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Show the furnace at about l649°C (3000F). The flue gases then flow down through the checkerwork on the right side of the furnace, heating the bricks. Every 25 minutes, valves, which are not shown in this picture, reverse the flow. This system was invented by Wilhelm Siemens in the 1850's. It led to the mass production of low cost steel, which in turn led to the industrial revolution. III - THE CRITERIA There are nine (9) questions we should ask when judging a waste heat recovery device. First - How much fuel will it save? Put another way, what is the temperature of the pre-heated air? It is obvious that the air temperature can be increased by increasing the heat transfer surface, but you must remember that the higher the air preheat, the lower the air-flue gas temperature differential. As the temperature differential decreases, the rate of heat transfer per square foot decreases. The bottom line is that an enormous amount of heat transfer is required to heat the air to within 111°C or 167°C (200 or 300 degrees)of the flue temperature. It is, therefore, generally uneconomical to build a recuperator that heats the air higher than 537°C or 649°C (1000 or l200F). Second - What is the installed cost of the device? In the good old days you could justify new equipment if the resultant savings over four (4) years equalled the installed cost. Today the payback time has to be between 10 and 18 months. Third - What is the maximum allowable flue temperature? Do you have to add dilution air to the flue gas to protect the recuperator? Fourth - How big is the heat exchanger? At the steel plant, the radiant tubes are on 71.1 cm (28 inch) centers and the individual legs of each "u" tube are on 30.5 cm (12 inch) centers. The best device in the world can't be considered if it won't fit in the available space. Fifth - What is the air pressure drop and the flue pressure drop across the heat exchanger? The steel company has 527 Pa (12 osi) combustion blowers. New blowers would add considerable cost. Sixth - Will the device run for long periods without plugging? If it does plug up, can it be cleaned easily? Seventh - What valving and controls are required? Eighth - How long will it last? If the payback period is 18 months and it fails in 18 months, the net savings are zero. Ninth - Is the flue gas path in the furnace altered? Recuperators do not alter the flue gas path, which is always from a fixed burner to fixed flue. Regenerators, on the other hand, cause the flue gases to change direction every cycle. In some furnaces, this is an advantage. Cycling regenerators have an enormous advantage over conventional burners and recuperators in radiant tube installations because they maintain more uniform tube temperatures. Thus, total heat transfer through the tube can be increased while reducing the temperature of the hottest spot on the tube. The net result is longer tube life. When you consider the above nine (9) criteria carefully, you conclude the regenerators are better than recuperators except for size, cost and control system. So you investigate what it takes to reduce the size and cost of regenerative beds and to Simplify the reversing valves. The traditional regenerator consists of 22.9 cm (9") brick stacked in an open checkerwork pattern. There are about 14.7 m2/m3 (4~ ft2 heat transfer surface/ft3) of regenerator volume. Suppose you use .317 cm (1/8") spheres instead of 22.9 cm (9") br~ck for the bed. Then you will get 1131 m2/m (346 ft2 heat transfer surface/ft3)of volume instead of l4.7(4~). Of course the pressure drop per 30.5 cm (lineal foot) of bed will be higher but that can be fixed by lowering the gas velocity. In addition to furnishing enough heat transfer surface area, you must also furnish enough bed weight to store the heat in the flue cycle until it is released in the firing cycle. The regenerator in the picture operates on· a 25 minute cycle. That meanS, if you are transferring 1.055 x 109J (1,000,000 btu) per hour into the air, you have to have enough bed to store M of 1.055 x 109J (1,000,000 btu), which is ~~39,935 x 108J (417,000 btu) into storage. But what if you cycled every 20 seconds instead of every 25 minute~., Then you need only enough bed to store ~x 1.055 x 109J (1,000,000 btu), which is 5.91 x 106J (5,600 btu) into storage, instead of 4.39,935 x 108J (417,000 btu). IV - THE SOLUTION In November, 1982, after a lot of thinking along these lines, the Gas Research Institute and the North American Mfg. Co. began a joint project to develop compact regenerative burners for both radiant tubes and direct firing. The direct fired burner models that resulted from this development are made in sizes from 439,500 W to 5,274,000 W (l~ million btu/h to 18 million btu/h). They have been sold for the following applications: Car Bottom Forge Continuous Slot Forge Box Forge Aluminum Melting Aluminum Holding Glass Melting |