Specifications

43
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. 22)
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. 22 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. 22 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 (Software Version 9 and Ear-
lier) — On all units with Frame 5 compressors and those Frame
4 compressors with the variable (split ring) diffuser option, the
PIC II adjusts the diffuser actuator position (DIFFUSER ACTU-
ATOR on the COMPRESS screen) based on the ACTUAL
GUIDE VANE POSITION. This is done in accordance with a
compressor build-specific “schedule” entered in the SETUP2
screen. The schedule consists of guide vane and diffuser posi-
tions 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. Diffuser actuator output is con-
trolled by a 4 to 20 mA output from CCM terminals J8-3(+) and
J8-4(–). Figure 23 shows the relationship between diffuser-
related parameters for a typical build.
Diffuser control output is enabled whenever the DIFFUSER
OPTION is enabled, whether the machine is running or not. As
shown in Fig. 23, 0% output 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). This coordinated guide vane-diffuser operation may be
tested in the Control Test selection “IGV and SRD Actuator.”
Note that the diffuser actuator should NOT be forced to a great-
er percent than the configured 25% Load Point (maximum)
value as the diffuser will be at a mechanical stop. The diffuser
opening can be incremented from fully open to completely
closed. A 0% setting is fully open; a 100% setting is completely
closed.
Split Ring Diffuser Control
— The schedule of DIFFUSER
LOAD PT versus GUIDE VANE % LOAD PT is printed on a
label on the back of the control panel cover. See Fig. 11.
25 % Load Point
50 %0 %
75 %
50 % Load Point
rotating stall
less likely
surge more likely
75 % Load Point
surge less likely
2 mA
(0%)
4 mA
(0.2%)
100%
Open
100%
Closed
Pinion Shaft Clockwise Rotation
DIFFUSER POSITION
(% CLOSED)
Control Output (Actuator Position)
GUIDE VANE POSITION (% OPEN)
* DIFFUSER FULL SPAN MA.
25 %
100 %
approx.
20 mA
Fig. 23 — Diffuser Control
a19-1861
-1
-0.5
0
0.5
1
1.5
2
2.5
3
46.4
46.6
46.8
SET POINT = 47
47.2
47.4
47.6
47.8
48
48.2
TOTAL ERROR
°F
EXAMPLE 1
ECW(°F)
ECW-10 sec
Error, Gain = 1
Error, Gain = 3
EXAMPLE 2 EXAMPLE 3
X
X
X
X
X
X
X
X
X
X
X
X
X
X
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Fig. 22 — Transient Example of ECW Gain
a19-1957