Objective To reduce energy consumption of natural gas holding furnaces for aluminium pressure diecasting machines from the current 90 kW to +- 9 kW/h by use of electric resistance systems. An attempt will also be made to reduce oxidation, metal losses and rejects thereby increasing machine yield. It is planned to develop, erect and test 20 electric resistance furnaces of between 350 to 700 kg providing annual total energy savings of 170 TOE.The project's innovative features are the use of resistance elements: these are cheap to produce and allow in-situ repairs in certain circumstances. A high degree of insulation capacity was also required to allow total power consumption of 3 kW/h.Since the furnaces did not have sufficient heat transfer ability of low temperature metal during production process, a 12,000 kg buffer stock furnace was installed between the main melting furnaces and the machine holding furnaces. This furnace would accurately control the supply source temperature to the various holding furnaces.The first nine furnaces supplied were of 300 and 400 kg capacity. During initial priming it was decided to adapt the Elmonta roof to adapt the embedded element design used by Strikfeldt & Koch in order to prevent damage to elements from metal splash. The advent of microprocessor based temperature controls enabled the control strategy to be changed from on-off switching to proportional three terms control. Thisgave the following advantages:- element temperature was reduced;- this reduced both energy consumption and corundum growth;- more accurate control of metal temperature: typically improvements of +/- 5% to +/- 2% were obtained;- the elements were not subject to severe temperature cycles.The proportionate control was fitted retrospectively on the first nine furnaces and embodied in all furnaces from this stage on.Plant utilisation was generally over 90%, the usual cause of lost time being for repairs to the associated casting machine.The three types of hot face lining - zircon castable, high alumina and aluminosilicate, all behaved differently.The zircon based lining exhibited superior non-wetting (6 months) and then survived for two months before refurbishment which could easily be carried out during a weekend with only minimal production loss. The high alumina lining (K3) performed satisfactorily with little need for major repair: such corundum growth as occurred was readily removed. The aluminosilicate lining deteriorated most quickly.It is likely that poor installation practice provided nucleation centres for the initial growth but after the first reburbishment with a zircon wash coat, their performance was similar to the Elmonta furnaces.One feature to emerge from the project is the paramount need for good housekeeping in the foundry.Prior to the initiation of the electric furnace prinicple, Metal Castings Ltd had concentrated on achieving maximum efficiency from its existing gas fired furnaces. Proportional control units had been fitted to control burner operations and sophisticated recuperative burners were utilised to minimise waste from flue gases and maintain flue temperature. Improvements of between 50 and 60% in gas consumption had been achieved by these means.Resultant savings based on the demonstration running at MC(W)Mtd, using the average industrial gas furnace efficiency rather than that achieved at MC(W)Ltd, would be in the order of 713 TOE.Existing natural gas-fired holding furnaces at 20 aluminium diecasting machines were replaced by 20 electric furnaces to test the influence of differing diecasting machine throughputs. Kanthal, a chrome-iron-aluminium alloy, is used as a heating resistance element in conjunction with thyristor controlled switching relay. Furnace shell, furnace material, heating zones, bale out and refractory lining were optimized, ensuring best possible thermal insulation and lowest radiation losses to the environment.Ten furnaces installed at regular intervals during 1983 were used, six 13 Kw Elmonta type - four of 600 kg capacity and two of 700 kg and four Striko of 500 kg capacity. The heating elements of all ten are protected by overheat thermo-couples preventing element temperature exceeding 850 deg. C.Five of the Elmonta type were lined with a hot face of calcium aluminate bonded zircon castable and the sixth (of 600 kg capacity) with a pre-cast, pre-fired assembly of nitride-bonded silicon carbide tiles. The addition of extra insulation to this unit made the thermal performance of all Elmonta units very similar. Of the four Striko furnaces one was lined with dense, castable Purotab giving case temperatures of 55-60 deg. C, one with Dycast 1600 giving case temperatures of 70-75 deg C, the other two using experimental high carbon blast furnace castable giving case temperatures of 55-60 deg C. One of these units suffered from excessive heat loss attributed to poor design and insufficient insulation. Normal casting temperature was 630 deg C-680 deg C and on-off temperature control achieved using a thermo-couple immersed in the melt. Machines with automatic ladling used higher temperatures to compensate for higher heat loss of mechanical metal delivery systems. Microprocessor temperature controllers enabled control strategy to be changed from on-off to proportional three-term (P.I.D.) control, giving the following advantages:- reduced element temperature ;- reducedenergy consumption and corundum growth ;- more accurate metal temperature from +- 5 deg C to +- 2 deg C ;- elements were not subjected to severe temperature cycling ;Considerable casting temperature variation and significant energy consumption increase took place, attributed to deterioration of refractory linings due to ageing of ceramic. Programme(s) ENG-ENALT 1C - Programme (EEC) of financial support for projects to exploit alternative energy sources, 1979-1984 Topic(s) 1.6 - INDUSTRY Call for proposal Data not available Funding Scheme DEM - Demonstration contracts Coordinator Cookson Precision Castings Ltd EU contribution No data Address Droitwich Road WR3 7JX Worcester United Kingdom See on map Total cost No data
