Ceramic Industry

KILN CONNECTION: Focus on Temperature Uniformity

May 1, 2005
Poor temperature uniformity is rarely the result of one factor-many elements contribute to firing consistency. This column will focus on the heart of the kiln-the burner and combustion system.

Figure 1. Entrainment can eliminate localized overheating of the ware and furniture.

Burners

When the high-velocity style of burner was introduced over 30 years ago, it was quickly embraced as a means of achieving temperature consistency. The idea makes sense-a burner developing a stream of 400 mph gases must provide a great degree of circulation, and the ability to penetrate the load with convective gases does, indeed, improve uniformity.

A high-velocity stream of gases also has another important quality: the ability to induce or entrain internal kiln gases into the stream. This causes far greater degrees of circulation-as much as 10 times the burner stream volume. And very importantly, it also reduces the temperature of the gaseous stream. Consider the burner operating in Figure 1. Even though the burner jet is very hot (1700ºC), the actual jet temperature is only 1245ºC (with a kiln temperature of 1200ºC). Clearly, the entrainment has eliminated the chance for extremely hot areas of burner splash and severe, localized overheating of the ware and furniture.

But entrainment is only a part of the story. It is also critical that the burners be mounted correctly, and this is an area where many kiln manufacturers miss the boat. Most of the entrainment occurs right at the outlet of the burner block; if the block is recessed back into the wall, then entrainment is severely limited. Suddenly, the entrainment is only two or three times the jet volume, and the jet stream above becomes several hundred degrees hotter, resulting in hot spots in areas close to the burner.

When you examine any kiln design, always look at the outlet nozzle of the burner. It should be mounted even with the interior wall of the kiln-not recessed into a port or opening. Some burner manufacturers suggest the use of a venturi block to offset the problem, but this is no solution. The blocks don't significantly help entrainment, and even worse, they block the outlet of the burner when they fail.

Combustion Systems

If entrainment is a good thing, then the kiln's combustion system should be able to insure that it occurs. As you know, there are three primary types of combustion systems: proportional, excess air and pulse.

In proportional systems, air and fuel are proportioned at the proper ratio. Temperatures in the kiln (or zone) are maintained by increasing the air and fuel in tandem. This is a problem, however, because the degree of entrainment always varies. If the burner is at 100% output, the picture looks like Figure 1. When the burner is operating at 50% of output, however, the velocity of the burner is half as much, and the entrainment is halved too. The resulting jet stream of temperatures becomes 1283ºC-enough to make for some hot spots. As the burner throttles down further, the situation becomes worse. The only plus for proportional systems is that they are efficient-minimal or no excess air is used, and this saves fuel.

Excess air systems maintain the combustion air at a fixed level and throttle the fuel to maintain set point requirements. If the air is set at a high level, then all is well-the velocity emanating from the burner is high, entrainment approaches the 10x volume, and the combined jet stream is very close to ware temperature. This would be a great system, if fuel were inexpensive. But the reality is that typical excess air systems consume anywhere from 20-40% more fuel than proportional systems. This may be fine in some specialized applications, but it is not cost efficient in most cases.

Pulse firing is decidedly different, in that it applies nearly all of the required heat input with the burner at maximum firing rate, at stoichiometric ratio. To maintain temperature control, either the pulse high-fire time is varied (preferable), or the time between fixed duration pulses is varied. In this way, a properly designed pulse system always provides the maximum entrainment capability.