In the situation of non-steady operation, for example, when starting up the installation causes fluctuating flue gas temperatures, necessary measures must be taken to ensure the reinforcement profiles can be heated evenly. The temperatures on the duct wall, as well as on the inside of the
reinforcement element, increase rapidly when the installation is started up, whilst the outside
of the profile remains cold at first and only heats up after a longer delay. This leads to temperature
differences, which can cause undue stressing of the component. The extent of the temperature
differences depends on numerous parameters.
A few examples follow:
■ The operating speed influences the speed at which temperature of the flue gas increases
and the temperature difference in the reinforcement element.
■ High temperature differences occur in the case of large profiles.
■ The shape of the reinforcement profiles influences an even temperature distribution. Thick walled profiles, for example, do not warm up as evenly as thin walls.
■ The different thermal conductivities of the materials used and the heat transfer rates lead
to an uneven temperature distribution.
To reduce the temperature differences, the insulation must be structurally designed to enable as much heat as possible to be transported by means of radiation and convection from the duct wall to the external flange of the reinforcement profiles.The following shows the design details for a profile insulation system.
This type of design is generally recommended for
profiles measuring up to < 10" (240 mm) in height.
In the case of profiles measuring above 10" (240 mm) in height, a covering sheet should also be installed. The heat transfer from the duct wall to the external flange is therefore not impeded and the cavities do not need to be insulated.
The profile insulation described leads to increasing heat losses through convection in the case of vertical steel girders. As a result, barriers - for example in the form of sheets welded into the reinforcement elements - must be fitted at intervals of approximately 10 to 16 feet (3 to 5 m) to reduce convection.
