×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*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.

# How to calculate Air exchange in a room?

## How to calculate Air exchange in a room?

(OP)
Hi All,
How would I calculate air exchange per hour in a room with supply and exhaust fans?
Thank you

### RE: How to calculate Air exchange in a room?

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.

### RE: How to calculate Air exchange in a room?

(OP)
Do I count the exhaust fan?

### RE: How to calculate Air exchange in a room?

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).

### RE: How to calculate Air exchange in a room?

Wouldn't the least capable fan dominate?

### RE: How to calculate Air exchange in a room?

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).

### RE: How to calculate Air exchange in a room?

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.

### RE: How to calculate Air exchange in a room?

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.

### RE: How to calculate Air exchange in a room?

(OP)
In my case it is easy- it is 100% outside air.

#### 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.

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:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!