Smart questions
Smart answers
Smart people
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Member Login




Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • 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!

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

Donate Today!

Do you enjoy these
technical forums?
Donate Today! Click Here

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

How to calculate Air exchange in a room?

zu56 (Mechanical)
19 Sep 07 14:57
Hi All,
How would I calculate air exchange per hour in a room with supply and exhaust fans?
Thank you
mechanicaldude (Mechanical)
19 Sep 07 16:41
Air changes per hour = CFM*60/Volume where cfm is the air flow rate (cubic feet per minute)and the volume is the volume (cubic feet) of the room.
zu56 (Mechanical)
19 Sep 07 17:30
Do I count the exhaust fan?
ChrisConley (Mechanical)
19 Sep 07 18:30
Air in = Air out

Ignoring infiltration and exfiltration the exhaust fan should be equal to the supply fan (plus a small varience for pressure control).
IRstuff (Aerospace)
19 Sep 07 21:45
Wouldn't the least capable fan dominate?

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

MintJulep (Mechanical)
20 Sep 07 7:17
It depends on what you mean by "dominate".

Fans in series add their static pressure curves together.

As Chris correctly points out, conservation of mass applies, since neither fan has the mystical ability to either create or destroy mass.

So, from the supply fan's perspective, it doesn't know that there is or isn't an exhaust fan.  It simply knows it's inlet and discharge pressure, and operates at the corresponding point on it's fan curve.

If there is an exhaust fan, the supply fan's discharge pressure is lower than it would be if there is not an exhaust fan, so the supply moves more air.

Same, but opposite, for the exhaust fan.

If the fans are seriously mismatched, say for example the intake fan >>> exhaust fan, then the intake fan will simply blow through the exhaust fan.  It may even turn the exhaust fan's motor into a generator, but both fans will still be operating on their system curves (although generator portion of a fan's operating envelope is generally not published).
daly1 (Mechanical)
1 Oct 07 12:14
Also make sure you account for the volume of the "stuff' in the space.  to many of we engineers are tasked by customers with "give me 10 ACH" in the space. Casework vendors did studies years ago and found that most labs are 18-22% of the volume occupied by "stuff". Take the V x 0.85 for most labs to generated the effective Ve in the ACH calcualtions.

Not doing this essentially makes your HVAC system not 10 ACH but 12, and adds  more fan HP, reheat, etc.
GMcD (Mechanical)
1 Oct 07 14:00
Also clearly define if the air change rate is with "outdoor air" or if recirculated air mixed with outdoor air is acceptable.  Failure to define "what is" specifically the "air" that's to be "changed" can lead to some interesting conversations with Lab Commissioning folks especially if you are doing Level 2 to Level 4 Containment Labs, Bio-Hazard Labs, or whatever.
zu56 (Mechanical)
1 Oct 07 15:46
In my case it is easy- it is 100% outside air.

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!

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close