Access to the air heater as well as the potential for vibration will impact on the length of the air heater tube. Long lengths may also require casing expansion joints to accommodate different rates of expansion between the tubes and the casing. Ideal tube length per pass is 3.0mtr and guide vanes should be provided to ensure that the airflow across the air heater is not compromised.
Where air or gas is laden with dust, it is recommended that the design specify the tubes to be arranged in line rather than staggered. Air heater tubes are generally expanded or flared into the tube sheet and some are also welded as well as flared for added rigidity. This method ensures that tubes can be easily removed if needed.
Flue gas contains water vapour, which condenses on the cool surfaces of the Air heater forming water droplets. These droplets eventually lead to corrosion failures especially around the air inlet section of the air heater where the air temperature is likely to be close to the ambient air temperature. For this reason, air heater designs comprising of multiple blocks or sections are more popular as they are easier and cheaper to repair. Single block air heater designs can be very expensive to replace and can lead to long down times.
Various methods of preventing vapour condensation can be employed.
When air heater tubes fail, the increased air temperature will not be transferred to the furnace leading to increased furnace fuel consumption. Multiple tube failures lead to poor fuel combustion in the furnace prompting air heater tube repairs.