4-1
62-11848
SECTION 4
CONTROL LOGIC AND TEMPERATURE CONTROL
4.1
SEQUENCE OF OPERATION
General operation sequences for cooling, null, and heating are provided in the following paragraphs. The micropro-
cessor automatically selects the mode necessary to maintain box temperature at setpoint.
4.1.1
Perishable Mode
The unit operates in the perishable mode with set points above 10°F (-12°C).
a. With return air temperature above setpoint and decreasing, the unit will be cooling with the compressor and
evaporator fans operating. (See
for a description of the refrigeration circuit during cooling.)
The condenser fan will operate for the first three minutes after start up then operate under the control of the
condenser pressure control switch (HP2). The green unit operating LED will operate in accordance with
b. Once temperature decreases to the setpoint, the unit will then enter the null mode. If the continuous air flow
parameter is set to ON, the evaporator fans will continue to operate with all other components OFF. If the
continuous air flow parameter is OFF, the evaporator fans and all other components will be OFF. A five min-
ute delay is required before restart is allowed.
c. If temperature increases during the null mode, the unit will restart in cooling.
d. If temperature continues to decrease, the unit will enter the heating mode with the compressor and evapora-
tor fans operating and the hot gas solenoid valve (HGS1) energized (open). The condenser fan and con-
denser pressure control valve (HGS2) will operate under the control of the condenser pressure control
switch (HP2). If the engine coolant option is installed, the optional heaters will also be energized. For a
description of the refrigeration circuit during heat and defrost, See
.
Figure 4.1 Operating Sequence - Perishable Mode
+5.4º F (+3º C )
or +3.6º F (+2º C )
or +1.8º F (+1º C)
SETPOINT
--1.8º F (--1º C)
or --3.6º F (--2º C)
or --5.4º F (--3º C)
(setting)
COOL
UNIT STOPPED
HEAT
