fine-tuning of kiln firing cycles can improve productivity while decreasing
firing process is arguably the most
important stage in ceramic and brick manufacturing.
Naturally, continuous kilns are the principal means of firing due to their
higher efficiency compared to intermittent kilns, but there is always room for
Each of the three functional zones (preheating, firing
and cooling) in a tunnel kiln can be considered a separate heat exchanger. In the preheating and
cooling zones, heat transfer occurs predominately by convention, while heat is
transferred by convention and radiation in the firing zone. The calculation of the
heat required for firing in each segment of the kiln and the comparison to the
heat absorption capability of the treated material is the basis for discovering
optimal process parameters.
When the ware being fired cannot absorb the essential amount of heat,
manufacturers must adjust the gas/air flows in the kiln. The fine-tuning of the
kiln’s firing cycles can improve productivity while decreasing overall costs.
Figure 1. Tunnel kiln parameters.
Tunnel Kiln Example
brick manufacturer was faced with unacceptable
rejects (up to 15%) and high energy consumption (1590 Btu/lb) with its tunnel
kiln firing process. The kiln parameters are shown in Figure 1. The kiln
measured 336 ft long x 11 ft wide x 5.8 ft high, and fired 3696 brick/hr
(13,948 lbs/hr). The brick being
fired measured 75/8 x 35/8 x 21/4 in., and
weighed 4 lbs each.
Each of the kiln’s 28 cars carried 12 packs of
616 brick each, and the decks were 12 ft 6 in. x 111/2 ft. The
total insulation mass was 800 lbs. Brick entered the kiln at 70ºF. It moved
into the firing zone at 1200ºF, and the cooling zone at 2000ºF, before
exiting the kiln at 120ºF.
Figure 2. Theoretical gas/air flows in the kiln.
independent calculation of the heat balance for each of the
functional zones of the tunnel kiln defined the theoretical required gas/air flows
in the kiln channel (see Figure 2).
3. Comparison of heat required vs. absorption capability in the kiln’s
functional zones for the original firing cycle.
as shown in Figure 3, the heat transfer/absorption capabilities of the existing
kiln zones did not provide the total heatwork needed for preheating the brick
(heat deficiency = 2.86 MBtu/hr). Similarly, the cooling zone lacked 3.36
MBtu/hr of capacity when cooling the fired brick to 120ºF.
4. The mathematical model recommended a new breakdown of gas/air flows in the
mathematical model recommended a new breakdown of gas/air flows without expensive kiln reconstruction (see Figure 4). The new firing cycle solved problems by increasing the input hot gas in the
preheating zone from 18,007 lbs/hr to 29,328 lbs/hr and the number of carts
from 8 to 10. In the cooling zone, the input of cold air was changed from 9124
lbs/hr to 13,245 lbs/hr.
5. Comparison of heat required vs. absorption capability in the kiln’s
functional zones following the updated firing cycle.
This approach allowed for the recovery of heat (Figure 5) in the
preheating and cooling zones. On the other hand, increasing the flow of cooling
air, which helped the preheating energy requirement, increased the energy input
into the firing zone. Despite the increased hot zone input, overall input based
on the load was reduced to 1011 Btu/Lb.
The implementation of these
recommendations reduced production rejects and decreased heat consumption from 1590 Btu/hr to 1011 Btu/hr. Total cost savings was
year without any additional investment for kiln modernization.