In order to meet design flux rates and target run-lengths, cracking heater lower convection coils, radiant coils and crossovers must be designed to operate at elevated temperatures. However, they are normally constructed and maintained at ambient conditions. As the pipes and tubes are heated from ambient to operating temperatures (in the range of 2000°F+ for the radiant coils), significant thermal expansion of the crossover and radiant tubes will occur and must be accommodated in the design of the supports. At operating temperatures, the radiant coil materials are near the mechanical limits for the metallurgies, exhibit limited mechanical strength, and are easily deformed. After several heat up and cool down cycles, the tubes will not return completely to their original ambient or "cold" positions because axial tube stresses during heat-up/cool-down and operation will "stretch" the tubes such that the overall length increases slightly. This irreversible process occurs over each operating cycle, with the net result that the coil's overall dimensions will progressively change over the coil life. This progressive length increase is known as "creep elongation" and must also be accommodated by the support system. The radiant coil support system is normally designed to nullify unbalanced forces and moments, and also to allow the crossover and radiant coil to expand and move as needed over any single warm-up (expansion) and cool-down (contraction) cycle. However, since the rates and extent of creep elongation will vary with operating parameters, it may be required to make adjustments to the support systems over the life of the coil, in order to maintain the range of free motion needed to avoid generating moments and stress on the coils. Such unbalanced forces and stresses will usually be evident as deformation (e.g. bending and bowing) of the affected vertical radiant tubes. In order to properly adjust and maintain the coil support systems, it is necessary to understand the requirements for coil expansion with respect to the amounts and directions of the expected movements. The direction and extent of coil expansion is tied to the coil configuration and support concept. Measurements must be taken and data collected in order to allow the coil creep behavior to be tracked and projected. With the coil expansion defined and the expected range-of-motion limits established, techniques and procedures to adjust and monitor the radiant coil support spring hangers can be developed.^
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