Friction loss for concrete duct

[alex1987]

Hi,

I found this formula in Carrier Handbook,
delta p = 0.03*f*(L/d^1.22)*(V/1000)^1.82

delta p = friction loss (in. wg)
f = interior surface roughness (0.9 for galvanized duct)
L = length of duct (ft)
d = duct diameter (in.), equivalent diameter for rectangular ductwork
V = air velocity (fpm)

Here is the problem, I could not find the interior surface roughness, f for concrete duct at anywhere else. May I know where does this formula derived from? I only can find Darcy, Colebrook, and Altshul-Tsal Equations on the internet. I need the interior surface roughness, f for concrete duct.

Thanks

 

[Drazen]

I have recommendtions in one handbook, though in SI units.

In practice, however, I would never advocate their use except if you do not have ample space in so constructed ducts, so that errors in assumptions will still leave much reserve.

Surface finish of concrete ducts is mostly created on-site, and that is the main trouble. You would need to establish link with civil engineer or architect and be able to directly influence that construction activity or to include specification within civil design.

In handbook I have roughness varies from 0,5 mm for smooth finish up to 3,o mm for rough finish.

Apparently that is incredibly large difference to make "rough" assumptions.

 

[alex1987]

Well, if I were still need to use this equation to calculate the friction loss, do you know what is the interior surface roughness? I don't think your 3.0mm rough finish is suitable for this equation, please correct me if I'm wrong.

Thanks

 

[Drazen]

I trust in Carrier, their articles are normally of good quality.

The trouble is, however, that they presented simplified formula, and I would suggest that you carefully read assumptions and limitations they applied to formula.

This formula is derived from general formula, but instead of lambda they have put 0,003*f as much as I can see.

As lambda is not proportional to f normally, and if you cannot have f for concrete applied to exactly that formula, I would suggest that you abandon formula and use general formula.

delta p = lambda * L/D * density * (second power of velocity)/2

You can find lambda from diagram, knowing f, velocity and density. I believe it is easily findable on Wiki.

It is not too much more complex than your formula, but what I saw often in practice - people make large mistakes by trying to use simplified formulas outside their boundaries and assumptions.