Tin Bath Roof and Annealing Lehr Heating Control

Bath Roof Heating

The multiple zones of a tin bath roof require a huge electrical heating system that takes care of hundreds of silicon-carbide (SiC) heating elements. In order to guarantee the lifetime of those electrodes combined with the highest energy efficiency they have to be controlled in a special way.

During heat up these types of electrodes run thru a non-linear resistance curve in which the SCR controller needs to protect the electrode against too high voltage and current.  Once the electrode arrives at operation temperature the SCR controller will have to switch to full burst mode in order to avoid the inefficient low power factor of the phase angle fining mode.

Last but not least, the overall tin bath roof heating system needs to avoid any peak power demands in order to assure the highest efficiency at the lowest power tariffs.

To meet all those complex control tasks, Eurotherm developed EPower, the most sophisticated multi stack SCR controller on the market. EPower provides dynamic switching of firing modes, special SiC control strategies and Predictive Load Management to prevent any peak power demand and providing load shedding in case of emergency power generation situations.

Annealing Lehr Heating Control 

Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process is carried out in a temperature-controlled Lehr.  Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to its durability. If glass is not annealed, it will retain many of the thermal stresses caused by quenching and significantly decrease the overall strength of the glass.

In the annealing process the glass is heated until the temperature reaches a stress-relief point, that is, the annealing temperature (also called annealing point) at a viscosity, η, of 1013 Poise = 1012 Pa·s, at which the glass is still too hard to deform, but is soft enough for the stresses to relax. The piece is then allowed to heat-soak until its temperature is even throughout. The time necessary for this step varies depending on the type of glass and its maximum thickness. The glass is then slowly cooled at a predetermined rate until its temperature is below the strain point (η = 1014.5 Poise). Following this, the temperature can safely be dropped to room temperature at a rate limited by the heat capacity, thickness, thermal conductivity, and thermal expansion coefficient of the glass. After the annealing process the material can be cut to size, drilled or polished.

Next to necessarily well controlled annealing process, the control system also needs to take care for energy efficiency. In most of the cases the incoming glass containers will introduce enough heat to the annealing lehr by themselves and therefore a well designed temperature and draft control will result in best annealed glass as well as in low energy consumption.

 

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