In indirect heating with the help of jet and flame tubes the heat is transmitted by radiation. In this process, the fuel and waste gasses must not get into contact with the atmosphere of the furnace since this atmosphere is frequently decisive for the success of the temperature treatment. However, the hot waste gasses can be used for pre-heating of the combustion gasses by routing these to a recuperative burner which is integrated into the steel pipe and works as a counter-current heat exchanger. This principle considerably increases the efficiency of modern burners.

A high thermal conductivity and thermal shock stability rank very high on the list of requirements put to the materials used. Steel and various ceramic materials fulfil these preconditions. In this context, ceramic materials entail the big advantage of a significantly higher net heating combined with a considerably lower density and, hence, a much lower thermal mass. Moreover, they can be used up to considerably higher temperatures. However, the porosity of many ceramics precludes its application since a diffusion of the combustion gasses has to be prevented. For this reason, a silicon-infiltrated silicon carbide (SiSiC) is very suitable since it does not have any remaining porosity. Furthermore, the net heating output is outstandingly high in case of SiSiC. The use of SiSiC entails the following advantages:

• No porosity, very good oxidation and corrosion resistance on account of this
• Very good thermal fatigue resistance
• Natural stability up to the threshold temperature for use of the material (high fatigue endurance)
• Gas tightness
• No pipe bending in use, reduced maintenance expenses on account of this
• Very good thermal conductivity (increase of the specific radiating capacity)
• Low mass
• High operational security and economic efficiency
• Excellent efficiency
• Very High net heating output