4 replies to this topic

[gvdlans]

Remember: the draft produced by a column of hot flue gas depends on the density difference between the hot gas and the ambient air. The draft you measured at the bottom of the furnace is measured at a different elevation than the draft measured at the arch section. Note that this draft is the pressure difference between inside the furnace and the outside ambient air at the same elevation. For example, when the arch section is 10 m above the bottom of the furnace, the outside air pressure at the arch section is 10 m of ambient air lower than the outside air pressure at the furnace bottom. 10 m of ambient air column is about 12 mm of H2O column.

Bottom line is that, although your gauge pressures increase between furnace bottom and arch, the absolute pressures decrease. This is the reason why flue gases will still flow from furnace bottom to arch!

Draft, in inches of water, can be calculated as follows:
Draft = (0.52)(Ls)(p')(1/Ta-1/Tga)

where:
Ls = stack height (ft)
p' = atmospheric pressure (psia)
Ta = ambient temperature (°R)
Tga = flue gas temperature (°R)

Furthermore, I would like to advise you to get a copy of following articles:

Herbert L. Berman, "Fired Heaters - III", Chemical Engineering, August 14 1978, pages 129-140.
Especially pages 139 and 140 are relevant.

Norman Wimpress, "Generalized method predicts fired-heater performance", Chemical Engineering, May 22, 1978, pages 95-102.

These articles are based on a natural draft furnace. However, for an induced draft furnace, stack has just been replaced by an induced draft fan, but same principles apply.

[mbeychok]

To: gvdlans

I just want to commend you for the excellent answer you gave to
Manika's question.

[gvdlans]

Milton, thank you for commending me. It was a pleasure writing the answer.

On the web, I found an article that shows the typical draft profile of a fired heater:
Optimized Fired Heater Operation Saves Money

Figure 4 shows the profile for a natural draft heater.