Heat/Cooling load calculations 1

[var10]

Hello everyone,

I need some help understanding this formula.

qc = Hc/(p*Cp*(t0-tr))

qc Volume of air for Cooling m3/s
Hc Cooling load W
t0 Outlet Temperature C
tr Room or housing Temperature C
cp Specific heat capacity of air J/kg. K
p density of air kg/m3

My scenario - I am basically trying to cool a PC case. The components inside the enclosure gets upto 70 to 100 deg C while overall inside temperature is around 50 to 60deg C. So I was planning to laser cut the bottom panel for fan cutouts and just blow some cold air into the enclosure. So now I basically need to decide on a fan size that will bring the overall temperature inside to around 40 deg C. Parts in development stages and I can do whatever mods I want to now.

What I don't understand in this formula is the cooling load units. I pretty much have all other constants and variables. When I calculate for Hc I get around 120 W for a 7CFM (0.00330363 m3/s) fan. What does this 120 W mean . Am I stuffing something up when I am doing the calculations? Its been a while since I have done thermodynamics so go easy on my silly question.

All I want to know is how much volume flow rate is required to keep the temperature down to 40 deg C. And based on that I can decide on how many fans I need.

Thanks,

V

 

[TD2K]

The 120 W is the heat load. Think of a 100 W incadescent light bulb, it gives off 100W of energy. Or a blow drier, that's up to 1500 W or more.

If you were to have a 100 W light bulb in a box and wanted to keep the air inside the box no more than 10C above ambient, then the equation would look like

qc = 100W / ( p * Cp* 10C ). If you substitute the density of air (p) and the heat capacity of air (Cp), you can calculate how many m3/sec of air you need to supply.

In your case, you need to esimate how much heat is being given off by your computerm, that's Hc.

t0-tr is the temperature rise you want to limit the rise to, looks like about 20C assuming room temperature is about 20C.

Solve for qc and convert to units that you can use to pick an appropriate fan.

 

[IRstuff]

Just offhand, I assume your box is either gaming or CAD? You're probably running closer to 200 watts, so you'd need double the flow.

TTFN
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[var10]

Its an interface unit. I am trying to work out how much power the unit draws at the moment. So if I find that I should have a volume flow rate. This volume flow rate is the flow required to maintain the temperature at tr - room or housing temperature that already exists?? And if I want to bring this temperature down, simply increase the flow rate by xxx% as it is directly proportional to the temperature.

Thank IR stuff & TD2K.

 

[dbill74]

In thermodynamic terms, your computer or the components within it are basically like little space heaters. As they do their thing, they generate heat. This heat energy given off by the components is measured in W = watts.

Worst case scenario, what's the capacity of the power supply? Size your fan for the power supply's power rating, e.g. 200W, 600W etc.