Investigation of the Influence of Proximal Radiation on the Thermal Stresses and Lifetime of Metallic Radiant Tubes in Radiation-Dominated Industrial Furnaces*

Abstract

The influence of surrounding (or proximal) radiation on radiant tubes inside a continuous hot-dip galvanizing line was investigated. The furnace chamber, the strip and the neighbouring tubes were considered as the surroundings. A coupled heat transfer model was developed and subsequently validated against experimental measurements. This model was used to calculate the radiation exchange between the tube and its surroundings, and to give the new temperature distribution on the tubes as its output. This result served as an input to an already validated FEM model, which was used to assess the creep behaviour and the corresponding stresses on the tube. Basis of the investigated setups were Alloy 602 p-type tubes operating under burner on/off firing. The results show an increase in creep deformation of the tube when the surroundings were taken into account. Highest creep deformations were observed for setups with a strip, even though these cases showed the lowest maximum tube temperatures. Furthermore, an opposing effect between creep deformation and stresses acting on the tube exists. This is supported by the fact that no definite pattern relating the creep and stresses of a tube was found. Local tube temperature gradients and transient cyclic loading due to burner on/off firing were observed to have a significant influence on the tube's service life.

Influence of Surroundings on Radiant Tube Lifetime in Indirect-Fired Vertical Strip Annealing Furnaces

“…In contrast, the temperature values showed no significant change at the burner-on point. Spatially resolved, the temperature distributions of the reference case (I) showed high temperatures at the first elbow and minimum temperatures at the pinned support (XYZ) [35]. The cases II to IX that were influenced by surroundings showed similar locations of the maximum and minimum temperatures.…”

“…None of the previous investigations, however, took the effects of the tube's radiation exchange with the surroundings on its thermomechanical behavior into account. The authors of the present work developed the first coupled heat transfer (CHT) model [35] to consider the effects of surrounding radiation on the lifetime of radiant tubes in a CGL, characterized by their creep deformation and corresponding thermomechanical stresses. The CHT model was validated against tube temperature measurements on a P-type radiant tube, operating with a 120 kW self-recuperative burner, in an in-house test bench.…”

“…The reported deviations were accounted for in the simplifications of the FVM model, chosen to keep computational times practicable. Secondly, the obtained final tube temperature distributions from the CHT model were validated against temperature measurements in the same in-house test bench [35]. The cooling sections of the test bench were set up to represent a strip, having a width of 825 mm and a uniform temperature of 600 • C, passing on both sides of the radiant tube.…”