The glass industry continues to demonstrate the centrality of an ancient material that can meet all the challenges of modernity, both environmental and technological. To highlight the industry’s progress, CD Automation presents the solutions and products developed to optimise and make efficient heating processes for float glass, also known as flat glass.
Float glass: definition and history
The name float glass (from the English “to float”) refers to the flotation system with which the material is manufactured, in particular to the stage of the process in which the molten glass ribbon, coming out of the furnace, floats on top of a layer of tin (the float bath), in a controlled atmosphere. The process, which has been in place since the 1950s, makes it possible to produce glass sheets that find numerous uses in the construction industry. The float glass production operation, on a commercial level, was devised by Alastair Pilkington and Kenneth Bickerstaff, with the aim of obtaining flat or smooth glass sheets without resorting to costly operations to eliminate abrasion.
Float glass has quality and strength characteristics that cover a large part of the world’s glass production, thanks to a production process that allows large quantities of material to be processed at low cost. This processing makes it possible to produce products with different colours, perfectly smooth and suitable for various processes, such as decoration, grinding, bending, layering and coking.
Production steps for float glass
The melting furnace
In this process, the raw materials used are:
- a vitrifying agent, i.e. silica sand (70/74%)
- a stabiliser, calcium carbonate (12/13%)
- a flux, sodium sulphate (12/13%).
In the inlet area of these elements, careful temperature control is essential in order to avoid rapid deterioration of the furnace’s firing thermocouples due to the high operating temperatures. Here, the mixture of raw materials, measured and humidified to obtain a vitrifiable substance, is conveyed by conveyor belts into the melting furnace, inside which the temperature reaches 1550°C.
The tin bath
On leaving the melting furnace, the viscous glass is passed over a bath of molten tin at a temperature of around 1000°C. As we anticipate, it floats on the liquid, flat surface and is pulled until it becomes a ribbon with parallel faces. The tin smoothes the lower surface of the glass by direct contact, while the upper part is flattened by gravity, as it is a material in a semi-melted state. On the edges of the belt, toothed wheels (top-rolls) stretch or retract the glass laterally to obtain the desired width and thickness. The thicknesses obtained are between 1.1 and 19 millimetres.
Annealing and controlled cooling
At the end of the tin bath phase, the temperature of the glass is about 600°C. It, now in a solid state, enters an annealing chamber. This process step serves to change the internal tensions of the material to make it more suitable for cutting. The float glass is then lifted and placed in a cooling tunnel, where it reaches room temperature.
The cutting
The ribbon is then cut into standard glass sheets, mostly measuring 6×3.21 metres, with the longitudinal edges removed. At the end of the line, the float glass sheets are positioned vertically on the backs by means of suction lifts.
CD Automation solutions for float glass production
This brief overview of the float glass production process makes clear the strategic importance of precise power and temperature control throughout the various processing stages. For this reason, equipping float glass production lines with top-quality SCR power controllers is one of the priority missions of CD Automation’s technical department, which has been studying processes for over thirty years in order to define the most suitable complete hardware and software system for each requirement.
And the company, based in Legnano, specialises in this application. It provides complete cabinet and thyristor unit solutions and first-class service throughout the life of the float glass production plant.
Typically, the system can have 30 to 35 zones, with a power range from 100 to 150 kW. For each zone it is possible to have a dedicated terminal to be positioned in front of the panel, connected directly to the unit. In the event of an emergency, the unit can be operated by setting the power in manual mode, after knowing the password or enabling it from the control room.
CD Automation’s static units make it possible to regulate the power of applications involved in the production of float glass that make use of heating elements and in particular: silicon carbide elements in the Tin Bath stage and normal three-phase connection resistors in the annealing and controlled cooling stage.
Monitoring of silicon carbide elements
Silicon carbide is a semiconductor material that has a much higher resistivity than metallic materials. The resistance value of these elements at room temperature is quite high, and drops with increasing temperature to a minimum value of approximately 600-900°C. Constant power regulation with a V to VxI transfer is therefore necessary. On three-phase loads with Silicon Carbide, the use of a VxI feedback is suggested to achieve constant power regulation. This is necessary to compensate for changes in the resistance value with temperature and age of the elements. The value at the end of their use is in fact four times the initial value.
REVO C power controllers for more efficient float glass production
The REVO C Series is the ideal solution for regulating power in this application involving silicon carbide elements. This high-performance unit, developed by CD Automation, is adaptable to all SCR applications, including inductive loads, high-temperature oven resistors, infrared or UV ovens. The REVO C SCR is equipped with an advanced microprocessor that makes it universal and fully configurable via software.
REVO C is the most complete unit thanks to:
- three-phase synchronisation, phase rotation diagnostics, phase angle firing, current limit, measurement accuracy;
- integrated Profinet communication and reduced integration time with TIA PORTAL libraries. IP Ethernet communication with libraries for reduced integration time;
- SCCR approval 100kA – 600V (Short Circuit Current Rating), according to UL508.
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Resistance control with the CD Automation Revo S SCR unit
Considering instead the annealing and controlled cooling of float glass, the REVO S Family up to 700A is used for applications with low-variation resistors. Where the REVO S Family consists of high-performance, reliable, flexible and compact SSRs with the following properties:
- nominal voltage 480V-600V-690V;
- SSR or analogue input already configured;
- Burst Firing (Fast Zero Crossing);
- Heater Break alarm to diagnose partial or total load failure or thyristor short circuit;
- internal fuses that reduce wiring time and cabinet size.
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Above 700A, however, it is necessary to resort to the REVO C Series, which reaches 2100A. The latter is also appreciated for the availability of serial communication and voltage control, which prevents mains voltage fluctuations. For energy savings, the REVO PC Series should be used instead.
In applications involving transformers with loads placed on the secondary, the main requirement is to prevent current peaks occurring when switching on the primary from damaging fuses or thyristors. The REVO C series, with current limit option, has been designed to provide different solutions for driving single-phase transformers. REVO C-3PH, on the other hand, has been designed to drive three-phase transformers.
You want to understand in more detail how REVO C and REVO S can make your float glass production lines more efficient and competitive? Contact us and book a free consultation session with one of our experts.
