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Show 635 C (1060 to 1175 F). Not only are the material requirements for the recuperator eased. but the size of the recuperator is also reduced somewhat. DIRECT FIRED HEATING OPERATIONS - There are many applications in industry where heat is applied by direct impingement of combustion gases. Kiln operations. heat treating. melting. and drying are some examples. The application of pulse combustors to the drying process is described below. Spray Dryer Applications - Drying of moist materials in moderate-temperature air flows occurs as a result of heat and mass-transfer phenomena working in concert. In a typical spray dryer. the material to be dried is sprayed into a chamber held at elevated temperature by consumption of fuel. The atomized material drifts downward through the dryer and emerges at the bottom in a dried state. Residence time and temperature are adjusted to accommodate the load. This is a mature technology and is adequately serving several industries. However improvements can be made by the application of advanced combustion techniques. Concepts showing promise include drying in acoustic fields. either artificially imposed or naturally resonant systems. and the use of high-velocity flows. By augmenting the heat and mass transfer. decreased drying time and/or enhanced energy efficiency will result. Pulse-Coabustion Drying - There are pulsecombustion dryers presently available for drying slurries and other materials capable of being dried in a spray dryer. Figure 8 shows a typical configuration for these devices. Firing rate for the gulse combustor is on the order of 1172 Kw (4 x 10 Btu/Hr) and energy requirements are typically in the range of 0.806 to 0.935 KwH/Kg (1250 to 1450 Btu/Lb) H20 removed(14). This compares well with conventional spray drying technology where energy requirements range from 0.920 to 1.16 KwH/Kg (1400 to 1800 Btu/Lb) HzO(15). Key advantages of the pulse combustor in this application include (1) increased heat transfer due to high velocity fluctuations and (2) mass transfer enhancement derived from the high velocity gases in the resonance tube. The slurry of particles to be dried is injected into the resonance tube where the high velocity gases accelerate the product to high velocity and partially strip away any surface water. The Battelle experiments described earlier confirm this phenomenon and showed an increase in heat transfer of up to five times over steady combust ion. Other Direct-Fired Applications - A mathematical treatment of the drying process is difficult to perform with accuracy. so it is difficult to predict the drying performance with pulse combustors and to compare the results to conventional technology. Laboratory and field data have been presented to describe the benefits obtainable. The same argument can be used for pulse-combustion applications in other areas where direct heating is used. Examples might include kiln operations. calcining. surface drying. car-bottom furnace operations. etc. As a general guideline. the heat-transfer enhancement resulting from the pulsations will decrease as distance from the pulse combustor discharge is increased. The greatest heat transfer enhancement occurs within the combustor, and is related to the acoustic intensity. The pressure and velocity fluctuations decrease as the gases expand outward from the discharge of the system. There is not an extensive body of data on the pressure and velocity fluctuations downstream from the combustor, but there are some indications that the pressure levels may decrease by 3 to 5 dB within 3 M (10 Ft) of the outlet(16). Although this is not a significant decrease in intensity, the temperature of the moving gases decreases considerably due to entrainment of surrounding gases. Measurements of this effect have shown that the exhaust gases drop from 675 to 230 C (1250 F to 450 F) in only 460 mm (18 in)(4). In many cases this is a desirable effect because furnace gases will be recirculated to lower hot spots and to promote uniform heat transfer. CONCLUSIONS Pulse-combustion technology has the potential to improve many industrial processes through heat-transfer enhancement. Experiments, field tests, and simplified mathematical treatments have shown that the advantages of pulse-combustion technology result in compact design, lowered material requirements, and reduced energy consumption. Before these advantages can be realized, pulse combustion must be developed to an industrial scale, and some of the problem areas associated with the technology, such as noise, need to be addressed. REFERENCES (1) Corliss, J.M., Putnam, A.A., Murphy, M.J., and Locklin, D.W., "HOx Emissions from Several Pulse Combustors." ASME Paper 84- JPGC-APC-2, 1984. 43 (2) Hanby, V. I. 196~. "Convect i ve Heat Transfer in a Gas-Fired Pulsating Combustor." Trans. ASME, J. of Eng. for Power. (1):48-51. (3) Blomquist, C. A., and Clinch J. M., 1982. "Operational and Heat-Transfer Results from an Experimental Pulse-Combustion Burner." Proceedings on Pulse-Combustion Applications. GRI-82/0009.2, Vol. 1, Gas Research Institute, Chicago. Illinois. PB 82-240,060. |