×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Curious about duct sizing

Curious about duct sizing

Curious about duct sizing

(OP)
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.

RE: Curious about duct sizing

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.

RE: Curious about duct sizing

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.
 

RE: Curious about duct sizing

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.

RE: Curious about duct sizing

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.

RE: Curious about duct sizing

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.

RE: Curious about duct sizing

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?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources