Preheated Combustion Air

"Technology is now available to increase efficiencies to 92 percent and better." I wrote those words in 1979 for a paper presented to the Mid-Year API meeting in May of 1979. The words are still true today. The single, most significant obstacle to overcome in this application is high flue gas dew points versus percent sulphur in both oil and gas fuels. In the years since 1979, the problem with flue gas dew point has been offset to some degree by the use of cleaner fuels. However in some applications, it is still a concern. The calculation of flue gas dew point temperature when sulphur is present is discussed on page 6 of this section.

There are basically two ways to obtain preheated combustion air and reduce the amount of fuel required. One process would use heat from an external source to preheat the combustion air, such as waste steam or flue gases from other sources. The other method would be to utilize the flue gas from the heater to preheat the combustion air.

In either case, the heat available in the preheated air is taken into account in the radiant heat transfer as described in section 4, Heat Transfer Concepts. In the second case, the additional heat removed from the flue gas in the air heater is taken into account when calculating the overall efficiency of the heater. The amount of heat available in the flue gas can be calculated using the enthalpy of the flue gas at the entering and exiting temperatures. This enthalpy can be obtained by using the calculator described in the Heat Transfer Concepts section. To use this calculator, click below.

qavail = (hin - hout) * Wg
Where,
 qavail = Heat available in flue gas, Btu/hr hin = Enthalpy of gas at inlet temperature, Btu/lb hout = Enthalpy of gas at outlet temperature, Btu/lb Wg = Flue gas mass flow, lb/hr