Tunnel kilns are the most efficient in the industry, being used regularly in factories with high continuous production, since the fuel production/consumption ratio is high.

However, if they are not set up correctly, they lose much of their efficiency.

One of the most important factors to consider, in order to avoid this, is the control of the flow of gases. We hope that here you can learn more about this concept, its importance and how we can help you manage things to maximize the capacity of your equipment.

Article content:
Gas flow dynamics in tunnel kilns
Pressure control systems
Oxygen profile
Other control parameters

Gas flow dynamics in tunnel kilns

A tunnel kiln is basically a long rectangular chamber, lined with refractory material or ceramic fiber, through which the products to be processed pass on some type of platform (usually an insulated kiln car), while the products of combustion and cooling air move in the opposite direction, as we can see in Fig. 1.

To achieve effective control of kiln gas flows, the internal pressure needs to be monitored and controlled properly.

Fig. 2 shows the general configuration of a tunnel kiln with the addition of the equipment that controls the flow of gases.

Fig. 3 shows how the movement of gases inside the kiln comes from the cooling end to the products of combustion exhaust fan (POC) at the front end of the kiln.

Pressure control systems

Since the flow always goes from higher to lower pressure, the pressure profile of the kiln (relative to the atmosphere) is similar to that shown in Fig. 4, where the neutral point of the kiln is usually near the front of the firing zone. There should only be one neutral point in a tunnel kiln. The kiln pressure control circuit must detect the pressure at point P and automatically adjust the flow through the products of combustion exhaust fan. Subsequently, when balancing the flows of the exhaust fans and injection fans, we finally optimize the flow of the kiln gases.

Some of the cooling air remains in the tunnel kiln and the remainder is expelled by the cooling exhaust fan. This air can sometimes be used in other applications such as dryers, preheaters, boilers, etc. With this gas flow pattern, the kiln can operate with greater efficiency.

Oxygen profile

At Nutec Bickley we adjust the oxygen level so that the burners operate with excess gas (better known as gas rich mixture). Although the burners are tuned with a gas rich mixture, the atmosphere is oxidizing, as the burners use the oxygen inside the kiln to complete combustion.

Other control parameters

(1) Kiln push rate (actual car movement vs. time)

(2) Fast cooling output

(3) Cooling air supply fans output

(4) Exhaust fans output

We use electric pushers to control the push speed using a frequency inverter. For heavier kiln cars, in the event that the tunnel kiln involves very dense loads, a hydraulic type is used that allows the push speed to be controlled by means of a flow regulating valve. Kiln car movement can be continuous or indexed.

We recommend using high-velocity burners in the kiln, as these provide better temperature uniformity.

At NUTEC Bickley we have extensive experience in tunnel kilns and we can help you tunning your equipment. We also have models in sizes from 15m to more than 150m long for all ceramic sectors: sanitaryware, ceramic colors, tableware, technical ceramics, refractories, abrasives, etc.

Contact our expert advisors and tell us about all your requirements so that we can assist.