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Figure 13.17 Buffeting in Rectangular Ducts (Fry, 1988)

Figure 13.17 Buffeting in Rectangular Ducts (Fry, 1988)

level data on noise generated in straight duct runs for straightened flow. Levels generally follow an 18 dB per doubling of velocity scaling law.

The cited data show a significant rise in level when the cross sectional duct dimension is equal to a wavelength, which for this example is about 500 Hz. This bump coincides with the establishment of full cross duct turbulent eddies illustrated in Fig. 13.17. Eddies form downstream of disturbing elements such as rods or dampers. By themselves eddies are not particularly efficient sound radiators; however, they can generate noise when they impinge on a flat plate or other low-frequency radiator. In an open duct, eddies cause the flow to alternately speed up and slow down, producing pressure maxima at points X and Z and a pressure minimum at point Y.

In air distribution design it is prudent to control the duct velocity by increasing the cross sectional area of the duct, thus slowing the flow as it approaches the space served. A duct layout is pictured in Fig. 13.18.

Figure 13.18 Typical Duct Run in an HVAC System

Figure 13.18 Typical Duct Run in an HVAC System

Table 13.9 Velocity Criteria for Air Distribution Systems

Maximum Air Velocities (ft/min)

Description NC Criteria

Slot Speed at Termination

Distance from Termination

< 10 ft (3 m) No Lining

> 5 ft (1.5 m)

> 10 ft (3 m)

> 20 ft (6 m)

< 5 ft (1.5 m) Lining*

Lining*

No Lining

Lining*

No Lining

Lining*

NC 15 supply

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