The thermal response of reactors, furnaces and Post Weld Heat Treatment (PHWT) arrangements is a result of multiple heat inputs and thermal losses. As part of both the mechanical and process design, it is necessary to understand the system behaviour for:
- Determining design temperatures of critical components
- Determining secondary stresses in components subject to non-uniform temperature distributions
- Understanding heating and cooling rate limits
- Developing control system theory for furnace management
- Inputs to the process design
- Creep assessment
In our most recent example, a long cylindrical reactor vessel had 6 separately controlled furnace elements. Operating at very high temperatures with large power draw, the secondary stresses caused by along shell and through wall temperature gradients was a limiting feature. With so many heat inputs and a varying reactor thermal duty, the associated control philosophy was critical to the mechanical design.
A coupled thermal mechanical transient analysis was completed with non-linear creep modelling. Extensive use of scripting and user subroutines was required to develop a functional model that permitted calibration and iteration to arrive at a functional design.
Radiation wash from between furnace zones was determined to play an important role in the thermal response. The control system required each furnace zone to evaluate neighbouring thermocouples to ensure overtemperature did not occur. Without an appropriately designed control system philosophy, shell hot spots were predicted.
These modelling techniques have application in the following fields:
- Furnace thermal control
- Reactor heating simulation
- Hydrogen reactors
- PWHT simulations
- Process design
An example of a furnace wall heated by radiative heating elements, controlled via a temperature control sensor. Poor placement of the temperature control sensor on the left end of the furnace results in regions of over-temperature (purple). Initial heat-up also generates significant through-wall thermal gradients and resulting stresses. Through modelling and iteration of these considerations, the control system and operating parameters can be selected to ensure the design is acceptable.
