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Control Training and Service Manual
2 Stage Compressorized Cooling – With Dead Band
Temp Set Point: 70°F
Proportional Band: 8°F
Dead Band: 2°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 Dead Band + 1/2 Proportional Band)
DB
Note: in the above example that the control band begins at the 70°F temperature
set point and has a length of 5°F, which is ½ of the programmed temperature dead
band value plus ½ of the programmed temperature proportional band value.
As the return air temperature increases Cooling 1 (lead compressor) is activated at
73°F or ½ of the dead band value plus 50% of the cooling control band. If the
return air temperature continues to increase Cooling 2 (lag compressor) will
activate at 75°F or ½ of the dead band value plus 100% of the cooling control
band.
When the return air temperature starts to decrease, Cooling 2 (lag compressor) is
deactivated at 73°F or ½ of the dead band value plus 50% of the cooling control
band and Cooling 1 (lead compressor) is deactivated at 71°F or ½ of the dead
band value plus 0% of the cooling control band.
Remember the temperature dead band value is used by the control to shift the
cooling on/ off operations away from the temperature set point.
Optional 4 - Stage Cooling, Two (2) Compressors with Unloaders
The basic temperature cooling control band is established at the temperature set
point with the length equal to ½ of the programmed temperature proportional band
divided by the number of cooling stages.
The Liebert DS units are supplied with two (2) compressors, each compressor is
rated at ½ of the unit capacity. Each compressor will be the semi-hermetic type
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