Section 6  Control Features And Functions; Standard Control Features And Functionality - Ingersoll Rand X8I Mode D'emploi

SECTION 6  CONTROL FEATURES AND FUNCTIONS
STANDARD CONTROL FEATURES AND
FUNCTIONALITY
PRESSURE CONTROL
Pressure control is achieved by maintaining the system pressure
within an acceptable range, or pressure band, which is defined and
programmed by the user. Pressure will rise in the band when system
demand is less than the loaded compressor's output. Pressure will
fall in the band when system demand is greater than the loaded
compressor's output.
Simply stated, pressure control is achieved by unloading and
loading compressors to closely match compressor output with
system demand within a specified pressure band defined by PL and
PH. See Figure 1.
Variable speed compressors also operate within the pressure band
and actively match compressor output with system demand by
speeding up and slowing down around a target pressure defined by
the exact midpoint of the pressure band defined by PT. See Figure 2.
a
b
Figure 1 — Typical System Pressure vs. Time
As pressure rises to point "a", the compressor will unload
based on the sequencing algorithm. System pressure is
then allowed to decrease due to the drop in supply until
point "b" is reached. Once point "b" is reached, the X8I will
load the next compressor in the sequence to match the
air demand. This cycle will repeat as long as the X8I is able
to keep the system air pressure between PH and PL.
Figure 2 — Typical VSD Pressure Control vs. Time
The variable speed compressors in the system will run
on their target pressure and smooth out the variations in
system pressure. This assumes that system demand does
not vary more than the capacity of the variable speed
compressor.
A variable speed compressor will be included in the
load/unload sequence and be controlled exactly as a
fixed speed machine with the exception of speed control
to maintain target pressure.
ANTI-CYCLING CONTROL
The most efficient way to utilize most air compressors is
either fully loaded or off, with the exception of variable
speed compressors which can operate efficiently at
reduced loading. Compressor cycling (start-load-unload-
stop, etc.) is essential to maintain pressure control.
Excessive cycling, however, can result in poor compressor
efficiency as well as increased maintenance.
Anti-cycling control is incorporated to help ensure
that only the compressors that are actually required
are started and operating while all others are kept off.
Anti-cycling control includes a pressure tolerance range
or band, defined by the user, which is outside of the
primary pressure band. Inside the tolerance band, an
active control algorithm continually analyzes pressure
dynamics to determine the last possible second to add or
cycle another compressor into the system. This control is
further enhanced by the ability to fine tune the tolerance
band settings and algorithm processing time (Damping).
TOLERANCE
Tolerance is a user adjustable setting that determines
how far above the PH setpoint and below the PL setpoint
system pressure will be allowed to stray. Tolerance
keeps the X8I from overcompensating in the event of
a temporary significant increase or decrease in system
demand.
Figure 3 — Tolerance in Relation to PH and PL
Tolerance (TO) is expressed as a pressure defining the
width of the band above PH and below PL in which
energy efficient control will be in effect.
When system pressure is in the tolerance band, the
X8I will continuously calculate the moment at which
compressors will be loaded or unloaded based on the rate
of change of system pressure. When the system pressure
strays outside of the tolerance band, the X8I will abandon
energy efficiency and begin to protect the system air
pressure by loading or unloading the compressors.
Loading will be delay controlled.
13