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Show hours. Brick kilns that typically produce 50 million units per year have run times from 30 to 40 hours, due to product temperature constraints. Tunnel kilns are capital intensive investments. Manufacturers faced with increasing demand are forced to extend the production of their existing facilities. One constraint to extending productivity is the temperature uniformity of the product in the firing zones. As the burners are increased to achieve higher production, the quality of the product sharply declines. It is not uncommon to find 5 to 6 % scrap product in the brick industry from a kiln that is trying to extend output. The percentage of scrap is m u c h higher in other industries, such as dinner ware. Recognizing that 1 to 2 % may be attributable to processes other than the firing of the kiln (i.e. handling, drying, cooling, etc.), it is still reasonable to consider engineering solutions to improve the uniformity in the firing zones. For example, a brick plant that produces 50 million units per year, at a cost of 15 cents per unit, stands to gain $75,000 for each percent of scrap product. Adding burners, or increasing the fire power of the existing burners, to extend the production of the kiln will almost assuredly result in greater product temperature differentials, unless burner excess air is adjusted. Increasing the excess air through the burners is particularly important in the lower temperature zones (preheat) where the rate of product temperature rise is high. By controlling the excess air, one can tightly hold the temperature difference between the product and flame. This temperature difference and the momentum issuing from the burner jet are two critical elements to ensure uniformity and penetration. Unfortunately, increasing the production of a kiln has the net effect of requiring greater percentages of excess air per burner. Consequently, excess air diminishes fuel efficiency and imparts an additional power demand on both the combustion air fan and the products of combustion fan. Potential Developing Fully Developed I Core Zone Zone Zone Figure 2 High Velocity Burner Jet 3.0 Jet Theory Combustion from a high velocity burner can be fundamentally considered a free, turbulent jet consisting of three zones (see Figure 2): the potential core, developing (transition) and the fully developed zones. Non-reacting, isothermal jets of this type are readily predicted using known principles of conservation of mass and momentum. Several key investigators have conducted experimental work that address s o m e of the special conditions associated with flame jets. Work done by Thring and Newby4 (1953) was one of the first published attempts at investigating mixing length and entrainment of flames. This work developed a scaling model to predict the effects of combustion/reaction on simple jets. The work also supported the idea that the characteristic length of a turbulent flame jet (jet diameter, d0) should incorporate a correction for the density difference between the jet and the surrounding fluid (non-isothermal case). This correction involves multiplying the jet diameter by the square root of the ratio of the different densities, i.e. d0(po/p-),/2. Ricou and Spalding5 (1961) investigated measuring jets of entrainment in a specially designed experimental facility. Their work confirmed previous claims about the effects of density (non-isothermal case). As shown in Figure 3, the mass entrainment increases as the jet becomes less dense. Burning of a jet (propane and hydrogen) was briefly investigated in the same work and the results showed mass entrainment was decreased (up to 3 0 % lower than the s a me jet not burning). 3 |