J1 PRODuct sizing by the equal friction method
How to size supply and return ducts with a constant friction rate, choose rectangular sizes from the equivalent diameter, and check velocity and noise limits.
The equal friction method sizes every duct section for the same pressure loss per metre. It is simple, self-balancing on compact systems, and the most common method for comfort ventilation and air conditioning.
Step 1: Choose a friction rate
Typical design values for low-velocity comfort systems are 0.8 to 1.0 Pa/m. Lower values (0.5 to 0.7 Pa/m) give larger, quieter ducts and a smaller fan. Higher values save ceiling space but cost fan energy every hour the system runs. Many project specifications fix the friction rate. Check yours first.
Step 2: Size each section
For each section's airflow, find the round diameter that gives the chosen friction rate (from the Darcy-Weisbach equation or a friction chart). Then convert it to a rectangular duct that has the same friction loss using the equivalent diameter:
Pick the width and height to fit the ceiling space. Keep the aspect ratio at 4:1 or less where possible. Flat, wide ducts use more sheet metal, leak more and lose more at fittings.
Step 3: Check velocity
Friction rate alone doesn't guarantee a quiet system. Check the velocity in each section against the project's noise criteria. As a general guide for offices:
| Duct location | Typical maximum velocity |
|---|---|
| Main duct in a plant room or shaft | 7 to 10 m/s |
| Main duct above occupied ceilings | 5 to 7 m/s |
| Branch ducts | 3 to 5 m/s |
| Final run-outs to terminals | 2 to 4 m/s |
Use lower values for studios, hotels, hospitals and anywhere the background noise level must be very low.
Step 4: Use the actual velocity
The equivalent diameter gives the same friction, not the same area. When you report velocity or calculate fitting losses, always use the real rectangular area, W × H. Using the equivalent-diameter area overstates velocity and fitting losses.
The J1 Pro Duct Sizer gives rectangular sizes by friction rate or velocity, in SI or IP units.
After sizing
Once the run is sized, calculate the external static pressure along the index run to select the fan. See how to calculate duct ESP.
Guides are for preliminary design and learning. Always verify final designs against project specifications, local codes and manufacturer data.