100% Outside Air System

[krb]

I have a case where we will build a building that requires 100% OA for process purposes. There is a requirement for 2 air changes per hour. The building is a typical metal building used in the US, and will be insulated. There will be about 10 people in the building normally, and some industrial equipment. My question is, based on the OA requirement, how does the equipment (internal) and external load impact the cooling load calculation? All air will be exhausted, so heat generated from these loads will be as well. Would these loads be added to the cooling load calculated for the Outside air or not?

 

[Drazen]

internal cooling load assumptions are based on realistic prediction of interior loads, you need to separate radiant load from convection load, and than to assess whether most of convection load can be exhausted locally, by exhaust point near the sources of heat.

 

[GMcD]

As Drazen said plus look at whether you are exhausting point source equipment and "source of heat" exhausts to get the total actual "cooling" load that needs to be applied for worker comfort.

 

[Mech110]

if you are performing calculations on HAP, I think the outdoor air parameters can be set to 100% and ACH to 2. HAP will run calculations and give cooling load incorporating your requirement.

 

[LittleInch]

To me, if I understand this correctly, you are cooling outside air to control inside air temp with a X2 air change only. The issue would seem to be temperature control within the building, but without some idea of numbers and relative amounts of heat inputs its not easy to say what might happen.

The bottom line is heat energy and capacity.

Your air enters at a certain temperature having been cooled and is then heated up by the radiant heat and alos by the people and the machines. That will raise the air temp to some temperature which you can calculate before it is exhausted out of the building.

Hence if you get the air distribution wrong, some people will freeze and others will or could get very hot if your machines emit a lot of heat.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[krb]

Thanks for the replies. I am getting a list of the equipment, the loads are mainly motors but in the space so will need to be included. The duct layout is not yet finalized, so we can localize any exhausts to maximize heat removal from the equipment at the proper location. Thanks again.

 

[MedicineEng]

If the 100%OA is driven by process, you might also want to consider spot cooling/heating.
A temperature that you can run a motor might not be the same to maintain an operator comfortable.

 

[willard3]

This is a control volume. Heat in = heat out

 

[chicopee]

"This is a control volume. Heat in = heat out" if it is steady state.

 

[lilliput1]

2 ACPH will probably be not enough for cooling. In a typical office supply air at 55F and average 1 CFM/SF (equal to 7.5 ACPH for space with 8 ft ceiling height) is needed to cool space to 75F. Can you make supply air mix of OA & RA?

 

[lilliput1]

If 100% OA is required you have to increase amount as needed to cool space to design. Consider adding exhaust air heat recovery.

 

[GT-EGR]

From what I read in your question it is two separate answers

Your interior space heat gain doesn’t directly impact the unit operation. Your unit is always just drawing in outside air and that air is independent of your building heat gains

That air change number is your minimum CFM, but then you have to calculate your internal heat gains, and then compare your target space temperature and humidity criteria to the air change number to determine if that amount of air will provide enough conditioning. If not, you’ll have to increase your supply air CFM until it does.

Once you determine how much CFM you need - the max of the cooling number or air change number - from there your load is just based on taking the airflow from peak outdoor design condition to the required supply air temperature you need.

 

[krb]

Thanks Gregory, That is essentially the same conclusion I had come to based on the conditions.