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Control Training and Service Manual
temperature is at least 8°F below the return air temperature, or 3°F lower than the
return air temperature for two consecutive hours.
When the microprocessor decides that the return glycol fluid temperature is cold
enough the first cooling band is the modulating valve control method, and the
second band, added to the first band, is for the compressors as in the normal
2-Stage or 4-Stage control method. If the chilled glycol fluid temperature is not
cold enough the valve control band is replaced by the compressor band. If the
chilled glycol cooling capacity is reduced by a rise in the glycol fluid temperature,
the control band shrinks proportionally. This allows the compressor band to move
down as well. The following shows the Glycool operation at 100% capacity and
the Glycool at 50% capacity.
Glycool at 100% Capacity – No Dead Band
Temp Set Point: 70°
Proportional Band: 8°
70 71 72 73 74 75 76 77 78 79
Increasin
g
Temperature
Band 1 Gl
y
cool Valve Band 2 Compressors
Valve
Closed
100%
Open
Cool 1 On
Cool 2 On
Temp Set Point + (1/2 Proportional Band + 1/2 Proportional Band)
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