As I understood there is no precise boundaries between air drafts. Only some guidance:
1. If oil or other durty liquid is used forced or mixed (forced + induced) draft is recommended. It provides better mixing in oil guns and so way less air excess is required.
2. If gas is used for burners and there is only 1-2 burners in furnance plenum forced draft can be a source of an accident. If even one burner is blown out gas can spread around the heater and explode. In case of natural draft explosion is possible only inside of heater shell.
3. In any case if forced draft is used there is a hazard of flue gas leak and personnel poisoning
4. In any case if forced draft is used there is a hazard of leak in radiant section (for example through observation door) and subsequent refractory lining supports or metal structures overheating.
4. Induced draft has more utility consumption than forced since air volumetric flowrate is more
5. Induced or mixed (forced + induced) draft is acceptable if air preheating is used.
6. In all cases forced draft requires less air excess than natural.
Edited by shvet, 11 November 2016 - 09:59 PM.