I have found several pages that all repeat the same:
Soft brick kiln with 9" walls requires 10,000-16,000 BTUS to reach cone 10 . The higher number will give you a firing of between 6-8 hours, while the lower BTU/CF figure will give you a 14-16 hour firing. Somewhere in between means a 10-12 hour firing.
OK, fine by me
Lets say we have a 250 l kiln (9 cf). We are going to need 16000 x 9= 144000 BTU
We know that 1 Kg of propane packs about 13.97 kWh or 47668 BTU
So, 144000/47668= 3.02 Kg (6.65 lbs) of propane per firing to cone 10?
(I am using weight and not the volume because volume can be anything if exact pressure and temperature are unknown)
Q 1: How close is this to actual gas usage?
About the downdraft kilns and actual stacking area.
Q 2: Is it safe to say that 250 l total inside volume will give us only about 155 l of stack space?
I am asking, because we like to make some firing cost calculations before we start taking bids for building a 250 l kiln... or do we actually need a 350+ l kiln