【正文】 d accordingly. It is particularly important to understand initial and future loads in detail. Otherwise, the stated initial and future loads could HAVC pounded safety factors or, in the worst casts, guesses. Data Equipment Power Trends and Cooling 4pplications (ASHRAE 2005a) identifies the following topics to consider when predicting future load:.Existing applications39。 floor spaceFig. 3 Projected Power Trends of Data Equipment (ASHRAE 2005a)Performance growth of technology based on footprintProcessing capability pared to storage capabilityChange in applications over timeAsset turnover Once initial and future loads are understood, as well as the part and lowload conditions, equipment can be selected. This includes gaining consensus from the project stakeholders regarding acceptable amount of disruption that can occur in an operating facility for upgrades such as cooling capacity or distribution.Ventilation and Infiltration For load calculation protocols relating to ventilation and infiltration, refer to Chapter 16 of the 2009 ASHRAE HandbookFundamental. Data facilities39。 outdoor air requirements may be lower than other facilities because of the light human occupancy load. In many casts, it is advantageous to precondition this air, with the space undo positive pressure, to allow for 100% sensible cooling in the space. If this approach is adopted, however, preconditioning system failure must also be addressed to avoid the potential for widespread condensation in the space.Electrical Equipment In some casts, power distribution units (PDUs) are located in the data equipment room as the final means of transforming voltage to a usable rating and distributing power to the data equipment. The heat dissipation from the transformers in the PDUs should be accounted for by referencing the manufacturer39。s equipment specifications.Lights Highefficiency lighting should be encouraged, as well as lighting controls, to minimize lighting heat gain. Additionally, depending on the means of fire suppression and ceiling type, unvented light fixtures should be considered.People Occupancy loads should be considered as light work. People often prise the only internal latent load in a data facility, which may be a factor in selecting cooling coils, especially if outdoor air is supplied through a dedicated outdoor air system.Building Envelope Heat gains through the building envelope depend on the buildings39。 location and construction type. More detailed design information on envelope cooling loads can be obtained from Chapter 18 of the 2009 ASHRAE HandbookFundamentals.Heating and Reheat The need for heat in electronic equipmentloaded portions of data facilities is typically minimal, because of the high internal heat gains in the spaces. Still, initial or first day loads in many data facilities can be low because of low equipment occupancy, so sufficient heating capacity to offset the outdoor air and envelope losses should be included in the design. Many puter room airconditioning (CRAG) units include reheat coils for humidity control. Reheat use for humidity control must be carefully monitored and controlled, because simultaneous heating and cooling wastes energy.Humidification Humidification and/or dehumidification is needed in most environments to meet both the remended and allowable humidity ranges specified for Class 1 and Class 2 data centers (see Table 1).In most casts, the predominant moisture load is outdoor air, but all potential loads should be considered. Where humidification is not provided, personal grounding is typically utilized to minimize electrostatic discharge (ESD) failures. Vapor retardant analysts should also be performed where humiditycontrolled spaces contain outside walls or ceilings. Refer to Chapter 27 of the 2009 ASHRAE HandbookFundamentals for additional design information.HighDensity Loads Heat density of some types of data equipment is increasing dramatically. Standalone server heat loads (or rack loads) can HAVC heat loads exceeding 30 kW. These increased heat densities require a design engineer to keep abreast of latest design techniques to ensure adequate cooling, and to pay close attention to the loads of installed electronic equipment and future equipment deployments. Ensure that local highdensity loads are provided with adequate local cooling, even when the overall heat density of the general space is below the highdensity threshold. Rack inlet conditions should be checked and verified as adequate to meet the manufacturer39。s requirements. Refer to Thermal Guidelines加r Data Processing Environments (ASHRAE 2008) for additional information and guidance. Increases in heat density HAVC made it more difficult to aircool puters, leading to increased interest in efficient liquidcooling techniques. Liquid cooling media include water, refrigerants, highdielectric fluorocarbons, or twophase fluids such as dielectrics. Some puter manufacturers HAVC already taken this approach, and there are also products available that use liquid cooling at the cabinet level. The reader is encouraged to keep abreast of researeh and development in this area. ASHRAE (2006) also published a book on liquid cooling, Liquid Cooling Guidelines for Data Equipment Centers. New data facilities and those slated for major renovation should consider adding appropriate infrastructure (piping taps, feeders, etc.) for future use and load increases. Retrofitting these backbones is typically much more expensive, disruptive, and risky. Note that local highdensity areas can HAVC power densities significantly higher than the average for the center. Ideally, highdensity puting equipment should be identified during design so appropriate cooling can be provided.HVAC SYSTEMS AND COMPONENTS It may be desirable for H VAC systems serving data facilities to be independent of other systems in the building, althou