Improving the overall energy efficiency of processes is necessary to reduce costs, lower the specific energy consumption and thereby reduce the direct or indirect emission of gases that contribute to climate change. In many metallurgical processes, a large amount of energy is lost with the off-gas. In metallurgical recycling processes, off-gas often can be used to preheat the to-be-recycled metal scrap, leading to significantly higher energy efficiency. However, the application of preheating has the disadvantage that it often requires more precise planning in the design and better control of the process. In this paper, a simplified look at a continuously charged scrap preheating aggregate for the widely used electric arc furnace (EAF) in the steel processing industry is used as illustration. Continuous scrap charging in EAF-type furnaces in general has much higher demands on process control and general process knowledge, which is why they are found only very rarely. General issues and basic modeling approaches to mitigate such issues allowing a better process control will be described. In particular, a fast, one-dimensional modeling approach for the determination of the temperature distribution inside a constantly moving scrap bulk, with hot air (or exhaust gases) flowing through it, will be described. Possible modeling applications, assumptions, possible enhancements and limitations are shown. The first results indicate that this approach can be used as a solid basis for the modeling of scrap bulks with thermally thin parts, consisting of materials with similar thermodynamic material properties. Therefore, as a basis, this approach may help improve the design and control of future or existing preheating devices in metal recycling processes.

Calculating the radiation exchange between arbitrary surfaces using view factors is a common practice in the fields of industrial furnaces, climate modelling, solar power, thermal building design and so on. Here, a new model was developed to calculate the view factors of arbitrary two-dimensional geometries. This model is based on Hottel’s crossed strings method paired with an optimized algorithm to efficiently detect shadowing effects between the surfaces. The model’s accuracy and discretization dependency was tested against an analytical view factor calculation method using an example of two concentric, infinitely extended cylinders. The model can be applied to arbitrary linear discretized two-dimensional geometries. In particular, it guarantees the highest possible accuracy within the chosen discretization. The need for developing customized view factor equations of different two-dimensional geometries is therefore no longer necessary, since the convergence of the model with decreasing mesh size on analytical results can be demonstrated. Additionally, the performance and validity of the newly developed shadowing algorithm was tested against a common brute force approach and significant speedups were achieved. Furthermore, the additional application of the net radiation method for the calculation of the heat fluxes exchanged by radiation within those geometries is shown.

Tube bundle recuperators are generally designed to operate with smooth tubes. Structured tubes can be used to increase the efficiency of recuperators. Compared to smooth tubes, the surface for heat transfer is increased and thus heat transfer is enhanced. This effect is accompanied by an increased pressure loss, which must be kept as low as possible. Four tube geometries with different honeycomb structures are examined. The results are compared with the performance of a smooth tube. The investigations were carried out both numerically and experimentally at different off-gas and combustion air velocities. The experimental results show that the highest heat transfer is achieved with the concave 6 mm structured tube. The greatest pressure loss also occurs here. The validation of the numerical model has shown issues in resolving the turbulence.

The effects of surrounding radiation—emanating from radiation exchange with neighboring partners in indirect-fired vertical strip annealing furnaces, such as the other radiant tubes, the passing strip, and the enclosing furnace chamber—on the radiant tube lifetime were studied. In-house developed and validated numerical models were used to calculate the thermomechanical behavior, especially creep deformations and the corresponding stresses as lifetime indicating parameters. Different setups of recirculating P-type radiant tubes were investigated, including a reference case of an isolated tube. The investigations could be broadly classified into the study of the effects of different tube arrangements, burner operations (synchronous/asynchronous on/off firing), and changes of strip parameters (width/temperature). Results showed higher creep deformation of the central radiant tube in the setup with three tubes arranged horizontally in a row compared to three tubes stacked in a vertical column, even though the respective characteristic temperature values in a firing cycle were similar. Furthermore, the cases with asynchronous burner firing resulted in lower creep rates than other cases, where the burners were operating in synchronous on/off firing modes. In addition, the change of strip width had a higher impact on radiant tube lifetime compared to locally changing strip temperatures across the furnace. Alternating temperatures, caused by burner operation or process changes, such as change of strip’s speed or cross-section, and local temperature gradients were observed to be the main factors influencing the tube’s service life.

In industrial plants, metal strips are quenched using convective heat transfer. This involves accelerating gas through a nozzle system onto the material to be quenched, resulting in a fast and uniform cooling process. The efficiency of the heat transfer is determined by the specific nozzle system. The presented work is focussed on the analysis of the heat transfer by forced convection through impingement jets both theoretically and experimentally. A unique method for determining the forced convective heat transfer coefficient is described and its application to industrial size nozzle arrays of annealing-line cooling sections is presented.