Consulting-Specifying Engineer - March 2009 - (Page 52) Noise and vibration To reduce the transmission of noise from an RTU into a building, Trane’s Muhlada recommends an integral plenum curb/ isolation rail. He said this blends the structural vibration reduction advantages of an isolation rail with the airborne acoustical advantages of a plenum and keeps duct breakout noise above the roofline. The cost of this option can be offset by the ability to use a common shaft for both supply and return ductwork. However, Sellers points out that, on and near the West Coast, engineers face an additional issue—earthquakes—and a standard isolation rail will not suffice. He points to a “working paper” published by Mason Industries entitled “A Pictorial Study of Seismic Damage and the Use of Proper Safeguards.” Muhlada said another way to avoid noise transmission is to use horizontal supply and return ductwork connections to the unit. Running ductwork above the roofline dissipates sound via breakout above the roofline. Muhlada describes a system of ducts with 90-deg turns to take advantage of end-reflection in the lower octave bands before entering the building. He said many consultants are using this strategy to successfully avoid sound transmission, though he warns that it will not eliminate structural vibration transmitted through the isolation rail. He also acknowledges that the raised equipment creates a line-of-sight issue and an access problem since you need to attach a ladder and a platform to the unit. Meanwhile, Sellers warns against the potential inefficiency introduced by 90deg duct angles. He provides two examples to underscore his point. Figure 1 shows that what might seem to be small differences in duct geometry can make a big difference in pressure loss. Figure 2 demonstrates that two closely spaced elbows can have significantly more pressure drop than the individual loss coefficients would predict. The long and winding road So what are we left with? Not surprisingly, the answer is a fair amount of disagreement among experts. Where some see the regulatory scene as a tangled web, others see it as an orderly process that has shown consistent progress driven by federal control. Perhaps the balance point is struck by Sellers, who emphasizes personal responsibility: “We all need to do the best we can with the available and emerging technology,” he said. “There are a lot of resources out there that we can use to help educate designers, owners, and end users, which will help guide and even drive the industry toward more sustainable equipment and systems.” Kronick is a Minneapolis-based freelance writer specializing in engineering and architecture. He is also a writing trainer who has presented more than 1,000 business writing and technical writing seminars on four continents. Ivanovich has been the editor-in-chief of Consulting-Specifying Engineer since 2007 and has a master’s degree in building systems engineering from the University of Colorado at Boulder. 0.25 0.20 Low velocity High velocity Pressure Drop, inches w.c. 0.15 0.10 Normal application range Flow Loss through two closely coupled 90-deg. mitered vaned elbows Flow Flow Loss through two separate non-interacting elbows 0.05 0.00 -0.05 0 1,000 2,000 3,000 4,000 5,000 6,000 Velocity, feet per minute Flow Loss through a 45-deg. offset with radiused elbows spaced to minimize interaction Figure 2: Pressure drop with three configurations of ductwork is shown. The method with 90-deg turns (close-coupled elbows) would have greater pressure drop, resulting in more noise and energy use. Source: David Sellers, based on ASHRAE data and data from AMCA 200-95 (R2007) Air Systems 52 Consulting-Specifying Engineer • MARCH 2009
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