While the utilization of the waste heat in the chimney gases for
preheating the combustion air supplied to the boiler furnace, by means of air
heaters, has long had a wide use in steamships, as, for example, in the Howden
system. By raising the temperature of the air supplied to the furnace, the
combustion temperature is raised, and the absorption of heat from the burners
flame by radiation and from the gases by conduction and convection is
The overall efficiency is improved, as less heat is lost in the funnel gases.