Variable-Speed Centrifugal Chiller Control for Variable Primary Flow (VPF) Applications
Variable speed operation offers the potential of increased centrifugal compressor efficiency at off-design conditions compared to variable geometry compressor control. However, this advantage of variable speed operation is offset by early (=low head) compressor surge. A combination of variable-speed and variable-geometry control is required to take advantage of the variable speed efficiency improvement without sacrificing low-flow high-head capability. Once two flow control methods are possible, optimal control becomes more problematic, where a change in one parameter may be theoretically beneficial for efficiency improvement but detrimental to stable operation. Understanding the physics and inter-relation of these parameters is critical to proper chiller control, Compounding this issue is the fact that building system control is sometimes optimized by variable primary water flow and the fact that most chiller systems are not equipped with water flow measuring devices, which, with the water temperature change, is the direct way of measuring compressor flow. Conventionally, with the know compressor mass flow, condenser to evaporator saturated temperature difference and water temperature rise, the position on the performance map can be determined, and in the case of chiller control, the margin from surge to the current operating point. This paper describes the aerodynamic reasons for the necessity of a multi-input, multi-output compressor control system with model-based feed-forward element if part-load performance is to be improved through the use of variable speed operation. The model-based feed-forward element of the controller determines as a function of required operating condition (pressure ratio and flow rate) which combination of speed and variable geometry will result in maximum compressor efficiency. Since compressor peak efficiency occurs close to the surge an accurate definition of the surge line as well as a precise measurement of the actual compressor operating point are required. Actual compressor operating conditions are normally determined from head and flow measurements. Compressor head can be calculated fairly precise from suction and discharge pressure measurements through instrumentation already available on the machine. Compressor flow rate is obtained indirectly through a heat balance over the evaporator, requiring knowledge of chilled water side flow rate and temperature drop over the cooler. The water flow rate for the traditional primary/secondary chilled water flow systems installations is constant, making the water-side temperature drop in the cooler the indicator of compressor flow. Since chillers are equipped with entering and leaving chilled water temperature sensors, the actual operating point of the compressor can then be determined without the need of additional water-side flow measurements. For VPF systems, the chilled water flow rate changes necessitates an additional flow measurement, which also has proven to be less reliable and accurate in actual field installation. A new control system that circumvents this problem and consequently is not affected by the variable primary flow changes is described in this paper.
As energy cost rises and end users become more sophisticated in there application of cooling on the building system level, variable speed motor and inverter technology is making ever stronger inroads into products that were traditionally operating in fixed speed mode and purchased on first cost considerations only. Equipment life cycle cost reduction has been the driving force behind this trend. With ever increasing energy costs and a focus on low carbon emissions, this trend towards more energy efficient operation but higher initial costs is only more likely to accelerate. With payback reaching 1 to 3 years for these upgrades, the attractiveness of variable speed operation is only greater.
The HVAC industry is experiencing this transition from fixed-speed to variable-speed operation, especially for commercial size equipment. Variable-speed fan/blower operation of commercial air-handling products is currently the standard product offering while every year a larger percentage of chillers have variable-speed compressors. The
International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008