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reduce the lift experienced by the compressor and help prevent a surge condition. Surge Prevention Algorithm with VFD — This is an operator configurable feature that can determine if lift con- ditions are too high for the compressor and then take corrective action. Lift is defined as the difference between the pressure at the impeller eye and at the impeller discharge. The maximum lift a particular impeller wheel can perform varies with the gas flow through the impeller and the diameter of the impeller. A surge condition occurs when the lift becomes so high the gas flow across the impeller reverses. This condition can even- tually cause chiller damage. When enabled, the Surge Preven- tion Algorithm will adjust either the inlet guide vane (IGV) position or compressor speed to maintain the compressor at a safe distance from surge while maintaining machine efficiency. If the surge condition degrades then the algorithm will move aggressively away from surge. This condition can be identified when the SURGE/HGBP ACTIVE ? on the HEAT_EX display screen displays a YES. The surge prevention algorithm first determines if correc- tive action is necessary. The algorithm checks two sets of oper- ator-configured data points, the lower surge point (MIN. LOAD POINT [T1,P1]) and the upper surge point (FULL LOAD POINT [T2,P2]). The surge characteristics vary between dif- ferent chiller configurations and operating conditions. The surge characteristics are factory set based on the original selec- ton with the values displayed inside the control panel of the chiller. Since operating conditions may affect the surge preven- tion algorithm, some field adjustments may be necessary. The surge prevention algorithm function and settings are graphically displayed on Fig. 21 and 22. The two sets of load points on the graph (default settings are shown) describe a line the algorithm uses to determine the maximum lift of the com- pressor for the particular maximum operating speed. When the actual differential pressure between the cooler and condenser and the temperature difference between the entering and leav- ing chilled water are above the line on the graph (as defined by the minimum and full load points), the algorithm operates in Surge Prevention mode. This is determined when the ACTIVE DELTA T is less than SURGE/HGBP DELTA T minus the deadband. When in Surge Prevention mode, with a command to in- crease capacity, the VFD speed will increase until maximum VFD speed is reached. At VFD MAXIMUM SPEED, when Ca- pacity still needs to increase, the IGV’s open. When in Surge Prevention mode, with a command to decrease capacity only the IGVs will close. Surge Protection VFD Units — The PIC II monitors surge, which is detected as a fluctuation in compressor motor amperage. Each time the fluctuation exceeds an operator- specified limit (SURGE DELTA % AMPS), the PIC II registers a surge protection count. If more than 5 surges occur within an operator-specified time (SURGE TIME PERIOD), the PIC II initiates a surge protection shutdown of the chiller. On VFD units, if a surge count is registered and if ACTUAL VFD SPEED is less than VFD MAXIMUM SPEED then motor speed will be increased by the configured VFD increase step. While the SURGE PROTECTION COUNTS are >0, a speed decrease will not be honored. The surge limit can be adjusted from the OPTIONS screen (see Table 2). Scroll down to the SURGE DELTA % AMPS parameter, and use the INCREASE or DECREASE softkey to adjust the percent of surge. The default setting is 10% amps. The surge time period can also be adjusted from the OPTIONS screen. Scroll to the SURGE TIME PERIOD parameter, and use the INCREASE or DECREASE softkey to adjust the amount of time. The default setting is 8 minutes. Access the display screen (COMPRESS) to monitor the surge count (SURGE PROTECTION COUNTS). Surge Protection (Fixed Speed Chiller) — The PIC II monitors surge, which is a fluctuation in compressor motor amperage. Each time the fluctuation exceeds an opera- tor-specified limit (SURGE DELTA % AMPS), the PIC II counts the surge. If more than 5 surges occur within an opera- tor-specified time (SURGE TIME PERIOD), the PIC II ini- tiates a surge protection shutdown of the chiller. The surge limit can be adjusted from the OPTIONS screen. Scroll down to the SURGE DELTA % AMPS parameter, and use the INCREASE or DECREASE softkey to adjust the percent of surge. The default setting is 10% amps. The surge time period can also be adjusted from the OPTIONS screen. Scroll to the SURGE TIME PERIOD parameter, and use the INCREASE or DECREASE softkey to adjust the amount of time. The default setting is 8 minutes. Access the display screen (COMPRESS) to monitor the surge count (SURGE PROTECTION COUNTS). HEAD PRESSURE REFERENCE OUTPUT (See Fig. 23) — The PIC II control outputs a 4 to 20 mA signal for the configurable Delta P (condenser pressure minus evaporator pressure) reference curve shown in Fig. 23. An output is avail- able on the ISM module [Terminal J8 (+), J8 (–) labeled spare]. For chillers with Benshaw Inc. solid-state starters terminal strip labeled J8 (+), J8 (–) located next to the RediStart MICROTM input/output card is provided. The Delta P at 100% (chiller at maximum load condition default at 35 psi), Delta P at 0% (chiller at minimum load condition default at 25 psi) and Mini- mum Output points are configurable in the EQUIPMENT SERVICE-OPTIONS table. When configuring this output en- sure that minimum requirements for oil pressure and proper condenser FLASC orifice performance are maintained. The 4 to 20 mA output can be used as a reference to control a tower bypass valve, tower speed control, or condenser pump speed control. DELTA P AT 100% DELTA P DELTA P AT 0% MINIMUM REFERENCE OUTPUT 0 mA 2 mA 4 mA (0%) 20 mA (100%) 4 T0 20 mA OUTPUT 40 Fig. 23 — Head Pressure Reference OutputPDF Image | 19XR,XRV Hermetic Centrifugal Liquid Chillers 50/60 Hz With PIC II Controls and HFC-134a
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