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Kiln Connection: Pressure Control in Tunnel Kilns, Part 2


Pressure curve anomalies (red) can be adjusted (black) to improve firing.

As discussed in Part 1 (CI, September 2011), internal kiln pressure in a tunnel kiln governs airflow direction and affects temperature uniformity, soaking time, and energy consumption. Management of the kiln pressure profile makes a huge difference in kiln stability.

Taking Measurements
The first step in managing kiln pressure is to develop a set of readings on your tunnel kiln. Select a manometer with sufficient sensitivity—0.001 in. resolution—so you can develop an internal pressure curve of the kiln. Try to measure the pressure in the kiln every 20 ft, and always measure the pressure at the same elevation (preferably close to the kiln car base). At the same time, measure the oxygen level at each of these locations; this supplemental data will be useful when analyzing necessary changes.

Figure 1 shows several pressure curves of tunnel kilns. The red curve represents kiln pressure before adjustment, and the blue curve is the percentage of O2. The black curve represents pressure after the kiln was adjusted, while the green curve is the corresponding percentage of O2.

The red pressure curve has some anomalies. The kiln is too negative in general, and as a result, suction of cold air into the kiln is prevalent for nearly all of the heating cycle, which wastes energy and affects temperature uniformity. In addition, the kiln pressure is barely positive in the hottest zones, which likely affects energy and the ultimate uniformity of temperature.

Finally, the pressure in the cooling zone is less than the pressure in the hot zone. This means that hot gases are being sucked into the cooling zone. This causes a number of undesirable effects, including loss of energy, an additional burden on cooling, and the exhaustion of combustion byproducts into the cooling zone (where they will affect products being dried by waste heat). Note the oxygen curve displays depressed O2 levels in the cooling zone—a further indication of reverse flow or “backdrafting.”

Making Adjustments
Fixing the curve in Figure 1 requires the adjustment of both the main exhaust system and the cooling exhaust system. First, the cooling exhaust should be reduced to raise the pressure within the cooling zone so that it exceeds the pressure in the hot zone. Next, the products of combustion fan suction should be reduced to shift the overall pressure curve upward. These steps will raise the kiln pressure and eliminate the flow of hot zone gases into the cooling zone. Because these adjustments are interdependent, they will have to be repeated a few times to attain the proper results.

 The black curve in Figure 1 is much better than the prior example. The kiln pressure rises progressively from the entry to the exit, indicating proper flow of gases down the tunnel. It is confirmed by the oxygen curve, which shows the typical decrease in oxygen in the heating zone, followed by an immediate rise after the hot zone.

An area of concern would be the relatively high pressures in the hot zone and cooling. Typically, the hot zone pressure (measured at the kiln car base) should be in the range of +.01 to + .02 in. WC. When pressures are higher, you’ll usually find that excessive leakage of hot gases from the kiln are penetrating the car-to-car interfaces and overheating the car running gear. If this were the case in this example, additional exhaust in both the cooling zone and the products of combustion fan would be indicated, reducing the pressures throughout the kiln.

When airflows within a tunnel kiln are out of balance, energy usage always increases and temperature uniformity is poorer than it should be. Managing kiln pressure and oxygen curves will tell you a lot about internal airflow and is the first step in correcting pressure anomalies.


Ralph Ruark is a registered professional engineer with degrees in ceramic engineering and business, and 37 years of experience in the ceramic industry. He formed Ruark Engineering Inc. several years ago and serves as a technical consultant to a number of ceramic manufacturers and kiln companies. He is dedicated to assisting ceramic companies with a variety of kiln and firing needs, leading kiln analysis efforts, providing training expertise, and improving operations. Ruark can be reached at (941) 730-2253, fax (888) 370-2546, email or online at

Any views or opinions expressed in this column are those of the author and do not represent those of Ceramic Industry, its staff, Editorial Advisory Board or BNP Media.

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