Equal Friction Method

The equal friction method of sizing ducts is easy and straightforward to use

The equal friction method of sizing ducts is often preferred because it is quite easy to use. The method can be summarized to

1. Compute the necessary air flow volume (m³/h, cfm) in every room and branch of the system
2. Use ¹⁾ to compute the total air volume (m³/h, cfm) in the main system
3. Determine the maximum acceptable airflow velocity in the main duct
4. Determine the major pressure drop in the main duct
5. Use the major pressure drop for the main duct as a constant to determine the duct sizes throughout the distribution system
6. Determine the total resistance in the duct system by multiplying the static resistance with the equivalent length of the longest run
7. Compute balancing dampers

1. Compute the air volume in every room and branch

Use the actual heat, cooling or air quality requirements for the rooms and calculate the required air volume - q.

2. Compute the total volume in the system

Make a simplified diagram of the system like the one above.

Use ¹⁾ to summarize and accumulate the total volume - q_total - in the system.

Note! Be aware that maximum load conditions almost never occurs in all of the rooms at the same time. Avoid over-sizing the main system by multiplying the accumulated volume with a factor less than one (This is probably the hard part - and for larger systems sophisticated computer-assisted indoor climate calculations are often required).

3. Determine the maximum acceptable airflow velocity in the main ducts

Select the maximum velocity in the main duct on basis of the application environment. To avoid disturbing noise levels - keep maximum velocities within experienced limits:

• comfort systems - air velocity 4 to 7 m/s (13 to 23 ft/s)
• industrial systems - air velocity 8 to 12 m/s (26 to 40 ft/s)
• high speed systems - air velocity 10 to 18 m/s (33 to 60 ft/s)

Use the maximum velocity limits when selecting the size of the main duct.

4. Determine the static pressure drop in main duct

Use a pressure drop table or similar to determine the static pressure drop in the main duct.

5. Determine the duct sizes throughout the system

Use the static pressure drop determined in ⁴⁾ as a constant to determine the ducts sizes throughout the system. Use the air volumes calculated in ¹⁾ for the calculation. Select the duct sizes with the pressure drop for the actual ducts as close to the main duct pressure drop as possible.  

6. Determine the total resistance in the system

Use the static pressure from ⁴⁾ to calculate the pressure drop through the longest part of the duct system. Use the equivalent length which is

• the actual length + additional lengths for bends, T's, inlets and outlets

7. Calculate balancing dampers

Use the total resistance in ⁶⁾ and the volume flow throughout the system to calculate necessary dampers and the theoretical pressure loss through the dampers.

Note about the Equal Friction Method

The equal friction method is straightforward and easy to use and gives an automatic reduction of the air flow velocities throughout the system. The reduced velocities are in general within the noise limits of the application environment.

The method can increase the numbers of reductions compared to other methods, and often a poorer pressure balance in the system require more adjusting dampers. This may increase the system cost compared to other methods.

Example - Equal Friction Method

The equal friction method can be done manual or more or less semi automatic with a spreadsheet as shown in the table below.

The table is based on the diagram above. Air flow and friction loss from a diagram is added. Minor pressure loss coefficients must be summarized for for the actual applications.

The pressure loss in each path is summarized on the right and pressure loss is added manually in the dampers to balance the system.

The excel template can be downloaded here!

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