Firing and Drying / Columns / Kiln Connection

KILN CONNECTION: Tunnel Kiln Killing You?

Figure 1. Kiln temperature data at various locations in the kiln setting.

Kilns should be the foundation of the ceramic factory, producing consistent results, creating zero defects and performing efficiently. Reasonable-sized facilities usually have the bulk of firing capacity invested in tunnel kilns, so I will address this class of kiln in this article.

Under certain conditions, you can improve the performance of your older tunnel kiln through well-engineered and executed modifications. What it takes is careful analysis of the causes of poor performance, thorough engineering analysis of the design improvements that are needed, and professional execution of the necessary changes.

Figure 2. The goal is to bring the top and bottom curves closer together so that they are similar to the curve in the middle of the kiln car.

Investigate the Problems

One of the great advantages to modifying an older kiln is that it is a known quantity, even if it is a poor performer. Kiln designers don't try to do a poor job, but building to the lowest possible cost can often result in bad performance. If you can define why the kiln is not working properly, however, you need only to correct those known flaws to make a significant improvement in performance.

If the kiln does a good job in certain areas, you can retain those elements of design and the related good performance in those areas. In other words, you needn't reinvent the entire kiln-you just have to identify areas needing modification and evaluate how to make them better.

The starting point for this effort is to evaluate the kiln structure. Clearly, you must have a sound kiln structure that will serve as a long-term asset. It doesn't make sense to invest in a kiln that is structurally flawed. Next, define your performance improvement objectives. For example, list firing defects that you commonly attribute to the kiln system and do your best to isolate them to specific firing curve characteristics. Buy a temperature logger so you can see the curve flaws and address them.

Figure 3. Low-velocity burners were incapable of generating enough velocity to flow under the load to heat the bottom of the setting.

Case in Point

Let's start with a terrible kiln-one of the worst that I have ever worked on-and evaluate what to do with it. It is a structurally sound kiln that produced ware with multiple defects. Figure 1 shows the kiln temperature data at various locations in the kiln setting. This kiln has poor temperature uniformity and undesirable heating or cooling rates through every section. As a result of poor design and performance, ware fired in this kiln suffers from the following defects:
  • Early preheat cracks due to the fast rate of temperature rise at the top of the load at entry
  • Later preheat cracks due to temperature non-uniformity from the top (level 1) to the bottom (level 6)
  • Oxidation problems at the lower levels due to the fact that the lower levels of setting are heated so slowly at the beginning of the kiln that they have limited time between the oxidation temperatures of 1200-1600°F
  • Variations in ware strength and absorption properties due to non-uniformity of temperature at soak
  • Fine cooling cracks on the lower levels due to extremely rapid cooling through the β quartz inversion when cooling through the temperature range from 1200-900°F
All kilns-whether they operate properly or not-do so for a reason. Improvements can be made by analyzing why the temperature uniformity is poor, etc., and making the proper corrections. When a kiln has performance as poor as this one, it is sometimes daunting to know just where to start. But like all projects, if you break the kiln into individual sections and consider what improvements to make, it becomes relatively easy. So, let's start at the beginning.

Early Preheat

Defects associated with the early preheat of the kiln consist mainly of ware cracking in the upper levels. Clearly, the kiln heats the ware very quickly (red and green curve) upon its entrance to the kiln. Sometimes this is because there are too many BTUs coming from the hot zone into the preheat of the kiln, but, in this case, it is clear that the hot gases traveling down the crown of the kiln are not being circulated and therefore do not heat the bottom of the load. It is also clear is that the bottom of the load is hardly being heated by the exhaust gases at all, and all of the energy in the exhaust gases is being applied to the upper part of the setting.

The solution is simple: add air jets to create circulation. These jets can be either cross- or counter-flow jets, depending on the geometry of the kiln and the space above the load. The end result is to bring the top and bottom curves closer together so that they are similar to the curve in the middle of the kiln car, as shown in Figure 2.

When designed properly, air jets can be very effective; it is possible to reduce the ΔT from more than 500°F down to less than 100°F. At the same time, the kiln's efficiency is improved because more of the load is preheated with the exhaust gases.

Figure 4. Placing a high-velocity burner far away from the hot face causes much of the burner velocity and all of the air entrainment to be lost when firing through narrow ports, resulting in very hot gas streams.

Late Preheat/Oxidation

As the ware advances through the kiln, it is clear that the bottom of the load remains much colder than the top of the load, with a ΔT of 350°F. Such a large deviation can create warpage, late cracking and oxidation problems.

Accordingly, the solutions were obvious: repair the car seals to eliminate cold air infiltration and properly install high-velocity burners to create circulation. This solution can reduce the ΔT from 350°F to less than 50°F. As a result, all cracking of ware is eliminated and oxidation issues at the bottom of the load disappear.

Hot Zone Temperature Uniformity

When firing traditional ceramics, the key properties of strength, size, absorption and porosity are developed during vitrification in the hottest zones of the kiln. To achieve consistency, the hot zone temperatures should always be as uniform as possible. Obviously, this kiln cannot produce accurate properties. Individual burner gases are impinging on the ware in the bottom levels of the kiln, causing a temperature spread of over 100°F as the ware passes each lower burner.

In the case of this kiln, the hot zone walls were 30 in. thick, and, consequently, the high-velocity burners used were set back in the wall roughly 20 in. from the hot face of the kiln lining. This burner installation is typical of many early retrofits of high-velocity burners into older kiln with thick walls.

Placing a high-velocity burner far away from the hot face negates the use of the high-velocity burner in two ways. First, the hot gas jet is narrowest at the point of discharge from the burner. The further away from the burner, the wider the effective hot gas envelope. The result is that hot gases impinge on the load. At the same time, much of the burner velocity and all of the air entrainment is lost when firing through narrow ports, resulting in very hot gas streams (see Figure 4).

The solution is simple: purchase longer high-velocity burners that are made for thick-walled kilns. In this case, air circulation is greatly enhanced, the burner gas temperature is reduced significantly due to entrainment, and the hot gases are directed entirely beneath the load and do not impinge on the product. Achievement of a hot zone temperature uniformity within a ΔT of 15°F is usually easily achieved when the correct burners are installed properly.

The difficulty with this solution is usually cost related; the longer versions of high-velocity burners usually cost at least twice as much as the normal models. Another approach to burner replacement is being evaluated and could solve the cost and performance issues in this area.


The cooling curve shown in Figure 5 is flawed from start to finish, including very poor temperature uniformity from top (hot) to bottom (cold). In traditional ceramics, proper cooling provides for very rapid cooling from soak temperature to 1200°F, and then slow cooling from 1200°F to 900°F through the quartz inversion phase. After 900°F, cooling can proceed at a moderate, linear rate to the exit of the kiln.

It is also important to achieve good temperature uniformity through the inversion, in particular, to minimize fine cooling cracks and highly stressed ware. A reasonable approach to the curve in Figure 5 is shown in red, which depicts faster rapid cooling and a slow rate through the inversion.

The selection of jet location and capacity are dictated by the thermal requirements of cooling. Normally, the rapid cooling section is automatically controlled, or even broken into two or more separate zones to ensure that fast cooling stops at around 1200°F to allow for a safe and smooth transition to the slow cool/inversion segment of the kiln.

Figure 5. The cooling curve shows multiple flaws.

Multiple Rewards

At first glance, fixing the performance of your old tunnel kiln might be something you don't want to tackle. Every kiln seems to have idiosyncrasies that make you wish you were a bulldozer operator rather than a kiln specialist.

However, analyzing the performance deficiencies and applying the proper engineering solutions can resolve age-old issues that may be hurting your product quality and yield. Examining every part of the kiln curve and applying the proper tools for correcting each problem can often pay back the cost of investment very rapidly. At the same time, it can be extremely satisfying to turn a temperamental liability into a precise asset.

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