ASHRAE Journal - August 2008 - (Page 56)

Figure 4 shows a rooftop air-handling unit schematic arranged to allow excess heat available from the data center hot aisle to sensibly reheat the air leaving the WBE at the required room dew point. Figure 5 shows that, while maintaining the required room 55°F (13°C) DP with humidiﬁed outdoor air, this air-handling unit conﬁguration would provide sensible reheat to a 75°F (24°C) cold aisle inlet temperature at a relative humidity of 50%. This condition now meets the Table 2.1, Best Practices, cold aisle inlet requirements for Class I and II data centers. From Figure 5, it is apparent that the WBE may now offer tonhour reductions until ambient conditions exceed the hot aisle enthalpy of 33 Btu/lb (76.8 kJ/kg) (95°F [35°C] DB and 69°F [21°C] WB). Let’s revisit Table 1, Zone 3, where refrigeration load may be reduced and humidiﬁcation eliminated. Now, based on Figure 5, the hours per year in Zone 3 may be greatly expanded in Boston, Chicago, and Atlanta. The annual hour percentage for Zone 3 in Boston increases from 16.5% to 30.5%, in Chicago from 17% to 34% and in Atlanta from 21% to 44%. An additional advantage of the unit conf iguration shown in Figure 4 is the location of the final filter downstream of the supply fan. This position of the filter ensures that both the outdoor air and recirculation air are cleaned before entry into the data center. 90% RH 70% RH 60% RH 50% RH % 60 40% RH % 55 Moisture Grains Per Pound of Dry Air 80% RH En alp th y tu )B (h Pe o rP d un of D r i yA r Sa tu ra ti on m Te p er u at re °F Zone 4 30% RH 50 45 20% RH 55°F Dew Point 40 Zone 4 – 90 hrs/Yr 10% RH 35 55° Zone 1 3,261 hr/yr 10 55°F DB Zone 2 2,530 hr/yr 15 FW et B ulb 62. 5°F We t Bu lb Zone 3 2,865 hr/yr 25 30 20 Enthalpy (h) Btu Per Pound of Dry Air Zone 1 – Has 3,261 hours per year where mechanical cooling and humidiﬁcation energy may be eliminated. Zone 2 – Has 2,530 hours per year above the 55°F supply air setpoint, when an air-side economizer would require refrigeration operation. In this zone the wet-bulb economizer provides all cooling and humidiﬁcation requirements for the data center. Zone 3 – Has 2,865 hours per year where the outdoor enthalpy is lower than the return air enthalpy and refrigeration energy costs may be reduced by the introduction of 100% outdoor air. Zone 4 – Has 90 hours per year where the wet-bulb economizer will position dampers for minimum outdoor air as required for space pressurization. The direct evaporative cooling device will be off. Figure 3: San Francisco wet-bulb economizer zones where free cooling and humidiﬁcation are available based on bin weather data and a 24/7 duty cycle. 1 OA DEC = Direct Evaporative Cooler/Humidiﬁer CWC = Chilled Water Coil BDD = Back Draft Damper CD = Control Damper VAV Supply Fan Relief Air Fan With BDD EA 5 CD DEC CWC Filter CD 2 3 4 5 Return Air Data Center Supply Air Figure 4: A schematic of a roof mounted central station air-handling unit designed to use a portion of the data center return air for reheat, at the required room dew point, to meet the Table 2.1 ASHRAE design guide cold aisle inlet air conditions speciﬁed for Class I and II data centers. Refrigeration Redundancy For Arid Climates Murphy’s Law dictates that refrigeration cooling systems will fail on the hottest day of the year, so Table 2 shows the data center hot aisle return air temperature and relative humidity with 56 ASHRAE Journal and without a WBE backup in ﬁve western U.S. cities. Cold aisle inlet conditions would be ambient dry-bulb temperature and relative humidity, ignoring fan heat, for the refrigeration only design. The adiabatic evaporative cooling/humidiﬁcation ashrae.org August 2008 http://ashrae.org

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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