Carrier Streamline Scroll 69NT20-531-300 - Manual (Page 58)

T-309 4-6
noid valve changing from economized operation to
unloaded operation. (As shown in Figure 4-5)
d. 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 temper-
ature drift from set point over time.
e. If the calculation determines cooling is no longer re-
quired, contacts TD and TN are opened to de-energize
compressor motor and condenser fan motor. The cool
light is also de-energized.
f. 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.
g. If the supply air temperature increases to 0.2_C
(0.4_F) above set point and the three minute off time
has elapsed, relays TD, TU and TN are energized to
restart the compressor and condenser fan motors in
unloaded operation. The cool light is also illumi-
nated.
CONTROL TRANSFORMER
POWER TO
CONTROLLER
SIGNAL TO
CONTROLLER
ENERGIZED
DE--ENERGIZED
FOR FULL DIAGRAM AND
LEGEND, SEE SECTION 7
NOTE: HIGH SPEED EVAPORATOR FAN SHOWN. FOR LOW SPEED
CONTACT TE IS DE--ENERGIZED AND CONTACT TV IS ENERGIZED
TD T6 PA PB
T6 PB PA
USV
ESV
TU
TS
Figure 4-5 Perishable Mode Heating
4.9.3 Sequence Of Operation --
Perishable Mode Heating
a. If the air temperature decreases 0.5_C (0.9_F) below
set point, the system enters the heating mode. (See
Figure 4-2). The controller closes contacts TH (see
Figure 4-5) 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 0.2_C (0.4_F) above
set point, contact TH opens to de--energize the heat-
ers. The HEAT light is also de--energized. The evapo-
rator fans continue to run to circulate air throughout
the container.
c. A safety heater termination thermostat (HTT), at-
tached to an evaporator coil support, will open the
heating circuit if overheating occurs.
4.9.4 Sequence Of operation -- Frozen Mode Cool-
ing
a. With supply air temperature above set point and de-
creasing, the unit will transition to economized cool-
ing with the condenser fan motor (CF), compressor
motor (CH), economizer solenoid valve (ESV), low
speed evaporator fan motors (ES) energized and the
COOL light illuminated. (See Figure 4-6.)
b.When the air temperature decreases to a predeter-
mined tolerance above set point, the in-range light is
illuminated.
CONTROL TRANSFORMER
POWER TO
CONTROLLER
SIGNAL TO
CONTROLLER
ENERGIZED
DE--ENERGIZED
FOR FULL DIAGRAM AND
LEGEND, SEE SECTION 7
TD T6 PA PB
T6 PB PA
USV
ESV
TU
TS
Figure 4-6 Frozen Mode
c. When the return air temperature decreases to 0.2_C
(0.4_F) below set point, contacts TD, TS and TN are
opened to de-energize the compressor, economizer
solenoid valve and condenser fan motors. The cool
light is also de-energized.
d. The evaporator fan motors continue to run in low
speed to circulate air throughout the container. The
in-range light remains illuminated as long as the re-
turn air is within tolerance of set point.
e. When return air temperature is 10_C (18_F) or more
below set point, the evaporator fans are brought to
high speed.
f. When the return air temperature increases to 0.2_C
(0.4_F) above set point and the three minute off time
has elapsed, relays TD, TS and TN are energizes to
restart the compressor and condenser fan motors. The
cool light is illuminated.
4.9.5 Sequence Of Operation -- Defrost
The defrost cycle may consist of up to three distinct
operations. The first is de-icing of the coil, the second is
a probe check cycle and the third is snap freeze.