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Show to the inside of the tube is followed immediately by a heating solvent dispersal/baking process. Traditionally this has been accomplished using Roof Mounted Ceramic Radiant Block Gas Burners burning an air/gas mixture. Once internally coated the tubes are individually loaded onto a rolling roller conveyor which passes under a radiant burner roof. The heating time is 3 minutes and a typical Oven of this type has a rated input of 3.2 Million Btu/hr with the roof line being some 6-12" from the tubes. Conversion of the gas energy to radiant energy has been calculated to be in the order of 40-55%, thus a high percentage of the preheat energy is wasted. With the ceramic roof burners response time to load change is poor and the whole system has considerable thermal lag taking I-I! hours to reach operating equilibrium before production can commence and taking longer to cool down before an empty conveyor can be stopped. A development here in the United States has enabled a novel approach to this production requirement by employing Ceramic Fibre Infrared Generators. Using an air/gas premix fuel at 4-5 inwc, these Generators incorporate a 1" thick Ceramic Fibre Matrix pad. The air/gas mixture burns in the outside edge of the matrix creating a face temperature of 1600oF. The burning fuel heating the multi-fibre structure produces a uniform radiant face and achieves a 65-70% conversion of Gas to radiant energy. The Matrix has no thermal mass and reaches temperature in approximately 3-4 seconds and is cool enough to touch in the same time span once the Gas flow has been shut off. -- A unique feature of the type employed on the application being discussed is the air plenum seal which surrounds the main air/gas manifold. This simple device when flooded with air at a pressure slightly above the air/gas mixture pressure purges the outside !" of the Matrix preventing any combustion taking place, thus keeping the Matrix containment strip plates and pins completely cool. In vertically mounted locations and positions, when downward firing is needed, the entire manifold can be fibre insulated. These Infra-red Generators have a standard width of 14" and the effective radiant width of 11". Length can be up to 18 feet and to accommodate different process width requirements they can be compartmented to fire centre cores with one or more wings on each side of the centre core. For a Fluorescent Tube Roller Conveyor Oven for a European Country, the production requirement was to process 4000 tubes an hour 275 to 1100oF, with two tube lengths of 5ft and 6ft. Thermal input capacity utilising conventional Ceramic Radiant Burner was 3.2 Million Btu/hr. Replacing this entire ceramic roof with 10 Ceramic Fibre Infra-red Generators, each 72" long, controlled in 2 zones of fi ve firing directly down onto the glass tubes on the conveyor. 3.2 Million Btu/hr capacity has proven very successful. The complete new Oven was Shop built and fully tested with product achieving full output on the 6 foot tubes at a rate of 4000/hr. Tube residence time in the Unit was 1 minute. Benefits deri ved from the use of the ceramic fibre generators can be summarized as follows:( a) Fuel consumption reduced by 30%. (b) Elimination of thermal roof mass enables Unit to come to temperature and full production some 30% faster. (c) With less thermal mass in the complete unit the load on the water cooled frame was reduced thus reducing the risk of the oven growth affecting the process equipment either side of the oven which had occurred in the past. (d) Shutdown was simplified and the low mass panels cool immediately. (e) Turndown requirement of 3:1 on power input was achieved. (f) Response time in control was considerably improved with the low mass ceramic fibre generators. (g) It is envisaged that maintenance will be reduced, coupled with much reduced risk to tube and conveyors from the previous hazard of ceramic/refractory roof particles. REGENERATIVE CERAMIC BURNER SYSTEMS Regenerati ve Ceramic Burner Systems are unlike any other System in the World. Maximum process efficiency must be a function of process furnace temperature. The available heat from the flame will decrease as the process furnace temperature approaches flame temperature. Typically a well managed furnace operating at 1400oC, and using cold combustion air would achieve approximately 25% combustion efficiency. Further improvement can be gained by using various Recuperators, giving up to 45% efficiency. However, these Recuperators can be expensive and limited by the quality of materials, at the higher temperature range. With increasing temperature, failure can occur unless dilution air is used. Regenerati ve Ceramic Burner Systems can achieve combustion efficiencies of up to 75% without sophisticated materials, even with contaminated waste gases. The Regenerator materials of refractory construction can even be removed, cleaned or replaced without process interrupti on. |
