Large flue gas ducts are fitted with reinforcement profiles to stabilize the duct. These can consist of double T-girders, hollow sections or reinforcing ribs and form potential thermal bridges. This may cause the following problems:
■ The thermal bridges cause an increased heat flow and lead to a temperature decrease on the inside wall of the ducts.
■ Temperature variations between the inner and exterior lead to stress in the profiles. If the tensile forces become too great, which can lead to deformations and breaking of the welding.
Preventing temperature drops on the inside wall
To prevent a drop in temperature on the inside wall in the area of reinforcement profiles, they must
always be insulated. The insulation thickness required depends on factors such as the size and
geometry of the profiles, the temperature level and rate of flow within the flue gas duct and the
operating method. Complex calculations may be required to determine the insulation thickness.
These are usually established by the plant manufacturer, who is aware of the installation
parameters.When starting up the installation, a brief drop in temperature will occur for the dew point ofthe flue gas on the inside wall of the duct.
Reduction of stress due to temperature in the reinforcement profiles
The operating method of the installation influences the problem of stress in the reinforcement profiles caused by temperature.
Generally, stresses due to temperature are not critical if the implementation principles outlined in the AGI guideline Q101 are observed:
■ The insulation thickness across the reinforcement elements should be of the same thickness as the insulation on the flue gas duct.
■ In the case of ducts with reinforcing ribs up to a height of 4" (100 mm), the thickness of the insulation layer across the ribs must measure at least one third of the insulation thickness
required for the duct.
