Cooling Math is screwed up.

socal70 said:

If ya' wanna' get down to the nitty gritty, here is the formula to calculate how much cfm you will need to cool your space.

Airflow(CFM)=watts x (3.413 BTUH/W) / 1.08 x delta T (exhaust temp - inlet temp)

Where:
-- 3.413 BTUH/W is the heat conversion from watts to heat.
-- 1.08 is the standard air density. (we could go on and on about humidity, yadda' yadda' yadda')

Example:

You have a 500W grow system. Your inlet temp is 72 degrees F and your exhaust 82 degrees F.

CFM= (500 x 3.413) / 1.08 x (82 - 72)
CFM= 1706.5 / 10.8
CFM= 158

There you have it. You don't really need to know how big the space is really. Because we are taking into account the difference between the inlet and exhaust temps. A small grow space with 500W would have a much higher delta T than a large grow room with 500W.

Thanks,

Click to expand...

come build my new grow room...kiss-ass

socal70 said:

You are correct that this isn't an end all be all calculation. There is much more that can be taken into account. This is just a good method to use since most of us know down to the watt how much power we are using. And we mostly know our temps. Sure there are volumetric calculations that can be taken into account along with how many times you want the air changed out per minute/hour. But I am waaayyy to lazy to go through all of that when I'm at home! hehe... I get enough of that at work!

Thanks,

Click to expand...

it's nothing short of a miracle that your butchered equation gives the correct CFM with every unit and constant wrong, but somehow it does... thats not the density of air, or the density of anything mind u. its either about 1.2 kg/m^3 or 0.0746 lb/cuft. (must be in lb/cuft for english conversion)
THIS is the exact equation, from a thermodynamics standpoint, no matter what u here:

Vrate (cuft/min) = [W (watts) x 0.0569 (BTU/min/W)]/[(0.0746 lb/cuft) x (0.24 BTU/lbR) x (T2 +460 - T1 - 460 R)]

Therefore:

Vrate = (3.178 x W)/(T2 - T1)

Vrate is in CFM
W is in watts
T's are in degree F

the density of air, assume atmospheric P and T are 14.65 psia and 70 F, is roughly 0.0746 lbm/cuft.
when using a 500 W source, 72 F intake temp, and 82 F exhaust temp, the volumetric flow rate of the air required to remove the heat input is 159 CFM. It's just by luck that socal70's equation somehow works out. thats of course with air running into and leaving the box in a steady flow; ur heat removal will be less because you are just sucking air from the top instead of completely exchanging it, hence a much larger fan will be needed to move all that air

U should seriously think about swapping out ur hood for either an inline one, or one with a 6" flange directly above the bulb, so u can hook ur exhuast fan and ducting to the top (venting and cooling at the same time), and still bring in fresh air with smaller fan

socal70 said:

If ya' wanna' get down to the nitty gritty, here is the formula to calculate how much cfm you will need to cool your space.

Airflow(CFM)=watts x (3.413 BTUH/W) / 1.08 x delta T (exhaust temp - inlet temp)

Where:
-- 3.413 BTUH/W is the heat conversion from watts to heat.
-- 1.08 is the standard air density. (we could go on and on about humidity, yadda' yadda' yadda')

Example:

You have a 500W grow system. Your inlet temp is 72 degrees F and your exhaust 82 degrees F.

CFM= (500 x 3.413) / 1.08 x (82 - 72)
CFM= 1706.5 / 10.8
CFM= 158

There you have it. You don't really need to know how big the space is really. Because we are taking into account the difference between the inlet and exhaust temps. A small grow space with 500W would have a much higher delta T than a large grow room with 500W.

Thanks,

Click to expand...

i think this need to be pind or sticked be this is info i love to find 100% helped me out a shit load this is perfect

socal70 said:

If ya' wanna' get down to the nitty gritty, here is the formula to calculate how much cfm you will need to cool your space.

Airflow(CFM)=watts x (3.413 BTUH/W) / 1.08 x delta T (exhaust temp - inlet temp)

Where:
-- 3.413 BTUH/W is the heat conversion from watts to heat.
-- 1.08 is the standard air density. (we could go on and on about humidity, yadda' yadda' yadda')

Example:

You have a 500W grow system. Your inlet temp is 72 degrees F and your exhaust 82 degrees F.

CFM= (500 x 3.413) / 1.08 x (82 - 72)
CFM= 1706.5 / 10.8
CFM= 158

There you have it. You don't really need to know how big the space is really. Because we are taking into account the difference between the inlet and exhaust temps. A small grow space with 500W would have a much higher delta T than a large grow room with 500W.

Thanks,

Click to expand...

WOW!!!

:::Bows down in awe:::

"Ventilation 101" from ICMag
"passive intake, math of many small holes?" from ICMag
"Understanding air flow" from ICMag