Design Methods

• Constant friction method

• Constant velocity method

• Gradual velocity reduction method

• Pressure recovery method

• Balanced pressure loss method

The design methods are normally a combination of one or more of the above methods.

TABLE 9.9 Recommended Air Velocities to Ensure An Economic Minimum Of Noise Production

Velocity, m sr1, in:

Allowable sound pressure level, dB(A), in the room

35 dB(A)

40 dB(A)

45 dB(A)

Main duct

6.5

7.5

9.0

Branch

5.5

6.0

7.0

Last branch

3.25

4.0

.5.0

• Balanced pressure loss method. This is a method which is used to improve the performance of a preliminary design of a duct system and reduce the number of control dampers.

9.7.2.1 Boundary Conditions

Apart from the energy constraints due to the friction losses, the main rea­son for limiting the air velocities in ducts is to reduce noise production. Some recommended air velocities are given in Table 9.9.

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