Duct Fittings Pressure Drop Values

[walz]

I always use Ashrae duct fitting database tables for the pressure drop calculations of duct fittings.
I just happened to do the same calculations for a different duct material this time.
Traditionally we always use duct material Galvanized Steel, for the pressure calculations, for duct as well as it fittings.

Now i am doing the same for a different duct material. Surface roughness of both Aluminum and Galv. Steel is not same.
I have used the value of Aluminum roughness to calculate the pressure drop of ducts. However, when i started doing for the duct fittings, this question came to my mind that the ASHRAE tables (giving loss coefficients) are based on traditional duct material, i.e., Galv Steel. May be we dont have them available for any other material.

Has anyone come across this problem. Is there any theoretical material available for this subject.
How do we do the duct fittings pressure drop calculation for any other material ?

Do we have to make use of same ASHRAE tables ?

 

[lukaiENG]

I have not come across this but (IMO) the pressure drop through the fittings is driven primarily by the geometry/flow changes rather than the surface friction. I think you would be safe to apply the ASHRAE tables.
If you're not confident, increase your safety factor.

 

[willard3]

You can also find the relative roughness of alum and alter steel numbers proportionally.

 

[lilliput1]

See SMACNA HVAC System Duct Design Table 14-1 Duct Material Roughness Factors and Figure 14-3 Duct Friction Loss Correction Factors (Graph of correction factor with velocity and duct diameter.

 

[joecampbell]

i think the ashrae tables have too many mistakes and typos in them to be used. smacna is better.

 

[Jamsith]

Actually it's same like lukaiENG said,
in the ASHRAE The pressure drop for the fitting given by the equivalent length of straight duct. So this will describe you the length only. but static loss values will not be same for the different materials of same sizes. for example if your sizing your duct for 300 l/s with the pressure drop of 0.49 Pa/m for the G.I duct the sizes will be 300X300. if you use the same flow rate and pressure drop the size may increase or decrease for the another material according to their roughness. So all about the pressure drop value.

I am right..