In the 1950s, British engineer Sir Alistair Pilkington came up with an idea that was to revolutionise the glass industry: allowing molten glass to flow onto liquid tin. To this day, the ‘floating’/tin bath forms the basis of industrial flat glass production and is one of the deciding factors for the subsequent quality of the glass.

A specialist in the manufacture of flat glass production plants, CNUD EFCO GFT tailors its tin baths to customers’ requirements. Together with its parent company, Grenzebach, which acquired CNUD EFCO GFT in late 2019, the Group is now a full-range supplier ‘from hot to cold’: from the tin bath to the annealing lehr to the cold end, and from the coating technology to the transport of the final product to the warehouse.

The tin bath – endless fascination

Molten glass is fed into the tin bath, which contains liquid tin in an inert gas atmosphere. On the molten tin, the glass mass flows and is formed into an endless ribbon. By matching the bottom of the glass to the completely flat surface of the tin and simultaneously heating from above, a plane-parallel, distortion-free glass ribbon of high optical quality is formed. The glass temperature drops in the tin bath from 1,050°C at the inlet to around 600°C at the outlet, and the glass leaves the tin bath shaped and pre-strengthened.

New generation tin bath thanks to process simulation

It’s good to know in advance what you’re going to get. Therefore the complete tin bath is simulated in advance, optimised and agreed with the customer for the best possible results. In this process, it is possible for the first time to simulate all four parameters – atmosphere, tin, glass and temperature – with their mutual dependencies.

In this way, valuable insights can be gained, which in turn are incorporated into the development and improvement of the technologies.

CNUD EFCO GFT’s tin bath has been redesigned in all areas based on process simulation: from the optimised bath bottom profile to the flow-optimised atmosphere distribution through the roof, all factors of the tin bath affecting efficiency have been further developed. The result is a revolutionary tin bath tailored to the customer’s needs with the highest yield and quality while saving on investment and operating costs.

De-drossing

The de-drossing pocket at the end of the tin bath is an important quality factor: it is placed on the side to remove the dross that accumulates in the outlet area. This prevents dross from sticking to the underside of the glass ribbon while it is being lifted from the tin bath toward the dross box.

Contaminated tin is passed through a refractory channel. The dross floating on the tin gets caught on a weir, is dammed up and can then be removed. The system is usually driven by a linear motor that generates electromagnetic fields and thus a flow in the direction of the de-drossing pocket. Alternatively, a paddle system can be used, which reduces the cooling effect – particularly important in the production of special glass where cooling inputs must be avoided. Thus, the de-drossing pocket contributes immensely to the quality of the glass; without it, defects in the final product are almost inevitable.

Insulated dross box

The dross box is another indispensable component in the glass production line: here, the pre-strengthened and shaped glass floating on the tin is lifted out by three motorised rollers and transferred to the annealing lehr. The dross box also has the important task of sealing the tin bath atmospherically. This is done by means of graphite strips pressing from below against the rollers conveying the glass. Above the glass, curtains are used to seal the transition between the tin bath and the annealing lehr.

CNUD EFCO GFT’s dross box features a particularly high degree of insulation. The result is less heat loss and better – more homogeneous – temperature distribution over the entire width of the glass. The temperature at the edge no longer deviates significantly from that in the centre of the glass ribbon. This ensures better glass quality and reduces the risk of transverse breakage.

The most important feature of the CNUD EFCO GFT dross box is that it is particularly tight, so it minimises oxygen ingress and thus prevents or reduces oxidation of the tin (slag, dross). This makes it possible to reduce the hydrogen content in certain areas, which in turn results in considerable savings in operating costs.

Compact dimensions

Coming up at 6 to 9 metres shorter than most, no space is wasted in Grenzebach and CNUD EFCO GFT’s tin bath design. The cooling area is optimised which means that, despite its compact dimensions, the bath produces the same yield. This space saving is particularly valuable for cold repairs in order to make optimum use of the available space.

The tailor-made design also contributes to efficiency. The desired tonnage, the range of thicknesses and the glass quality; everything is taken into account in the design. As a result, customers are able to maintain productivity at the same level – or even increase it – while using fewer resources.

Advantages at a glance

Lower CAPEX: CNUD EFCO GFT’s compact tin bath reduces investment costs across the board. A shorter building, less steel construction, less space and less tin (10–15%); all this adding up to considerable savings.

Lower OPEX: In addition to the initial investment, consumption of resources in operating the plant is also reduced, resulting in lower maintenance costs. Less heat loss, less inert gas required, less loss due to atmospheric leakage and fewer flaws in the glass, as well as a smaller free tin surface.

Higher production capacity: tonnage of the glass production line can be increased in the event of a cold repair; that is, with an existing glass plant and available space – which means more output can be produced with the same size of tin bath.

CNUD EFCO GFT will check and individually calculate exactly what and how much is feasible in each specific case.

United power to create new synergies

Grenzebach and CNUD EFCO GFT have been supplying equipment for the production and further processing of industrial float glass from the very beginning. Grenzebach has been a technology innovator since the 1990s. CNUD EFCO GFT was originally born of a merger between three pioneering companies: CNUD, EFCO and GFT.

• CNUD has supplied annealing lehrs since the early days of float glass production.

• EFCO has delivered groundbreaking work in tin bath development, with a special focus on the tin bath roof.

• GFT is the expert in tin bath technology, but also in the entire glass manufacturing process, including media supply and the waste heat recovery system.

Today, well over half of all glass panes installed are produced with technology from Grenzebach and/or CNUD EFCO GFT. Deep process knowledge and decades of experience can produce even better results: Grenzebach and CNUD EFCO GFT jointly create added value for the international glass industry. Bundled competence for flat glass production means innovative technologies, a competitive edge through digitalisation and energy-efficiency, creating the benefits of tomorrow