OMM TGZ-1 TGZ 040A through TGZ 190A 37
Controller Operation
Calculations
Control Band
The Control Band defines the temperatures around the Cool Leaving Water Temperature set
point where compressors will be staged on or off. The Control Band is calculated as
follows:
Control Band = Evap Delta Temperature Set Point * 0.3 Four compressor units
Control Band = Evap Delta Temperature Set Point * 0.2 Six compressor units
If the Unit mode is Cool:
When the Cool Leaving Water Temperature set point is more than half the Control
Band above 39.0° F the Stage Up temperature is calculated as follows:
Stage Up Temperature = Cool LWT + (Control Band/2)
The Stage Down temperature is calculated as:
Stage Down Temperature = Cool LWT – (Control Band/2)
If the Cool Leaving Water Temperature set point is less than half the Control Band above
39.0° F the Stage Down temperature is calculated as:
Stage Down Temperature = Cool LWT – (Cool LWT - 39.0
°
F)
Stage Up temperature is calculated as:
Stage Up temperature = Cool LWT + Control Band – (Cool LWT – 39.0
°
F)
In all other Unit modes the compressor staging temperatures are calculated as shown below:
Stage Up Temperature = Cool LWT + (Control Band/2)
Stage Down Temperature = Cool LWT – (Control Band/2)
The Start up and Shutdown temperatures are calculated from the Control Band. The Start
Up temperature determines when the first compressor on the unit will start. The Start Up
temperature calculation is shown below:
Start Up Temperature = Stage Up Temperature + Start Up Delta Temperature
The Shutdown temperature defines when the last running compressor will shutdown. The
Shutdown temperature calculation is:
Shutdown Temperature = Stage Down Temperature – Shutdown Delta Temperature
Leaving Water Reset
The leaving water reset input uses a 4-20mA signal to reset the leaving water set point to a
higher value. The adjustment varies linearly from 0 to 10
o
F, with a reset of 0 for a 4mA
signal and a reset of 10 for a 20mA signal.
Leaving Water Set Point
(Evap LWT)
Control
Band
(Control Band/2)
(Control Band/2)
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