Pic Iii System Functions; Capacity Control; Ecw Control Option; Control Point Deadband - Carrier AquaEdge 19XRV series Operation, Maintenance And Installation Manual

PIC III System Functions

IMPORTANT: Words not part of paragraph headings and
printed in all capital letters can be viewed on the ICVC (e.g.,
LOCAL, CCN, RUNNING, ALARM, etc.). Words printed
both in all capital letters and italics can also be viewed on the
ICVC and are parameters (CONTROL MODE, TARGET
GUIDE VANE POS, etc.) with associated values (e.g.,
modes, temperatures, pressures, percentages, on, off, enable,
disable, etc.). Words printed in all capital letters and in a box
represent softkeys on the ICVC (e.g.,
See Table 4 for examples of the type of information that can
appear on the ICVC screens. Figures 18-24 give an overview
of ICVC operations and menus.
CAPACITY CONTROL — Generally the chiller adjusts ca-
pacity in response to deviation of leaving or entering chilled
water temperature from CONTROL POINT. CONTROL POINT
is based on the configured SETPOINT (in the SETPOINT
screen: LCW SETPOINT or ECW SETPOINT or ICE BUILD
SETPOINT), and CONTROL POINT is equal to this SET-
POINT plus any active chilled water reset value. A reset value
may originate from any of the three chilled water/brine reset op-
tions configured in the ICVC Service/Equipment Service/
TEMP_CTL screen (see page 53) or from a CCN device. The
default reset value is 0° F. CONTROL POINT may be viewed or
manually overridden from the MAINSTAT screen.
Minor adjustments to the rate of capacity adjustment can be
made by changing PROPORTIONAL INC (Increase) BAND,
PROPORTIONAL DEC (Decrease) BAND, and PROPOR-
TIONAL ECW (Entering Chilled Water) GAIN in the Service/
Equipment Service/SETUP2 screen. Increasing the PROPOR-
TIONAL INC BAND or PROPORTIONAL DEC BAND, or de-
creasing PROPORTIONAL ECW GAIN will reduce the ampli-
tude of the capacity control response (within limits). See also
Proportional Bands on page 45.
Factors and variables used in the capacity control determi-
nation are displayed in the Service/Control Algorithm Status/
Capacity screen and in the Status/COMPR screen. Viewing this
data will aid in troubleshooting and understanding current
operation.
Variable Speed (VFD) Application — The PIC III controls
the machine capacity by modulating both motor speed and
inlet guide vanes in response to chilled water temperature devia-
tion from the CONTROL POINT (see above). During operation,
when the CONTROL POINT is not met within 1/3 of the width
of the CHILLED WATER DEADBAND, the controller will cal-
culate a GUIDE VANE DELTA which will effect a percentage
change to either the guide vane position or TARGET VFD
SPEED. Factors considered in the capacity control algorithm in-
clude: (1) the sign and magnitude of GUIDE VANE DELTA
(based on deviation from CONTROL POINT adjusted for the
error trends and CHILLED WATER DEADBAND), (2) ACTU-
AL GUIDE VANE POS (Position), (3) ACTUAL VFD SPEED,
and (4) surge prevention mode. Generally the controller will
maintain the highest inlet guide vane setting at the lowest speed
to maximize efficiency while avoiding surge.
First, the calculation of GUIDE VANE DELTA is performed.
If GUIDE VANE DELTA is positive, the response will be an IGV
or VFD position increase (within limits). If GUIDE VANE DEL-
TA is negative, the response will be an IGV or VFD position de-
crease (within limits). Next, the surge prevention mode is deter-
mined based on location of the present operating point on the
CHILLED WATER DELTA T/ACTIVE DELTA P map relative
the configured surge prevention line. This mode will either be
Normal or Surge Prevention. The following table indicates
which output is modulated first. When the first output reaches its
and ).
ENTER EXIT
44
limit (ACTUAL GUIDE VANE position reaches maximum),
the second output is modulated. See Table 5.
Table 5 — Guide Vane Delta Modes
NORMAL
CONTROL
GUIDE VANE
MODE
DELTA
IGV
From +0.2 to Increase
+2.0 1st
Decrease
From –0.2 to when VFD
–2.0 speed =
min
Normal Control mode occurs when ACTIVE DELTA T
> SURGE/HGBP DELTA T.
Surge Prevention Mode occurs when ACTIVE DELTA T
 SURGE/HGBP DELTA T.
The VFD GAIN parameter allows for additional adjustment
of the VFD response. Increasing VFD GAIN will increase the
rate of speed change.
Generally for the case of line voltage equaling motor volt-
age (460 volts), VFD output (motor) current is a few percent
higher than line current at full speed (60 Hz). As drive speeds
decrease from maximum, drive output voltage decreases
linearly with output frequency, and motor current continues to
increase relative to line current.
The TARGET VFD SPEED, ACTUAL VFD SPEED and the
VFD GAIN can be viewed and modified in the CAPACITY
display screen. The TARGET VFD SPEED can be manually
overridden by the operator from the COMPRESS screen. The
VFD MINIMUM SPEED, VFD MAXIMUM SPEED, VFD
GAIN and VFD INCREASE STEP can be selected and modi-
fied in the SETUP2 display screen. TARGET and ACTUAL
VFD SPEED can be viewed in the COMPRESS screen.
ECW CONTROL OPTION — If this option is enabled, the
PIC III uses the ENTERING CHILLED WATER temperature to
modulate the vanes instead of the LEAVING CHILLED
WATER temperature. The ECW CONTROL OPTION may be
viewed on the TEMP_CTL screen, which is accessed from the
EQUIPMENT SERVICE screen.
CONTROL POINT DEADBAND — This is the tolerance
range on the chilled water/brine temperature control point. If
the water temperature goes outside the CHILLED WATER
DEADBAND, the PIC III opens or closes the guide vanes until
the temperature is within tolerance. The PIC III may be config-
ured with a 0.5 to 2 F (0.3 to 1.1 C) deadband. CHILLED
WATER DEADBAND may be viewed or modified on the SET-
UP1 screen, which is accessed from the EQUIPMENT SER-
VICE table.
For example, a 1 F (0.6 C) deadband setting controls the
water temperature within ±0.5 F (0.3 C) of the control point.
This may cause frequent guide vane movement if the chilled
water load fluctuates frequently. A value of 1 F (0.6 C) is the
default setting.
A deadband is a span of measurement in which a controller
takes no action. In the PIC controls it is a temperature range
centered on the CONTROL POINT. If the LEAVING
CHILLED WATER TEMP falls within the CHILLED WATER
DEADBAND, the guide vanes will not move.
The purpose of the deadband is to prevent slight fluctua-
tions in ENTERING CHILLED WATER TEMPERATURE
from keeping the guide vanes in constant movement.
Adjusting The Deadband — The default setting of the
CHILLED WATER DEADBAND, is 1° F, which is 0.5° F above
SURGE
PREVENTION
MODE
VFD IGV VFD
Increase
only if VFD
speed =
Increase
max and if Increase
when IGV
hot gas 1st
= max
bypass is
present
and open
Decrease
Decrease —
1st