How to calculate Air exchange in a room?

[zu56]

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

 

[mechanicaldude]

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]

Do I count the exhaust fan?

 

[ChrisConley]

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]

Wouldn't the least capable fan dominate?

TTFN

FAQ731-376

 

[MintJulep]

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]

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]

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]

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