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Show System Principles (Diagram 1) The direct connection of a Burner, to each Regenerator and the cycling of the Burners, eliminates hot valves working at full exhaust gas temperature. The reversing valve is a purpose built unit, requiring a supply of compressed air for actuation and an electrical supply for the logic circuit to programme reversal based on a combination of time and exhaust temperature. This reversing system is completely independent of other furnace controls, with the exception of flame protection to which it requires to be interlocked for burner start sequencing. The fuel supply is switched to the firing burner by an electrically actuated valve, timed from the reversing system. Each burner allows approximately 3% of the maximum fuel rating to remain firing during the exhaust mode. It is unnecessary to relight at each reversal. Fuel/air ratio is typically controlled using a differential signal, from an orifice plate in the combustion air line and a multiplying air regulator to supply a flow based loading signal to a conventional air/gas regulator. Air and exhaust flow for the unit are controlled by tandem linked valves, driven from the process temperature controller. A fine trim on exhaust flow can be provided by a motorised valve, mounted on the outlet of the suction fan, controlled from furnace pressure. The exhaust fan is protected against overheating caused by the system malfunction. Heat Recovery (Diagram 2) The two Sankey diagrams demonstrate conditions for a typical process operating at 1400oC. The first diagram indicates that for cold combustion air, an energy input of 397.6 units, would be required to achieve 100 units to the process. The balance of 297.6 units being the mass of exhaust gas at process temperature. On the second diagram in a steady state condition, the process requirement of 100 units can be seen to be satisfied by a gross input of only 133.8 units. The unit content of exhaust gases, less system leakage is 73% transferable to the incoming combustion air as pre-heat giving a 49% boost to the nett fuel input. The overall balance being 74.7% efficiency at 14000C process temperature. Possible Fuel Savings (Diagram 3) The high level of energy recovery provided by the (RCB) will produce fuel savings of 55-60% over cold air systems at a process temperature of 13000C and 20-25% over typical recuperated systems at the same temperature. 276 APPLICA TIONS Glass Tank I The first commercial Regenerative Burner System in the United Kingdom was deliberately installed on an open Soda Lime Glass Tank fitted with a pair of 1.0 Million Btu/hr burners providing an operating environment of known difficulty where hot corrosion and potential blockage of conventional metal recuperators could occur. The furnace operates daily at 14000C for melting for a period of 11 hours and at 11800C for conditioning and working (13 hours). No corrosion difficulties have been experienced and trials have demonstrated ease of removing, cleaning and recharging generator packing when batch carryover due to overenthusiastic charging has occurred, the cleaning operation taking a maximum of 20 minutes per packing. Glass output 1.0 Ton/day. Fuel savings on this installation are not as high compared to equivalent installations as this installation is not operated continuously, but is dependent on loading and utilisation. Fuel savings of 25-40% have been achieved. Glass Tank II Located in West Germany, the furnace temperature is 1350-1400oC. The conventional gas firing system using ambient air was replaced with a pair of 3,000,000 Btu/hr each. Output on a 24 hour production basis is 10-12 tons of high quality filament glass. Fuel savings between 50 and 60% were recorded immediately but insufficient consideration had been given to the carryover of Borax in the waste gases passing through the regenerator units. Borax in this application, "condenses" at 800°C which naturally lead to the generators rapidly blocking up. Rapid modifications to the orientation, flow patterns and physical characteristics of the regenerator units themselves were carried out. These modifications were progressive, but rapidly achieved as furnace temperature and production had to be maintained. This installation once again highlighted the vital necessity of appreciating all factors involved on any combustion engineering project, including relevant production characteristics/conditions over and above thermal and efficiency considerations. SUMMARY 1. Considerable improvements in productivity and product quality can be obtained by examining current working practices and adopting the latest techniques and combustion equipment. |