Water/Brine Reset Ð Three types of chilled water/ brine reset are available, Reset Type 1, Reset Type 2, and Reset Type 3. They can be viewed or modi®ed on the CON- FIG screen (accessed from the EQUIPMENT CONFIGU- RATION table). See Table 2, Example 6.
The LID default screen status message indicates when a reset is active. The WATER/BRINE CONTROL POINT tem- perature on the STATUS01 table indicates the chiller's cur- rent reset temperature.
To con®gure a reset type, input all con®guration informa- tion for that reset type on the CONFIG screen. Then activate the reset type by entering the reset type number in the SELECT/ ENABLE RESET TYPE input line.
RESET TYPE 1 (Requires an optional 8-input module) Ð Reset Type 1 is an automatic chilled water temperature reset based on a 4 to 20 mA input signal. The value for Rest Type 1 is user con®gurable (DEGREES RESET AT 20 mA). It is a temperature that corresponds to a 20 mA signal. (4 mA corresponds to 0° F [0° C]; 20 mA corresponds to the tem- perature entered by the operator.)
This reset type permits up to ±30° F (±16° C) of auto- matic reset to the chilled water/brine temperature set point, based on the input from a 4 to 20 mA signal. The signal is hardwired into the No. 1 eight-input module.
If the 4 to 20 mA signal is externally powered from the 8-input module, the signal is wired to terminals J1-5(+) and J1-6(±). If the signal is powered internally by the 8-input module (for example, when using variable resistance), the signal is wired to J1-7(+) and J1-6(±). The PIC must be con®gured on the SERVICE2 screen to ensure that the appropriate power source is identi®ed. See Table 2, Example 9, 20 mA POWER CONFIGURATION.
RESET TYPE 2 (Requires an optional 8-input module) Ð Reset Type 2 is an automatic chilled water temperature reset based on a remote temperature sensor input.
This reset type permits ±30° F (±16° C) of automatic re- set to the set point based on a temperature sensor wired to the No. 1 eight-input module (see wiring diagrams or cer- ti®ed drawings). The temperature sensor must be wired to terminal J1-19 and J1-20.
Con®gure Reset Type 2 on the CONFIG screen (Table 2, Example 6). Enter the temperature of the remote sensor at the point where no temperature reset will occur (REMOTE TEMP [NO RESET]). Next, enter the temperature at which the full amount of reset will occur (REMOTE TEMP [FULL RESET]). Then, enter the maximum amount of reset re- quired to operate the chiller (DEGREES RESET). Reset Type 2 can now be activated.
RESET TYPE 3 Ð Reset Type 3 is an automatic chilled wa- ter temperature reset based on cooler temperature differ- ence. This reset adds ±30° F (±16° C) based on the tempera- ture difference between entering and leaving chilled water. Reset Type 3 is the only reset available without the need for a No. 1 eight-input module. No wiring is required for Reset Type 3, because it already uses the cooler water sensors.
Con®gure Reset Type 3 on the CONFIG screen (Table 2, Example 6). Enter the chilled water temperature difference (the difference between entering and leaving chilled water) at which no temperature reset occurs (CHW DELTA T [NO RESET]). This chilled water temperature difference is usu- ally the full design load temperature difference. Enter the difference in chilled water temperature at which the full amount of reset occurs (CHW DELTA T [FULL RESET]). Next, enter the amount of reset (DEGREES RESET). Reset Type 3 can now be activated.
Demand Limit Control Option (Requires Optional 8-Input Module) Ð The demand limit may be externally controlled with a 4 to 20 mA signal from an Energy Management System (EMS). The option (20 mA DE- MAND LIMIT OPTION) is enabled or disabled on the CON- FIG screen (Table 2, Example 6). When enabled, the control is set for 100% demand with 4 mA and an operator con®g- ured minimum demand set point at 20 mA (DEMAND LIMIT AT 20 mA) .
The EMS demand reset input is hardwired into the No. 1 8-input module. The signal may be internally powered by the module or externally powered. If the signal is externally powered, the signal is wired to terminals J1-1(+) and J1-2(±). If the signal is internally powered, the signal is wired to terminals J1-3(+) and J1-2(±). When enabled, the control is set for 100% demand with 4 mA and an operator con®g- ured minimum demand set point at 20 mA (DEMAND LIMIT AT 20 mA).
Surge Prevention Algorithm Ð Surge occurs when lift conditions become so high that the gas ¯ow across the impeller reverses. This condition can eventually cause chiller damage. Lift is de®ned as the difference between the pres- sure at the impeller eye and the impeller discharge. The maxi- mum lift that a particular impeller wheel can produce varies with the gas ¯ow across the impeller and the size of the wheel.
The surge prevention algorithm is operator con®gurable and can determine if lift conditions are too high for the com- pressor. If they are, the PIC takes corrective action. The al- gorithm also noti®es the operator, via the LID, that chiller operating conditions are marginal.
The surge prevention algorithm ®rst determines if correc- tive action is necessary. This is done by checking 2 sets of operator con®gured data points: the minimum load points (MIN. LOAD POINTS [T1/P1]) and the maximum load points (FULL LOAD POINTS [T2/P2]). See the SERVICE1 screen or Table 2, Example 8. These points have default set- tings. Information on how to modi®y the default minimum and maximum load points can be found in the Input Service Con®gurations section on page 54.
Figures 18 and 19 graphically display these settings and the algorithm function. The 2 sets of load points (default set- tings) describe a line that the algorithm uses to determine the maximum lift of the compressor. Whenever the actual differential pressure between the cooler and condenser and the temperature difference between the entering and leaving chilled water are above the line on the graph (as de®ned by the minimum and maximum load points) the algorithm goes into a corrective action mode. If the actual values are below the line, the algorithm takes no action.
Corrective action can be taken by making one of 2 choices. If the optional hot gas bypass line is present, and the op- erator selects the hot gas bypass option on the SERVICE1 screen (selects 1 for the SURGE LIMIT/HGBP OPTION), then the hot gas bypass valve can be energized. If the hot gas bypass option is not present, then the SURGE LIMIT/HGBP OPTION is on the default setting (0), and the guide vanes are held. (Also see Table 4, Capacity Overrides.) Both cor- rective actions reduce the lift experienced by the compressor and help to prevent a surge condition.
