## 5 Results and Discussion
Two asynchronous firing cases were calculated, one with the furnace chamber temperature of the burner-on point and one at the burner-off point, in order to evaluate their influence. It appeared that the furnace chamber temperature had a marginal effect on the creep rate. A correlation between the creep rates with the local temperature cycles of the tube could be inferred (Figure 11a). With asynchronous firing, the temperature cycle had a value of 28-29 \({}^{\circ}\)C at location EL. This magnitude was only half as that seen for the reference (61 \({}^{\circ}\)C) or the synchronous firing (65 \({}^{\circ}\)C) cases. This supported the observed behavior,  
Figure 10: Creep strain magnitudes at location EL (= evaluation point on second elbow) for cases used to investigate the strip influence: (**a**) presence or absence of a strip; (**b**) different arrangements of two or three adjacent tubes. linking cyclic changes in thermal loads to rates of creep strain. Higher the local tube temperature cycle values, greater seemed to be the creep rates. In addition, the effect of full-load firing burner operation was investigated for the tubes in setup (b) (same as for case VI). The resulting creep rate had expectedly the lowest magnitude compared to all other burner firing modes studied in this work. For operating a CGL, this was challenging to adapt since the heating section's energy input was controlled by burner on/off firing. Further studies must be carried out, including the infrastructure and controls of the furnace. The Von Mises stresses were evaluated at location EL for all cases and are summarized in Figure 12. The opposing effect of creep strain and associated thermomechanical stresses could be observed when comparing the reference (I) with the case without a strip (II). On the one hand, the former case resulted in higher tube stress than the latter one, with the creep strains magnitudes following an opposite pattern. Similar observations could be made for the cases with asynchronous firing modes (III-IV) as well. On the other hand, all cases with the presence of a strip in synchronous burner operation (V-IX), regardless of their arrangement, showed different behavior. Despite higher creep strains in these arrangements, higher stresses occurred compared to the reference. The comparison of creep strains and stresses was carried out not only at the primary evaluation point EL but also at three other secondary evaluation points, as shown in Figure 9b. These points were located on the first leg (LG1), the first elbow (EL1), and, likewise to point EL, on the second elbow (EL2). They represented the next three local maxima. The results of the creep strains and stresses for all nine cases are shown in Figures 13 and 14, respectively. All values were normalized to the reference case (100%). Figure 11: Results of burner firing operations: (a) temperature-time-cycle and (b) creep strain at location EL. Figure 12: Von Mises stresses and corresponding creep strains at location EL after 400 h of process time for all nine cases. The creep strains of the cases with asynchronous firing laid below the reference, as already stated at location EL. The results were also consistent for the synchronously firing cases without a strip and with a strip (two horizontal tubes, wide strip, three vertical tubes). Despite these consistencies, deviations could be seen while taking a look at the case with the narrow strip (V). Its resulting creep strain showed similar behavior as the wide strip case (VI), but, in contrast to previous observations, the stresses were lower than that of the reference. A similar behavior could be seen for the three horizontally adjacent tubes (VIII-IX), regardless of the local strip's temperatures. Taking a closer look at the evaluation point on the first leg, it could be observed that all cases showed creep strains lower than or of equal magnitude to the reference. The partly contrary results showed that a simple correlation of stresses and creep strains with the given boundary conditions was not possible.