Water-Cooled Panels and the EAF Shell: How Heat Load Design Actually Works
Open the side access door of any modern EAF and the first thing you see is panels - rows of water-cooled copper or steel boxes bolted to the shell, framing the slag line and the hot face above the taphole. Get those panels wrong and the furnace does not run. Overheat them and you crack a copper panel mid-heat, water hits molten steel, and you have an emergency. Under-design them and the steel shell warps, the refractory fails, the bottom of the upper shell bows inward, and the whole furnace comes down for a reline weeks ahead of schedule.
The water-cooled panel is the EAF's first line of defense. Here is how the heat load numbers get set and why they matter.
Start with the basic heat balance. A 60-ton UHP EAF with a 60 MW transformer dumps roughly 45 to 50 MW into the bath during the flat bath phase. Maybe 60 percent of that energy goes into the steel and slag as useful heat - melting, superheat, chemistry reactions. The other 40 percent is losses: offgas, water cooling, refractory surface radiation, and conduction through the shell.
The offgas takes the biggest single share - typically 15 to 20 percent of the input. Water cooling takes another 8 to 12 percent. Refractory losses are 5 to 8 percent. Radiation and conduction through the uncooled shell account for the rest.
In a well-designed EAF, water cooling should never exceed 12 to 15 percent of total input. Above that, you are paying to heat cooling water instead of melting steel. The cooling water then needs to dump that heat somewhere - typically a cooling tower or a plate heat exchanger. The cost of pumping and treating that water, plus the energy to cool it back down, becomes a real operating expense.
Panel design starts with the heat flux.
Heat flux on an EAF sidewall panel depends on position. The slag line - the band of refractory and paneling directly in contact with the foamy slag - sees the highest flux. Values of 250 to 400 kW per square meter are common during flat bath operation. The hot spots near the electrode arcs can spike to 600 kW/m^2 or more during bore-in. The lower shell, below the slag line, runs much cooler - 80 to 150 kW/m^2 - because it is lined with refractory brick or monolithic material.
The delta region - the curved transition between the cylindrical shell and the dished bottom - is the toughest area to cool. Heat pools there because the geometry resists natural convection in the slag. Most EAF designs use extra-heavy panels in the delta, often with internal baffles to force higher water velocity.
Panel material matters. Copper has the highest thermal conductivity of any practical panel material - around 390 W/m-K - which means thinner panels can carry higher heat flux without overheating. Steel panels are cheaper but conduct heat at less than a third of copper's rate. For high-heat-flux zones - slag line, delta, hot spots near electrodes - copper is the standard. Steel panels are common on the upper shell and the roof, where the heat flux is lower and the panel mass helps with stiffness.
A typical EAF copper panel is 25 to 40 mm thick, with internal water passages sized for 2 to 4 m/s water velocity. Higher velocity means better heat transfer, but also more pumping power and more erosion of the panel internals over time. 3 m/s is a common design point. Below 1.5 m/s, you risk localized boiling at high heat flux - and boiling on the hot face of a copper panel is the start of a leak.
Panel cooling water circuits are usually separate from the rest of the plant cooling water. The reason is control. EAF panels need consistent inlet temperature (typically 35 to 45 degrees C) and consistent pressure. They also need treated water - demineralized, with low dissolved oxygen and low chloride - to prevent internal corrosion and scaling. Mixing panel water with general plant cooling water invites fouling and corrosion that show up as leaks three years into a campaign.
The leak detection question is its own topic. A leaking panel is dangerous because water hits molten bath. Most EAFs install conductivity sensors on the drain side of the panel circuit, plus flow switches on each panel loop. A drop in flow or a rise in drain conductivity triggers an alarm. Some modern EAFs use differential pressure monitoring - a sudden pressure drop on a single panel suggests a breach.
When a leak happens, the response depends on the heat and the panel position. A small leak in the upper shell can sometimes be managed by reducing power and finishing the heat. A leak at the slag line is an emergency - tilt the furnace, drain the bath into the ladle, and shut down for panel replacement. Most EAF operators have a written emergency response plan that covers this scenario.
Panel life varies enormously with operating practice. A copper panel on a well-controlled EAF running foamy slag can last 1500 to 2500 heats before needing replacement. On a poorly controlled furnace with frequent bore-ins, panel life drops to 600 to 1000 heats. The difference is almost entirely about slag practice. Buried arcs distribute heat across the bath and the slag. Unburied arcs focus energy on a small area of the sidewall.
The refractory behind the panels also matters. A typical EAF sidewall has a layer of magnesia-carbon brick or monolithic magnesia between the shell and the panel hot face. The refractory insulates the panel, reducing the heat flux it sees - but if the refractory wears thin, the panel takes more heat. Some EAFs use laser-based or thermal imaging systems to monitor refractory thickness in real time, alerting the operator when a zone wears to a critical level.
Refractory wear is not uniform. The hot spots near the three electrodes wear fastest. The slag line, where chemistry is most aggressive, wears second fastest. The lower shell, below the slag, wears slowest. A common design strategy is to use higher-grade, more expensive refractory in the high-wear zones and standard grade in the low-wear zones. The cost difference is justified by the campaign life extension.
Upper shell panels see different conditions than slag line panels. The roof and the upper shell experience radiant heat from the arc and convective heat from the offgas, but they do not see direct contact with the bath. Heat flux on the upper shell is typically 50 to 150 kW/m^2 - lower than the slag line. Steel panels work fine here. Roof panels see the highest radiant flux, particularly around the electrode ports, and they often use a combination of refractory and water cooling.
The water system itself is the silent partner. Pump pressure must stay high enough to maintain flow through every panel, even when the pump curve shifts due to fouling or air entrainment. Most EAF water systems run at 4 to 6 bar panel pressure. Inlet temperature should be monitored at every panel bank. Outlet temperature should be monitored too - a sudden rise on one panel suggests a flow restriction.
Water quality has a real impact on panel life. Dissolved oxygen above 50 ppb accelerates pitting corrosion in copper panels. Chloride above 20 ppm attacks stainless steel piping and brass fittings. Scaling from hardness ions reduces internal passage size over time, dropping flow and raising panel temperature. The water treatment system - typically a demineralizer plus a side-stream filtration unit - is part of the critical path of EAF operations. Skimping on water treatment shows up as panel failures two to three years into a campaign.
Author: MONTE INTELLIGENCE electric arc furnace engineering team. For sidewall and water system audits, contact helenxu@cnlymonte.com.
