Diffuser Control - Carrier AquaEdge 19XR series Start-Up, Operation And Maintenance Instructions Manual

in order to keep the ENTERING CHILLED WATER TEMP at
the ENTERING CHILLED WATER (ECW) SETPOINT.
The ECW GAIN affects the size of the LCW CONTROL
POINT change in proportion to the difference between the
ECW SETPOINT and ECW TEMPERATURE.
NOTE: Before enabling ENTERING CHILLED WATER CON-
TROL and before tuning the ECW GAIN, the LCW PROPOR-
TIONAL BANDS and LCW DEADBAND should be adjusted
satisfactorily.
Increase the ECW GAIN if the ENT CHILLED WATER
TEMP drifts away from the ECW SETPOINT. Reduce the
ECW GAIN if the ENT CHILLED WATER TEMP swings
above and below the ECW SETPOINT. Because the water must
travel around the entire loop before the controls receive feed-
back on the effect of the LCW CONTROL POINT, the chilled
water loop should be given the opportunity to stabilize before
the gain is adjusted. The following example shows how the En-
tering Chilled Water Control works to move the vanes based on
the rate of change of the ENT CHILLED WATER TEMP as
well as the by the difference between ENT CHILLED WATER
TEMP and ECW SETPOINT.
Effect of Proportional Entering Chilled Water Band (ECW
Gain) (See Fig. 24)
Error = the contribution of the ECW control to the total er-
ror that inputs to the guide vane control. Positive error drives
the vanes open. Negative error drives the vanes closed.
ECW set point = 47 F
ECW = Entering Chilled Water
ECW-10 = Entering Chilled Water 10 seconds previous
Example 1 — The first section of Fig. 24 shows the entering
water dropping with a constant rate. The ECW algorithm is
reducing its effort to open the guide vanes. After the entering
water temperature drops below the setpoint the error drops
below zero and thus is trying to close the vanes.
Example 2 — The second section shows the water temperature
dropping but at a decreasing rate as shown by the reduction in
the difference between ECW and ECW-10. The error value is
leveling at zero but takes a dip because the entering water tem-
perature drops below the set point.
Example 3 — The third section of Fig. 24 shows the entering
chilled water temperature increasing. Again the error starts to
level off or drop as the temperature change over 10 seconds
becomes smaller.
DIFFUSER CONTROL — On all units with Frame 5 com-
pressors and those Frame 4 compressors with the variable (split
ring) diffuser option, the PIC II adjusts the diffuser actuator
48.2
48
47.8
47.6
47.4
47.2
SET POINT = 47
X
46.8
X
46.6
46.4
a19-1957
X X
X X
X
X
X
X
EXAMPLE 1 EXAMPLE 2
Fig. 24 — Transient Example of ECW Gain
position based on the ACTUAL GUIDE VANE POSITION,
then adjusts from that position using Carrier's Variable Diffuser
Optimization (VDO) algorithm. Configurations for VDO are
produced by the Carrier's chiller selection program and are en-
tered into the VDO_SRD screen. The starting point diffuser
schedule consists of guide vane and diffuser positions for three
points (designated as the 25%, 50%, and 75% Load Points). In
order for the schedule to be valid, the guide vane values must
be ascending and the diffuser values must be descending for
the three points. Figure 25 shows the starting point relationship
between diffuser related parameters for a typical build. 0% out-
put corresponds to a full open diffuser. The minimum closed
position (25% Load Point value) will be at less than 100% for
most diffusers (depending upon the model).
Diffuser control output is enabled whenever the DIFFUSER
OPTION is enabled, whether the machine is running or not.
The diffuser position adjusts from the specific relationship to
the guide vane position, and is adjusted based on several fac-
tors: guide vane position, compressor lift, rotating stall and
surge conditions. The diffuser and guide vanes will both move
in Guide Vane Control Test but the relative positions do not
represent the relative position of these actuators during opera-
tion; the diffuser and the guide vane operate independently. Us-
ing the Controls Test, the diffuser can be incremented from ful-
ly open to completely closed. A 0% output is fully open; the
setting is completely closed when the value equals that of the
Diffuser 25% Load Point value. The configurations for VDO
Diffuser Control are found on a label on the back of the control
panel. See Fig. 13. If you do not have the correct settings for
VDO Diffuser Control contact a Carrier Engineering Repre-
sentative.
The diffuser actuator should NOT be configured to a per-
cent that is greater than the factory set Diffuser 25% Load Point
value as the diffuser will be at a mechanical stop for its fully
closed position.
IMPORTANT: Do not alter the factory settings for VDO
Diffuser Control without consulting with Carrier Engineer-
ing. Incorrect settings could cause the machine to repeatedly
shut down on rotating stall alarm or surge.
A diffuser pressure transducer is installed on compressors
with a split ring diffuser. The CCM monitors pressure fluctua-
tions at the inlet to the inner diffuser ring. Excessive pressure
fluctuations may indicate that the impeller is experiencing pres-
sure pulsations known as rotating stall. A Diffuser Position
Fault (Alarm 247) is declared if pressure fluctuations exceed
acceptable limits.
X
X
X
X
EXAMPLE 3
46
3
2.5
2
E C W (°F )
1.5
E C W -10 sec
1
E rror, G ain = 1
0.5
E rror, G ain = 3
0
-0.5
-1