Heatcraft Mohave, Installation & Operation Manual, Page: 42 / 76

42
Refrigeration Operation
Refrigeration Operation
The refrigeration operation of the Mohave Hot Gas System is very similar to a
standard refrigeration system. An external thermostat is connected to the hot
gas control board at the terminal block connections labeled T-Stat and C (for
common). When the normally open contact inside the thermostat closes (a
call for cooling), the hot gas control board responds by activating a series of
solenoids and contactors (described below) in order to initiate and maintain
a refrigeration cycle. Later, when the thermostat contact opens, the hot gas
control board deactivates the solenoids in a preset manner in order to safely
turn off the refrigeration process and maintain an Off condition.
At initial power up, the system defaults to the
OFF
mode for a minimum
of two minutes. Following the two-minute hold off period, the control circuit
examines the state of the thermostat input. If the thermostat input signal is
activated (closed between T-Stat and C), the system begins the refrigeration
startup process. Full refrigeration mode (or
COOL
mode) is achieved when
the control board has activated the solenoids necessary to provide refrigerant
flow between the evaporator(s) and the condensing unit (Suction Solenoid
and Liquid Line Solenoid), activated the compressor contactor, deployed the
appropriate control over the condenser fans, and turned on
the evaporator fans.
Refrigerant Solenoid and Compressor Contactor Control: The Suction
Solenoid is initially activated following the power-up two-minute hold off time.
It is maintained in the
ON
state until a defrost cycle is initiated. The timing of
the liquid line solenoid (LLS) activation is based upon the saturated suction
temperature (SST) which is calculated from the suction pressure value. If the
SST is greater than 15ºF, the compressor contactor is activated before the
LLS is activated in order to decrease the suction pressure prior to startup.
When the SST drops to -5ºF, the LLS is activated. If the SST does not fall
to -5ºF within 2 minutes,
Er12
is activated and the system goes to
OFF
mode.
If the SST is 15ºF or less, the LLS is activated immediately. When the
SST rises to -10ºF for medium temperature applications or -20ºF for low
temperature applications, the compressor contactor is activated. If the SST
fails to rise in 2 minutes,
Er11
is activated and the system goes to
OFF
mode.
When the thermostat signal is deactivated, the liquid line solenoid is turned
off immediately. The compressor contactor will stay activated until the suction
pressure falls below the pre-programmed cut out pressure. The compressor
contactor and all condenser fan contactors will be deactivated simultaneously.
The evaporator fans will continue running.
Condenser Fan Control:
When the Condensing Unit Model is selected in
the Program Review Menu; the program automatically activates the default
Head Pressure Control Scheme.
Method:
Pressure Fan Cycling (
PRES
) All fans
Minimum condensing temperature: 65ºF.
The default
ON
and
OFF
settings are optimized to maximize energy
efficiency while still providing adequate pressure for the thermostatic
expansion valve(s) to work properly. The parameters are refrigerant specific.
Fans are staged to minimize fluctuations in head pressure during operation.
During refrigeration operation, the hot gas controller monitors liquid pressure
to determine if each fan should be
ON or OFF .
These settings and other fan control options may be modified by turning on
the Expert Mode (
XPRT
) in the Program Menu. See Program Review and
Optional Controls for more information.
Evaporator Fan Control: After initial power-up, the evaporator fans
will be turned off. When the system initiates the first cooling cycle, the hot
gas controller monitors the temperature value of the evaporator defrost
termination sensor mounted on the evaporator suction headers. When the
controller determines that the suction header has reached the refreeze
setpoint, the evaporator fans will be activated. If there are two evaporators,
the fans will be energized by the first sensor to achieve setpoint. Once
activated, the evaporator fans will continue to run until either a defrost cycle
is initiated, or if the system is placed in
SERVICE mode.
Anti Short-Cycle Protection: During cooling mode, the control board
is programmed to allow a minimum system ON time of 1 minute and a
minimum OFF time of 2 minutes.
Pump Down
At the end of each cooling cycle, when the box temperature is met, the hot
gas control system will pump down and turn off the compressor.