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Digital Scroll Compressors
A compressor with a suction cutoff unloader can only modulate its capacity between two distinct lev-
els: fully loaded and half loaded. 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 15.
Figure 15 Digital scroll capacity modulation, 10-100% variable
On single and dual digital scroll compressor systems, the first compressor is started at 25% calculated
output from the temperature proportional band and stopped at 10%. On dual digital scroll compressor
systems, the second compressor is started at 35% and stopped at 20%, see Figure 16. When a com-
pressor is started, the solenoid is energized longer than it is de-energized to match the call for cooling.
When the call for cooling increases to 100%, the solenoid is de-energized for the entire 15 second cycle.
Figure 16 Single and dual digital scroll compressor activation points
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
Temp Setpoint: 70°F
Proportional Band: 8°F
Deadband: 2°F
½ Dead-
band
Increasing Temperature
Comp 1
Off
½ Proportional Band
757473727170
0%
Cooling
+ 100%
Cooling
Comp 2
Off
Comp 1 On Partially
Loaded
Comp 2 On Partially
Loaded
Comp 1 & 2 On
Fully Loaded