Fresh Air Exchanges 1

[TEGGER]

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

 

[quark]

If total flow rate is 115000 cfm then outside air flow (35%) will be 0.35 x 115000 cfm. Room area is 87000 sq.ft and assuming a height of 20 ft for library, the total volume comes out to be 87000 x 20 cu.ft.

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,

 

[TEGGER]

Yes most of the ceilings on first floor are 20ft with second floor ceiling at 12-14ft except for a rotunda area. I wondering if 35% is a good number to use at each AHU. We have been using this percentage for the past 5 yrs. So if I understand you right, we are getting 1.39% air exchanges per hour? Thanks

 

[dysung]

I believe that 1.39 is the number of air change per hour.
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

 

[TEGGER]

Thank You Dan for your explanation! It really helps!

 

[danmsnyder]

If I were you I would install CO2 monitors in each AHU. They usually cost about $200 a piece and will more than likely pay for themselves the first year. The Outside Air requirement is for the the volume or the number of people. I doubt that the library is at 100% for every minute of its operation. The monitors can send a signal to open or close the OA damper to the appropriate setting based on the Parts Per Million concentrations for CO2.
Hope this helps.

 

[ChasBean1]

danmsnyder - CO2 control is a good concept that I've seen fail in the execution phase in about 95% of the projects I've been on. ATC contractors buy and install them, establish communication, and then wash their hands of any calibration issues. Calibrating and maintaining calibration is absolutely required and almost always neglected. Be sure you (or someone) has the initiative to have a zero and span gas and calibrate the sensors after installation.

 

[danmsnyder]

ChasBean1 - You're absolutely right. But of course maintenance is they key to every design! Too many engineers specify bells and whistles and have no concept of what's involved in maintaining them. I would recommend a semi-annual calibration. Also you need a good contractor to know what locations to place the monitors. And as a safeguard you could always allow for a cushion, ASHRAE 62-1989 requirement of 15 cfm of air, so I would set them for 20 to 25 cfm. And you must also remember to maintain your dampers and acutators. It's very easy for the rods to slip, blades get stuck, or the mounting bracket to come undone. But no one every said energy efficiency was easy.

 

[Vicsidhu]

Tegger:

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