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19
Two Compressors With Unloaders—Four-Step
The first two-step compressor is started unloaded at 33% calculated output from the temperature pro-
portional band and stopped at 17%. At 80% Compressor 1 will be loaded, at 70% unloaded.
The second compressor starts unloaded at 63% and stops at 47%. At 100%, Compressor 2 will be
loaded, at 90% unloaded (see Figure 14).
The four stages of cooling are accomplished in the following manner:
• 1 stage: One compressor, unloaded - Cool 1
• 2 stages: Both compressors, unloaded - Cool 2
• 3 stages: One compressor, loaded and one compressor, unloaded - Cool 3
• 4 stages: Both compressors, loaded - Cool 4
Figure 14 Two compressors with unloaders (four-step)
Cool 2
On
Temp Setpoint: 70°F
Proportional Band: 8°F
Deadband: 2°F
½ Dead-
band
Increasing Temperature
½ Proportional Band
757473727170
0%
Cooling
+ 100%
Cooling
Cool 4
Off
Cool 2
Off
Cool 1
Off
Cool 4
On
Cool 3
On
Cool 1
On
Cool 3
Off