Help with modeling bathroom exhaust airflow

[Swanny76]

I am trying to find a resource to help me model the airflow in a vertical bathroom exhaust stack serviced by a rooftop fan, with seven floors, each with two bathrooms. I am assuming that there is an integral that could be used to describe the rate of flow at each floor, but I am having trouble figuring out how to set it up. Does anyone know of any books, etc. that contain this kind of information? Any help would be appreciated.

 

[willard3]

I can't figure out what you're asking? Don't you know what the volume is in each bathroom? Are you sizing the duct?

If you do and are, this is a simple air friction problem.

 

[berkshire]

Maybe this will help,

http://www.engineeringtoolbox.com/flow-velocity-air-ducts-d_388.html

B.E.

 

[Swanny76]

Each room is supposed to flow 45 cfm after balancing. What I want to calculate is how much airflow I am getting through the vents when they are unrestricted, and then calculate how much I need to block off the vents to achieve this.

 

[GMcD]

Swanny- in real life we just specify a balancing damper on each branch so you can regulate the airflows- set them once, lock them in place and leave them. Size the exhaust fan for the system static pressure resistance and off you go. I'm not sure why you need the precision or "CFD style" approach to your application.

 

[willard3]

You sound verrrry early on the curve of duct analysis. Get books and read first.

You will need a pitot tube and a grille hood and a copy of SMACNA or similar.

You will then measure velocity as instructed in duct cross-section. Flow (cfm) = V (ft/min) X A (ft squared) whan all the rules are applied.

Better to get a guy who does balancing for a living and he'll measure a duct in about a minute. Use a locking duct-mounted balancing damper.

 

[DrRTU]

Your total flow is only 630CFM. Spend your time on the proper use of fire dampers or subducts to maintain the floor and shaft rating. 5” runouts from the riser to the grille box should be fine if it’s a short run. I prefer to use subduct to tap the riser. Balance the system at the tap or runout with a locking damper (not at the face). A system balanced at the face gets destroyed when the end user at the top opens his damper. Use a belt drive fan so you bump the static up or down to get the most restrictive room to perform. 45 CFM per inlet is a very low rate; make sure you are meeting all local codes.