Molybdenum Electrodes in the Forehearth and Flow Hole of Fiberglass Tank Furnaces

The main melting section of a fiberglass tank furnace gets most of the attention, yet forehearths and flow holes are where temperature problems are more likely to appear. These two locations are cramped and the glass flows fast; once temperature is uneven, the bushing break rate rises.

The flow hole connects the melting section to the forehearth, where glass passes from the high-temperature zone into a slightly cooler one under strong scour; the forehearth then distributes glass to each bushing position and requires stable temperature along its length. Electric heating in these two locations does not raise overall temperature but makes up local heat loss and holds the set temperature curve. Such arrangements are part of furnace electric heating solutions and must be considered together with the overall tank design.

The task of forehearth electric heating

Molybdenum electrodes or molybdenum plates are usually installed above the forehearth, compensating heat loss through Joule heating. The electrode form and mounting depend on forehearth structure: some are inserted from the top, some mounted horizontally through the side wall, and some use molybdenum plate as a large-area heating element. Ease of replacement must be considered at the design stage, since forehearth space is limited and hot work is difficult. On power distribution, the end near the flow hole loses more heat and often needs higher power, while the far end is trimmed section by section to bushing requirements. Specifications available in the product catalogue should be determined against the forehearth dimensions.

Service conditions at the flow hole

The flow hole has a small cross-section and high velocity, so scour of refractories and electrodes by the glass is pronounced. If temperature there is low, viscosity rises, flow decreases and glass may even solidify; if it is high, erosion accelerates and the downstream temperature regime is affected. Electrode layout should avoid the direct scour path, or a scour-resistant configuration should be adopted. In operation, watch the pressure differential across the flow hole together with temperature; a change in differential often appears before a change in flow.

The block structure at the flow hole is more complex than in the forehearth, and electrode position is constrained by openings and joints. When an electrode has to be replaced during maintenance, inspect erosion of the hole walls at the same time and arrange block repair where necessary; otherwise a new electrode will still sit off-axis because of an irregular opening.

Temperature uniformity and electrode life

Uneven forehearth temperature shows directly in fiber diameter. Uniformity cannot be judged from a single thermocouple; place several measuring points on the same cross-section and compare the transverse temperature difference. Local electrode overheating accelerates wear, and the changed end profile then affects current distribution, closing the loop. If one electrode carries persistently high current, first check whether refractory has spalled or glass flow is abnormal in that area.

Inspection methods

• Measure temperature at each point regularly and record trends in transverse and longitudinal differences.
• Watch electrode current and voltage and compare deviation within the same group.
• Check cooling water flow and temperature; if abnormal, look for scaling in the water jacket or a blocked pipe.
• During a furnace shutdown, measure remaining electrode length and end profile, and update the wear ledger.

Electrode consumption in the forehearth and flow hole is small, yet it directly affects yield. Reading temperature, current and flow data together is more reliable than relying on one signal alone. The dimensional records taken at shutdown are the basis for judging wear rate next time and should be kept long term.

Summarized quarterly, these records show whether electrode consumption rate is changing and provide the basis for the next spare parts plan.

The fiber drawing section of a fiberglass line is sensitive to temperature swings, and small deviations on the forehearth side are amplified stage by stage.