Outdoor Air Calculation

[esfo71]

Hello,

I have a question with concerns to Outdoor Air calculation. I have a Dictation Room within a hospital that is being reconfigured by the architects. This room has no exterior walls and is in the basement. The Volume of the room is about 3100 cubic feet and the Floor Area is about 400 square feet. I calculated the Heat Losses as follows:

Qlights = 3.41* (27*26W) *1*1*1 = 2400 BH
Qdesktop-lights = 3.41 * (5*50W)*1*1*1 = 900 BH
Qpeople = 5*250*1 = 1250 BH
Qequipment = 3.41 * (5*200W) *1*1*1 = 3500 BH

Qtotal = 8050 BH

CFM = 8050 / (1.085*(75-55)) = 371 CFM

The OA as per ASHRAE is recommended as 20CFM per person. Since there will be 5 people in the room, 100 CFM will be the amount the OA.

My question is the 100 CFM of OA added to the 371 CFM due to Heat Losses to make a total of 471 CFM of supply air or is 100 CFM part of the 371 CFM, that is, 371 CFM will be delivered to the space.

I would greatly appreciate it if someone could help me on this.

 

[KiwiMace]

It is included in the 371 cfm.

You also need to account for the cooling load of the 100 cfm of outdoor air which will vary depending on the conditioning or not of this air prior to it being supplied to the space.

 

[esfo71]

Thank you KiwiMace!!!

You are correct. The OA will be mixed-in with the room return air (271 CFM 75'F / 50%RH of Room air mixed with 100 CFM of 90'F DB / 75'F WB of OA) where it will then be treated by a fan-coil unit (which is located in the ceiling space of the room) and then redistributed to the room. I calculated:

Q = 4.5 * 371 CFM * (31.4 - 22.6) = 14692 BH = 1.2 Tons of Total Cooling

Again, thank you.

 

[tys90]

What year of ASHRAE 62.1 are you referencing? Using year 2004 , or 2007, the calculation for 5 people in an office of 400 square feet results in 49 CFM of OA. 5 CFM per person + 0.06 CFM per square foot. This is assuming the ventilation effectiveness is 1.0 in both cooling and heating.

 

[esfo71]

You're right tys90. I was referencing an old version of ASHRAE...I believe 1990...it was just for a quick calculation. Your method is more up-to-date.

 

[317069]

There is a space load wich is used to determine the cfm required for the space.
and there is a coil load wich is used for equiment selection.
OA is a part of the coil load not the space load.
it doesn't effect on the cfm required for a particular space.
if you need a 2000 cfm for a space, you still need this 2000 cfm either you need 100 cfm or 200 cfm or.. of OA.

- your equation:
Q = 4.5 * 371 CFM * (31.4 - 22.6) = 14692 BH

[air flow] is in cfm
[delta T] is in C degree
[Q] I guess is in BTU
what is 4.5 factor?.
and don't forget the latent load too