C6.3.2/0416-0716/E
7
2.6.3 Digital control
Capacity modulation is achieved by energizing and de-energizing the solenoid valve. When the
solenoid valve is de-energized, the Digital bank capacity is 100%. When the solenoid valve is
energized, the Digital bank capacity is zero. Therefore, the capacity achieved is the time average
capacity.
Example: In a 20-second cycle, if the solenoid is de-energized for 16 seconds, and then
energized for 4 seconds, the resulting capacity will be approximately 80%.
For 4M*D Stream compressors, one bank of the compressor remains loaded 100% while the
Digital bank will modulate 0 to 100% to provide the additional capacity. The capacity is 50 to
100% as one bank is modulating.
On a 6M*D Stream compressor with blocked suction, the blocked suction bank will unload when
demand capacity is less than 67% and load when the demand capacity is higher than 67%. The
Digital bank will continue to load and unload providing continuous capacity across the 33 to
100% range.
2.6.4 Recommended application settings for modulation on Stream Digital compressors
The Digital modulation recommended cycle time is 20 seconds. For other values, check with
Application Engineering.
Figure 6: The digital signal from the controller activates the unloading
The load and unload times will give the compressor an operating range during a 20-second
cycle. Minimum percent capacity would be 50% for 4M*D and 33% or 67% for 6M*D, depending
on compressor configuration. The compressor can also operate at a 100% load for the full
modulation sequence.
2.6.5 Digital solenoid valve / Gaskets
Due to the high life cycle requirements in a hot gas environment, a special valve has been
developed. Due to reliability requirements, only Emerson solenoid valves may be used. All
compressor warranties are null and void if the Emerson valve is not used. Solenoid coils will be
sold separately for all Stream Digital compressors.
The solenoid coil is available for several voltages: 24V, 120V and 240V.
Stream Digital compressors use special head and valve plate gaskets which have modifications
to ensure gas flow for proper Digital modulation. Only Emerson gaskets may be used. All
compressor warranties are null and void if the Emerson gaskets are not used.
2.6.6 Compressor cooling
Compressor motors must always be cooled, and cylinder head cooling may also be needed at
certain operating conditions.
All Stream Digital compressors are suction-gas cooled. With suction-gas cooled compressors,
the motor is cooled by refrigerant gas that is led over the motor. An additional fan may be
required
depending
upon
the
operation
conditions
(see
Select
software
at
).
2.6.7 Unloaded start
With direct starting the motor of a compressor is switched directly into the mains by means of a
switch. The resulting breakaway starting current amounts to multiple times the rated motor
current operating maximum without consideration being given to transient phenomena.
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