Classification Based on Heat Transfer
The generators classified according to heat transfer in the boiler may be characterized by convection, radiation, or indirect heating.
First of all, note that boiler is understood to be the bulk of steam-generating tubes in its entirety, that is, the tubes where the transformation of water into saturated steam takes place. Therefore, the economizer and air heater, as well as the potential superheater and reheater, are included in the steam generator but are not part of the boiler. From this point of view, there are generators that may be correctly called radiation generators. Instead, it would be wrong to speak of convection generators,
given the fact that in all generators any smaller or greater amount of heat will be transferred to the boiler through radiation.