OM AGSB-5
53
If the leaving chilled water temperature is above the Leaving Chilled Water Setpoint plus
one-half of the Control Band plus the adjustable Start-up Delta-T, the controller will select
the refrigerant circuit with the lowest number of starts as the lead circuit and initiate a start
request. The controller will perform a
PreOpen
function, opening the EXV and initiating
the pre-open timer. After the pre-open timer times out, the controller will start the
compressor and energize the evaporator oil return solenoid and the display will show
Opened EXV
. A green light under the Enter key will illuminate.
If additional cooling capacity is required, the controller will energize the additional cooling
capacity by activating the first compressor’s capacity control solenoids. As the system load
increases, the controller will start the lag refrigerant circuit when the lead circuit reaches
75%, some other capacity limit is reached, and the interstage timers are satisfied. The
compressors and capacity control solenoids will automatically be controlled as required to
meet the cooling needs of the system. The electronic expansion valves are operated by the
MicroTech II controller to maintain precise refrigerant control to the evaporator at all
conditions.
Standard FanTrol Condenser Fan Control
The first condenser fan stage will be started along with the first compressor to provide
initial condenser head pressure control. The MicroTech II controller will activate the
remaining condenser fans as needed to maintain proper condenser pressure. The MicroTech
II controller continuously monitors the condenser minus evaporator lift pressure and will
adjust the number of operating condenser fans as required. The number of condenser fans
operating will vary with outdoor temperature and system load. The condenser fans are
matched to the operating compressors so that when a compressor is off, all fans for that
circuit will also be off.
Pumpdown
As the system chilled water load requirements diminish, the compressors will unload. As
the system load continues to drop, the electronic expansion valves will be stepped closed
and the refrigerant circuits will go through a pumpdown sequence. As the evaporator
pressure falls below the pumpdown pressure setpoint while pumping down, the
compressor(s) and condenser fans will stop. The unit has a one-time pumpdown control
logic; therefore, if the evaporator pressure rises while the refrigerant circuit is in a
pumpdown mode, the controller will not initiate another pumpdown sequence. The
controller will keep the unit off until the next call for cooling occurs.
The chilled water pump output relay will remain energized until the time schedule’s "on"
time expires, the remote stop switch is opened, the system switch is moved to the stop
position, or a separate communications panel such as the Remote Monitoring and
Sequencing Panel or an Open Protocol interface deactivates the chilled water pump output.
Liquid injection
Liquid injection is turned on automatically at 185
°
F or discharge superheat above 60
degrees F. It is turned off at 170
°
F or discharge superheat below 40 degrees F.
Summary of Contents for AGS 206A
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