Waterford Crystal wanted a small furnace for the production of high quality 30% Pb0 glass for their new prestige production plant in Ireland. The specification was tough, 800-900kg/day production during an 8 hour shift, production range to be 1-11kg gobs from an automatic feeder. Glass quality to be the same as produced from their previous Glasstronics 25 tpd furnaces. This meant rejects for glass defects had to be less than 4% for a 5kg gob. The furnace was also to have a hand gathering bay for production of wine glasses and special pieces.
Traditional pot furnace technology and day tanks were considered for the project but to achieve the glass quality required we decided to base the design on continuous melting technology, the same technology as their previous furnaces but much smaller and with more flexibility designed in. The larger 25 tpd tank furnace designs that were supplied to Waterford were cold top electric melting furnaces and produced very high quality glass but with limited flexibility. The lack of flexibility was due to the control of the batch blanket, this is very slow to change when increasing or decreasing the glass temperature which is the precursor for changing the pull rates, pull variations of 10-20% / 24 hours were achievable with these furnaces. Maintaining stable pulls was easily manageable as the furnaces had 11 feeders but for the new small furnace with only 1 feeder this had to be improved, the 25 tpd furnaces operated for 6 plus year campaigns and efficiency of the cold top melting is very good operating at less than 0.7 kva/hr per kg of glass melting pelletised batch and 50% cullet and 1 kva/hr per kg of glass including all 11 forehearths.
For the new project we decided that the best philosophy of operation was to maintain a small pull overnight of 40-60 kg/hr with an increase to 100-120kg / hr during the 8 hour production period, a 50% change in pull rate in a short time !!. To make this continuous flow rate cost effective the furnace was designed to operate with a high cullet ratio of 60-80%, this eliminated glass wastage and also improved efficiency by reducing melting temperature and power. To manage the large change in pull rates it was necessary to stabilise the convection currents in the bottom of the furnace. We achieved this by streamlining the design and introducing a lower level heating zone, both this and the melting electrode level were current controlled using Eurotherm Epower thyristors. The complete process being controlled via a PLC and SCADA system which enabled remote alarming and interface.
To achieve flexibility of the melting zone we needed to eliminate the critical batch blanket issues and we did this by designing the furnace with a closed superstructure. We had considered a gas burner to maintain a constant superstructure temperature but decided against following model testing. The advantage of having no burner was improvements in efficiency and also very low pollution.
The furnace was to be fed with 20-40% pelletised batch and this was pre-mixed in 750kg bins with small cullet (>10mm) which we load onto the furnace via twin screw chargers. This feeding method was chosen as it could be used for cold, warm or hot top operation and the two loading points resulted in a good surface distribution.