12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

The main focus of the proposed paper is on the design and efficient operation of metallic recirculating radiant tubes installed with recuperative or regenerative burners in vertical strip galvanizing lines. Premature tube failures may result in loss of production capacity arising from reduced heat input or, worse, unplanned plant shutdowns. As a galvanizing line furnace equipped with around 200 radiant tubes is not an uncommon case, the factors influencing tube life need to be thoroughly examined. From the perspective of the suppliers and end users of both the radiant tubes and the galvanizing line furnaces, the following aspects influencing tube life form the central themes of the paper:

- burner control and tube structural design,

- tube installation and process conditions in the furnace, and

- tube and burner maintenance.

Creep and the corresponding thermal stresses play a dominant role as a limiting factor in the lifetime of these radiant tubes. This is not only due to their exposure to high temperatures (> 900 °C) during operation, but also because of the resulting inhomogeneous surface temperature distributions and also, their own dead-weight. Recent investigations conducted in the Department for Industrial Furnaces and Heat Engineering in the field of radiant tube technology show that it is possible to determine an optimal burner position and tube geometry in order to maximise the recirculation of gases inside the tube and thereby, minimize the thermal stresses in the tube arising from temperature gradients on it. In addition, these tubes sustain alternating temperatures during the furnace operation, e. g. during burner on/off-firing or process-related changes such as change of the strip’s speed or cross-section. The effects of these temperature changes are much more detrimental than previously thought. Furthermore, the influence of the tube’s surroundings, i.e. neighbouring tubes or strip, is seen to exacerbate the material weakening phenomena and lead to a further shortening of its service life.

The results are from multiple public-funded research projects as well as collaborations with different burner manufacturers and furnace operators. Numerical simulations have been validated against experimental measurements from in-house test benches. Particular emphasis is on best-practices for furnace operation and on the tubes’ structural design (including its end support) for the improvement of its service life. Concluding remarks also include an outlook on future research topics.