4 replies to this topic

[jrtailor09]

wanttolearnprocess

Individual economic and environmental constraints should determine whether natural or forced draft furnaces will be used. In either case draught should be sufficient to supply to the burners 50% of excess air at maximum ambient temperature and 110% of design load.

Use of forced draft burners and combustion air preheat is preferred where economically justifiable.

Regards,

Edited by jrtailor09, 23 September 2012 - 11:17 PM.

[Flame]

I will give some examples to illustrate:

1) A small heater of say 5.0 Gcal/h fired on refinery fuel gas in a refinery in the U.A.E (middle east): In this heater, it may not be justifiable to put in extra investment in the form of Air preheater, FD / ID fans, ducting, extra supporting etc. for realising fuel savings. Since local fuel gas cost is low, and the simple payback period for extra investment may come out to be say > 7 years. Hence not so profitable. It is better to specify a natural draft heater.

2) A reformer with a long convection section: A steam methane reformer has lots of heat recovery sections in the convection portion. These leads to extra resistance (in the form of draft loss) in the flue gas path. So in this case, it is apt to use an ID fan in order to push the flue gases to the disposal stack. However, in case the combustion air circuit is relatively resistance free, then air will be sucked naturally into the burners with the help of stack effect created by the tall radiant sections.

3) A large CDU heater employing recuperative flue gas cast & steam air preheaters: The flue gas circuit and air circuits are equally circuitous, (APH , STAPH, ducts, convection draft losses, air pressure drop across burners etc.). Fan on both air and flue gas circuit is necessary. More so, as the heater duty is large, reasonable air pre-heating (by recovering heat from flue gas) results in substantial savings, thereby amounting to a lower pay-back period (> 3 years) and justifying extra investment.

Hope i have helped you understand a little bit.....


Regards,

[TS1979]

There are essentially two draft systems: natural draft system and forced draft system. Natural draft system includes no fan - the air flowing through the burner by natural convection.

Forced draft system normally includes forced draft (FD) fan and Induced draft (ID) fan. Some systems either FD fan or ID fan may be provided. If the system is installed with air preheater, normally the system will be installed with forced draft system.

The natural draft system is normally used with the furnace duty less than 4 MW.