Calculating CFM

[InsaneBongHit]

I'm wanting to accurately calculate the CFM for my grow room. I Plan on getting a grow tent which will house 2 1000watt'ers with cool tubes. When calculating CFM do i calculate just for the tent or for the entire room which the tent will be in ?

 

[tip top toker]

i don't quite follow. 1. what are you trying to calculate the CFM of? if you have a fan, and that fan is rated at 100cfm, then under ideal conditions it will move 100cfm. if you are trying to work out the CFM with regard to pressures, possible airflow, flow restriction, maintained fan efficiency etc then you're in for a rather complicated and tedious calculation. 2. why do you need to work it out?

 

[InsaneBongHit]

If you don't know what I'm talking about then don't post.

 

[tip top toker]

necessity does not alter the complication of an issue

you need to work out all your variables, what your available intake will be based on the power of the outtake, the corresponding negative pressure, the pressure drops caused by airflow restrictions, how your plants will effect the airflow from intake to outake (also causing pressure to alter) the air temperature etc, then you can factor all of those into an equation and you'll find an answer.

alternatively you can just think right, a 100cfm fan, a 100cf area to ventilate, so that will refresh the whole tents air in one minute, so a 50cfm fan will replace the air in 2 minutes. just find the size of your tent and you can then easily solve for fan power. a vague but useful enough answer.

 

[Oneton]

find the cubic foot of the tent ........length*breadth*height
u want to totally replace the air every 3 mins so times it by 3 and that will tell you what size fan u need
this is assuming there is no resistance to the fan of course
does that help dude?

 

[tip top toker]

i know exactly what i'm talking about bonghit so i'll damned well post way to edit your post after someone points out it's flaws to try and cover your tracks...

 

[InsaneBongHit]

Ahh, I was rushing around this morning, I thought u where just some clown who didn't understand what I was trying to say. hehe Sorry.
Yes you nailed it well. I don't blame you for assuming complication would put me off.

Nor should complication stand in the way of necessity, hehe.

Ok so my room is roughly. 13 x 9 x 8 ft L,W,H. or 4 x 2.8 x 2.5 meters L,W,H. The tent is 2 x 2 x 2.4 meters. Which will (eventually) house 4 flood tables and two 1000 watt'ers I will also have a sectioned area for clonebox and mothers in the same room as the tent.
"what your available intake will be based on the power of the outtake" So when I calculate what my outtake needs to be I can then calculate from that what my intake needs to be, is what you're saying ?...

I plan to intake from another room in the house, and outtake into the loft( should be pretty constant ?,) <-- I say this because I get stuck at the bit where you mention negative pressure. I plan to use a 6" inch ruck fan thermostat controlled for the cool tubes and want to keep the grow room temperatures at about 27 degrees using maybe an 8 inch exhaust fan and 6 inch intake, but that's what I'm still trying to figure out.

Edit: I'm not sure how air tight these tents are, and when I open and shut the doors/curtains. How that effects things.

 

[tip top toker]

All is forgiven

I'm a bit groggy so maybe not sounding that clear Basically if there is not adequet intake vents or fans, then the exhaust fan will end up trying to pull more air out of the tent than is readily available, which causes a pressure drop inside the tent. this pressure drop will decrease the efficiency of the fan, so ust as a wild example, a 250cfm fan might only move 200cfm due to part of it's energy being spent elsewhere. So you can either open up some flaops on the tent to aid in the intake, or add a fan blowing air into the tent.

You seem to have it pretty much correct, it is however very difficult to work it all out on paper. trial and error, while it can be expensive, is the best method for working out your exhaust fan needs. it should however be fairly easy to fine tune, for example if your exhaust is too powerful, then you can just cover up part of an intake, if too weak, you can just open another flap or throw in an intake fan etc.

on the whole though, i guess you can never have enough exhaust power and it can do no real harm unless you're sucking the plants through the cooltubes

 

[NLXSK1]

I plan to use a 6" inch ruck fan thermostat controlled for the cool tubes

If you are running a cool tube with separate air intake and exhaust for the lights you will want this running the entire time the lights are on.

You may want to have a separately controlled exhaust to handle heat inside the tent.

unless you're sucking the plants through the cooltubes

The cool tubes should be completely isolated from the grow room. If you are sucking plants through them then you are doing it wrong...

 

[InsaneBongHit]

I see what your saying tip top.. I wonder what pressure diff there would be outside the tent compared to inside, would the pressure diffs be so great ? are they pretty air tight ?
If I open a flap on the tent, too much outside light may get in at lights off, Even if i sort that, I want it to not leak any light either if poss.
So it needs to be totally self contained in that sense ideally

If you are running a cool tube with separate air intake and exhaust for the lights you will want this running the entire time the lights are on.

You may want to have a separately controlled exhaust to handle heat inside the tent.

So what do people usually use thermostat controllers for ? intake/outtake fans maybe ? Never worked with anything over a 600watt or a cool tube before. Is a 6 inch ruck fan constantly pushing through a 1000watt'er (or 2) still likely to leave the temp inside the tent higher than 27 degrees ?
Reason I'm now muddled about the thermostat is that, the only option left is that it must be to control intake and or outtake fans so as to take care of heat. But then what about the CFM air exchange thingy ? If I have it right those thermostats will shut the fan(s) on and off according to temperature. But if you want constant air exchange then you can't have those fans going on and off they would need to be set to a 24 hour constant schedule ?

Edit: So I guess my question is what do i use the thermostat for and how do you go about controlling temperature and air exchange at the same time ? thinking about it, the only option left is that the thermostat must control normal circulating/desk fans ? hmmm someone help I'm muddled

 

[InsaneBongHit]

Just found this on another site, not sure if I'm allowed to post which site on here ?

1.) Convert all inches into feet
Quote:
inches ÷ 12 = feet
38÷12=3.16'ft W (rounded up to 3.2)
28÷12=2.3'ft D (rounded up to 2.4)
42÷12=3.5'ft H
2.) Find the volume, feet cubed, of your cab.
Quote:
multiply Width × Depth × Height
W × D × H = feet cubed
3.2'ftW × 2.4'ftD × 3.5'ftH = 26.88'ft3
3.) Find for Exchange CFM rate. We want to echange the volume of the cab 4 to 5 times a minute. I will use the value of 5.
Quote:
feet cubed × 5 = Exchange CFM
26.88'ft3 × 5 = 134.4CFM
4.) Find for Additional Electrical Cooling CFM rate. Add all the watts of all equipment inside the cab.
Quote:
total watts of all equipment ÷ 4 = Additional Electrical Cooling CFM
250 watt light + 50 watt additional equipment = 300 Total Watts
300 Total Watts ÷ 4 = 75 AEC CFM
5.) Add the two CFM rates to get Total CFM
Quote:
Exchange CFM + AEC CFM = Total CFM
134.4 Exchnage CFM + 75 AEC CFM = 209.4 CFM
6.) Find the CFH (Cubic Feet per Hour) from the Total CFM rate.
Quote:
Total CFM × 60 = CFH
209.4 Total CFM × 60 = 12564 CFH

7.) Find for the amount of carbon needed for the filter.
Quote:
CFH × grams per foot of activated carbon (0.36) = total amount of carbon required.
12564 CFH × 0.36g per ft = 4523.04g
8.) Divide the amount (in grams) of total carbon by the length to yield the radius quotient. This is done with 2.6cm thickness as a constant.

I understand it all apart from number 8.) Anyone know what it means ? Length of what ? yield the radius ? 2.6cm thickness of what ?

 

[iHearAll]

Just found this on another site, not sure if I'm allowed to post which site on here ?

1.) Convert all inches into feet
Quote:
inches ÷ 12 = feet
38÷12=3.16'ft W (rounded up to 3.2)
28÷12=2.3'ft D (rounded up to 2.4)
42÷12=3.5'ft H
2.) Find the volume, feet cubed, of your cab.
Quote:
multiply Width × Depth × Height
W × D × H = feet cubed
3.2'ftW × 2.4'ftD × 3.5'ftH = 26.88'ft3
3.) Find for Exchange CFM rate. We want to echange the volume of the cab 4 to 5 times a minute. I will use the value of 5.
Quote:
feet cubed × 5 = Exchange CFM
26.88'ft3 × 5 = 134.4CFM
4.) Find for Additional Electrical Cooling CFM rate. Add all the watts of all equipment inside the cab.
Quote:
total watts of all equipment ÷ 4 = Additional Electrical Cooling CFM
250 watt light + 50 watt additional equipment = 300 Total Watts
300 Total Watts ÷ 4 = 75 AEC CFM
5.) Add the two CFM rates to get Total CFM
Quote:
Exchange CFM + AEC CFM = Total CFM
134.4 Exchnage CFM + 75 AEC CFM = 209.4 CFM
6.) Find the CFH (Cubic Feet per Hour) from the Total CFM rate.
Quote:
Total CFM × 60 = CFH
209.4 Total CFM × 60 = 12564 CFH

7.) Find for the amount of carbon needed for the filter.
Quote:
CFH × grams per foot of activated carbon (0.36) = total amount of carbon required.
12564 CFH × 0.36g per ft = 4523.04g
8.) Divide the amount (in grams) of total carbon by the length to yield the radius quotient. This is done with 2.6cm thickness as a constant.

I understand it all apart from number 8.) Anyone know what it means ? Length of what ? yield the radius ? 2.6cm thickness of what ?

I know this is old af but he/she was refferring to the filter thickness and radius when forming the filter into a cylinder... pretty sure.....

 

[WeedFreak78]

3) Find for Exchange CFM rate. We want to echange the volume of the cab 4 to 5 times a minute. I will use the value of 5.
Quote:
feet cubed × 5 = Exchange CFM
26.88'ft3 × 5 = 134.4CFM

Stupid old posts, No wonder new growers have so much trouble, wrong old info keeps getting drug up.

Exchange rate is total room CFM Divided by 5 (or 3 or 4,depending if you want more or less air exchange)

If yout want to exchange air 3 times in a 150 cu ft room you need a 50 CFM fan: 50 cu ft/min times 3 mins = 150cfm.

 

[Achalemoipas]

>Exchange rate is total room CFM Divided by 5 (or 3 or 4,depending if you want more or less air exchange)

That makes 1 air exchange every 5 minutes. You just failed the 4th grade.