ASHRAE Journal - August 2008 - 54

recovery and humidiﬁcation 90% RH 70% RH 50% RH R provided by the WBE system 60 80% RH 60% RH 40% RH % is most signiﬁcant in colder climates such as Boston, ChiMixed Air 64°F/55°F With 55 31.4% Outdoor Air cago, Atlanta and Denver ir yA r (Table 1). In cold climates, an D of 30% RH 50 °F d air blender may be needed in Space Supply Air re un tu Po 55°F/55°F ra the mixing box section (Figr pe Pe m 40°F/35°F Typical tu ure 1) to ensure proper winter Te )B n Winter Outdoor h 45 tio y( mixing of the warm return air ra Temperature 20% RH alp tu th Sa with frigid outdoor air. Room 75°F at 50% RH En 55°F Dew Point Figure 3 uses San Francis40 co, bin weather data to point out additional beneﬁts of the 10% RH 35 55° WBE in more arid climates. FW et B Zone 1 is the area where the ulb WBE uses heat recovery for 30 10 15 20 25 free humidif ication while keeping refrigeration off. 75°F DB 55°F DB 64°F DB Enthalpy (h) Btu Per Pound of Dry Air Zone 2 in Figure 3 is the ambient conditions in San Figure 2: The wet-bulb economizer process is illustrated at a typical 40°F ambient outdoor winter Francisco when an air side condition. Both room dew-point (humidity) and setpoint supply air temperature are generated with economizer, in arid climates, the heat recovered from the data center return air by the rigid media cooler/humidiﬁer. Fan heat is introduces 100% outdoor air not included. with a requirement for both refrigeration and humidiﬁcation. During Zone 1 Zone 2 Zone 3 Zone 4 these hours, a WBE provides both free Hours/Year Hours/Year Hours/Year Hours/Year Location % % % % cooling and humidiﬁcation and keeps refrigeration off. Boston 59 8.5 16.5 16 Zone 3 contains the hours per year where the WBE controls the room delivChicago 56 7 17 20 ery DP (humidity) with reduced refrig34 9 21 Atlanta 36 eration by introducing 100% outdoor air to the adiabatic device. Refrigeration is 59 30 11 Denver 0 required to develop the setpoint supply 35 22.5 38.5 Sunnyvale, Calif. 4 air temperature. Location of the refrigeration coil last in the direction of airﬂow Table 1: Percentage of annual hours that reside in each zone of Figure 3 for ﬁve North American cities based on bin weather data and a 24/7 duty cycle. provides three beneﬁts: Cold condensate from the coil drains into the DEC sump reducing makeup water requireWith a better understanding of how the WBE works, let’s ments and recovering some of the latent cooling effect. examine its application in meeting current mission critical Leaving air off the coil is effectively saturated so that facility best practices design requirements. room DP temperature (absolute humidity) is the same as the DB temperature. Best Practices for Datacom Facilities In arid climates where ambient conditions are often below Now that the data center room dew point has been established the room DP, refrigeration requirements may be reduced with the WBE, the abundance of heat generated by the electronby 15% to 17% compared to a system where the cooling ics may now be used to add sensible reheat to our supply air coil is located upstream of the rigid media DEC. system. The recently published ASHRAE design guide, Best In Zone 4 of Figure 3, outdoor air dampers close to their Practices for Datacom Facility Energy Efﬁciency, provides minimum position required to maintain data center positive Table 2.1 on Page 19, which lists recommended cold aisle inlet pressure control. The variable air volume (VAV) relief fan dry-bulb temperatures and relative humidity ranges. Class I and shown in Figure 1 is off during the hours shown in Zone 4. II inlet conditions from the cold aisle into the equipment may Refrigeration is required, but the DEC recirculation pump is range from 68°F to 77°F (20°C to 25°C) DB and between 40% off. The static pressure loss and dirt loading of the rigid media to 55% RH as recommended in ASHRAE’s Thermal Guidelines are reduced in this mode. for Data Processing Equipment. 54 ASHRAE Journal ashrae.org August 2008 Grains Moisture Per Pound of Dry Air

ASHRAE Journal - August 2008

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