44 AGZ 010B through 034B IOMM AGZB1
Transitions:
T1 – Transition from Off to Start
Requires any of the following
• Unit state = Auto
• LWT < Freeze setpoint - 1
T2 – Transition from Start to Run
• Flow ok for time > evaporator recirculate time
T3 – Transition from Run to Off
• Unit state = Off AND LWT > Freeze setpoint
T4 – Transition from Start to Off
• Unit state = Off AND LWT > Freeze setpoint
T5 – Transition from Run to Start
Requires all of the following
• Evaporator water flow loss for time > evaporator flow proof setpoint
Condenser Fans
Condenser fans are staged up and down based on the fan stage setpoint. These setpoints
define pressures at which fans will start or stop.
Fan 1 will start with the first compressor when the ambient temperature is greater than
75°F. Below 75°F, this fan starts when the condenser pressure gets up to the Stage #1 On
setpoint. Fan 2 will start when the condenser pressure gets up to the Stage #2 On setpoint,
and fan 3 will start when the condenser pressure gets up to the Stage #3 On setpoint.
Fan 3 will stop when the condenser pressure drops to the Stage #3 Off setpoint, and Fan 2
will stop when the condenser pressure drops to the Stage #2 Off setpoint. Fan 1 will stop
when the pressure drops down to the Stage #1 Off setpoint.
Low OAT Start
In order to avoid low pressure alarms at startup, low OAT start logic allows for running at
low pressures for a longer time than normal as well as multiple start attempts.
A low OAT start is initiated if the condenser saturated temperature is less than 85°F when
the compressor starts. Once this happens, the circuit is in this low OAT start state for a
time equal to the low OAT start timer setpoint. During this time, the freezestat logic and
the low pressure events are disabled. The absolute limit of 5 psi is still enforced.
At the end of the low OAT start, the evaporator pressure is checked. If the pressure is
greater than, or equal to, the low evaporator pressure unload setpoint, the start is considered
successful. If the pressure is less than the unload setpoint, the start is not successful and
the compressor will stop. Three start attempts are allowed before tripping on the restart
alarm. So if on the third attempt, the start is not successful, the restart alarm is triggered.
The restart counter is be reset when either a start is successful or the circuit is off on an
alarm.
Circuit Capacity Overrides
The following conditions override the automatic capacity control when the chiller is in
Cool Mode only. These overrides keep the unit from entering a condition in which it is not
designed to run.
Low Evaporator Pressure
If the evaporator pressure drops below the Low Evaporator Pressure Hold setpoint, the Low
Evaporator Pressure Inhibit event is triggered. This can occur with either one or two
compressors running. When triggered, the second compressor will not be allowed to start if
only one is currently running. If both compressors are already running, no action is taken.
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