Absolute roughness for textile ( fabric ) duct material

[ub313]

Hi All,

I am attempting to estimate friction losses for a large duct work which uses textile fabric (in place of traditional galvanized steel sheet).

I require help in finding out the ABSOLUTE ROUGHNESS FACTOR FOR FABRIC MATERIAL, Can some one please direct me to this info? Even an order of magnitude is sufficient as textile ducts are available in many patterns.

The manufacturers of such duct material do not seem to have authentic data on these values.
ASHRAE shows 0.0003 ft ( Epsilon value) for GI and 0.01ft for fabric and wire flexible duct, (the type used for connectiions to air terminals) which is too high, since the fabric duct material, has no wires on the inside is much smoother and always inflated when fan is operational.]Thanks in anticipation

 

[Michael09]

I assume that you need the roughness to calculate the fan duty.

I would recommend that you involve a ductwork textile manufacturer early in design.
They normally have computer packages which could emulate the air flow pattern within the space and give you advice on what type of textile duct is most suitable for your application.
Once you agree with them the solution to be adopted they can tell you what is the fan static pressure required at the begining of the textile duct.

 

[fredt]

Don't forget permeability. It has a significant effect on fan rating.

 

[ub313]

Dear Michael Thanks for your suggestion.
For fabric ducts, since we maintain constant cross sectional area, its really amazing to find out that the static regain, more than compensates the friction losses in straight ducts and more often i need to add a damper to reduce the duct pressure to have a control on air dispersion through the slots or fabric. (in metal ducts we always reduce sizes as needed and the losses at transformations are more than the static regain).
Of course the above experience is by seeing installations and measurement at site. I need theoretical calcs to show that the static regain is adequate to take care of friction losses.
The suppliers of such duct materials dont either give the absolute roughness factors or disclose the textile manufacturer's info..

Fredt, T
hanks for your information.
This is news to me that permeability can affect fan duty. can you direct me to any published info on this? (i had just considered designed permeation as normal air dispersion and nothing to add on). I understand, for permeation its a mix of an exit loss for air which leaves the duct & friction losses for remaining air which flows further down. any more info available?

 

[Zesti]

ub313,

What is the purpose of your textile duct?
Is it used to supply/distribute air in a room?

Most of these ducts need 80-100 Pa of pressure to work (push the air through the textile) and so a couple of Pa flow resistance over the length of the textile duct does not matter much.

I think it is no coincidence that such a relatively high pressure is needed to push the air through the cloth: the pressure loss of the flow from entering the duct until the end of the duct is of very small consequence because of this high pressure. As a result, more or less the same amount of air will flow from every section of the duct.

So, if you are using the textile duct to distribute air in a room, I would not worry about a bit of flow resistance over the length of the duct (which would be different over the length of the duct anyway because air is leaving the duct everywhere).

(PS: I would be more worried if I had a textile duct supplying air to one room and just a regular diffuser in the next room: then you would have to dampen 80 Pa before the diffuser, which could make quite some noise...)

 

[AbbyNormal]

perhaps the duct sox manufacturer can tell you how much pressure is needed to inflate their product

Take the "V" out of HVAC and you are left with a HAC(k) job.