4-8
T-268-07
c. Cooling with Single-Speed Compressor
(See Figure 4-3.)
NOTE
Evaporator fan motors will run in high speed.
(Contactor EF energized)
NOTE
A pressure control system has been
incorporated by means of a condenser pressure
transducer (CPT) and condenser pressure
control (CPC) logic to maintain discharge
pressures above 130 psig in low ambients.
The condenser fan will cycle off if the
condenser pressure is below 130 psig. If the
condenser pressure goes above 200 psig, the
condenser fan will cycle on.
With supply air temperature decreasing, and if the
supply air is above set point, the unit will be cooling
with the condenser fan motor, compressor motor and
evaporator fan motors energized. Also, at this time, the
cool light is illuminated.
When the air temperature decreases to a predetermined
tolerance above set point, relay TI energizes and the
in-range light is illuminated. (Refer to section 3.1.4,
Code 30.)
If the air temperature continues to fall, modulating
cooling starts at approximately 2.5
_
C (4.5
_
F) above set
point. The modulating valve will have a variable current
up to 1.30 amps at full modulation.
During this cooling mode, a running sum of the
temperature differential (supply air temperature -- set
point) is kept. When the supply air falls below set point,
the differential is negative. The longer supply air
remains below set point, the greater the negative
differential in the running sum.
When the supply air temperature decreases to 0.2
_
C
below set point and the running sum is less than --250
degrees C-seconds, relays TN and TC de-energize
shutting off the condenser fan and compressor motors.
Also, the cool light is de-energized.
The evaporator fan motors continue to run to circulate
air throughout the container. The in-range light remains
illuminated as long as the supply air is within a tolerance
of set point, and the 15 minute override is met.
If the unit is in the holding mode (neither heating nor
cooling) and the supply air temperature increases to
0.2
_
C (0.4
_
F) above set point, and providing a six
minute off time has elapsed, relay TC energizes to
restart the compressor. Also, at this time, the condenser
fan motor starts and the cool light is illuminated.
