Carrier AquaEdge 19XRV series Operation, Maintenance And Installation Manual page 45

and below the CONTROL POINT. The CONTROL POINT set-
ting range is 0.5 to 2° F. If temperature control is satisfactory
and the guide vanes are stable, do not change the setting.
When very close temperature control is required, as for
some process applications, the deadband may be reduced.
When constant small load changes occur in a system and
the vanes will not stabilize, increase the size of the deadband.
The deadband must always be smaller than the allowable drift
in leaving water temperature.
PROPORTIONAL BANDS — The proportional bands con-
trol how far the guide vanes will move in response to a specific
change in leaving water temperature. Increasing the propor-
tional band increases the amount that the water temperature
must move away from the control point in order to move the
guide vanes a specific amount. Decreasing the proportional
band allows the vanes to move the same amount with a smaller
change in water temperature.
If the proportional band is too large, the leaving water tem-
perature will increase or decrease slowly enough that the tem-
perature moves away from the control point by an unaccept-
able amount. A proportional band set too low will cause the
leaving temperature to overshoot the control point and cause
the guide vanes to "hunt."
The PIC controls have separate proportional bands for in-
creasing and decreasing capacity. The PROPORTIONAL IN-
CREASE BAND should be set as described above. The PRO-
PORTIONAL DECREASE BAND should be set at a typically
smaller value than the increasing band so that the guide vanes
can close quickly enough on a sudden drop in load to prevent a
low temperature safety trip.
Proportional Entering Chilled Water Gain — When Entering
Chilled Water Control is enabled the controls are resetting the
Leaving Chilled Water (LCW) control point every 10 seconds
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.
48.2
48
47.8
47.6
47.4
47.2
SET POINT = 47
X X
46.8
46.6
46.4
a19-1957
X X X
X X X X X
X X X X X X
X X X
X X X
X
EXAMPLE 1 MPLE 1 EXAMPLE 2
Fig. 25 — Transient Example of ECW Gain
NOTE: Before enabling the ENTERING CHILLED WATER
CONTROL and before tuning the ECW GAIN, the LCW
PROPORTIONAL 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. 25)
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. 25 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 set point 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. 25 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.
X X X
X
X
X
X X X
EXAMPLE 3
45
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