OMM 1008-1
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With the IGV fully open the compressor will now follow the ModBus Demand sent from the compressor
controller to pursue the target temperature.
Lag Compressor Start
As the lead compressor achieves Full Load Status (either by low evap pressure, or bumping a high RLA, or
in normal operation by exceeding 98% of the Maximum Speed for 90 seconds), the lag compressor is
cleared to start at its discretion. The lag controller looks at the evaporator leaving water temperature and
slope, to determine if it should start.
Once the lag compressor has determined that it will start, it sends a signal to the lead compressor
requesting it to unload. The unload period is set by the Unload Timer setpoint. As this period expires, the
lag compressor will set its Start Speed at a percentage of the lead compressors Min and Max speeds
calculated for the present conditions.
As described with the lead compress the lag compress will ramp speed until it achieves the Start Speed
minus one thousand RPM. As the RLA exceeds 8%, the lag compressor Start Speed is backed down to the
Minimum speed calculated by the lag compressor. At this point there is Modbus control of Demand and
the lag compressor can unload with speed if the target temperature has been over run..
The Lead compressor will continue to unload until one of the following occurs:
•
The Lag KW exceeds the Lead compressors KW.
•
The Lead compressor’ Suction Super-Heat exceeds 95 degrees F.
•
The Lead compressor has unloaded for a period ten times the Unload timer.
After the Lag compressor has exceeded the KW of the Lead, and is working to get its IGVs fully open, the
Lead compressor will begin loading up based on the Lag compressors demand feedback signal.
Unloading compressors:
The setpoint of ‘Nominal Capacity’ is used for defining the point to unload a compressor on a single or two-
chiller system. With each compressor having its ‘Nominal Capacity’ defined, then the network, which is load
balanced, continues to unload at 0.2 tenths or more below setpoint. Each compressor keeps computing the
spare capacity of the network. When the designated ‘Next Off’ sees enough spare capacity, it will turn off.
Then similarly, in about 40 seconds, a new compressor will be designated as the ‘Next Off’ and the spare
capacity will continue to be calculated between the remaining compressors. Compressors continue to unload
and stage off until there is only one compressor running. It will shut off when the water temperature reaches
the LWT Setpoint minus the Shutdown Delta T.
Operating the Chiller Control System
Interface Panel On/Off
The Operator Interface Panel is turned on and off with a push-push switch located at the upper-left corner on
the rear of the panel. ON is the outermost switch position and a white band will be visible on the switch
stem. Off is innermost and no white is visible.
The screen is equipped with a screen saver that blackens the screen. Touching it anywhere reactivates the
screen. If the screen is black, touch it first to be sure it is on before using the ON/OFF switch.
Start/Stop Unit
There are four ways to start/stop the chiller. Three are selected in SETPOINT\MODE\SP3, the fourth way is
through panel-mounted switches:
Operator Interface Panel (LOCAL)
Home Screen 1 has AUTO and STOP buttons that are only active when the unit is in "LOCAL CONTROL".
This prevents the unit from being accidentally started or stopped when it is under control from a remote
switch or BAS. When these buttons are pressed, the unit will cycle through its normal starting or stopping
sequence. On dual compressor units, both compressors will be stopped and normal dual compressor starting
procedure will be in effect.
