Equal Friction Method 3

[ASSADEQ]

To design air duct by Equal Friction Method, the friction factor of 0.1 in wg. Is used as a rule of thumb for all supply duct sizes. And 0.8 is used for sizing the return duct.
When energy cost is high and installed ductwork cost is low, the low friction rate design is more economical.


My question is
Up to which limit can we go lower than this friction factor? Also what are the tips should we follow to avoid any improper flow rate thru duct?

Apreciate your response

 

[cme]

That is the rule of thumb of the rule of thumbs ..... when using a residential grade ahu use lower friction rates ..... when designing vav or say hospital cv systems, use .20" wg/100' and max 2000 fpm for the ductwork upstream of the boxes (smoke and fire/smoke dampers have max 2000 fpm rating) ...... in dual duct systems you can go higher ...... straight duct runs are not the major souce of pressure drops; its the elbows and fittings and s & f/s dampers

 

[AEBuck]

I would emphasize that compound fittings are among the most abused and costly of all. Duct fitting loss cefficients in ASHRAE, for example, are based upon 6 hydraulic diameters of straight duct upstream of the fitting. With one exception, there is no data upon which to base calculations of pressure loss for close-coupled fittings. The same goes for smoke and fire/smoke dampers, which I believe are tested by some manufacturers with up to 10 straight upstream diameters. One other common mistake with potentially significant losses is underestimating or neglecting system effect at fan inlets/outlets. Very few fans are installed as tested.

FYI Fire/smoke dampers are available up to 3,000 FPM. See Ruskin for example.

ASSADEQ ... Recommend you gain some experience with using the factors you quoted, which are typical for a large percentage of "normal" HVAC applications - i.e. those applications without other special requirements such as grease or particle exhaust. Exceeding these norms on the high side should be done with caution and experience - noise being another factor not yet mentioned. To learn why equal friction is not the best method of sizing primary air ductwork in systems with terminal units of any size, experiment on a "sample" project with a decent static regain duct sizing program. This procedure establishes an initial velocity that provides sufficient energy to account for nearly all static losses in the critical run. Look at the resulting friction factors throughtout the system and the relatively constant inlet static pressure at all boxes (varies by no more than 0.25" to 0.5" WG). Short of becoming a balancing or commissioning agent for a few years, this is the next best way to learn.

 

[cme]

SOME of those dampers are rated to 3000 fpm ..... but BEWARE the pressure drop at those velocities!

 

[lilliput1]

We only used the static regain method on high velocity ductwork - 3000 to 4000 FPM initial velocity. But with energy cost and noise concerns, we have not done any high velocity ductwork in the last 11 years! I don't think it is that good as claimed by AEBuck. The run from the fan to the 1st takeoff of each branch is balanced by varying the sizes of the branch, after that the duct size usually wants to remain the same size. It takes the most loss from the fan to the first takeoff. Good if there is long run & multiple branches with the 1st takeoff in each branch remote. If a branch with takeoff is much closer than others you have to make the duct small, max 4000 FPM and you may have noise problems. Of course the architects locate conference rooms next to the duct shaft! With equal friction method, we limit maximum velocity to 2000 FPM. What is good w/ equal friction is you can easily gauge which system is worst. So if you dont have time to calculate ductwork of all systems you just do the worst of the group & assume similar system require the same static to come up w/ a conservative fan hp estimate. If you have a good ductwork program, you can modify the equal friction method to have equal loss to the 1st takeoff off each branch from the fan, but usually time pressure would not allow you to do that. VAV systems with pressure independent boxes can compensate between about 0.3"wg to 3" wg and can come furnished with integral attenuator section so you don't need to use static regain in sizing low velocity ductwork.