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18
3.3.2 Compressor Control
Depending on the type of Liebert air conditioning unit you have your unit may contain one or two
compressors with or without unloaders.
Compressor Proportional Bands
One Single-Step Compressor Without Unloaders—One-Step
One single-step compressor, Cool 1, is started at 100% call for cooling from the temperature propor-
tional band and stopped at 0% (see Figure 12).
Figure 12 One single-step compressor without unloaders
Two Single-Step Compressors Without Unloaders—Two-Step
First single-step compressor, Cool 1, is started at 50% calculated output from the temperature propor-
tional band, and stopped at 0%. The second compressor, Cool 2, starts at 100% and stops at 50% (see
Figure 13).
One Compressor With an Unloader—Two-Step
The two-step compressor is started unloaded at 50%, Cool 1, calculated output from the temperature
proportional band and stopped at 0%. At 100% the compressor starts fully loaded, Cool 2, and returns
to unload operation at 50% (see Figure 13).
Figure 13 Two single-step compressors without unloaders or one compressor with an unloader (two-step)
Temp Setpoint: 70°F
Proportional Band: 8°F
Deadband: 2°F
75
½ Proportional Band
0%
Cooling
+ 100%
Cooling
71 72 73 7470
½ Dead-
band
Cool 1
Off
Cool 1
On
Increasing Temperature
Temp Setpoint: 70°F
Proportional Band: 8°F
Deadband: 2°F
75
1/2 Proportional Band
0%
Cooling
+ 100%
Cooling
71 72 73 7470
1/2 Dead-
band
Cool 1
On
Cool 2
On
Increasing Temperature
Cool 2
Off
Cool 1
Off