Operation
20
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 15).
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 15 Two compressors with unloaders (four-step)
Digital Scroll Compressors
A digital scroll compressor can modulate its capacity anywhere between 10-100%. This variable
capacity modulation allows cooling units to control an environment more precisely.
Digital scroll capacity modulation is achieved by energizing and de-energizing a solenoid valve on the
compressor. When the solenoid valve is de-energized, the compressor capacity is 100%. When the sole-
noid valve is energized, the compressor capacity is zero. Therefore, the capacity of the compressor
depends on how long the solenoid is de-energized for. If the solenoid is de-energized for 10-seconds,
then energized for 5 seconds during a 15-second cycle, the resulting capacity will be 66% as shown in
Figure 16.
Figure 16 Digital scroll capacity modulation, 10-100% variable
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
Call for Cooling: 66%
100% Loaded
0% Loaded
Solenoid
De-energized
Solenoid
De-energized
Solenoid
Energized
0 5 10 15 20 25 30
Solenoid
Energized
15-Second Capacity
Modulation Cycle
15-Second Capacity
Modulation Cycle
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