Ventilation Calculation

[Zeroman]

Hi all,

I'd appreciate it if someone could set me straight.

I am working on designing an exhaust fan for a small building. I have calculated the heat gain for the room and know that the outdoor design temperature is something like 28*C while the desired inside temp is about 20*C. Using the formula Q = 1.2*CFM*deltaT I should be able to calculate the CFM. But I have one problem with this, How can I cool a room to 20*C if I am pulling 28*C air into it (through louvers)? It just doesn't make sense to me, I would think that I could pull all sorts of 28*C air into a room and it would only drop and stay at 28*C.

Thanks.

 

[MintJulep]

Sounds logical to me. Can't cool down a room with hot air. One of those pesky laws of thermodynamics.

By the way. The 1.2 constant in your equation should really be closer to 1.1. And using that equation, T must be in degrees F.

 

[Zeroman]

Thanks MintJulep,

I was just about to use that equation then I thought "wait a minute". So in this case I would have to use some type of cooling ie. glycol cooling coil or something. Right?

Thanks again.

 

[MintJulep]

Yep. Or something.

 

[friartuck]

The simple way to think of it that you need a delta T to drive the heat flow.
If you supply air into a room and the room is at 20 and the supply air is at 20, then the supply air will neither cool nor heat.

But...if you supply warmer air, you will warm the room and if you supply cooler air you will cool the room....Dead simple.

Now, if you have a room in a hot climate...and you do notthing, it will become very warm, i.e. typically above the outdoor temp. So the room might be 30 if outside is 28.

NOW, if you supply outside air into the room, it will cool it (only slightly, because it is 2 degrees cooler than outside)

In effect, you can't get something from nothing so logically you need to put a cooling system in (If you can afford it)


Friar Tuck of Sherwood

 

[Zeroman]

Thanks Friar !

 

[chicopee]

May be you could use an air conditioning unit in the room to cool down to the 20 deg C level.

 

[HVAC68]

If you are in a place where the wet bulb temperature is low, and you don't mind humidity inside the conditioned area, and you if are after a cheaper solution (both initial and running), then try an air washer system.

HVAC68

 

[imok2]

With a dry climate it sounds like a good candidate for an evaporative cooler!