Airflow / Temperature / Wattage table (and calculation).
We've all seen the room volume/air flow calculation* and, hopefully, we've all realised that it's next to useless for controlling temperature when high-Wattage equipment is used in small spaces.
To address that, I made a table which shows how much air flow is required to maintain a given temperature rise above ambient.
It's calculated using the basic heat transfer/dissipation equation, which is a standard method for estimating the cooling requirements of hot things in small enclosures (it's often used in relation to electronic devices, for instance).
I hope it's useful
Short (memorable?) link to this post: http://is.gd/airflow (I made it as easy as I could!)
If you've found this post helpful then, please, do pass that on to when people ask the age-old, "how do I work out air flow?" question.
Short link directly to the table: http://tinyurl.com/airflow-temperature
*The room-volume method is derived from calculations used for building ventilation. For those who don't know it, it's 20 x room_volume per hour.
My original post is below here. I'm leaving it because, it has good keywords for the search engines and, it may be useful if people want to copy/paste the formulae but, I've removed the old tables to reduce the page size a bit.
[edit 26-10-11]
To make it easier for everyone I've created a table which can be used to look up lamp Wattage and temperature rise and then read off the required air flow.
<snipped table>
And here's one to convert CFM to m³/Hour...
<snipped table>
Hope it helps.
[/edit]
Calculating the airflow required to maintain a stable temperature, and so determine the size of fan needed, is surprisingly simple.
It boils down to CFM = 3.16 x Watts / DT(°F)
Watts is the lighting power (and any other 'hot' things in the grow space) in Watts.
DT is the allowable temperature rise within the enclosure (i.e. desired temperature minus ambient temperature) in °F.
So, to work out the size of fan required (in CFM) simply plug your lighting Wattage into the equation along with the DT value.
An example is:
Ambient temerature = 20 °C
Target temperature of the enclosure = 25 °C
DT in °C = 25 - 20 = 5
DT in °F = 5 x (9 / 5) = 9
Lighting Wattage = 250 Watts
Plugging these values into the equation gives:
CFM = 3.16 x 250 / 9 = 87.77
This represents the actual throughput required but it doesn't take account of the static pressure necessary to overcome the system impedance (how hard the fan has to suck or blow). But for a free-air system with no ducting or filters it should be fairly accurate.
For those who want a bit more info on how to work it out, here it is.
First, you'll need to know the amount of heat that needs to be dissipated.
The general equation for heat transfer is:
q = Cp x W x DT
where:
q = amount of heat transferred
Cp = specific heat of air
DT = allowable temperature rise within the enclosure
W = mass flow
Mass flow is defined as:
W = CFM x Density
DT is the difference between the ambient air (room) temperature and the target temperature inside the grow space in °F.
At sea level the density of air is 1.2041 kg/m3 (at 20°C) and the specific heat capacity (under typical room conditions) is 1.006 kJ/kgC. After doing some substitution and conversion this gives:
CFM = 3.16 x Watts / DT(°F)
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