Melting Furnace Control

The chain of command for air/fuel ratio controls is usually as follows The burner or zone input control responds to a T-sensor (or steam pressure sensor in the case of a boiler). The burner input control (also termed furnace input control, kiln input control, etc.) may actuate a burner or zone air valve ( air primary air/fuel ratio control ) or a burner or zone fuel valve ( fuel primary air/fuel ratio control ). Air primary air/fuel ratio control is more common with smaller burners. Many problems are avoided if each burner is equipped with its own ratio control. Where multiple burners are ganged in parallel downstream from a single air/fuel ratio control, if one burner has a problem with its ratio, all parallel burners of that zone will have the opposite difficulty, the intensity of which will be divided by the number of burners in the zone. 

Furnaee engineers and operators must understand the many aspects of air/fuel ratio control for safety and for equality. Mass flow control is essential if the combustion air is preheated. Changing air temperature affects the weight of air passing through 

Individual ratio controls at every burner make it easy to modify the input profile pattern up and down or across a furnace without having to reset the ratio of each burner afterward. 

Small burners without preheated air are generally controlled by cross-connected air/fiiel ratio regulators (one for each burner). This arrangement is ideal because it saves the operator from constantly having to adjust the ratio—until the paint is worn off the hand dial— because of changing maldistributions of flows in either air or fuel manifold. 

Air and Fuel Manifolds. It is difficult to correct bad manifold designs therefore, it is important to be generous in initial air and fuel manifold sizing, and get it right the first time. (See fig. 6.10.) Designers should think of manifolds as plenums that should be sized for low velocities. A nonuniform air or fuel distribution often changes its maldistribution as burners are turned up and down. An easy, safe design has the manifold cross-sectional area equal to the sum of the cross-sectional areas of all of its offtake pipes. (See references 54 and 60.) 

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Stephens, W.A., Stolzenburg, T.R., Stanforth R.R., and Etzel, J.E. May 1984. "Use of Iron to Render Sludge from Ferrous Foundry Melting Furnace Emission Control Waste Nonhazardous." Presented at the 39th Annual Purdue Industrial Waste Conference. West Lafayette, Indiana. 

When this is done, the calcium hydroxide is included in the melting furnace slag, and the unspent calcium carbide is either used or oxidized in the melting furnace. Little testing has been done to determine the actual fate of the sulfur. Most of it may be included in the slag, but it may also be emitted to the air as sulfur dioxide, or, for foundries with wet emission control systems, it may be dissolved in the water. 

In addition to these more practical problems of catalyst preparation, there are also severe theoretical problems associated with the prediction of the chemistry in the fluid state of a compound. The motion of all structural elements (atoms, ions, molecules) is controlled by a statistical contribution from Brownian motion, by gradients of the respective chemical potentials (those of the structural elements and those of all species such as oxygen or water in the gas phase which can react with the structural elements and thus modify the local concentration), and by external mechanical forces such as stirring and gas evolution. In electric fields (as in an arc melting furnace), field effects will further contribute to nonisotropic motion and thus to the creation of concentration gradients. An exhaustive treatment of these problems can be found in a textbook [6] and in the references therein. 

Spinosa, W.C. Stephen, P.M. Schorr, J.R. Review of Literature on Control Technology Which Abates Air Pollution and Conserves Energy in Glass Melting Furnaces, Nov 11, EPA-600/2-77-005,/2-76-269,/2-76032b Corning, Inc. Battelle, Columbus, OH, 1977. 

The melting furnace operation has to be controlled continuously. Present glassworks are equipped with modern measuring and recording instruments, and some parameters may be controlled automatically. The introduction of computer control of furnace operation has recently become highly topical in large glassworks. 

The control of glass melting furnaces is based in particular on correct setting and maintenance of the temperature conditions. The temperature maximum usually occurs at one half or two thirds of the melting zone and the melting itself should be completed before this point is reached. Convection can be promoted and stabilized by eletric boosting or bubbling. 

Operation and Control of Plant. One great feature of this system is that the pyrolysis furnace is separate from the ashmelting furnace, so that each furnace can be independently designed to be optimum for its intended use. Another feature is that even when the rather less dependable ash-melting furnace fails and needs to be shut down, it does not basically interfere with the operation of the pyrolysis furnace for disposing refuse. 

One more advantage is that the system allows for simple batchwise operation controlled separately from the ash-melting furnace. Figure 6 shows the increase of temperature inside the pyrolysis furnace as it proceeds from the shut-down state to a steady state, as determined through experimentation in the test plant. As the figure shows, the pyrolysis furnace, after having been shut down for about 10 hours, proceeds to a steady state in approx. 30 minutes after the supply of air for combustion is resumed the refuse in the furnace is brought to the shut-down state in about one hour. 

Clean feeders can be directly melted in the melting furnaces. Because of oxides and other inclusions, the possible amount of recycling is limited. Specialised equipment for exact analytical and metallographical control is necessary. 

Fig. 6.9. Direct-charged aluminum melting furnace with cascaded temperature control and regenerative burners. On the next 20-sec cycle, two air valves, two exhaust valves, and two fuel shutoff valves will reverse positions. Ma = milliamps. Se = suction exhaust. SP = setpoint. T/s = temperature sensor. Courtesy of North American Mfg. Co. 

H. Abbasi, D. Fleming and H. A. Abbasi, "Development of NOx control Methods for Glass Melting Furnaces," Institute of Glass Technology, 1987. 

Castable technology is more popular for the fining of holding and melting furnaces. The shaft between the bricks is a weak point in the lining. The porosity of the mortar in the shaft may be even twice higher than in the brick, and the pore size distribution is not controlled. Also, it is necessary to take the human factor into account, such as the variations in the thickness of shafts, some remaining cavities, and so forth (Fig. 3.9). 

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