation
Heating
Cooling,
Modulating
Controller Set Point ABOVE
ï
10
q
C (+14
q
F),
or ï 5 q C (+23 q F) optionally
ï
.20
q
C
ï .50 q C
ï .20 q C
Air Circulation
Heating
Cooling
Pull Down
+5
q
C
(+9 q F)
Cooling,
Modulating
Falling
Temperature
Rising
Temperature
Perishable Mode
(Only Applicable to Perishable Mode)
Set
Point
Set
Point
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