13-17-613 Page 27
4.2 Variable Flow Compressors
A variable flow compressor can be defined as a machine which uses a fixed speed motor (with a
traditional full voltage or wye-delta starter) but can regulate the air flow by using mechanical devices
such as a turn valve to change the size of the compression chamber in the airend or a variable inlet
valve to restrict the intake of air. The Gardner Denver Electra-Saver line of compressors are an
example of variable flow machines. In the case of a compressor with an inlet valve and a turn valve,
two separate PID algorithms are used which control each device based on the package discharge
pressure, in other words, the
Plant Pressure
is compared to the
Target Pressure
. So that the two
independent algorithms do not attempt to cancel or fight each other, each is used in succession while
the compressor is operating in different flow regions. The
Target Pressure
setting is compared to the
air storage receiver or
System Pressure
signal on units with the optional communications module that
are run in Sequence mode of operation. In a typical Gardner Denver machine, the turn valve is used to
control the flow between 100% and typically 40% of full capacity while the inlet valve is used to control
the flow at even lower capacity levels. The AirSmart G2 Controller can also be programmed to use only
the inlet valve for flow control over the full capacity range of the machine.
At full capacity, the inlet valve is fully open and the turn valve is in the fully closed state. As the air
demand decreases, The AirSmart G2 Controller will open the turn valve to decrease the capacity of the
airend to match the demand and keep the
Plant Pressure
value at the
Target Pressure
setting. When
the turn valve is fully open and as the demand continues to decrease, the AirSmart G2 Controller will
begin to close the inlet valve to further decrease the air flow after the
Plant Pressure
has risen to the
Target Pressure
+ 3 PSI. The 3 PSI offset helps keep the two PID controllers from interfering with
each other. Decreasing the demand for air from this point will cause either the inlet valve to fully close
or the pressure to rise to the
Unload Pressure
setting both of which will cause the compressor to
unload. If the demand increases the inlet valve will open followed by the closing of the turn valve to
meet the new demand. If the demand does not increase, the compressor package will eventually
unload and stop. The compressor will start again when the
Plant Pressure
value falls below the
Load
Pressure
setting.
4.3 Fixed Speed Compressors
A fixed speed compressor is the simplest type of machine in the Gardner Denver product line and uses
a fixed speed motor along with a binary inlet valve (open or closed) for flow control. In a simple fixed
speed compressor, the
Target Pressure
setting in the AirSmart G2 Controller is ignored due to the fact
that there is no way to partially restrict the air flow. The controller loads (starts) and unloads (stops) the
compressor based only on the
Load Pressure
and
Unload Pressure
settings. The
Load Pressure
and
Unload Pressure
settings are compared to the air storage signal.
Содержание AIRSMART
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