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HVAC Noise Control

HVAC Noise Control. Architectural Acoustics II January 31, 2008. Resources. MJR Chapter 9 Long, Architectural Acoustics , Chapters 13 and 14 Cavanaugh, Architectural Acoustics , pp. 126 – 147 Egan, Architectural Acoustics , Chapter 5 Manufacturers Trane Industrial Acoustics

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HVAC Noise Control

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  1. HVAC Noise Control Architectural Acoustics II January 31, 2008

  2. Resources • MJR Chapter 9 • Long, Architectural Acoustics, Chapters 13 and 14 • Cavanaugh, Architectural Acoustics, pp. 126 – 147 • Egan, Architectural Acoustics, Chapter 5 • Manufacturers • Trane • Industrial Acoustics • Many others • ASHRAE (American Society for Heating, Refrigerating, and Air-Conditioning Engineers)

  3. Basic HVAC Functionality • Room air is blown over a heat exchanger through which heated liquid (hot water) or cooled liquid (cold water or other refrigerant) liquid is circulated. • Unwanted thermal energy is released outdoors • This requires…

  4. Fans (to move the air) Axial Centrifugal Propeller Compressors (to convert gas to liquid) Piston Rotary Scroll Centrifugal Screw Pumps (to circulate liquids) Diffusers and Ductwork (to distribute air) Turbulent aerodynamic noise “Break-out” noise Main HVAC Noise Sources From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  5. Other MEP Noise Sources • Waste and Rain Leader Piping • Transformers • Dimmer Racks • Lights & Ballasts • Elevator Equipment

  6. Noise Control Approaches • Location of equipment • Sealing penetrations • Resilient mounting of equipment & connected services • Flexible connections to equipment • Lower fluid velocities • Internal duct lining and duct attenuators • Routing of ductwork and piping • Enclosing ductwork and piping From Kirkegaard Associates

  7. Fan Coil Units • Opportunity for significant noise issues: • Fan and coil in close proximity: high turbulence • Applications: typically close to “listeners” (hotel rooms, etc.) • Water flow noise From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  8. Packaged Air Handler • Includes fan or fans • Heating coil • Cooling coil • Air filters • Humidifier • Air dampers and controls From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  9. Packaged Air Handler From Kirkegaard Associates

  10. Typical Air-Handler Design MJR Figure 9.3, p. 192

  11. Equipment Location: Rooftop From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  12. Equipment Location: Mechanical Equipment Room • Noise inside the MER • Noise outside the MER • Duct Breakout • Active Noise Control From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  13. Isolator Types Elastomeric Pads

  14. Isolator Types Elastomeric Pads

  15. Isolator Types Neoprene-In-Shear Floor Mount

  16. Isolator Types Neoprene-In-Shear Floor Mount

  17. Isolator Types Neoprene-In-Shear Floor Mount

  18. Isolator Types Open Spring Floor Mount

  19. Isolator Types Open Spring Floor Mount

  20. Isolator Types Restrained Open Spring Floor Mount

  21. Isolator Types Restrained Open Spring Floor Mount

  22. Reciprocating and Centrifugal Chillers Noise • Reciprocating chillers tend to be quieter than centrifugals for the same load From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  23. Fan Noise Components • 1 duct length • 3 duct length • 5 duct length • Aerodynamic noise • Blade-passage noise • fB = (RPM/60) ·N • N = number of blades From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  24. Fan Noise Fan noise depends on the fan operation point on the fan curve From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  25. Fan Noise Fan noise depends on the fan operation point on the fan curve From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  26. LW = fan sound power level KW = fan specific value Q = volume flow rate (cfm) P = static pressure (in H20) BFI = blade frequency increment C = efficiency correction Estimating Fan Noise • η= Hydraulic efficiency of the fan = Q·P/(6350 · HP) • HP = nominal horsepower of the fan drive motor From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  27. Estimating Fan Noise US Army TM 5-805-4 Technical Manual, “Noise and Vibration Control”, Table C-13

  28. Diffuser Noise • Flow sets the noise level at a given static pressure level forcing the flow • Good aerodynamics are important to low noise from air terminals From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005 (Long Fig. 13.23, p. 474)

  29. Indoor Diffusers • Linear or Slot Diffusers • Round or Rectangular Diffusers • Grilles • Registers From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  30. Specifications for Diffuser Noise • Ideal: sound power data in octave bands versus static pressure & CFM • Reality: most manufacturers only provide the NC “rating” at a fixed “room effect” (typically 10 dB) • Sound power from NC: • Sadly, this only provides a noise estimate based on a perfect NC curve (diffusers are typically high-frequency elements, therefore this tends to over-estimate low frequency power) • 400 sabins • 12 feet From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  31. Estimating Diffuser Noise • LW = sound power level (dB re. 10-12 Watts) • SD = cross-sectional face area of diffuser (ft2) • UD = flow velocity prior to the diffuser (ft/s) • ξ = normalized pressure-drop coefficient • ΔP = pressure drop across the diffuser (in. H20) • ρ0 = density of air (0.075 lb/ft3) Long, p. 475

  32. Estimating Diffuser Noise • Octave-band power levels can be calculated from the overall level LW for round diffusers for rectangular diffusers Generalized Diffuser Spectrum peak frequency NB(x) = octave-band number of frequency x (32 Hz = 0, 63 Hz = 1, 125 Hz = 2, …) Long, Fig. 13.24, p.476

  33. Plant Rooms 5m/s Aud. Shafts 4m/s Within Aud. 2.5m/s Branch Runouts RC-35 2.75 m/s RC-25 2 m/s RC-15 1.25 m/s Recommended Velocity Limits Terminal velocities are critical because there is nothing after the diffuser to provide additional attenuation! From Kirkegaard Associates

  34. Unlined Ducts • Not much attenuation in unlined ducts • Little absorption from surfaces (although some energy is lost to break-out noise) • Plane-wave propagation → no spreading loss • Plane-wave propagation when duct dimensions (not length) are less than half a wavelength

  35. Attenuation in Unlined Ducts MJR Figure 9.6, p. 193

  36. Duct Liner MJR Figure 9.5 and 9.7, pp. 193 and 194

  37. Duct Liner • Attenuation in lined rectangular ducts can be approximated with this equation • P = duct perimeter (ft) • S = duct cross-sectional area (ft2) • t = thickness of lining (in) Octave-Band Center Frequency (Hz) Long, Eq. 14.12, p. 487

  38. Duct Liner x x x x x x x x x x x x Data from Long’s equation x MJR Figure 9.5 and 9.7, pp. 193 and 194

  39. Duct Liner Data http://www.owenscorning.com/comminsul/documents/FiberglasDuctBoardLiner.pdf

  40. Duct Liner Internal Fiberglass Duct Lining From Kirkegaard Associates

  41. Airflow: Turbulent Noise in Ductwork From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005, (MJR Fig. 9.12, p. 198)

  42. Airflow: Turbulent Noise in Ductwork From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005, (MJR Table 9.1, p. 197)

  43. How Ductwork Radiates Noise (Break Out) From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  44. Duct Shape and Noise Control • Stiffness of round ductwork reduces break-out noise since motion of the duct walls is restricted • However, this means that more noise energy stays within the duct and may produce higher noise levels at the outlet From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  45. Duct Shape and Noise Control • The ratio of perimeter to cross-sectional area is also important, and can be used to approximate duct attenuation. • P = perimeter (ft) • S = cross-sectional area (ft) • l = duct length (ft) • f = octave-band center frequency between 63 and 250 Hz Long, p. 486

  46. Duct Shape and Noise Control • For octave bands above 250 Hz • P = perimeter (ft) • S = cross-sectional area (ft) • l = duct length (ft) Long, p. 486

  47. Duct Shape and Noise Control • Data for circular duct from Long, Table 14.1 • Data for square duct from previous equations with P/S = 4 Long, p. 486

  48. Discharge Noise High noise levels near the discharge of the AHU From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  49. Discharge Noise Control • Stiffen the initial 25-50 ft of the discharge duct • Often done by wrapping the duct with gypsum board or loaded vinyl From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

  50. Duct Lagging Make the ducts stiff using lagging, typically fire-rated drywall. From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

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