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Debating Burner PlacementI read the article by Jim Feese ("Optimizing Burner Placement," Ceramic Industry/Brick & Clay Record, August 2005, pp. 27-30) with great interest because burner placement is often overlooked in kiln design. Using Fluent(r) computational fluid dynamics (CFD) software is helpful, but the reported results seem to conflict with my experience. I have evaluated more than 300 Datapaq curves for tunnel kilns and have never seen the recessed mounting of burners achieve the uniformity of flush-mounted systems. The modeling software, though accurate, depends on many variables that must be selected by the user. These variables, which are often "guessed," can make a tremendous difference in the calculated results.
Air entrainment in kiln burners is helpful in multiple ways. Most importantly, though, entrainment significantly reduces the burner jet temperature and therefore promotes temperature uniformity and freedom from excessive hot spots in the kiln. An additional improvement in NOx emissions is a further benefit.
The article notes that all recessed installations promoted "far more entrainment...than the flush-mounted burner," but the temperature uniformity reported does not support this conclusion. The flush-mounted system produced a reported delta T of 120°F in the load. The recessed delta T values for 19 in. and 6 in. recessed burners were reported to be 280°F (worse) and 70°F, respectively, despite both burners having higher calculated entrainment ratios. This inconsistency in the data should be further evaluated.
I believe that the difference between flush-mounted vs. recessed has more to do with the fact that different burners were used in the flush vs. recessed models. The two burners selected have around a 10% difference in output, as well as different mixing characteristics, as indicated by the extreme differences in the excess air (XSA) capabilities of the two burners.
Ralph Ruark, Ruark Engineering
Bradenton, Fla., email@example.com
The Author Responds:As mentioned in the article, the entrainment results will depend on tunnel geometry. If the curves evaluated were with burners substantially more recessed instead of only 6 in. recessed, I would expect the uniformity to be worse, as shown. All modeling parameters used for the article-including relevant boundary conditions such as the furnace and tunnel geometry, zone temperatures and flue gas from previous zones-were from an actual furnace. However, as Ralph points out, this is a steady-state model-i.e., the benefits of furnace atmosphere stirring due to pulse firing are not accounted for-so the model does have limitations.
With regard to the apparent difference between the temperature uniformity and entrainment values, temperature uniformity is a function of the heat release profile and burner location, as well as the heat transfer in the furnace-not just entrainment.
There may be some differences in results between the recessed vs. flush-mounted burner cases as noted, since two different burners were modeled. However, modeling the two burners at the same average burner exit velocity mitigated the magnitude of such a difference. In the future, we plan to run the flush-mounted case using the same burner as the recessed cases to further validate the results.
Jim J. Feese, Director of Product Development
Hauck Manufacturing Co., Cleona, Pa. firstname.lastname@example.org