HVAC & Air Duct Noise Control with Flat Oval Ducts
Do you have a version of the venerable “Trane Ductulator”? It’s okay, we know you use them, even though we are all supposed to be using static regain duct design programs by now. Look on the right-hand side in the red-orange area of the wheel, where “Air Volume – CFM” overlaps “Velocity – FPM”. Did you ever notice how there is a darker shading on velocities greater than 2000 FPM and a note to verify that your velocities are correct? That’s because rectangular ducts are not recommended to be used over 2000 FPM velocity.
Those square corners in rectangular duct cause far more turbulence than the rounded profiles of round and flat oval duct, and turbulence increases dynamic pressure losses and noise. For ducts suspended above an acoustic ceiling to meet a design RC of 35, the recommended maximum velocity for rectangular duct is 1750 FPM versus 3000 FPM for round and flat oval (2015 ASHRAE Handbook – HVAC Applications Chapter 48).
If you move the same mass of air but can do so at a higher velocity by changing from rectangular to round and flat oval, what does that mean? It means you could be using smaller ducts. Yes, higher velocities result in higher pressure drops, even with round and flat oval ducts. Overall, the fittings are more efficient already. Reducing leakage allows you to reduce your needed volume. Lastly, using a static regain design program lets you really start getting a smaller, more balanced system without needing those other noisy things you put in rectangular duct systems – balancing dampers.