Figure 15 Digital Scroll Capacity Modulation, 10-100% Variable; Figure 16 Single And Dual Digital Scroll Compressor Activation Points - Liebert ICOM User Manual

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

Call for Cooling: 66%
100% Loaded
0% Loaded
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

Solenoid
De-energized
0 5
15-Second Capacity
Modulation Cycle
Temp Setpoint : 70°F
Proportional Band : 8°F
Deadband : 2°F
Comp 1 On Partially
Loaded
½ Dead-
band
70 71 72
0%
Comp 1
Cooling
Off
Comp 2
Off
½ Proportional Band
Solenoid Solenoid
Energized De-energized
10 15 20
15-Second Capacity
Modulation Cycle
Comp 2 On Partially
Loaded
73 74
Increasing Temperature
19
Solenoid
Energized
25 30
Comp 1 & 2 On
Fully Loaded
75
+ 100 %
Cooling
Operation