Carrier 69NT20-531-300 manual Frozen Mode

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.

ENERGIZED
DE--ENERGIZED	CONTROL TRANSFORMER
FOR FULL DIAGRAM AND
LEGEND, SEE SECTION 7

POWER TO

CONTROLLER

T6	PB		PA
TD	T6	PA	PB
SIGNAL TO
CONTROLLER	TU	USV
TS			ESV

NOTE: HIGH SPEED EVAPORATOR FAN SHOWN. FOR LOW SPEED CONTACT TE IS DE--ENERGIZED AND CONTACT TV IS ENERGIZED

Figure 4-5 Perishable Mode Heating

4.9.3Sequence 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.

4.9.4Sequence 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.
ENERGIZED
DE--ENERGIZED	CONTROL TRANSFORMER
FOR FULL DIAGRAM AND
LEGEND, SEE SECTION 7
POWER TO
CONTROLLER
T6	PB		PA
TD	T6	PA	PB
SIGNAL TO
CONTROLLER	TU	USV
TS			ESV

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.

T-309 4-6