Duct Sizing: V vs. dP

[CuriouslyGeorge]

Am I correct if I conclude that sizing a duct based on rec'd maximum velocities (V) will lead me to accepting greater total dP; hence, needing a more robust fan in my AHU. As opposed to minimizing total dP even if my ducts become slightly larger than the velocity sizing method would pick?

And, isn't the latter the cheaper alternative?
Thanks,
- CuriouslyGeorge

 

[CRG]

And, isn't the latter the cheaper alternative?

Is this first cost, or life cycle?

If it is first cost then smaller ducts means cheaper and easier to install because of their size.


If life cycle cost is evaluated, you will pay for the dP in the utilities bill for the life of system.

 

[CountOlaf]

George,

Olaf here again. You should try to minimize total dP by sticking with those starting point numbers that most of the responders agreed on. I only mentioned the "upper limit" velocities because frequently when running ductwork, you're going to be in tight spots "architecturally" and are going to be thankful for other options. Besides, you don't have to stick to a method for every duct in the system. It is common to size some of the large trunks at higher DP and velocities so that you can get them through the various areas. They will only be for a finite length anyway. Once you start branching off, you can revert to the pressure drop "rules of thumb" for sizing. Obviously if space is not an issue at all, run really large ductwork--see if I care!

Regarding the last responder's comments (CRG), this may be true in some cases, however; if the motor, fan and air handler get too big due to excesive dP, even if the ductwork is small, the project first costs may be larger in addition to the life cycle costs.

 

[CuriouslyGeorge]

Hmm...wasn't thinking about the long-term costs. Also, in this case, I should mention this is rectangular, galvanized ducting...if that factors into your thinking.

 

[lilliput1]

If you look at SMACNA chart of friction loss with CFM, velocity and duct sizes you will see that the opposite is true. As your system handle more air and as you increase the duct size to maintain the velocity, the friction loss in in.wg/100' of duct goes down.

 

[25362]

What lilliput1 has said is a fundamental fact in hydraulics.
Take water at 60 deg F flowing in pipes of various sizes running at V=10 fps. The friction drops, ft/100 ft, for pipe diameters, D, 2-, 4-, 6-in would be about 23, 8, 5, respectively.

The friction drop for a given length of pipe would be proportional to f V²/D, where f is the friction factor. Keeping V constant, and f values dropping with increasing diameters, we clearly see that the pressure drops by friction at equal velocities would become lower for larger duct diameters.