Fresh Air Exchanges
Fresh Air Exchanges
(OP)
How would you calculate for % of outside air into a 87,000 sq/ft library using 4 AHU's delivering a total of 115,000 cfm into the building. We currently are using 35% OS air at the dampers. What would the air exchanges be p/hr? Thank You





RE: Fresh Air Exchanges
Therefore, no. of fresh air changes = (Flow rate/volume)x 60 = (0.35 x 115000)x60/(87000 x 20) = 1.39
Just check the room height.
Is this what you are asking for?
Regards,
RE: Fresh Air Exchanges
RE: Fresh Air Exchanges
it can be interpreted as the entire volume of air in the library will be replaced with outside air 1.39 times per hour.
ASHRAE standard 62-2001 "Ventilation for acceptable indoor air quality" manual has a table that shows required outside air amount per person. For libraries it's 15cfm per person and occupancy rate is 20 people/1000 SF.
WIth this guideline, you can come up with the number of people in the library and total required amount of outside.
87000 SF * 20 people/1000SF = 1740 people
1740 people * 15 cfm/people = 26100 cfm
115000 cfm * 0.35 = 40250 cfm
It seems there is enough outside air being provided in your application(may be too much in the point of energy conserving view). To get the minimum amount of outside air to save energy you can set the outside air damper as follows:
% of outside air = 26100 / 115000
which gives about 23%. If i were you i'll set the damper to allow 25% outside air.
hope this helps..
Dan
RE: Fresh Air Exchanges
RE: Fresh Air Exchanges
Hope this helps.
RE: Fresh Air Exchanges
RE: Fresh Air Exchanges
RE: Fresh Air Exchanges
Have you checked if the unit can deliver 35% OA. This is on the extremes. I once ran into a problem just saying that on the drawings, but the model I specified could not do it. Big Mistake. It was a high density small restaurant and I had to increase the make up air fan to accomodate the high ventilation air. I certainly did not look good as the unit was bought.
Vic