PIC II System Functions
NOTE: Words not part of paragraph headings and printed in all capital letters can be viewed on the CVC/ICVC (e.g., LOCAL, CCN, RUNNING, ALARM, etc.). Words printed both in all capital letters and italics can also be viewed on the CVC/ICVC and are parameters (CONTROL MODE, TARGET GUIDE VANE POS, etc.) with associated values (e.g., modes, tempera- tures, pressures, percentages, on, off, enable, disable, etc.). Words printed in all capital letters and in a box represent soft-
keys on the CVC/ICVC (e.g., ENTER and EXIT ). See Table 2 for examples of the type of information that can appear on the CVC/ICVC screens. Figures 14-20 give an overview of CVC/ICVC operations and menus.
CAPACITY CONTROL FIXED SPEED — The PIC II con- trols the chiller capacity by modulating the inlet guide vanes in response to chilled water temperature deviation from the CON- TROL POINT. The CONTROL POINT may be changed by a CCN network device or is determined by the PIC II adding any active chilled water reset to the SET POINT. The PIC II uses the PROPORTIONAL INC (Increase) BAND, PROPOR- TIONAL DEC (Decrease) BAND, and the PROPORTIONAL ECW (Entering Chilled Water) GAIN to determine how fast or slow to respond. CONTROL POINT may be viewed or over- ridden from the MAINSTAT screen.
CAPACITY CONTROL VFD — The PIC II controls the machine capacity by modulating the motor speed and inlet guide vanes in response to chilled water temperature deviation from the CONTROL POINT. The controller will maintain the highest inlet guide vane setting at the lowest speed to maxi- mize efficiency while avoiding surge. The CONTROL POINT may be changed by a CCN network device or is determined by the PIC II adding any active chilled water reset to the to the SET POINT. CONTROL POINT may be viewed or overridden from the MAINSTAT screen. The PIC II uses the PROPOR- TIONAL INC (Increase) BAND, PROP DEC (Decrease) BAND, and the PROPORTIONAL ECW (Entering Chilled Water) GAIN to determine how fast or slow it takes the system to respond. The VFD GAIN allows for additional adjustment of the VFD response. At start-up, the inlet guide vanes (IGV) start in the closed position and the VFD ramps to its minimum speed setting.
The PIC II controller then initiates the Capacity Control al- gorithm to maintain the chilled water temperature at the CON- TROL POINT. During operation when the CONTROL POINT is not met, the controller will establish a GUIDE VANE DELTA which will either affect a percentage change to the GUIDE VANES or the VFD TARGET SPEED. Any change that will be made to the IGV position or the VFD SPEED will depend on whether the GUIDE VANE DELTA is positive or negative, and the status of the Surge Control Algorithm. The Surge Control Algorithm determines if the chiller should operate in Normal Mode or Surge Prevention Mode. The logic for how the IGV’s and VFD SPEED will be affected by the GUIDE VANE DEL- TA and the Surge Control Algorithm can be seen below:
| | NORMAL | | SURGE |
GUIDE VANE | CONTROL | PREVENTION |
DELTA | MODE | | MODE |
| | IGV | VFD | IGV | | VFD |
From +0.2 to +2.0 | 1st | 2nd | 2nd | | 1st |
From –0.2 to –2.0 | 2nd | 1st | 1st | | — |
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 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, MAXIMUM SPEED, VFD GAIN and INCREASE STEP can be viewed and modified 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 II 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 II opens or closes the guide vanes until the temperature is within tolerance. The PIC II 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 SETUP1 screen, which is accessed from the EQUIPMENT SERVICE 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.
DIFFUSER CONTROL — On 19XR FRAME sizes 4 and 5 compressors equipped with a variable discharge diffuser, the PIC II adjusts the diffuser actuator position (DIFFUSER ACTUATOR on the COMPRESS screen) to correspond to the actual guide vane position (ACTUAL GUIDE VANE POS on the COMPRESS screen).
The diffuser control can be enabled or disabled from the SETUP2 screen. See Table 2, Example 19. In addition, the dif- fuser and guide vane load points may be viewed and modified from this screen. These points must be correct for the compres- sor size. The diffuser opening can be incremented from fully open to completely closed. A 0% setting is fully open; a 100% setting is completely closed. To obtain the proper settings for Diffuser Control, contact a Carrier Engineering representative.
PROPORTIONAL BANDS AND GAIN — Proportional band is the rate at which the guide vane position is corrected in pro- portion to how far the chilled water/brine temperature is from the control point. Proportional gain determines how quickly the guide vanes react to how quickly the temperature is moving from the CONTROL POINT. The proportional bands and gain may be viewed or modified from the SETUP2 screen, which is accessed from the EQUIPMENT SERVICE table.
The Proportional Band — There are two response modes, one for temperature response above the control point, the other for the response below the control point.
The temperature response above the control point is called the PROPORTIONAL INC BAND, and it can slow or quicken guide vane response to chilled water/brine temperatures above the DEADBAND. The PROPORTIONAL INC BAND can be adjusted from a setting of 2 to 10; the default setting is 6.5.
The response below the control point is called the PRO- PORTIONAL DEC BAND, and it can slow or quicken the guide vane response to chilled water temperature below the deadband plus the control point. The PROPORTIONAL DEC BAND can be adjusted on the CVC/ICVC from a setting of 2 to 10. The default setting is 6.0.
NOTE: Increasing either of these settings causes the guide vanes to respond more slowly than they would at a lower setting.
