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A Review of Thermal Acoustical and Special Project Requirements Data in Designing a Duct System
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The primary foci in designing a duct system is: 1) Delivery of the correct amount of air; 2) Delivery of air at an acceptable temperature with minimum temperature drop; 3) Delivery of air quietly; and, 4) A system which is applicable to the project conditions. The Sheet Metal and Air Conditioning Association (SMACNA) and the Thermal Insulation Manufacturers Association (TIMA), have done considerable testing on air loss and temperature drop on operating HVAC systems. It is important to note that these tests show that air leakage through unsealed joints is the most significant factor in heat loss. No amount of insulation can make up for a 24 percent air leakage rate in an unsealed, rectangular sheet metal system. Acoustical data is not as readily available. ASHRAE is currently testing acoustical companies; however. this information is not yet available. Based on testing by Owens/Corning Fiberglas, duct attenuation in dB's per lineal foot at 500 HZ of various insulating materials can range from 2.9 dB's t o 3.7 dB's. The last area to consider in designing a duct system is project conditions. This area is not as technically oriented as the other three; however. it is crucial in designing a duct operable system. Items to consider are susceptibility to abuse, concealed or exposed duct, how critical the acoustics are, duct clean out requirements, climate conditions. residential versus commercial construction, and new versus retrofit construction. Without revealing the properties of each system, a failure or a less than acceptable environment for occupants could occur.
Lebens, A. F. (1986). A Review of Thermal Acoustical and Special Project Requirements Data in Designing a Duct System. Energy Systems Laboratory (http://esl.tamu.edu); Texas A&M University (http://www.tamu.edu). Available electronically from