Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations KootK on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Ductwork Sizing for VAV Box System

Status
Not open for further replies.

rlafrance

Mechanical
Dec 5, 2019
3
I’m working on a VAV system and I had a question on how to reduce pressure drop on my taps to the VAV terminal units. Generally the inlet of the VAV boxes have a high velocity, but, if I’m sizing my main for 0.1 in wg friction that’s relatively low velocity so there’s a huge pressure drop of trying to go low velocity to high velocity off a tap. Should I be sizing my main for something when I’m using a VAV box system? If we go medium pressure duct construction what’s the standard for sizing the duct is it velocity or friction based? Do the noise levels decrease if we use a medium pressure duct construction or is there a rule of thumb for sizing medium pressure duct for noise concerns? Thanks ahead of time for the help!
 
Replies continue below

Recommended for you

Your post raises a lot of questions/concerns. I would not dive into trying to design a VAV system without an experienced engineer guiding you. The VAV box itself is the transition from medium to low pressure - not the ductwork in between. Medium pressure ductwork is sized based on multiple parameters, including friction and velocity. Look up the Static Regain Method as a start.
 
I guess that’s the issue I’m having, I’m still relatively new and I was told size all supply for 0.1” friction or 2000 fpm and to tap the main and the next highest even duct size as the vav box and transition to vav inlet 3 equivalent diameters before the inlet for a vav box. So I’ve been researching and looking at pressure losses and it didn’t physically make sense to me why we have a branch at relatively high velocity tap off a main at lower velocity seems like the static loss in that is huge.
 
The tap from your medium pressure supply to your VAV doesn’t have to be at a lower velocity, doesn’t gain you anything. You can stay in those same velocity and pressure drop guidelines (or even higher than 0.1) as long as you transition to the required VAV box inlet size with the required straight length.

The duct main being limited to 2,000 fpm is a typical value on the medium pressure side, to keep noise to a minimum assuming the duct is above a ceiling. You’ll find a lot of different duct sizing rules from a lot of engineers, but when people aren’t overly concerned with fan power this is a common number.

The VAV inlet is all about providing a VAV box and it’s air measuring sensor a velocity that will work across the range of air flows it may vary between. So it has to account for more than just its max airflow. The manufacturer will give you a table showing airflow ranges that work for each inlet size. As far as the noise that is generated, it is a relatively small sound source in a relatively limited area, but depending on your full airflow range needed you probably shouldn’t be pushing that inlet too much higher than 2,000 fpm or you are going to get Boise out of the box itself and also excessive VAV box pressure drop too. Manufacturers will provide tables to show how much noise is associated with each VAV box size across a range of air flows.
 
I've seen 0.08"/100' used for the distribution system and 0.1"/100' for downstream of the VAV box. What is the pressure drop you are seeing and what is it in the grand scheme of things? The 0.1"/100-ft is an equal friction value that, at one time, was based on a good balance based on economics and performance. Since energy codes continually clamp down on fan power, it may be worth looking into lower friction factors (will result in larger ducts and higher first cost) but will help you reduce external static pressure (energy use). Alternatively, you could look into other duct sizing methods as GT-EGR suggested with the static regain method.
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor