We use devices called “Burners” to fire our downdraft kilns. This page provides a description of how they work.
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First, look over Figure 1. That may seem like just a big jumble of pipes, but it’s actually all the parts to one complete burner. It’s a “forced air”, natural gas burner, and there’s two behind each of CAW’s downdraft kilns. Together, they can pump in enough heat (500,000 BTU/hr) to raise a fully loaded kiln up to 2360oF (ie, cone 10) over the course of a day. The temperature of the gas flame itself is around 3500oF.
Now just focus on the parts labeled in the Figure: The Fan (located up top) blows gas and air down the large 2” Burner Pipe into the Nozzle down at the end, where it’s ignited by the Pilot. There’s separate controls for gas and air, so the mixture going to the main burner can be adjusted (more on that below). The controls allows us to set the kiln atmosphere to any desired level of oxidation or reduction at any point in the firing.
Forced air burners like ours are a great choice for powering a kiln. They don’t rely on gas flow or chimney draft to mix in primary air, so weather conditions don’t affect how they burn. In fact, our kiln chimneys are only slightly higher than the kilns themselves. Forced air burners also produce a longer, more powerful flame, so the kiln can run on just two burners, rather than the four to six that would be needed using venturi burners.
Now, check out Figure 2 and you’ll see what’s on the inside of those burner nozzles (ie: viewed from the inside of the kiln). The main nozzle consists of a central 1.25” diameter hole surrounded by 12 small jets that focus the main flame, preventing it from “blowing off” the pipe. The oval pilot burner sits below the main burner, its flame curves upward and continuously ignites the combustible gas/air mixture shooting out of the main burner. Both burners are set back from the rear wall of the kiln by about 2 inches, so the entire 22.5 in2 rectangular port is available for secondary air entry.
Here’s a view of the burner and pilot injecting flame during a firing.
Lighting the Kiln
Firings begin by lighting the pilot. It’s the small venturi style burner below the main burner. In venturi burners, suction caused by the gas jet pulls in enough fresh air to mix with the gas, so no fans or electricity are needed for it to operate.
In order to light the pilot, the operator pushes a spring-loaded red pilot button (top label in Figure 3) while holding a flame in front of the burner. A flame sensor detects when the pilot ignites, and then allows pressure to reach the main gas control valve. After that, the push button is released. If the flame ever goes out, the sensor detects the loss and shuts off all the gas.
There’s also an electrically-operated “shut off valve” in the pipe upstream of the burners so that, in an emergency, gas can be shutdown from a switch on the wall in front of the kiln.
Next, we have to get the main burner going. As mentioned earlier, there’s two controls: one for Primary Air and one for Gas. Primary air blows downward from the gray fan on the top (see the blue arrow in Figure 4); flow is regulated by opening/closing the movable flap shown.
Gas sprays in from the small 0.25” side pipe (see red arrows). Gas flow is controlled by the faucet handle labeled in the pic. As the handle is turned, pressure can be read on the gage ( 0 to 5.0 Inches of Water ) .
The gas and air mix together as they travel down to the nozzle, and ignite when they reach the pilot flame. Typically, we start up with a gas pressure of about 0.3 in water and an air flap opening of 0.25 inch.
Based on the pressure meter reading, the air flap position, and the color and sound of the flame, the operator can judge the degree of combustion.
During a firing, there are periods of oxidation and reduction, and a desirable profile to ramp kiln temperature slowly upward, so the controls are monitored on a regular basis to trim the settings.