The CNUD flat / float glass lehr has a long history which is absolutely worth to be studied because it explains at the same time the current lay out of a lehr. In the past, it was general practice for flat (rolled) glass to work with a ceramic tunnel (and radiation cooling) down to 300°C. Below 300°C, forced convection (F-zone) was used to cool further the glass to a temperature, which can be handled by the cold end. This ceramic tunnel was rather expensive and did not allow a good temperature control.
1962: where it all began
CNUD was in 1962 the first to develop a steel lehr, insulated with mineral wool. Also the air cooled heat exchangers were constructed from steel. This principle was also applied for float glass in 1967. However, lehrs were in relation to the glass velocity rather long because a serious safety factor was included.
In 1974, CNUD started with programming a calculator for the lehr. FORTRAN was chosen as programming language and the forced convection part was first programmed. Later on, the more difficult radiation part was included together with a calculation of the residual stress (1979), where the thermal history was taken into account. This thermal history theory became published in 1974 and CNUD was the first to include this (Belgian) knowledge into its lehrs.
The principle of radiation cooling down to 300°C caused rather long lehrs and in 1976, the RET was put on the market. RET stands for “Recirculation on temperature”, which is one of the few French abbreviations in the glass industry. In this zone, forced hot air is used to cool the glass below 400°C, where the air is heated by the glass by using an insulated zone. From that moment, cooling rates of 65°C/min for 6mm glass could be used which induced a serious shortening of the lehr.
Later on, the moveable heater was installed to heat the edge of the ribbon. Today, architectural glass lehrs only use these moveable heaters without the heating drawers. For rolled glass lehrs with low load, CNUD started to combine the AB –zone to reduce the investment. This step could be taken thanks to the residual stress calculation. In 1990, CNUD has built the longest lehr for 930 TPD at that time.
A further shortening of the lehr became possible by using more efficient F zones. The typical slot nozzle cooling became replaced by the more efficient hole nozzle system, which was well known in the EFCO tempering furnaces. Also wider lehrs became used with 7 control zones.
Achieving a leading position through innovation
In the cellular glass world, mold foaming and forced convection annealing on a belt was the standard. However, also in this world, theoretical calculation has removed the old dogma that the annealing time of cellular glass should need extreme long lehrs. As a consequence, the more efficient continuous foaming was developed followed by radiation cooling. CNUD installed the first continuous foamed glass lehr for a pilot line in 1999 and the first production cellular glass lehr in 2007. Today, after 70 years mold foaming, the CNUD EFCO radiation lehr became the standard in the cellular glass world.
The introduction of EMISSHIELD® on the market is a major opportunity for CNUD EFCO. Indeed, the stainless steel heat exchangers, selected for temperature stability and corrosion resistance are in fact poor heat absorbers. The EMISSHIELD® coating improves the heat exchangers and allows an improved glass surface and an improved cooling of on-line low emissivity glass. The same coating lengthens also the life time of the heating elements in an SO 2 – rich atmosphere, removing the need for shielded elements.
To improve the energy consumption of a lehr, CNUD EFCO has redesigned the F (and RET) zone in order to reduce the pressure drop. An energy consumption reduction up to 50% became possible. The noise production of the F zone was also studied and by working with an in situ noise absorber, it is possible to work below 80 dB at 1m distance.
Internet and the WWW with the PHP-technology allowed CNUD EFCO to allow their customers to simulate their lehrs on the CNUD EFCO servers.
Read more about our float glass lehrs.