Ashrae Journal - October 2008 - (Page 36) Figure 1 (left): Perimeter computer room air handlers (CRAH). Figure 2 (right): Rooftop units. Q Demand λJ βJ = Architecture Specific Total Chiller Load (kWh) = Power Ratio Chiller, Consumed/Load (kWh/ kWh) = Bin Data Condenser Water Temperature (hr) Equations Used applies to fans or pumps (2) (3) (4) General Considerations for Comparison The architectures considered here include CAHUs, perimeter floor mounted CRAHs, and in-row air handlers (IRAHs). This study focuses only on sensible cooling requirements for the IT equipment and excludes considerations regarding space humidity control (dehumidification and or humidification). The reader should be cautioned that architectures using CAHU and CRAH equipment have lower sensible heat ratios than the IRAH, and likely require additional energy consumption to maintain space humidity requirements. Ultimately, the primary metric driver becomes the characteristic efficiencies of the three air delivery and distribution methods used by the specific architectures. The theoretical data center used for this evaluation has an actual heat release by IT equipment and lighting set at 0.75 MW. The chilled water cooling source for the IT loads, lighting, and air-handlers is supported by a vapor compression chiller, using screw compressor technology outfitted with an inverter drive. This chiller supplies a constant 45°F (7°C) chilled water supply for all three architectures considered. The heat of rejection of said chiller is removed by cooling tower water. 36 ASHRAE Journal Figure 3: Row-based cooling. Row-based cooling architecture, as depicted, uses a free air discharge without ducting or any containment of hot or cold airstreams. The water temperature from the cooling tower can track ambient environmental wet-bulb temperature down to a minimum tower leaving fluid temperature of 55°F (13°C). The leaving tower temperature for given wet-bulb bin temperatures is determined from cooling tower performance curve for 100% design flow with 10°F (–12°C) temperature range line.1 The combination of inverter compressor drive and lower condenser water temperatures allows for significant chiller efficiency gains during periods of low chiller lift. For the purpose of this comparison, the bin wet-bulb hours for St. Louis shall be considered. The resulting condenser water leaving fluid temperature bin hours are depicted in Table 1. While many condenser water systems may vary the condenser water flow as a function of chiller load, this study shall maintain full design condenser water flow to further enhance performance/efficiency of the chiller. The duty cycle of the ashrae.org October 2008 http://www.ashrae.org
Table of Contents Feed for the Digital Edition of Ashrae Journal - October 2008 Ashrae Journal - October 2008
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.