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Controls Training and Service Manual
The temperature controller deactivates the second stage of cooling (lag
compressor) when the return air temperature decreases to 50% of the cooling
proportional control band value. The first cooling stage (lead compressor) is
deactivated when the return air temperature decreases to the temperature set
point value or 0% of the cooling proportional control band value.
2 Stage Compressorized Cooling – No Dead Band
Temp Set Point: 70°F
Proportional Band: 8°F
70 71 72 73 74 75
Increasin
g
Temperature
Cool 1 On
Cool 1 Off
Cool 2 On
Cool 2 Off
Temp Set Point + (1/2 Proportional Band)
Note: in the above example that the control band begins at the 70°F temperature
set point and has a length of 4°F, which is ½ of the programmed temperature
proportional band value.
As the return air temperature increases Cooling 1 (lead compressor) is activated at
72°F or 50% of the cooling control band. If the return air temperature continues to
increase Cooling 2 (lag compressor) will activate at 74°F or 100% of the cooling
control band.
When the return air temperature starts to decrease, Cooling 2 (lag compressor) is
deactivated at 72°F or 50% of the cooling control band and Cooling 1 (lead
compressor) is deactivated at the temperature set point of 70°F or 0% of the
cooling control band.
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