Electric Boosting in Daily and Packaging Glass Furnaces: Molybdenum Electrode Selection

Daily and packaging glass is dominated by container products, with batch formulations mostly of the soda-lime-silica system. Melting temperatures are relatively moderate, but single-furnace output is large and continuous campaigns are long, and fuel costs account for a substantial share of total cost. With natural gas price volatility compounded by energy-saving and carbon-reduction requirements, inserting molybdenum electrodes into the melting and refining zones so that electric power carries part of the flame load is increasingly being added to container producers' retrofit lists.

Energy Structure of Container Furnaces and the Role of Electric Boosting

Electric boosting does not convert a flame furnace into a fully electric melter; it adds a controllable block of electric energy to the existing melting regime. A typical boosting ratio is between 5% and 20%, depending on output, batch formulation, bath depth and insulation condition; these figures are typical values / reference ranges, subject to actual operating conditions and the contract. Set the ratio too high and the burden on the bath walls and electrodes rises in step; set it too low and a stable temperature support is hard to establish in the hot spot area.

For container furnaces, electric boosting works at three levels. First, it supplements heat in the hot spot area to maintain the convection loop. Second, it holds temperature in the refining zone, reducing striae and bubbles. Third, it quickly restores the temperature field after night shifts, batch changes or short stops, shortening the recovery period. Lightweight bottle designs are more sensitive to wall-thickness uniformity, so the benefits of a stable temperature field show up directly in yield and in gob weight control.

Sidewall versus Bottom: How the Two Layouts Divide the Work

Sidewall insertion is the more common approach: the electrode passes through the sidewall blocks into the glass melt, is relatively easy to install and replace, and has little effect on the bottom structure, making it suitable for most container furnace retrofits. Vertical bottom insertion distributes heat more symmetrically and suits furnaces with a greater bath depth where heat should be supplied evenly along the depth, but port sealing and the risk of downward melt leakage require more careful design and inspection.

On standards, electrode products are controlled to YS/T 1312-2019 Molybdenum Electrodes; molybdenum rods follow ASTM B387 / GB/T 17792; molybdenum plates and discs follow ASTM B386 / GB/T 3876. Acceptance is based on the quality certificate and measured records.

The two methods can be combined: sidewall electrodes in the hot spot area to strengthen local convection, and bottom electrodes in the refining zone or at the front of the throat to make up the lower-layer temperature. Before combining them, calculate the power demand of each zone, then decide port positions and electrode length, so that the scheme does not have to be changed after the ports have been drilled.

Power Allocation and Current Density Values

Zone power is usually allocated by bath area, output and target boosting ratio, with the hot spot area taking the larger share and the refining and throat areas supplemented as needed. The power of a single electrode should not be pushed to its upper limit; leave adjustment margin, otherwise there is no room to maneuver when the batch formulation changes or output increases.

Current density should be held in the reference range of 1 to 3 amperes per square centimeter, taking the lower limit for highly corrosive formulations and approaching the upper limit for ordinary soda-lime-silica formulations. Electrode diameter is commonly 50 to 100mm, and length is determined by sidewall thickness, insertion depth and external connection space; all of the above are typical values / reference ranges, subject to actual operating conditions and the contract. Diameter and current density are paired: at the same power, a larger diameter lowers current density, but occupies more sidewall ports and also affects the local flow field.

Adjustment Under Lightweighting and Output Fluctuation

Lighter bottle designs reduce gob weight, and output changes with it, so the heat demand of the melting and refining zones is no longer fixed. If boosting power is left unchanged for long periods, the hot spot drifts: heat is insufficient at high output, and localized overheating appears at low output. The safer approach is to relate power to output, melt temperature and gob weight, record it by shift and re-check it at each product change.

After a product change or short stop, do not chase temperature by sharply raising electric power. Heating too fast imposes additional differential thermal stress on the bath walls and electrodes, and the glass melt flow field also needs time to stabilize. Restore power in stages, watch the changes in bubbles and striae in the refining zone, and return to the normal operating range once the temperature field is confirmed stable.

Documentation Before Retrofitting and Supply Rhythm

Before the retrofit scheme starts, prepare the furnace drawings, existing port positions, bath depth and width, output, batch formulation, current melting parameters and water-cooling conditions. With complete documentation, electrode specifications, quantity and layout can be fixed in one pass, reducing rework after the ports are drilled on site.

Container producers' retrofits are mostly tied to furnace repair windows, with large purchase volumes, concentrated delivery schedules and high demands on batch consistency. The supplier needs to machine to drawings and support parameter adjustment in the early stage of commissioning. Give the furnace documentation, power targets and repair schedule to the supplier at the same time, and selection and delivery dates are easier to align.