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Electrode Holders – Considerations for a Successful Electrode Heating System

Grahame Stuart* discusses the increasingly crucial role electrodes and electrode holders play in the industry’s aim to meet net zero goals.

As the industry moves towards hybrid and all-electric furnaces to meet net zero goals and reduce reliance on fossil fuels the part played by electrodes and electrode holders is becoming increasingly critical to operational success. Unsurprisingly there is much to consider when selecting not only electrode holders but also the water system and the monitoring system.

The key purpose of the electrode holder.

Electrode holders for molybdenum electrodes provide physical support for the electrode and cooling that prevents glass seepage around the electrode. However, their primary purpose is the part they play in providing cooling to the molybdenum electrode where the electrode is not protected by a glass seal and would otherwise be subjected to atmospheric temperatures where oxidisation may occur. Oxidisation will lead to ‘necking down’ of the overall electrode diameter eventually leading to breakage. This ‘necking down’ results in increased localised temperatures, accelerated refractory wear and greater risk of glass leakage.

Heat extraction.

The heat extracted from the furnace by the electrode holders must be considered when selecting these for any project as the objective is to cool the molybdenum electrode and not the surrounding refractory and glass. Excessive external cooling will not benefit the molybdenum electrode but will cause greater thermal stress on the electrode blocks leading to increased cracking and refractory wear as well as pulling heat from the melting process.

Additionally, if an electrode holder design is employed that extracts more heat than is necessary it is likely that a considerable amount of energy will need to be re-applied to the furnace to overcome this. For example, in a system with 24 electrode holders each extracting 2kw of energy more than is required the nett melting energy to produce 2 tonnes/day of glass is being lost, – it is easy therefore to see why care must be taken when selecting electrode holders to ensure they are not creating an unseen operating cost.

All Electroglass electrode holders feature external and internal low thermal mass insulation to minimise the heat extracted from the furnace whilst maximising the cooling around the electrode.

Figure 1- A batch of Molycool holders ready for dispatch.

Dissimilar Metal Contact.

Contact between dissimilar metals in glass can lead to a galvanic reaction and the generation of DC voltages which may result in the generation of bubbles at the junction between the two metals. These bubbles, typically of oxygen, can impact on glass quality, but more seriously can cause oxidisation to the molybdenum electrode at the point of generation leading to electrode failure.

It is important to consider electrode holders that have design features to prevent dissimilar metal contact helping to prevent DC voltage generation, as found on all electrode holders we produce.

Electrode Advancing.

Whilst a well-designed electrode heating system will minimise electrode wear, the oxidising properties of many glass compositions result in electrode wear and the need for periodic advancement of electrodes to compensate. The process of electrode advancing causes some degree of thermal shock on the electrode holder with the stopping and starting of cooling water flow and if the electrode holder is not designed with this in mind, holder life can be reduced.

Design considerations such as continuous, unwelded cooling coils and compressed air-pre-cooling facilities help reduce the risk of electrode holder failure whilst advancing, as can proper operator training during commissioning by the manufacturer’s engineers.

Figure 2- Typical electrode holder water cooling system

Water Cooling.

The biggest risk to electrode holders is poor water system design and badly maintained water quality. It is always our recommendation that a separate dedicated cooling system be installed for the electrode holders. A good water system should have redundancies in-built in case of pump or power failure. As a minimum we recommend that all our electrode holder customers consider the following features on their electrode holder water cooling systems.

• Duty and stand-by circulating pumps with automatic changeover.
• A high-level header tank capable of storing sufficient water to gravity feed the electrode for a minimum of 25-30 minutes of normal operation.
• An automatic filling loop to provide city water in the event of prolonged power or pump failure
• A heat exchanger or chiller capable of extracting sufficient heat at the highest annual ambient temperature for the plant.
• Water flow monitoring to each individual electrode holder cooling coil/circuit.
• Needle valves and magnetic strainers at the entrance to each cooling coil/circuit.
• A water softener operated on a loop to soften any top-up water or for use following any episodes operating on city water.
It is recommended that the water within the system is tested weekly and that the Ph, hardness, conductivity, and holding and header tank temperatures are recorded and any corrective action to keep parameters within recommended specifications be carried out.

System Monitoring.

Another area key to the success of any electrode heating system is monitoring of all operating parameters. Comprehensive metering, recording and associated alarm facilities are critical as is the operators understanding of this.

A well-designed monitoring and control system will give the operator an overview of system operation showing electrode holder temperatures, phase currents and voltages, status of transformers and key water system parameters such as individual water flows, pump status and tank water levels.

Other critical information that should be displayed and monitored includes individual electrode currents to ensure they are not exceeding design recommendations, earth current to ensure there are no earth faults within the system that may lead to localised heating and possible glass leak, and individual electrode volts-to-earth to help locate any earth faults and to assist with electrode immersion determinations.

All critical operating parameters should have alarm and trip set points to ensure any operational changes cannot go unnoticed and, where necessary the power to the transformers can be automatically stopped to prevent serious damage.

Figure 3- Typical control and monitoring system screens

Electrode Advancement Determinations.

Maintaining correct electrode immersions is critical to ensuring efficient system operation whilst maximising furnace life. There are many factors that will determine how often electrodes require advancing. These include glass chemistry, temperature, power applied, electrical connection arrangement, electrode layout and current loading.

It is important that electrode advancing is not treated as routine maintenance with advancements taking place at time-based intervals rather than when wear has actually taken place. Routinely advancing electrodes that do not require it will result in over advanced electrodes that can limit system power and possibly create convection that is detrimental to furnace operation leading to a reduction output and/or glass quality.

A far better approach is to monitor operating data and plot changes in electrode current, amps-per-volt and volts-to-earth and advance by the correct amount per electrode only when required. However, there are other factors that that will affect glass conductivity and therefore must be considered before determining electrode immersions. These include changes in glass temperature and glass composition.

At Electroglass we encourage our electrode heating system customers to work with us in determining electrode immersion and advancements by forwarding operating data on a regular basis. We will then review the various parameters and advise when and by how much to advance each electrode. Although this service is not unique to Electroglass the fact that we make no charge for it likely is.

Our Electrode Holder Ranges.
Electroglass offer three ranges of electrode holders to cover all melting situations. All of these are manufactured exclusively within our own workshops in England and every holder goes through extensive testing and inspection prior to its dispatch. Every holder can be customised in length to suit each individual installation with holders regularly produced ranging in length from less than 400mm to over 1650mm.

The Molycool Holder.

Figure 4- A holder undergoing manufacture in our own workshops in the UK

Our longest established electrode holder range is the Molycool which is primarily used for vertical and angled installations but can also be used for sidewall installations. It offers the lowest level of heat extraction from the electrode block through the design of its cooling coils and two stage thermal insulation. The Molycool range includes holders to suit 32mm (1.25”), 50mm (2”) and 63mm (2.5”) electrodes and, in common with all Electroglass holders features no welds on any of the cooling circuits.

Molycool holders also incorporate an air-pre-cooling circuit that allows the use of a small volume of low pressure compressed air to initially cool the electrode holder prior to starting the cooling water flow, helping to reduce thermal shock on the holder’s cooling coil.

The VS Splashguard Holder.

For glass types in which regular electrode advancement is expected we recommend the Vertical Splashguard (VS) range of holders. The VS range is manufactured to suit 50mm (2”), 63mm (2.5”), 76mm (3”) and 102mm (4”) diameter electrodes and features no welds on any of the cooling components and no sealed water-cooling circuits that could be ruptured by high pressure steam generated when cooling is re-applied following electrode advancing.

The VS range of holders is also recommended for applications where there are concerns about cooling water quality or consistency as they are extremely resistant to thermal shock and their internal construction means blockage of cooling ways due to calcium deposits is very unlikely.

The HS Splashguard Holder.
Horizontal Splashguard (HS) holders are manufactured to suit 32mm (1.25”), 50mm (2”), 63mm (2.5”) and 76mm (3”) electrodes and feature weld-free cooling components, no sealed water-cooling circuits and a high level of resistance to thermal shock during water disruptions or following electrode advancing.
Additionally, the HS range features removable cooling ways to allow the cleaning away of any sediment or calcium build up that might occur.
Important Choices

All of the above are important points to consider when investing in electrode holders and can mean the difference between success or failure of the entire furnace project.

With over 45 years’ experience in electrode holder development, design, and manufacture, and with several hundred produced each year for customers worldwide, Electroglass ensures that every holder it produces meets the exacting standards required to ensure reliability and long life.

ABOUT THE AUTHOR:
Grahame Stuart is Project Sales Engineer at Electroglass Ltd

T: +44 (0)1268 565577

F: +44 (0)1268 565594

4 Brunel Road, Manor Trading Estate,
Benfleet, Essex, SS7 4PS, England