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Operation Manual
D
–EOMWC00A07-16_02EN
7
UNIT FUNCTIONS
7.1
Calculations
7.1.1
LWT Slope
LWT slope is calculated such that the slope represents the change in LWT over a time frame of one minute with at least
five samples per minute for both evaporator and condenser.
7.1.2
Pulldown Rate
The slope value calculated above will be a negative value as the water temperature is dropping. For use in some control
functions, the negative slope is converted to a positive value by multiplying by
–1.
7.2
Unit Model
The unit model can be selected between the four available for this application. Depending on the model temperature ranges
and refrigerant type are selected automatically.
7.3
Unit Enable
Enabling and disabling the chiller is accomplished using set points and inputs to the chiller. The unit switch, remote switch
input, and Unit Enable Set Point all are required to be on for the unit to be enabled when the control source is set to local .
The same is true if the control source is set to network, with the additional requirement that the BAS request must be on.
Unit is enabled according to the following table.
NOTE
: An x indicates that the value is ignored.
Unit Switch
Control Source
Set Point
Remote Switch
Input
Unit Enable Set
Point
BAS Request
Unit Enable
Off
x
x
x
x
Off
x
x
x
Off
x
Off
x
x
Off
x
x
Off
On
Local
On
On
x
On
x
Network
x
x
Off
Off
On
Network
On
On
On
On
All of the methods for disabling the chiller, discussed in this section, will cause a normal shutdown (pumpdown) of any
running circuits.
When the controller is powered up, the Unit Enable Set Point will be initialized to ‘off’ if the Unit Status After Power Fai lure
Set Point is set to ‘off’.
7.4
Unit Mode Selection
The operating mode of the unit is determined by setpoints and inputs to the chiller. The Available Modes Set Point
determines what modes of operation can be used. This setpoint also determines whether the unit is configured for glycol
use. The Control Source Set Point determines where a command to change modes will come from. A digital input switches
between cool mode and ice mode if they are available and the control source is set to local. The BAS mode request
switches between cool mode and ice mode if they are both available and the control source is set to network.
