Compressed air in the glass industry

In the production of glass, a powder-like mixture of hygroscopic substances passes through a melt in a furnace and is then shaped into a specific form by a wide variety of production processes. Just as there are different types of glass, their manufacturing processes also differ. Flat glass, which is produced by the float process, is given its smooth and reflective surface by applying the glass mass to a hot tin bath. Hollow glass is pressed and blown into shape and must be robust and tasteless, depending on further finishing and subsequent use.

Requirements for compressed air quality in glass manufacturing

Regardless of the glass type, finishing or subsequent application, compressed air is required at various stations in glass production for transport, shaping, cooling or control. Nothing must be allowed to clump, clog or contaminate in order to deliver the highest quality and ensure continuous long-term production. A production stop is a real problem for the furnaces with a heating phase lasting up to two weeks.

It is therefore particularly important that the compressed air used is oil-free, particle-free and dry in order to keep the production lines trouble-free. To ensure cost- and energy-efficient production, it is necessary to individually adapt the compressed air to the required requirements and ambient conditions.

With its standard sheets, the VDMA provides important information on compressed air quality for various industries. For the glass industry, it defines quality class 3 in terms of particle content, class 4 in terms of humidity (vaporous) at a temperature above 10°C, class 2-3 below 10°C, and class 2 in terms of total oil content. These classes apply to both control air (e.g. for pneumatic drives) and conveying air (e.g. for sorting cullet, batches or for cooling).

*Source VDMA standard sheet 15390-1

Applications in glass production

Glass sorting

A high proportion of the glass consists of waste glass, which is either purchased or fed into the melt from the company's own batches. Since cullet of different glass colors cannot be mixed, the cullet must be sorted by color. This is done using high-resolution camera systems, light and compressed air. Light shines through the cullet, a camera with sensor detects the color, and conveying air sorts out the cullet in predetermined directions.

Not only must the conveying air be oil-free and clean so that there are no deposits on the glass or conveyor belt - the cameras must also be kept clean by means of compressed air and protected from contamination so that they do not fail or there is a loss of quality in the sorting process and must not become dirty or smudged. This "rinsing air" must be free of particles, moisture and oil vapors to reliably protect the optical systems.

Transportation

The ingredients of the batch (sand, lime, dolomite, soda, sulfate), which is later melted in the kiln, consists of hygroscopic substances, which are delivered by raw material suppliers and pumped/blown into silos. This is done by dry compressed air, which must be free of foreign substances and oil, or oil vapor, so that the powders do not stick/clump together and as a consequence clog the pipes or form deposits in the silos.

The transport of the end products also presents special challenges. This often takes place through extensive power systems from the compressor stations to the receiving points in the production halls. Often also over long distances outdoors and in the factory halls through numerous temperature zones and routes that promote condensation.

Please also read our case study special glass

Pneumatic drive

Many machines in glass production are operated or controlled pneumatically. Processes interlock seamlessly and never stop. Glass production runs around the clock 365 days a year and an interruption or stop in production would be a major economic and energy problem. Therefore, it is important to keep all pneumatically controlled production steps active. Valves must remain free and there must be no leaks that affect performance. The decisive factor here is the supply of constantly and reliably dry compressed air. Excessive humidity would lead to emulsions in the valves of the machines within a short period of time - i.e., to slime and, as a consequence, to standstill, production interruptions

Read our application report crystal glass

Shaping

In hollow glass production, when the glass mass, which has a temperature of approx. 1200°C, is portioned and placed as individual drops in the corresponding mold, a press is first used to form the base before the next station uses compressed air at up to 4 bar to form the body and blows the glass into shape.

Constant pressure and clean compressed air are necessary here to ensure that the quality of the end product is not affected by impurities, deposits or inclusions.

Cooling & Tempering

On its way through production, the glass mass is cooled slowly and in a controlled manner - depending on the type of glass and the intended end product. This is usually done either by ambient air on the way to the next processing station, or in special "chambers" in which a certain temperature and pressure prevail due to supplied tempered air. Too rapid cooling would increase the stress of the glass to such an extent that it would become unstable and brittle.

In cold tempering, substances such as polymers or waxes are applied to the glass to make it scratch-resistant and shiny, and to ensure that it does not become dull even in the dishwasher. The application is carried out using compressed air and must be thin, even and, above all, free of foreign substances such as oil, moisture or particles that may be present in the ambient air drawn in.

Packaging

The finished jars are usually automatically placed in the appropriate cardboard boxes with vacuum grippers and then foiled. Compressed air is also frequently used here, e.g. to generate the vacuum. It must also be free of impurities, on the one hand to ensure trouble-free production, and on the other hand to prevent the finished products, which have been kept clean up to this point, from becoming contaminated or polluted in this final step.

Please also read our case study hollow glass

References