Make up air

[Mech]

IMC - 2006 and 2009

I have a small school with four classrooms, a few administrative offices and a "cafeteria". I am dealing with air conditioning make up air for people only, no kitchen type equipment for food prep.

How do I calculate the total Outside Air required?

Is it:

1. classroom OA + office OA + cafeteria OA+ corridor OA?

2. classroom OA + office OA + corridor OA? (Can I assume that the cafeteria is incidental to the classrooms and will hold the occupants from the classrooms and offices and therefore do not need additional OA for the cafeteria?)

Thanks in advance.

 

[Mule]

403.2.2 Transfer air. Except where recirculation from such

spaces is prohibited by Table 403.3, air transferred from

occupied spaces is not prohibited from serving as makeup

air for required exhaust systems in such spaces as kitchens,

baths, toilet rooms, elevators and smoking lounges. The

amount of transfer air and exhaust air shall be sufficient to

provide the flow rates as specified in Sections 403.3 and

403.3.1. The required outdoor air rates specified in Table

403.3 shall be introduced directly into such spaces or into

the occupied spaces from which air is transferred or a combination

of both.

I wouldn't omit the cafeteria because it is an idependent space and needs it own fresh air.

I would calculate the entire area of each area to get the required OA needed. Now that is just my opinion and there are a lot more people out there a lot smarter than me!

So I vote #1

 

[klarenbeek]

I agree with Mule. Even if they say the entire school will be in the cafeteria at the same time now, that doe'snt mean they won't decide to change the schedule in the future and have 3 classrooms and the cafeteria occupied at the same time, or as the school grows come up with creative scheduling to make use of the space they havebefore having to do an addition.

 

[TJacobs]

What Mule said.

 

[pyrguy]

Never assume. Go with #1. If they want to limit the outside air automatic controls could be used to control the outside air in the cafeteria when not fully occupied.

 

[Dr. J]

The OP has both too much info and not enough info.

RE: Too much info - you said 2006 and 2009. Which is it?

RE: Not enough info - Is this a 100% outside air system (you said make-up, is this what you mean) that delivers OA to each space, then each space is conditioned by a separate AC/Furnace or Fan-coil? Or is it a central AHU/RTU that delivers OA mixed into the SA to all spaces?

If the 100% OA system, then it is a simple matter of adding up the OA CFM for each space. If the central AHU system, then you use the multiple spaces equations.

2006 and 2009 are very much different for outside air calcs. In 2009, a CFM/sf factor is used as well as CFM/Person, the multiple spaces equations are completely different, and a diversity factor is allowed to account for the fact that the same people in the classrooms are in the cafeteria at different times. To use diversity in the 2006, you would have to go with the "approved statistical data" exception to 403.3.

 

[RJJ]

Real good points DrJ! AND yes is it individual units or one big mother!