Curious about duct sizing 1

[ScottI2R]

Hi all,
I have noticed in restaurants and bars (those with open ceilings) that the ducts that travel throughout go so far and then reduce in diameter for another distance, and, depending on the required length, they may reduce again. All the while having vent outlets either directly on them , or a duct that branches out each way an equal distance.
I know nothing about this , but I assume this is to keep an even output through all vents over the lenght of the duct distributing the air / heat. Is this correct? If not, why is this done?

Just wondering and thanks,
Scott



In a hundred years, it isn't going to matter anyway.

 

[quark]

Many times, I maintained in these forums that electrical engineers have better logic and I am right this time too. You are right that the reduction of duct size after a branch off or an outlet is to facilitate proper distribution.

The most commonly used method of duct sizing is equal friction method, in which, a cetain fixed pressure drop is considered for 100' length of the ducting. When you open an outlet, the flow in the succeeding duct reduces and thus the pressure drop. So, you can reduce the duct size to maintain constant pressure drop.

 

[willard3]

Actually, keeping the main trunk duct the same size, called an extended plenum, gives better air distribution and is much easier to balance.

I have been in discussions with contractors on whether, considering there are no expensive fittings, the extended plenum is less expensive to build.

 

[quark]

I disagree with that idea. Ideally, the resistance to flow in your extended plenum will be low and the chances of imbalanced(and increased) flow will be higher unless you have tight control downstream the first opening. The same principle was used in piping(loop) systems (earlier). Either you go with a reducing(supply) increasing(return) header or use a reversed return system(incase of common size). These methods provide constant pressure drop across the equipment and no short cycling happens.

 

[lilliput1]

The extended plenum is generally used in residential application. Each room outlet will typically be connected with a 6" duct to the plenum. Opening or closing the volume damper at the room register then would then have minimal impact on the other taps connected to the extended plenum blower discharge which is typically located in the basement.

 

[willard3]

I last used an extended plenum in an Ag Research building for a large multi-story VAV system.

Worked like a charm, balanced like a charm.

 

[lilliput1]

If it ia a VAV system I assume you have VAV air terminal boxes which would be pressure independent and control for the correct airflow. So what does the extended plenum do for you?