Consulting-Specifying Engineer - December 2008 - (Page 41)

approach A GREEN TO AIR CONDITIONING SYSTEMS Low-flow/low-temperature, high-efficiency systems can deliver greener HVAC solutions for commercial buildings. BY EUGENE “SMITTY” L. SMITHART, PE, Trane, LaCrosse, Wis. T here’s never been a better time for HVAC products, systems, and services that are environmentally responsible to businesses and their stakeholders. The U.S. Environmental Protection Agency 1 has always encouraged HVAC technologies that maximize efficiency with low emissions. Additionally, owners are encouraged to sustain and document the performance over the lifetime of the application. In some circumstances, it can be a challenge to meet the performance requirements in a cost-effective manner. The use of low-flow/low-temperature, high-efficiency chilled water systems (LLH) is one way for building owners and engineers to deliver cost-effective, environmentally responsible comfort and added value without excessive added costs. When the systems are integrated with the entire HVAC system, an added benefit is improved IAQ and airside acoustic characteristics. LLH systems are not a new concept. They have been used by innovative system engineers for more than 20 years in new construction and renovation/retrofit projects. The fundamentals of an LLH system are similar to traditional systems design, with several notable differences. LLH systems use low supply temperatures and a larger temperature differential. This results in lower flow rates, which inherently can reduce the size of the supply fans, ductwork, pumps, piping, and other HVAC equipment. This provides an opportunity to reduce the first-cost investment in the system infrastructure with reduced operating costs. LLH systems are not optimal for every building application. It is highly recommended that the system designer uses a comprehensive computerized system simulation program with lifecycle cost analysis to evaluate economic and environmental alternatives. The computer model must include all aspects of the building design to properly evaluate the interaction of individual component modifications. Conventional VAV systems are typically designed for 55 F supply air temperature. In a larger system, this traditionally is accomplished using a water-cooled chiller with standard design conditions of 44 F leaving water temperature and a 10 F temperature differential for both chilled water and condenser water. The following discussion will compare a traditional system design to an LLH system. The LLH system will use 41 F leaving chilled water temperature, and 16 F and 15 F, respectively, for the chilled water and condenser water temperature difference. A reduced supply air temperature also will be evaluated in conjunction with the lower chilled water temperatures. The lower air temperature will reduce humidity levels, helping to reduce IAQ by mitigating the potential for mold. Chillers When the leaving water temperature is reduced, the kW consumption on the chiller(s) will increase. First, in this example, two 400-ton chillers with screw compressors are used to deliver 800 tons. At ARI Standard 550/590 conditions2, these chillers will produce 44 F water, and will consume 464 kW at full load. When the leaving water temperature is reduced to 41 F, the energy consumption increases to 490 kW, a difference of 26 kW at full load. This is a single component of the system and must be balanced against other improvements in the performance of other equipment. While the chillers consumed 26 more kW at 16 F, the increased temperature difference reduces the water flow, which results in a smaller pump and motor. The LLH design leverages components that will improve the most in overall system efficiency over the efficiencies of individual components. From a coefficient of performance perspective, chiller efficiencies have improved more than 70% in the past three decades, which allows unlimited flexibility to maximize the overall system performance. Traditional building HVAC designs will have at least two chillers for redundancy. If Consulting-Specifying Engineer • DECEMBER 2008 41

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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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