One of the ways we have to differentiate between kilns of different design is to examine
the heat transfer characteristics of those different designs. It is, after all,
the ability of a kiln to transfer heat uniformly and efficiently that makes it a good or bad kiln.
Clearly, if a kiln is not constructed well that will greatly affect the value of the kiln.
However, no matter how well a kiln is constructed, if it fails in its primary duty of transferring
heat properly, it will fail in its mission.
For the purposes of this article, we will discuss only the heat transfer.
Every fuel fired kiln has inside a wind of some magnitude. That fact is obvious
when you consider that whenever you burn a fuel you have products of combustion released.
Those products of combustion have to go somewhere, so we create a chimney to remove them.
As those products of combustion move from the fire to the chimney, they create a wind.
The strength of that wind depends upon the strength of the burning mechanism.
A fire in a grate (burning coal or wood) creates a relatively low wind.
A fire in an aspirator burner also creates a relatively low velocity wind.
An aspirator burner draws air into the kiln around the burning flame much the same as burning fuel in a grate.
The strength of the wind is dependent upon the amount of fuel being burned.
More wood on the grate or more gas in the aspirator burner will create
more wind until the burning limit is reached. As you know there is a point
where you can not add more wood or coal to a grate without beginning to smother the fire.
The same is true for the aspirator burner.
When air and gas are mixed at or before the burner, much greater wind can be generated.
Burners of this type are universally used in industrial kilns.
Some forms of this type of burner create so much wind that they are called "jet" or
"high velocity" burners. In these kilns the strength of the wind is dependent upon the force from the burners.
For a larger dimension inside a kiln a stronger burner is required.
Conversely a less strong burner is needed for a smaller dimension.
Therefore, burner selection depends, partly, upon the kiln's interior dimensions.
Heat is transferred from the burners to the load by radiation of the CO2 and water vapor
in the products of combustion. The wind is made from the above gasses as well as
the N2 and O2 in the products of combustion. Since both N2 and O2 are bipolar molecules,
they absorb as much energy as they emit. Hence, they do not contribute to the transfer of heat.
Although it is widely felt that the strong wind inside the kiln transfers heat by convection,
a quick calculation shows that only about 10% of the heat at temperatures as low as 400o F
is transferred in that manner. For one thing the wind is strongest in the open space in front of the burner,
when it reaches the load it is very weak. A low velocity wind will contribute little to heat transfer.
At higher temperatures the percentage of convection heat transfer is even lower.
Heat is also transferred to the load by radiation from the interior walls and arch.
At low temperatures about 40% of the heat is transferred in this way and another 40%
is transferred from the radiation of products of combustion. As can be seen, the better the emissivity
of the interior refractories, the better efficiently they will transfer heat.
That is why some refractory coatings decrease fuel consumption and why some refractories
are better choices than others.
A kiln design is improved when the designer finds a better way to move
the wind around the load. To begin with one of the best things a designer can do
is quite and old idea. The older designed round "beehive" kilns were designed as downdraft kilns.
That forced the wind to go through the load before it could escape the kiln.
The best modern day kilns use the same idea of a downdraft flue system.
By placing the burners in "fire lanes" outside the load, the wind then
swirls around in that open space and tends to even out the air temperature there.
The opening for the flue is then placed in the center of the load.
The downdraft flue pulls the hot air (that has been evenly heated) across the load,
leading to and even distribution of radiating gasses all over the kiln.
The temperature should lowest at the point where the gasses go into the flue.
They should be even all around the outside of the load.
Once the desired temperature is reached on the outside of the load,
it is held there while the inside toward the flue gradually heats up.
Placing cones or fire checks at various locations around the load should show that
the heat treatment of the outside and inside of the load is the same at the end of soak.