T-334
4--7
4.10.2 Sequence of Operation - Perishable Mode
Cooling
NOTE
In the Conventional Perishable Mode of Opera-
tion, the evaporator motors run in high speed. In
the Economy Perishable Mode, the fan speed
is varied.
a. With supply air temperature above set point and de-
creasing, the unit will be cooling with the condenser
fan motor (CF), compressor motor (PA, PB or CH),
evaporator fan motors (EF) energized and the COOL
light illuminated. (See Figure 4--5.) 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.
24 VOLT POWER
ENERGIZED
DE-ENERGIZED
TN
USV
ST
F
PA
PB
HPS
TCP
PA
PB
IP--CM
TC
CF
TE
IP--EM
EF
IP--EM
HTT
TH
HR
TU
ESV
TS
XBSV
TT
LIV
TQ
TV
EF
ES
NOTE: High-speed, normal operation, single phase
evaporator fan shown. For three phase motor units,
the compressor is independently operated by an addi-
tional (CH) relay. For full diagrams and legend, see
Section 7.
Figure 4--5 Perishable Mode - Cooling
b. The controller will monitor operation conditions and, if
required, contacts TT will close to energize the XBSV
and allow additional liquid refrigerant to bypass the
main TXV.
c. When the air temperature decreases to a predeter-
mined tolerance above set point, the in-range light is
illuminated.
d. As the air temperature continues to fall, modulating
cooling starts at approximately 2.5C (4.5F) above set
point. (See Figure 4--3.) At set point, contacts TS are
de-energized to close the economizer solenoid valve
and contacts TU energizes, which opens the unload-
er solenoid valve, changing from economized opera-
tion to unloaded operation.
e. The controller monitors the supply air. Once the sup-
ply air falls below set point, the controller periodically
records the supply air temperature, set point and
time. A calculation is then performed to determine
temperature drift from set point over time.
f. If the calculation determines cooling is no longer re-
quired, contacts TC and TN are opened to de-ener-
gize compressor motor and condenser fan motor.
The cool light is also de-energized.
g. The evaporator fan motors continue to run to circulate
air throughout the container. The in-range light re-
mains illuminated as long as the supply air is within
tolerance of set point.
h. If the supply air temperature increases to 0.2C (0.4F)
above set point and the three minute off time has
elapsed, contacts TC, TU and TN are closed to re-
start the compressor and condenser fan motors in un-
loaded operation. The cool light is also illuminated.
4.10.3 Sequence of Operation -
Perishable Mode Heating
a. If the air temperature decreases 0.5C (0.9F) below
set point, the system enters the heating mode. (See
Figure 4--3). The controller closes contacts TH (see
Figure 4--6) to allow power flow through the heat ter-
mination thermostat (HTT) to energize the heaters
(HR). The HEAT light is also illuminated. The evapo-
rator fans continue to run to circulate air throughout
the container.
b. When the temperature rises to 0.2C (0.4F) above set
point, contacts TH open to de-energize the heaters.
The HEAT light is also de-energized. The evaporator
fans continue to run to circulate air throughout the
container.
c. The safety heater termination thermostat (HTT),
which is attached to an evaporator coil support, will
open the heating circuit if overheating occurs.
24 VOLT POWER
ENERGIZED
DE-ENERGIZED
TN
USV
ST
F
PA
PB
HPS
TCP
PA
PB
IP--CM
TC
CF
TE
IP--EM
EF
IP--EM
HTT
TH
HR
TU
ESV
TS
XBSV
TT
LIV
TQ
TV
EF
ES
NOTE: High-speed, normal operation, single phase
evaporator fan shown. For three phase motor units,
the compressor is independently operated by an addi-
tional (CH) relay. For full diagrams and legend, see
Section 7.
Figure 4--6 Perishable Mode Heating
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