Glass melting tank

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The glass melting tank ( tank ) is required for producing the glass melt in the production of glass .

The glass raw materials are fed to the glass melting tank in batches or continuously. The components ( the mixture ) are melted to a liquid glass melt. In addition to the basic components, the batch also contains cullet made of recycled glass to save energy (each 10% cullet approx. 2% energy saving). Depending on the requirements of the desired glass color, the proportion of broken glass can be up to approx. 85% - 90% (green glass). When changing the color of the glass (re- coloring ), the entire process in large glass melting tanks often takes several days. For economic operation, the glass melting tanks with so-called bulk glass (hollow glass, flat glass) are operated around the clock all year round. From one to max. Apart from two smaller planned interim repairs in which the furnace is taken out of service, a so-called furnace journey ( campaign ) up to the general repair (rebuild) can take up to 16 years and more (depending on the product group).

Day tubs

This historical design of glass melting tanks produces batchwise ( discontinuous ); this melts glasses that are only required in small quantities. The melting area of ​​day tubs is a maximum of 10 m 2 , the melting capacity between 0.4 and 0.8 t / m 2 of melting area.
The harbor furnace is one of these. The furnace consists of a fireproof brick basin 40 to 60 cm deep ( lower furnace ), which is covered with a vault with a 70 to 80 cm diameter ( upper furnace ).

Today, day tubs can still be found in some tumblers and craft workshops as well as in some special glass tubs in which small amounts of high quality glass are melted, e.g. B. with some optical glasses . These day tubs are not necessarily switched off at the end of the day, since the refractory material typically cannot withstand large temperature changes and this leads to increased corrosion (consumption) of the same. The temperature of the day tub is then reduced significantly overnight. It can, however, be put out of operation for a longer period, similar to the continuous glass melting tank. To do this, however, defined cooling / heating regulations (procedure over two to several days) that are precisely adapted to the refractory material must be observed. Smaller ovens ( studio ovens ) in craft workshops are excluded . There the refractory lining is carried out accordingly.

Continuously operated glass melting tanks

Continuously operated tanks consist of two sub-areas, the melting tank and the working tank. These are separated by a passage or a constriction (float glass). The batch is melted and refined in the melting tank . The melt then passes through the passage into the working tub and from there into the feeder ( forehearth ). The glass is removed there. The processing machines below are charged with glass droplets (hollow glass, sheet glass) or a float bath (flat glass without structure: e.g. window glass, car glass) or a roller (flat glass with structure) supplied at outlets.

Melting tanks are made of refractory materials and consist of the groups of alumina (Al 2 O 3 ), silica (SiO 2 ), magnesia (MgO), zirconia (ZrO 2 ) and combinations thereof for the production of other refractory ceramic materials. Furnaces (melting tanks with regenerative chambers) in the glass industry can contain 2000 t (hollow glass) up to 9000 t (flat glass) of refractory material.

Natural gas, heavy and light oil and electricity, which are often combined with one another, are used as energy sources. There are also glass melting tanks operated with oxygen. There, the atmospheric combustion air is replaced by pure oxygen. Because of the lower volume to be heated during the combustion or the lower mass, energy savings and, in the best case, savings in operating costs are achieved. However, the oxygen-operated glass melting tanks are usually not profitable because of the high costs associated with generating oxygen (including the price of electricity) in the production of so-called bulk glass such as hollow glass and flat glass.

In order to save energy in the glass melting process, the combustion air is generally preheated in a regenerative or recuperator system in addition to the highest possible proportion of waste glass.

In the most frequently used regenerative system, the exhaust gases are in a chamber through a latticework, created with fireproof rectangular stones or spec. Shaped stones, headed. This so-called grid is heated in the process. After this warm-up period (storage of the thermal energy of the exhaust gas), the direction of the exhaust gas flow is reversed and fresh or cold air required for combustion flows through, instead of the now heated latticework of the chamber. The combustion air is preheated to approx. 1200 ° C - 1300 ° C. This results in considerable energy savings in the melting process. After combustion, the exhaust gases enter the grating of another chamber and heat the grating, which has now cooled down, again. The process then repeats itself by reversing the direction of the air and exhaust gas flow periodically at intervals of 20 to 30 minutes. The chambers are operated alternately, discontinuously.

Depending on the intended use, there are different designs of glass melting tanks.

The capacity can be from approx. One ton to over 2000 tons and the daily throughput can be from a few kilograms to over 1000 tons. The temperature in the tub is approx. 1500 ° C, the exact temperature depends crucially on the melting temperature of the respective glass.

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