Airflow and thermal guidelines

How well do you know your ventilation system?

If we try to 160m3 / h variant of the formula (described in more detail in the quote below):

Q = qv * d * cp * (thaw-tr)
400W = (160m3 / h / 3600 s/ h) m3 / s * 1.2 kg/m3 * 1000J/kg/K * (thaw-tr) K
400W / (160/3) (J) / (s) / K = 7.5 K temperature difference.

If you have a 20 ° C on air flowing in 27.5 ° C will be the temperature in the culture, which of course is fine.
If you have 25 ° C, the temperature in your garden will be 32.5 ° C which is more problematic for a single culture, but something that can be beneficial for some advanced techniques.

With the larger the fan, the difference is 3.25 ° C

Now the temperatures will of course depend on more than just the fan, lamp and the total air change rate but it can at least provide some guidance, from which one can try things out.

This is very good if you want to calculate how large / many fans you need to lower the temperature to a certain degree.
To know how much air you can employ to get the desired temperature, you can use the following formula to figure it out.

Q = qv * d * cp * (thaw-tr)

Q = heat output of your lights in W
qv = air flow in m3 / s
d = air density = 1.2 kg/m3
cp = the specific heat capacity (of the air) = 1000 J / kg / K
thaw = desired air temperature
tr = room temperature: the intake air
qv's what you want, so we re-package the formula a bit

qv = Q / (d * CP * (thaw tr))

Example:

You have a 400W HPS, that is Q = 400
For example you wish that you have 25 degrees in the culture, thaw = 25
Let’s say the temperature in your room is 20 degrees, tr = 20

qv = 400 / (1.2 * 1000 * (25-20)) = 0.067 m3 / s (cubic meters per second)

Take that times 1000 and you have the flow in liters / s

Thus 0.067 * 1000 = 67 l / s

If you count m3 / t, it should look something like this 0.067 * 3600 = 241.2 m3 / h
Or if you prefer cfm (Cubic Feet per Minute) take 1.7 cfm per each m3 / t, so 241.2 / 1.7 = 141.9 cfm.

And now that you know the air flow you need you can start watching for the great fans