Cooling Math is screwed up.

andrewcrestline

andrewcrestline

Active Member
Im not sure if I under Calculated? I have a 3 foot by 3 foot by 5 1/2 foot box. Mylar all around the box. I have a 600 watt HPS and a 210 cfm fan pulling from the top. My temp is pushing 100. What the hell? Any help would be huge!!!
 
What temperature is the intake? Where is the exhausted air going to? It needs to be well away from where the air is entering the box, into another room or preferably outside.
What type of fan is it? A lot of axial fans boast impressive cfm ratings for the price, but that is free air movement, from one side of a wall to another, and the actual rate of flow drops off sharply if you link it to any ducting or a filter. A centrifugal, or vortex fan works much better, but you should still add 5% per air cooled light or 10-15% per non air cooled light, 20% if you are using a carbon filter, and 25% if the ambient temps are over 80f for much of the time the lights are on.
Try running the lights during the night to maintain a more even temperature, too.
 
I live at 10,000 feet, and the intake is between 60 - 70. I think that doing it at night with a lot lower intake temp would make a huge difference. I will check tonight and repost that temp.
 
Make sure the AREA of the intake is atleast 2x the AREA of the exhaust. If you have any restrictions on your intake (i.e. lightproofing, filter, duct bends), then add more to be safe. Remember, the area of a circle is 3.14xr^2
 
beginningbotanist420 said:
Make sure the AREA of the intake is atleast 2x the AREA of the exhaust. If you have any restrictions on your intake (i.e. lightproofing, filter, duct bends), then add more to be safe. Remember, the area of a circle is 3.14xr^2
Click to expand...

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,
 
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...

Hey mate, you're fuckin' good.
 
socal70 said:
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
Click to expand...


This isn't an end-all of conclusive numbers, its more like a general rule. I think for a 600w exhaust temps would be over 90. It will work, but is it completely useful. Delta T depends on the CFM of the fan, so its entirely relative.

how big is the room the tent is inside of?
 
question - formula tells me I need 157cfm... but how do I translate that? In a room that's only 120 cubic feet, does that mean I need to fully exchange my air more than once a minute?
 
Yes it does, and with hid lighting most grows will need to exchange the air more than once a minute, unless they have low intake temperatures or have a small bulb for the size of the room.
Also don't forget to factor in ducting, and any filter you may have. Both will affect the actual cfms that you are moving, for a filter add 20% to the size of the fan, I'm not sure on the ducting as different types vary, but I'd say at least 5% for every 6' of ducting (anyone knows better feel free to correct me there)
I would make sure to do these calculations based around the hottest temperatures you encounter, to make sure of having an adequate fan even if the outside temp soars.
 
Hairy Bob said:
Yes it does, and with hid lighting most grows will need to exchange the air more than once a minute, unless they have low intake temperatures or have a small bulb for the size of the room.
Also don't forget to factor in ducting, and any filter you may have. Both will affect the actual cfms that you are moving, for a filter add 20% to the size of the fan, I'm not sure on the ducting as different types vary, but I'd say at least 5% for every 6' of ducting (anyone knows better feel free to correct me there)
I would make sure to do these calculations based around the hottest temperatures you encounter, to make sure of having an adequate fan even if the outside temp soars.
Click to expand...


ok let me see if i have this right:
BTW I have a 400w hps.

400*3.413=1365.2
1365.2/10.8=126.407

therefore I will need 126.407cfm? so in order to replce my air 3-5times per minute i will need a fan with 127cfm roughly?

for fans with a scrubber add 20%.

127*20%=25.4
25.4+127=152cfm to scrub the air 3-5 times per minute?

so as long as a i have a fan with a rating of 152cfm i will be ok?

I have a foothiils filter/scrubber and fan in one. It has a 6 3/4" axial aluminum fan with a rating of 240cfm will this be ok for my lil closet grow?

closet dimensions are (4Lx2Wx6H)
 
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...

So how does a [BTU/h]/(kg/m^3)(degree F) work in a formula with no conversions?? Think about it, nothing cancels out, density has to be in lb/cuft, and BTU/h has to be BTU/min. ALSO your missing 1 very important term in your equation, the entire formula is suppost to be divided by Cv (specific heat of air under constant volume), which is in units of BTU/lbm R (or kJ/kg K for metric). You're missleading this entire forum with false formula.

Unless you have an inline air-cooled hood or are constantly pumping in and pulling out air with a couple fans, you will always have a heat issue. My suggestion is to add another fan, same as u already have, to bring fresh air (preferably cool) into the box while it's being exhausted 24/7. most ppl have an AC unit to cool things down, but u might be limited to grow space, so this isn't an option.
 
Hairy Bob said:
There's a simpler formula here https://www.rollitup.org/grow-room-design-setup/217025-my-new-grow-room.html#post2801285 that is less controversial.
Click to expand...


i looked at the link you provided and i have a lil reservations, but i might just be paranoid.

4x2x6=48 cubic feet

48/5minutes=9.6cfm

now my contention is if i am dividing by 5 isnt this wrong, rather i ought to multiply by 5 to get the amount of cfm's for every 5 minutes? I may wrong, but please clear this up b/c i see a lot of ppl either dividing the cu ft or multiplying it.

the faq's, for example, show to multiply it under ventilation
 
dbo24242 said:
This isn't an end-all of conclusive numbers, its more like a general rule. I think for a 600w exhaust temps would be over 90. It will work, but is it completely useful. Delta T depends on the CFM of the fan, so its entirely relative.

how big is the room the tent is inside of?
Click to expand...
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,
 
switchone said:
ok let me see if i have this right:
BTW I have a 400w hps.

400*3.413=1365.2
1365.2/10.8=126.407

therefore I will need 126.407cfm? so in order to replce my air 3-5times per minute i will need a fan with 127cfm roughly?

for fans with a scrubber add 20%.

127*20%=25.4
25.4+127=152cfm to scrub the air 3-5 times per minute?

so as long as a i have a fan with a rating of 152cfm i will be ok?

I have a foothiils filter/scrubber and fan in one. It has a 6 3/4" axial aluminum fan with a rating of 240cfm will this be ok for my lil closet grow?

closet dimensions are (4Lx2Wx6H)
Click to expand...
What is your inlet and outlet temps? You used 10.8 as your temperature delta T You need to know those tow temps to get an accurate calculation. 10.8 was based on my example of having 72 deg inlet and 82 deg outlet. (82 - 72=10) then multiply that by 1.08 which is the air density.
 
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'm having a problem with your formula.

First, it appears that as your delta T value gets larger the smaller the fan required becomes. This seems counter-intuitive, but I'm no expert.

Second, you seem to be ignoring the fact that he is growing at 10,000 feet above sea level. This would change your 1.08 standard air density value to (I believe) .77, which would amount to about a 30% increase in the cfm's required.

All that aside, I'm also wondering if he's doing some kind of CO2 supplementation; I don't know if a MJ plant would have as much trouble taking in CO2 at 10,000 ft as a human would have taking in oxygen?

It would be interesting to see his results...
 
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