Preparing a Forced-air Wood Kiln

Here's a short video showing my wood kiln, which is almost ready to fire for the first time.  This is the first forced-air wood kiln I've ever seen.  Forced-air means that a fan is used to create the draft.  Normally a wood fired kiln does not use a fan, but instead relies on a tall chimney to create a suction or draft through the convection process of heat rising through the tall chimney.

The fire box is set slightly off-center (to the right) which was done to creat a swirling vortex.  This vortex extends the flame path through the firing space, providing more complete combustion and more even temperatures.

Initially this kiln was a gas-fired forced air kiln, built as a prototype under a grant I received from the New York State Energy Research & Development Authority (NYSERDA).  Originally it had 3 burners, each capable of producing 750,000 Btu per hour.  I built another conventional rectangular kiln of the same size, same wall thickness, same burners, etc., for the purpose of comparison.  My kiln proved to be 37% more efficient than the rectangular kiln, which is really quite an improvement.  This kiln is a downdraft, meaning that the hot flue gas vents at the bottom center of the kiln.

I adapted the original gas-fired design to a wood-fired configuration simply to see if it would work.   This is one big experiment: if it works well, it should fire faster than a conventional wood kiln.  A conventional wood firing usually takes around 3 days.  I'm hoping to cut that time substantially.  We'll see!

Kilns and furnaces

We’re back looking at applications for this block system. So far we’ve looked at architectural applications, water storage tanks, desalination spheres and (possibly?) oil spill containment methods. Today we’ll look at kilns and furnaces.

Kilns and furnaces made with this system offer the benefit of a radial design. Whether a sphere or a cylinder, a radial design uses less surface area per unit volume than any rectangular or square arrangement. A sphere provides the minimum surface area to volume ratio possible.

This is important because surface area is how heat is lost to the outside: by conduction, convection and radiation cooling. If we minimize surface area, we minimize heat loss.

Furthermore, if combustion is used to heat the kiln/furnace (as opposed to electrical heating) then a round structure greatly increases the efficiency of combustion. This is done by extending the flame path in the firing space. When the flame path is extended, the chemical reaction of combustion (combining fuel with oxygen) is more thoroughly accomplished. Combustion in a round kiln (or furnace) produces a swirling vortex, a “whirl wind” of burning flame. This vortex is how the flame path gets extended in the firing space. It also creates a much more even temperature within the firing space. A square or rectangular kiln will create “hot spots” where the flame impinges on walls and corners; it also creates cool spots where the flame is less evenly applied.  In the kilns I built, I used a 'downdraft' design, where combusted fuel is vented out the bottom of the kiln.

By using simple unit shapes, as described earlier in this blog, the cost of constructing a kiln is minimized. Blocks do not need to be custom cut and precision matched to their specific location: blocks are interchangeable.

I received two awards from the New York State Energy Research & Development Authority (NYSERDA) to build and test large scale kilns using this design. I built a standard rectangular kiln of the same volume, same wall thickness, same burners, etc. for the purpose of comparison. The round kiln was 37% more efficient than the rectangular kiln, and demonstrated a much more even temperature distribution. In the rectangular kiln, temperature varied by as much as 120 degrees F; within the round kiln temperature varied by only 5 degrees (at 2,300 degrees). Even temperature distribution is important to industry, where items should all be fired at the same temperature.

This technology can be used to fire ceramics, to make glass, for heat treating wood (kiln drying), and any other processes that require heat treatment. The same principals make this system appropriate for a number of consumer goods: including ovens, barbeques, smokers, etc.

Next time we’ll take a look at glass block, and how this masonry system can be used with glass block.