Glass International 2011
Recent improvements in electric furnace boosting
There seem to be advantages and disadvantages of both phase angle firing and burst firing modes when it comes to electrical boosting. René Meuleman* favours burst firing in multiple zone boosting systems in combination with predictive load management to avoid flickering, as he explains for Glass International.
In recent years we have witnessed increased interest in glass furnace electrical boosting systems. Is this becoming part of 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 metre. This increased pull rate may wear out a furnace more quickly.
Our objective is therefore not to have a furnace lifetime of +12 years, but to accept a lifetime of +6 years while 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 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 of course the maximum allowable crown temperature.
This is where electrical boosting comes into the equation. Of course, we can increase the amount of cullet, but for that cullet has to be available. We could also start using less energy-consuming recipes but again this will have its price.
There are also improved burner technologies to consider. In fact, electrical boosting is one of the major considerations when it comes to achieving an increase in furnace pull rate. Imagine waste heat recovery that feeds electrical furnace boosting…
Boosting considerations
There are several methods of applying electrical energy to a furnace, such as multi-tap switched or slide wire controlled transformers; both of which have their specific advantages and disadvantages. Another method makes use of silicon controlled rectifiers (SCRs) or what we call thyristors to control the power.
SCR-controlled boosting systems are considered to be the latest, solid state technology, although the principle has existed for more than 50 years (proposed by William Shockley in 1955). We have to understand that the SCR is actually just a solid state current switching device. The trick behind those SCRs is the complicated and sophisticated algorithms controlling them, which became available after the introduction of ultra-fast micro and digital signal processors.
With this technology, we are now capable of controlling SCRs so that they can adapt to all kind of situations and run these applications at the highest efficiency and best power factors possible. That is why we do not intend to fire SCRs in phase angle firing mode anymore but instead run them in burst firing mode where the specific
application allows it.
Firstly, let’s 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 a very smooth control but unfortunately, it generates a lot of harmonics and normally runs at unacceptable power factors if not running above at least 70% of set-point.
