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Show Af~the burner has operated in the fire mode for 20 seconds, the cycling gas valve closes and the exhaust flapper valve goes wide open (Fig. 9). This causes the flow to reverse. The products of combustion are pulled through the heat reclamation bed by the eductor, producing a negative pressure in the radiant tube. The burner pictured has an input of 8G,000 W (300,000 btu/h) when used on a 980°C (1800F) furnace. This will produce approximately the same heat transfer to the furnace as a conventional cold air burner with an input of 176,000 W (600,000 btu/h). The burner is equipped with an observation port that sights through the fuel tube. A dial thermometer, located in the backplate, indicates bed performance. A thermostat shuts off both burners' of the pair in the event of excessive temperature. The exhaust valves and cycling gas valves are rated for 5 million cycles. Limiting Orlflce cas Valve .L-______ ~A~ ____ ~ : . Cross-connected l _____________________ J Regulator Combustion Air Blower Fig. 10 - Radiant Tube Piping Schematic Fig. 10 shows the piping schematic for one pair of burners. When the furnace temperature is above set point, both gas valves of the pair are shut off but the exhaust valves continue to cycle, thus maintaining a negative pressure in the radiant tube without overheating the cold end of the burner. Cycling air only through the radiant tube wastes very litLle heat. The cold air that is cycled through the radiant tubes is first heated by one bed to near furnace temperature, then flows through the tube, then is cooled by the second bed, so that a negligible amount of heat is carried out of the furnace when the burners are in the off position. To date, 52 burners have been installed on the test site furnace. Fig. 11 shows some of them. The results at the test site are: 1. Gas consumption reduced to slightly less than half the consumption with cold air burners. 69 2. The tube is operating with a negative instecrl of a positive pressure. 3. The tubes show no sign of deterioration after 12 months of operation. 4. The ambient temperature around the TwinBed burners is about 32°C (90F). 5. The steel company has estimated the payback time, based on fuel savings -only, to be about 18 months. Fig. 11 - Regenerative Burners on Strip Furnace 'Jhe steel company has placed an order for 120 additional burners. Fig. 12 shows the comparative fuel savings for 537°C (lOOOF) and 1038°C (1900F) air pre-heats, assuming a l149°C (2l00F) flue temperature,lO% XSA and natural gas. We are continuing to improve both the direct-fired and radiant tube burners. The new radiant tube model will have the following features: 1. More compact. 2. Less expensive. 3. Suitable for firing either alloy tubes or high temperature silicon carbide tubes, to replace electric heating elements. 4. Reduced NOx emission. 5. More uniform tube temperature to produce a 30% increase without shortening tube life. |