Consulting-Specifying Engineer - January 2008 - (Page 48) Building envelope leakage Higher than expected heat and moisture migration across the building envelope will impact the internal temperature and RH by increasing the enthalpy of the indoor air. The primary driving mechanism is outside air infiltration, while moisture migration driven by vapor diffusion (from high to low vapor pressure areas) accounts for only small a fraction. Depending on the climate, building leakage can increase the energy use of the facility and increase or reduce too much the indoor moisture content of the air. National Institute of Standards and Technology, CIBSE, and ASHRAE studies investigating leakage in building envelope components show that building leakage is underestimated by a significant amount. Also, there is not a consistent standard on which to base building air leakage. In the absence of a well-controlled building pressurization system, air will pass into the interior of the building through cracks and other unsealed openings in the building envelope. The primary areas of building leakage tend to be at gaps around windows and doors, joints in building façade elements, wall/roof junctures, and other areas where it is difficult to develop an air-tight seal. Knowing that the actual air amounts that enter (or exit in some cases) the building are mainly related to pressure differences driven by wind, the following items need to be taken into consideration: •There is a high correlation between leakage rates and fluctuations in indoor RH—the greater the leakage rates, the greater the fluctuations. •There is a high correlation between leakage rates and indoor RH in the winter months—the greater the leakage rates, the lower the indoor RH. •There is low correlation between leakage rates and indoor RH in the summer months—the indoor RH levels remain relatively unchanged even at greater leakage rates. •There is a high correlation between building leakage rates and air change rate—the greater the leakage rates, the greater the number of air changes due to infiltration. Expanding the range So what if the range of humidity was expanded to allow for higher and lower moisture levels in the data center? Would this capture even more savings? What is NEBS? Understanding data center requirements. Network Equipment-Building System, or NEBS, is a set of technical requirements and objectives that originally were developed by AT&T’s Bell Labs under the title of Technical Publications with the purpose of making network switches robust and reliable. After the 1984 divestiture of AT&T, NEBS ownership passed on to Bellcore (currently Telcordia), the research arm of Regional Bell Operating Companies (RBOC), for maintenance and upgrades. Since then Telcordia has renamed GR-63-CORE Issue 2 • Fire resistance • Temperature and humidity (operating, storage, and transportation) • Shock, vibration, and earthquake (operating, storage, and transportation) • Airborne contaminates (corrosive and hygroscopic dust) • Acoustic • Altitude • Lighting • Floor loading, physical and spatial requirements the publications Generic Requirements (GR) and published many new ones. The NEBS requirements and test methods are specified in the Telcordia GR-63-CORE, GR-1089-CORE, GR-3109, and GR-3208 documents. Table 2 lists the general topics of each document. Compliance to NEBS is demonstrated by testing products in a Nationally Recognized Testing Laboratory (NRTL) sanctioned by RBOCs or in the manufacturer’s test facility, supervised and witnessed by an NRTL. Depending on the application and the customer, compliance can be demonstrated on a tiered system defined in the Telcordia SR-3580 document as NEBS levels 1, 2, and 3, where level 3 is for Central Office grade equipment. NEBS level 1 is mandated by the Federal Communication Commission for co-location of Competitive Local Exchange Carrier equipment in the Incumbent Local Exchange Carrier spaces. GR-3208 • Equipment cooling classifications • Room cooling classification • Environmental criteria • Heat release targets • Equipment specifications GR-1089-CORE Issue 3 • Electrical safety • EMI emissions • EMI immunity • Lighting immunity • ESD (operation and installation) • Bonding and grounding • AC power fault GR-3109 • Fire resistance • Temperature and humidity (operating, storage, and transportation) • Shock, vibration, and earthquake (operating, storage, and transportation) • Airborne contaminants (corrosive and hygroscopic dust) • Salt fog exposure • Acoustic • Altitude • Lighting • Applicable GR-1089 requirements Table 2: Source: “Thermal Design and NEBS Compliance,” Majid Safavi, Lucent Technologies. Electronics Cooling, February 2006. 48 Consulting-Specifying Engineer • JANUARY 2008
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