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# Outdoor Air Rate Determination

## Outdoor Air Rate Determination

(OP)
Hi Folks,

I have a Supply Fan Delivering 22,500 CFM of Supply Air. A Return Fan bringing back: 20,200 CFM. And a washroom exhaust fan in the space exhausting 1000 CFM.
To Summarize:
SA: 22,500 CFM
RA: 20,200 CFM
Washroom Exhaust: 1000 CFM

What is my Outdoor Air rate?

### RE: Outdoor Air Rate Determination

Written just like a homework problem. Simple conservation of mass equation will yield the answer.

### RE: Outdoor Air Rate Determination

If this is an HVAC situation, the supply air will probably be around 55F. The return and exhaust would be around 75F. The air volumes have to be calculated at their temperatures.

Bill
Engineering Professionals, Inc.

### RE: Outdoor Air Rate Determination

The outdoor air must at least be that required for ventilation (used to be about 15 CFM per person but now you have to follow ASHRAE ventilation calculation) or be equal to the sun of exhaust air plus pressurization air (about .05 CFM/SF to offset infiltration. You would need to relief air or add additional exhaust instead if the sun of initial exhaust + pressurization air is less than that required for ventilation.

For your case The outdoor air = 22500 SA - 20,200 RA = 2300 CFM
Confirm if this OA is adequate for ventilation and for sum of exhaust + pressurization.

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