MET 150

1
Air System Balancing Procedures

• MET 150

2
Performing a Duct Traverse

• A traverse can be performed with an anemometer or
  a pitot tube and manometer.
• Use average velocity to determine actual CFM
• Airflow is calculated as follows
• Q V x A
• Where
• Q actual airflow, cfm (ft3/min)
• A duct cross-sectional area, ft2
• V average velocity, fpm (ft/min)

3
Duct Traverse

• Not less than 16 readings per duct
• Not more than 6 centers
• Need straight duct before and after reading
  points

4
Duct Traverse

• Divide duct into equal areas
• For round ducts, these are equal annular areas
• Should have 10 diameters upstream and downstream
  of straight duct for highest accuracy

5
Round Traverse Constants

• Calculate reading points from table below or see
  chart on page 11.28.
• For example, for a 14 duct, reading 1 would be
  at 14 x 0.1581 2.2134 from the duct center

Duct Diameter
9 or less 10 12 14 or more
6
Flat Oval Duct Traverse

• Take 5 measurements in each hemisphere and treat
  center of duct as rectangular (no more than 6
  between measurements)

7
Performing a Duct Traverse

• Record all velocity pressures
• Negative pressures indicate excessive turbulence
  and should be counted as a reading of 0.
• Divide sum of all velocities by total readings
  (including those recorded as 0)

8
Example

• The following readings were found in a traverse
  of an 8 duct. What is the flow rate in the
  duct?

9
Example
10
Example

• Vaverage (1710014175)/12 2606 fpm
• A pd2/4
• p(8in x 1ft/12 in)2/4 .349 ft2
• Q V x A
• 2606 ft/min x 0.349 ft2
• 910 cfm

11
Plot a Velocity Profile
12
Example

• The following readings were found in a traverse
  of a 12 duct. What is the flow rate in the
  duct?

13
Example
14
Example

• Vaverage
• A pd2/4
• Q V x A