duct design
duct design
(OP)
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?
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?





RE: duct design
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.
RE: duct design
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.
RE: duct design
RE: duct design
I beleve there are some limitations for the neck velocity to avoid noise.
thanks for your help
RE: duct design
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