ASHRAE Journal - August 2008 - (Page 53)

had annual average concentrations that still meet the ASHRAE When the DEC water recirculation pump is activated, the requirements. By increasing the ﬁltration effectiveness from mixing box dampers blend outdoor air with data center return ASHRAE 40% to 85%, particle concentrations in data centers air to achieve a wet-bulb condition which, after the addition of using air-side economizers may be reduced to levels associated fan heat, results in the setpoint supply air temperature and dew with the minimum outdoor air design. point required to cool the electronics. Because of California’s mild climate, the humidity levels Figure 2 shows a typical winter outdoor air condition of 40°F in data centers served by air-side economizers were within (4°C) dry bulb (DB) at 60% relative humidity (RH) mixing with ASHRAE recommended levels. During cold ambient tem- a room return at 75°F (24°C) DB and 50% RH. To simplify this peratures, if the mixed air delivery temperature to the cold illustration, the saturation efﬁciency of the rigid media pad is aisle results in a lower than 40% relative humidity entering assumed to be 100%. Therefore, the supply condition at 55°F the electronics, humidiﬁcation may be required to achieve the (13°C) DB would also yield a 55°F (13°C) WB and 55°F (13°C) desired room dew point. dew point (DP) supply condition to the space. Refrigeration is One data center using air-side economizers was found to not required. Since data centers add heat but no moisture to the have a 30% mechanical cooling power reduction when the space, the room load line is shown to be horizontal. economizer was active. Annual savings at this center Direct Evaporative Cooling Final Filter Section, Rigid Media Type With were estimated to be within Refrigeration VAV Plenum Fan 100% Efﬁciency and With Cooling Coil the range of 60 MWh to 80 Water Recirculation Pump Pre-Filter MWh per year. VAV Relief Air Fan With such compelling evidence that air-side economizMixing Box ers save energy in data center Dampers cooling systems, why is the Roof design community reluctant to Outdoor Air Sump specify this design solution? With air economizers, peoReturn Air 75°F at 55°F Dew Point Supply Air 55°F DB/55°F WB Relief Air ple perceive that the opera55°F Dew Point tion of the data center hardware will be compromised by Figure 1: A roof mounted air-handling unit with a wet-bulb economizer blends outdoor air with return the introduction of increased air to produce the required supply air dry-bulb temperature to the space. For simplicity, the ﬁgure shows levels of pollutants, both performance based on a 100% saturation efﬁciency for the adiabatic device. Fan heat is not included. particulate and gaseous, and Space positive pressure is maintained by the variable air volume relief air fan. by reduced humidity levels in By blending 31% outdoor air with 69% return air (Figure 2) cold weather. A wet-bulb economizer (WBE) may be used to provide both stable room humidity (dew point) control and we are able to deliver 64°F (18°C) DB and 55°F (13°C) WB to reduced room particulate and gaseous concentrations. The lat- the wetted pad which then produces the setpoint 55°F (13°C) ter is true, since the direct evaporative cooling device scrubs DB, 55°F (13°C) DP required for the space sensible cooling the air passing through the wetted pad. load. Note that different room supply air setpoint conditions may be generated by this system simply by changing the proportions The Wet-Bulb Economizer of outdoor air and return air delivered to the mixing box (Figure Figure 1 shows a rooftop central station air-handling unit 1). Current design criteria for cooling Class I and II data centers layout, which uses a wetted rigid media direct evaporative cooler allows dry bulb delivery temperatures as high as 68°F to 77°F (DEC) section to extend data center economizer cooling hours (20°C to 25°C), which would greatly expand the free cooling and provide humidity (dew-point) control. Unlike air-side and hours of this design. Dew-point (absolute humidity) maximums water-side economizers, the WBE uses the heat generated by of 63°F (17°C) for Class I and 70°F (21°C) for Class II data the electronics inside the space to evaporate water off a wetted centers would set the upper limit for the delivery condition off rigid media pad during cold ambient conditions. the adiabatic cooler/humidiﬁer system. The 12 in. (305 mm) deep rigid media pad should be selected An examination of Figure 2 shows that an air side economizer, at 400 ft/min (2.03 m/s) face velocity which yields a 90% on the same winter day, would also produce the required 55°F saturation efﬁciency (sea level) with only a 0.l4 in. w.g. (35 Pa) (13°C) DB room setpoint, but at a 40°F (4°C) DP. To achieve the static pressure loss. The sump water recirculation pump would 55°F (13°C) DP absolute humidity condition in the data center, be sized to circulate 1.5 gpm/ft2 (1.0 L/[s·m2]) of horizontal 26 grains of moisture per pound of air (3.72 g/kg) would be media pad area. required to be added by a separate humidiﬁcation system. Heat August 2008 ASHRAE Journal 53

Table of Contents Feed for the Digital Edition of ASHRAE Journal - August 2008

ASHRAE Journal - August 2008 Contents Commentary Industry News Letters Meetings and Shows Maintain to Sustain—Delivering ASHRAE’s Sustainability Promise Ultraviolet Germicidal Irradiation: Current Best Practices Improving Humidity Control With Energy Recovery Ventilation Single- or Two-Stage Compression Data Center Cooling: Using Wet-Bulb Economizers Building Sciences InfoCenter Practical Pointers Products Emerging Technologies Washington Report People Special Products Classified Advertising Advertising Index

ASHRAE Journal - August 2008

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