Csa chimney

I'm planning my next rmh, I have a rectangular chimney measuring 15x30 cm (6"x12"), what csa I consider?

In theory:
SQRT(7.5 cm * 15 cm) = 10.6 cm radius.
Which gives fi around 21 cm.

That might depend if you intend to use the actual chimney or how the chimney is constructed, ie is it rough stone or brick or do you plan to insert an insulated flue. Other factors might be … high the chimney is, what is its overall condition, is it straight up or have bends?
What style of stove you want to use and how much draw you will require, ie if it is a batchbox with a 4’ high riser or a small riserless core that need a strong draw through an insulated chimney?

Fox James wrote:That might depend if you intend to use the actual chimney or how the chimney is constructed, ie is it rough stone or brick or do you plan to insert an insulated flue. Other factors might be … high the chimney is, what is its overall condition, is it straight up or have bends?
What style of stove you want to use and how much draw you will require, ie if it is a batchbox with a 4’ high riser or a small riserless core that need a strong draw through an insulated chimney?

Hi, I intend to use the chimney how It Is: in brick, not isolated, 5 meters high, straight. I thought of a stratification bell, I have not yet decided whether to make a batch or a shorty, of the maximum power that I can

The CSA is just the cross section as far as I understood.
In a rectangle A=a*b
So 30*15=450cm^2.

In a circle A=r^2*pi
Or r=squareroot of (A/pi)= sqrt (450/pi)= 12 cm.
So an equivalent round pipe would be 24cm diameter to have the same CSA.

For the streaming of fluids it does make a difference how the circumference to area ratio is.

Round is ideal, a square has almost the same hydraulic diameter as a round. The longer you make one side of a rectangle, the more it differs from a round with the same CSA.
They call that hydraulic diameter dh. The formula for a rectangle is dh=(2a)/(1+a/b)
If a=b as in a square than the result is dh=a, meaning a square has the same hydraulic diameter as a circle with the diameter a.

For your example it’s 30 and 15.
so dh=60/(1+2)=20cm.
Instead of 24cm it’s 20cm.

And then you would have to consider how rough the surface is. But 20cm is a good approximation which is also a standard round stove pipe size and also roughly a 8” system.

thank you so much for the explanation
now I can start designing the rest

One more thing, before connecting a new burning device it’s always a good idea to do a thorough inspection and a chimney sweep. This also helps your stove work correctly.
Better safe than sorry