Consulting-Specifying Engineer - December 2008 - (Page 43)

does not go down, it goes up, typically 0.5 to 1 F for every 1 F reduction in entering water temperature. Actual performance depends on the type and circuitry of the coil. It is also important to note that the MBH delivered in this example remained the same, leaving the supply air temperature unchanged. The same MBH can be delivered with substantially reduced water flows. If the water flow was held constant and the entering water temperature is reduced, additional capacity can be obtained. These alternatives can provide significant opportunities to either reduce pumping BHP or address cooling capacity problems. Low air temperature applications Typically, at least half of the HVAC system’s energy consumption is from the airside equipment of the system. This is due in part to the longer run time of fans and accessories in many chiller applications. Designers have been hesitant to extend LLH beyond the chilled water side of the system due to concerns over condensation. The LLH principles can be extended to the air side if the designer uses supply air temperatures in the 45 F to 48 F range for chillers and 50 F to 52 F for packaged equipment. This allows designers to optimize the energy consumption of both the air and refrigeration side of the system. Historical data indicate that the combination of an efficient centrifugal chiller, and a supply air temperature (SAT) of approximately 45 F is a good balance point. If somewhat less efficient screw chillers are used, 48 F SAT offers the best balance. If scroll compressors are used, especially in air-cooled applications, like rooftop/VAV, the right SAT may be in the range of 52 F. For example, a new office-warehouse building in Dallas required an efficient HVAC system for its 60,000-sq-ft two-story space. A 45 F low-temperature air distribution system, complete with ice storage, was installed. The system provided a comfortable indoor environment by controlling not only dry bulb temperatures, but indoor humidity levels, which were critical, based on the outdoor climate conditions. The owner saved approximately $17,000 per year, i.e., about 18%, of its annual operating costs compared to a traditional HVAC system. This was accomplished by leveraging the combination of reduced air-side and low chilled water off-peak utility rates. Building automation system An integrated control system is critical to the success of LLH systems. It allows the building owner to sustain maximum building performance. The control system also provides critical data necessary to proactively respond to performance fluctuations and report historical data. Five common strategies that enable building owners to achieve all the benefits associated with LLH are: 1. Fan pressure optimization, which is a control strategy that will poll the VAV boxes to identify the most static-demanding box. Based on the box position, the controls will reset the system static pressure to the lowest possible pressure required to satisfy the air flow requirements. This can reduce fan energy consumption by as much as 20% to 40% and will diminish VAV box noise due to over-pressurization in the duct system. This also can aid the reduction of duct and VAV box leakage. 2. Ventilation reset control strategies, such as demand control ventilation and the “Z factor,” will control the correct amount of ventilation on a real-time basis. This will minimize or eliminate costs associated with excess outside air. Reducing cold downdrafts and condensation Cold downdrafts and condensation are two critical elements that must be properly managed in low air temperature applications. The following are suggestions to mitigate these issues. Cold downdraft recommendations: • The use of parallel, fan-powered VAV boxes at the perimeter is a cost-effective method to control drafts efficiently. An energy benefit of these devices is the ability to mix warm plenum/return air with the cold primary air to satisfy the space needs prior to engaging reheat. These units should be selected to meet acoustical requirements and properly controlled to provide required air flows during low loads, which will eliminate on/off sound fluctuations due to fan cycling. • Cooling-only variable air volume boxes can be used within interior spaces. Using a supply air reset schedule is recommended to allow for additional hours of economizer usage in addition to minimizing reheat. The interior boxes will need to be designed to meet their loads using this higher reset temperature. • To accomplish proper air distribution, use linear slot diffusers with aspirating characteristics, described as the “Coanda” effect. Properly selected diffusers will induce room air to mix with the supply air at the ceiling before it is dispersed into the room. Condensation recommendations: • Cold surfaces must be kept inside the humidity controlled envelope. • Insulate supply air ductwork and piping. • Consider the space temperature and humidity level. Compare this to the dew point of any cold service under suspicion. Insulate the surface or change the space parameters to avoid condensation. • Night setback and morning pull-down can be controlled by an interior dew point sensor. In a pull-down mode the process should occur in gradual steps to avoid overcooling. • Positive building pressure is critical to control the interior environment. Ensure the building automation system has the ability, ideally on a floor-by-floor basis, to maintain a slight positive building pressure. • Design the “P” traps and pitch the condensate drains correctly because LLH systems produce a good deal of condensation. Collecting this condensation by piping the condenseate lines and running them to a tank is an excellent way to conserve water for use in irrigation or make-up water for cooling towers. Consulting-Specifying Engineer • DECEMBER 2008 43

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Consulting-Specifying Engineer - December 2008 Contents Viewpoint News M/E Roundtable ARC Awards New Construction Gold New Construction Silver New Construction Bronze Renovation Gold Renovation Silver A Green Approach to Air Conditioning Systems Turning Down the Lights Case Study New Products Equipment Lifecycles Advertiser Index Green Space Gas Technology Pure Power Supplement

Consulting-Specifying Engineer - December 2008

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