duct calcs

[multisync]

we got given a spec for a duct addition to a landlords 250mm dia supply. The spec states the supply is
5.0 l/s/m2 @ 30pa

However we have to give the calc to show the additional will have on the pressure drop. Can anyone tell us what 8m of 250mm spirial duct + 4 90 elbows with a jet nozzle on the end works out at?
We also have to install it to DW144 standards

[Toosh]

we got given a spec for a duct addition to a landlords 250mm dia supply. The spec states the supply is
5.0 l/s/m2 @ 30pa

However we have to give the calc to show the additional will have on the pressure drop. Can anyone tell us what 8m of 250mm spirial duct + 4 90 elbows with a jet nozzle on the end works out at?
We also have to install it to DW144 standards

Hi Multi, You will find a duct calculator here

http://www.fridgetech.com/calculators/

Norm

[frank]

Is the additional ductwork going to be on the Index circuit? If not then it probably won't add any additional static pressure.

[multisync]

Is the additional ductwork going to be on the Index circuit? If not then it probably won't add any additional static pressure.

Umm err what a err pardon?

[frank]

If you draw out a ductwork system on paper, the index circuit is the most tortuous route that the air has to travel, either input air or extract air, Ignoring all lesser branch ducts, the system resistance is calculated on this route.

So, for instance, if you had a a 3 storey building with the fan on the roof extracting air from each floor, the fan would have to work harder to 'pull' the air from the ground floor than it would from the 3rd floor. In this case, the Index circuit would be the path to the ground floor, ignoring the branch to the 3rd floor grille.

If the fan is capable of drawing air from the farthest/most difficult route, it can easily draw air from closer/lesser restrictive routes.

Fluids will always take the path of least resistance, so if the grille nearest to the fan is wide open and the grille on the ground floor is partially closed, most of the fan volume will be drawn in from the nearest grille.

The trick is to try and design a ductwork system where the most tortuous route has the least resistance, allowing you to select a smaller fan.

Got to say though, not many people do this in reality.

[Brian_UK]

As an old commissioning engineer to possibly another old one, well described there Frank.

Also the index is the terminal, air or water, that is fully open back to the fan/pump' no dampers or valves regulated on that route.