Consulting-Specifying Engineer - December 2008 - (Page 42)

Parallel chillers Series chillers 44 F 232 kW 218 kW 47 F 44 F 54 F 232 kW 56 F 246 kW 40 F Figure 1 Series chillers can offset the efficiency impact of producing lower chilled water temperature. Source: Trane these chillers are piped in a series configuration, there is an advantage from cascade cooling (see Figure 1). In this case, the full 26 kW increase can be recovered using lower leaving water temperatures. Cascade cooling simply means the first chiller will have a higher leaving water temperature and works with a higher suction temperature combined with the downstream machine. This configuration will produce the design 41 F leaving water temperature; this configuration can deliver first cost savings of 2% to 4% and energy savings typically in the range of 1% to 4%. Series chiller configurations are well suited for variable primary flow systems. The extra pressure drop, and thus greater pumping energy, of placing the two evaporators in series can be offset with variable speed pumps in a variable primary flow pumping system. This configuration allows for significant pump energy savings at partial loads. Pumps Often, the reason for reducing the water temperature is not due to coil performance; temperature might be lowered to reduce chilled water pumping energy consumption by reducing the water flow requirements. In our example, a conventional 800-ton design would require 1,920 gpm (2.4 gpm/ ton). Assume a system pressure drop of 110 ft with pump and motor efficiencies of 80% and 95%, respectively. The energy consumption of the motor would be 52 kW. On this same system, if a variable frequency drive is applied to the pump, and the chillers producing a supply water temperature of 41 F, the required gpm would be reduced from 1,920 to 1,200 gpm (1.5 gpm/ton). The total pressure drop is reduced to 49 ft, which can be calculated using the pump affinity equation, which states that power varies by the cube of flow reduction. The energy requirement would have been reduced by 36 kW, representing a 70% reduction in pump energy consumption. In variable flow applications the designer should use a critical zone reset strategy, which allows the system pressure be driven by the most pressure-demanding valve. Motor efficiency should be analyzed over the range of modulation to optimize efficiency and power factor performance throughout the operating curve to meet projected performance. Cooling towers As a conventional system this 800 tonload would require 2,400 gpm at 10 F temperature difference, or 3 gpm/ton. A typical cooling tower fan for this application would require about 40 BHP. In comparison, the LLH system design would require only 1,600 gpm at 15 F difference or 2 gpm/ton, and use a 30 BHP fan motor. This can result in a smaller tower with a smaller footprint. However, the higher temperature difference will cause the chiller to work at a higher head pressure and consume more energy. System designers need the computer model and a lifecycle cost analysis program to establish the optimum balance between lower tower energy and the increased chiller energy consumption with respect to overall system optimization. Cooling coils This example of a traditional system uses a chilled water coil differential temperature of 10 F. The entering temperature would be 44 F, and the leaving temperature at 54 F. These conditions would produce 504 MBH (41 tons) of cooling and require 101 gpm of water. If the system designer selected a 16 F temperature differential, the same coil, now with 41 F entering water temperature, could produce the same 504 MBH with 63 gpm or 37.5% less water. When the enter water temperature is reduced, the leaving water temperature 42 Consulting-Specifying Engineer • DECEMBER 2008

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Consulting-Specifying Engineer - December 2008 Contents Viewpoint News M/E Roundtable ARC Awards New Construction Gold New Construction Silver New Construction Bronze Renovation Gold Renovation Silver A Green Approach to Air Conditioning Systems Turning Down the Lights Case Study New Products Equipment Lifecycles Advertiser Index Green Space Gas Technology Pure Power Supplement

Consulting-Specifying Engineer - December 2008

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