Supply Air Flow 1

[afa13]

Hey Guys,
I need your help with the following:
I am starting a design and at the moment i'm just brainstorming. I got the outdoor design conditions as well as the indoor conditions. I managed to determine the exfiltration flow rates for each room. But what's been busting my head is how to determine the total air flow (mixed flowutdoor and recirculated air) so that i can input them into HAP to get my final loads. can someone help me with this please?

 

[ZDR1985]

Supply air will be calculated by HAP based on your building heat load.

Once you input your desired outdoor air rate, HAP will calculate the total coil load for you.

 

[afa13]

Thanks man... what got me confused is a design I saw and it had its own HAP. In there, the designer had mistakenly input the supply air flow as the OA requirement.
So to recap, after calculating the fresh air requirements (min. as recommended by ASHRAE) I input them in the OA, is that it?
And I have another question regarding construction: I saw a duct that is slightly bent/damaged due to transportation but then the contractor managed to smooth the surface however there is still a gap. The contractor insisted that it can be fixed by installing clamps on both ducts in order to tighten them together, is this allowable? According to my experience this would work but I would like to hear what u guys have to say. . .

 

[RandomUserName]

afa13,

OA Airflow in HAP:

As you create "spaces" in HAP, each space automatically calculates required OA per 62.1 as you input people loads and total square footage based off you selection on the "general" tab for the OA Ventilation Requirements. As you create a "system". under the "system components" tab for ventilation air, there is a dropdown selection box for ventilation sizing method (either sum of space OA airflows or your current ASHRAE 62.1

Supply airflow is based on two items:

1. the difference between the supply air temperature and the room air temperature at summer design (delta T)
2. the sensible heat load that is required to be rejected to maintain room air temperature at summer design (Qs)

Qs = (1.08)*(CFM)*(delta T)

Assuming Qs is fixed, if you can increase delta T (drop supply air from 58F to 55F, or increase summer design room air temperature setpoint from 70F to 75F) you can decrease CFM required to meet the load, and vice versa.


As for the ductwork damage, lots of questions. Is it exhaust or supply air? Is it going to be insulated? Does it carry something toxic?

 

[afa13]

RandomUserName,
Thanks for the insight. But I saw a design where the engineer had input in the OA tab the exfiltration rate (leakage from roof and exterior walls) and in the space usage tab it is "user defined" and in the space usage defaults' tab it ASHRAE 62.1. So apparently I have to calculate the leakage from the space and input it as my OA requirement. I don't understand why but from now on I will do it like that.
As for the duct it is a supply air duct and it carries no toxic materials whatsoever and it is used to offset the sensible heat of an unmanned building where the temp in most spaces is to be maintained at 27 C (of course the duct is insulated with vapor barriers as well).

 

[mirza057]

The air and water side calculations will be done by HAP. All you need to do is give the correct inputs.

With regard to the damaged duct, It will be required to carry out a smoke or light test on the duct and the contractor will be obliged to change the piece of duct if it fails the test.

Just ask the contractor not to insulate (if any) the damaged piece before carrying out the test.

 

[ZDR1985]

afa13,

I think you are talking about pressurization.

Adding all the leakage rate from doors, windows, external walls, and floor. Compensation of exhaust air as well. Then using this as your OA requirement.

Estimated leakage rates can be found in BSEN 12101-6.

 

[afa13]

ok guys! thanks a lot!