Figure 3−6 Perishable Mode Cooling - Carrier 69NT40-561-201 Operation & Service Manual

Two timers are activated during dehumidification to pre-
vent rapid cycling and consequent contactor wear:
1. Heater debounce timer (three minutes) - The heater
debounce timer is started whenever the heater
contactor status is changed. The heat contactor re-
mains energized (or de-energized) for at least three
minutes even if the set point criteria are satisfied.
2. Out-of-range timer (five minutes) - The out-of-range
timer is started to maintain heater operation during a
temporary out-of-range condition. If supply air tem-
perature remains outside of the user selected
in-range setting for more than five minutes, the heat-
ers will be de-energized to allow the system to recov-
er. The out-of-range timer starts as soon as temper-
ature exceeds in-range tolerance value set by Cd30.
3.3.9 Perishable Mode Cooling - Sequence of
Operation
NOTE
In Standard Perishable Mode, the evaporator
motors run in high speed. In Economy Fan
Mode, the fan speed is varied.
a. When supply air temperature is above set point and
decreasing, the unit will cool with the condenser fan
motor (CF), compressor motor (CH), and evaporator
fan motors (EF) energized, and the blue COOL light
illuminated. (See Figure 3−6). Also, if current or
pressure limiting is not active, the controller will close
contacts TS to open the economizer solenoid valve
(ESV) and place the unit in economized operation.
ENERGIZED
DE-ENERGIZED
F
HPS
IP-CM
IP-EM1 IP-EM2
HTT
ESV
TS
Figure 3−6 Perishable Mode Cooling
T-364
ST
24 VOLT POWER
TC CH
CF
TN
EF
TV ES
TE EF
TH HR
b. When supply air temperature decreases to a prede-
termined tolerance above set point (Cd30), the green
IN RANGE light is illuminated.
c. As air temperature continues to fall, unloaded cooling
starts (DUV pulses opens) as the supply air temper-
ature approaches set point. (See Figure 3−5).
d. When unloaded cooling starts, EEV control will trans-
ition from a full cool superheat set point to a lower
modulated cool superheat set point. Once unloading
starts, the EEV controls evaporator superheat based
on the system duty cycle where instantaneous super-
heat will vary.
e. When the supply air temperature has fallen to within
1.9_C (3.4_F) of set point temperature and the aver-
age capacity of the system has fallen below 70%, the
unit will open contacts TS to close the ESV and take
the unit out of economized operation.
f. The controller continuously monitors supply air tem-
perature. Once the supply air temperature falls below
set point, the controller periodically records supply air
temperature, set point and time. A calculation is then
performed to determine temperature drift from set
point over time. If the calculation determines that
cooling is no longer required, contacts TC and TN are
opened to de-energize the compressor motor and the
condenser fan motor. In addition the controller will
close the EEV.
g. The evaporator fan motors continue to run to circulate
air throughout the container. The green IN RANGE
light remains illuminated as long as the supply air
temperature is within tolerance of the set point.
h. If the supply air temperature increases to 1.0_C
(1.8_F) above set point and three minutes have
elapsed, contacts TC and TN close to restart the
compressor and condenser fan motors in standard
mode (non-economized) operation. The white COOL
light is also illuminated.
i. If the average system capacity has risen to 100% dur-
ing unloaded cooling and three minutes off time has
elapsed, relay TS will energize to open the ESV, pla-
cing the unit in economized mode.
j. If the supply air increases more than 2.5_C (4.5_F)
above set point temperature, the microprocessor will
transition the evaporator superheat control from
modulation back to full cool control.
3−6