Engineered Systems - September 2008 - (Page 76)

Data centers are expensive propositions, so incorporating some free cooling is financially attractive as well as green. However, reaching the real savings involves more than just incorporating economizers. You need the controls to execute an overall sequence of operation that handles the transitions in chilled water plant performance, too. BY WILLIAM GEMBINSKI, P.E., KRIS JOHNSON, AND MARK J. LEPORE, P.E. few circumstances are fueling the move to incorporate sustainable designs into data center infrastructure systems. These include the high energy demands of data centers, rising energy costs, and changes in many building codes to require some form of economizer as part of the mechanical system design. As a result, outside air economizers (free cooling) are being considered as one approach to meeting more sustainable design criteria. Outside air economizers offer a significant potential for reducing hours of mechanical cooling operation. The use of outside air economizers introduces concerns about reliability as systems transition between free cooling and chilled water plant operation (DX cooling is not addressed here). In order to address these issues, we must understand when the economizer is in operation as well as the load on the chilled water plant during economizer operation. ECONOMIZER OPERATION Outdoor air economizers utilize the hours in which the enthalpy of the outdoor air is less than that of the return air. During these periods, the use of outdoor air will reduce the load at the AHU when compared with the load associated with return air. The economizer presents three options for operation: • No economizer — Outdoor air enthalpy greater than return air enthalpy: During this period no outside air is introduced to the AHU. The AHU and chiller plant see the full load of the data center. 76 En gi neer ed Sy stem s September 2008 A Full mechanical cooling is required. • Partial economizer — Outdoor air enthalpy lower than return air and higher than supply air setpoint: During this period, the load on the AHU is reduced but some mechanical cooling is still required. • Full economizer — Outdoor air enthalpy lower than return air and supply air setpoint: During this period the supply air setpoint is met by either 100% outside air or by mixing outside air with return air. No mechanical cooling is required during this period. Successful implementation of design aspects such as hot aisle/cold aisle, expanded thermal envelope criteria, and elevated supply air temperatures have shown significant increases in the hours of operation of the outside air economizer in the data center environment. The elevated supply air temperature has led the designer to consider economizer in climate zones where it is not an energy code requirement. New thermal guidelines may result in supply air temperatures in the range of 68°F (20°C) db. Chilled water temperatures will elevate with the increase in supply air temperatures. The resulting available hours of economizer operation in southern U.S. climates can be greater than 50% and up to 100% in northern climates. It is important to consider any minimum ventilation requirements for support spaces that may require a lower chilled water temperature for dehumidification. RELIABILITY When specifying control systems for the mission critical industry, there are many factors that need to be accommodated. First and foremost

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Engineered Systems - September 2008 Contents Editor's Note HVAC Challenge Back2Basics Case In Point Commissioning Building Automation HVACR Designer Tips Application Checklist LEED®, Or Get (It) Out Of The Way? BACnet Supplement Race To Market Steam Humidification In Pharmaceutical Facilities Outside Air, Inside Data Centers Forecast: Increasingly Cloudy And Under Control Products Computers & Software Literature Glossary Classifieds Advertiser Index Tomorrow's Environment

Engineered Systems - September 2008

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