Electric Furnace Boosting

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To enhance the glass melting process, increase throughput, improve glass quality and to reduce CO2 emissions electrical furnace boosting is often used.

It also provides a useful control parameter for advanced glass temperature control strategies and pull rate fluctuation compensation.

Eurotherm EPower controller based solid state electrical heating systems provide 100% freedom of control at optimum efficiency, achieving best power factor with minimum harmonic distortion. The award winning Predictive Load Management strategies will eliminate peak power demand when running in full cycle mode.

The Load Tap Changing capabilities of the EPower controller provide the highest efficiency when operating in phase angle firing. For electrical power control and monitoring in fiber glass, float glass, containers and insulation applications, you can rely on us to provide complete systems including the electrical cabinets, power controllers and the necessary transformers.

Introduction

In recent years there has been an increased interest in glass furnace electrical boosting systems. Is this becoming part of a new business strategy in glass manufacturing? Instead of aiming to increase furnace lifetime an alternative business case may be to produce more tons per square meter. This increased pull rate may wear out a furnace more quickly. The objective therefore would be not to have a furnace lifetime of +12 years, but accept a lifetime of +6 years whilst achieving double the throughput. In fact, this carries some further advantages such as a faster return of investment and the opportunity to innovate more often.

Such a concept of course faces the constraints of what the infrastructure such as refractory material is capable of handling when it comes to more extreme high temperatures; one of these being the maximum allowable crown temperature. This is where electrical boosting comes into the equation. Of course the amount of cullet can be increased, but for that cullet itself has to be available. Also, less energy consuming recipes could be used but again this will have its price. There are also improved burner technologies; if these have not been applied then that would be a priority anyway. In fact electrical boosting is one of the major things to consider when it comes to achieving an increase in furnace pull rate. Imagine waste heat recovery that feeds electrical furnace boosting?

Furnace Boosting Considerations

There are several methods of applying electrical energy to a furnace, like multi tap switched or slide wire controlled transformers, both having their specific advantages and disadvantages. Another method makes use of SCR’s (silicon controlled rectifiers) or what we call thyristors, to control the power. SCR controlled boosting systems are considered to be the latest, solid state technology although the principal has existed for more than 50 years (proposed by William Shockley in 1955). It is important to understand that the SCR is in fact only a solid state current switching device. The trick behind SCR technology is of course the complicated and sophisticated algorithms controlling the SCRs which became available after the introduction of the ultra fast micro and digital signal processors

Having this technology available it is now possible to control SCRs so that they are capable of adapting to all kind of situations and running these applications at the highest efficiency and best power factors possible. That is why there is no more intention to fire SCRs in phase angle firing mode anymore but instead of that it is preferable to run them in burst firing mode where the specific application allows it.

But let’s take a step back and have a look at both phase angle and burst firing modes to understand what they mean and what advantages and disadvantages they have. Phase angle firing is the oldest method of controlling electrical power with SCRs since this firing method could be handled by analogue circuitry. It offers very smooth control but it unfortunately generates a lot of harmonics and normally runs at unacceptable power factors if not at least running above 70% of set point.

An alternative method of controlling power with SCRs is what is called “burst firing”. For this purpose a duty cycle needs to be defined: a window of a specific amount of sine waves, in which single or multiple sine wave packages are managed, to control the total amount of power inside that duty cycle. The major advantage is that such a burst firing system runs at very good power factors and minimizes harmonics. However, a major disadvantage with high power loads or multiple power loads is that burst firing may cause “flickering”.

Overcoming common disadvantages of SCR control

With the EPower controller Eurotherm has a solution for the poor power factor and harmonics generation of a phase angle fired system called ‘Load Tap Changing’ (LTC). Eurotherm can also offer a solution for flickering disadvantage of multiple burst fired systems which is called ‘Predictive Load Management’ (PLM).

Even a combination of both load tap changing and burst firing is possible, and will normally give the best result. However, for that solution, multiple SCRs on multiple tapped transformers are required. However, this would introduce additional cost on both the power control system as well as on the transformers. Nevertheless, an acceptable return on investment can normally be achieved and should always be calculated when on the subject of a power system layout consideration.

Normally high power values are running through a glass furnace boosting systems, therefore power factor, harmonics and flickering need to be considered. With simple phase angle firing we avoid any flickering issues. However, in phase angle firing there are normally some constrains:

Give up the majority of control freedom or run the system with a bad power factor thereby introducing harmonics?

That is why Eurotherm introduced burst firing in multiple zone boosting systems alongside predictive load management to avoid flickering. The result of such a system is optimum power factor, minimum harmonics and due to the intelligent ‘predictive load management’ distribution of sine wave packages over the total duty cycle, no flickering. This is in fact the best of both worlds at an achievable price.

Recently Eurotherm Glass installed a 9 zone furnace boosting system which runs in burst firing mode. This installation has now been running for several months operating successfully and demonstrating the benefits detailed above. One of the biggest advantages of running a solid state EPower controller system is of course that it needs no maintenance and is not subject to any wear. Even constantly controlling power fluctuations or active glass temperature control will not harm the system at all. In fact, it provides an additional controlled parameter to your glass making process which will become important as soon as you consider the use of advanced process control methods. Simply switch it on and forget about it!

Fundamentals

For better understanding it is necessary to know that a single SCR will behave as a diode with the only difference that it starts conducting only and if the gate is triggered by a pulse. Therefore, to be able to control an alternating current, we need to put two SCRs anti parallel as in the diagram below. One SCR triggering for the positive sine wave and the other SCR triggering for the negative part of that sine wave.

To operate such a system, trigger signals to both SCRs need to be applied using the sine wave zero crossing as reference. In fact, for each SCR there is a firing angle of 1800 and the longer the delay to trigger the SCR(s) the shorter time it will be in conduction, consequently the lesser power is applied to the load. By controlling the firing angles of both SCRs the applied power to the load is controlled.

The drawings below show in more details how phase angle controls the power and the effect the firing angle has on the power factor. Running a phase angle fired system below 80% of the set point may already result in an unacceptable low power factor.

The drawing below illustrates the difference between phase angle firing and burst firing at equal power levels. Since the sharp waveforms of phase angle firing cause the harmonics and reduced power factor, it is obvious that burst firing, due to controlling only full sine waves, will not have those drawbacks.

Conclusions

There are many different considerations which have to be evaluated during the design of a furnace boosting system. Initial costs, return of investment, freedom of control and mean time before failure, are only some of them. Using burst firing modes in combination with predictive load management provides a smooth constant control, a very good power factor and minimum harmonics at an achievable price level. Last but not least – it is maintenance free.