Fig. 3 : Sankey diagram of low efficient heat treatment line

The visualization of the energy balance for the aforementioned furnace system is illustrated in Fig. 3 by a Sankey diagram. Theoretically it would be sufficient to introduce into this diagram only the heat which is necessary for the austenization of the work pieces. This theoretical heat is best reflected in the heat that is released by the parts during the quench process. Therefore, this value of heat is taken as a reference point of 100% and all other losses and energy inputs will be referenced to that metallurgically necessary heat.

The unfortunate result of this energy balance indicates that the heat input into the described heat treatment system is nearly 8.5 times more than required from the metallurgical standpoint only. From the energy and environmental perspective this configuration has to be considered as an "energy wasting machine".

The firing system(s) for carburizing furnaces are designed to heat up the charge uniformly to the required temperature while maintaining an even temperature distribution within each control zone. The heat transfer is mainly achieved by radiation from radiant tubes, which also prevents the heat treated goods from direct contact with waste flue gases. A fundamental approach to reducing energy consumption

the use of recuperator burner systems operating at reduced excess air values [2]. To maintain flame stability and low NOx emissions, it is advised to keep the excess air ratio in a range between 1.1 and 1.2. Also, it is important to note that the recuperation configuration has some practical limitations. Due to space requirements and material limitations, the maximum air temperature is approximately 1100°F (600°C). For single ended radiant tubes, the recuperator is directly integrated into the burner as shown in figure 4. Ceramic recuperators manufactured from SiSiC material offer the advantage of extended life and higher permissible heat fluxes (almost double of that of metal alloy).

Other radiant tube designs require different types of recuperators, which may not be as energy efficient. In figure 5 a comparison of the efficiency between recuperative and non recuperative burners is given. At a commonly applied heat treat temperatures of 1700°F (930°C), a non recuperative burner has an efficiency of 53%, whereas a recuperative burner offers efficiencies in the range of 78%. This difference results in potential gas savings in excess of 32%. The efficiency gain may vary with the particular design of the recuperator. The presented figures reflect a recuperator with 65% recuperator efficiency, a widely used design.