duct design 2

[ASSADEQ]

how many supply branch ducts can be taken off of the main duct before reducing the main duct size?

can I size the main duct at 0.1 in wg/100 ft, then size the branch duct for noise sensitive room at 0.05 in wg/100 ft.

what is the lowest friction rate that I should not select?

 

[ChrisConley]

The reduction in main duct size has to do with saving money in installation more that anything. If you wanted to, you could run your main duct at the same size throught the building.

It is absolutely acceptable to size branch ducts at a reduced friction rate, and is common when controlling noise.

There is no 'minimum' friction rate, if materials had no cost all ducts would be sized very large for next to no friction losses. In the real world we all live in 0.1 in/100' is a good rule of thumb for friction loss in a low velocity system.

 

[ASSADEQ]

ChrisConley:

thank you for your input.

regarding the min friction rate, I am confuised with your reply. what I understand that air flow should be with sufficiant velocity to conevy air through the duct . if firiction rate is too low, the air flow will not be distrbuted in the zone propelry due to lack of velocity.

if may understanding is wrong please correct me.

Thanks in advance for any help.

 

[briand2]

If I know that I want to move 1 m3/sec of air along a duct, I can either install a narrow duct (cheap, easy to install, but high resistance hence more powerful fan required) or a wide duct (expensive, hard to install, but low resistance hence less powerful fan required). The normal compromise is to select a duct size such that the pressure drop will be perhaps 1 Pa/m (or about 0.2in/100ft as Chris stated). Once you've selected the duct and then selected a fan to suit the system resistance, your air will flow; it's got no choice!

 

[ASSADEQ]

briand2: I agree with you that when fan is selected to suit the system the air will flow to the latest part of the duct whatever the velocity is, but my concern is the terminal or (neck) velocity near the diffuser.

I beleve there are some limitations for the neck velocity to avoid noise.

thanks for your help

 

[briand2]

assadeq:
You are absolutely correct. Although we size the duct typically on pressure drop per unit length, we must always ensure that velocities are "reasonable" (which means, for HVAC applications, the velocity is not so high that noise is excessive). We want to limit the "neck velocity" of the terminal to some value (maybe 500 feet per minute / 2.5 metres per second) and also make sure that the duct velocity in the region of the terminal connection is also about the same velocity. Here in the UK, we typically have branch duct velocities of 3 - 4 m/s for residences or 6 - 8 m/s for fairly busy public buildings. There is no absolute value; it all depends on circumstances.

I hope this helps!

Brian