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© Achenbach Buschhütten GmbH & Co. KG, Kreuztal
Exhaust air purification in the industry
Projektinfo 05/2017

Injected smoke shows the barrier effect of the air blade.
© Achenbach Buschhütten GmbH & Co. KG, Kreuztal

Proportion of auxiliary units relative to the installed plant output
© Achenbach Buschhütten GmbH & Co. KG, Kreuztal

Construction of a cold roll stand for producing foils. The supply and exhaust air gaps for the demonstration system are indicated schematically.
© Achenbach Buschhütten GmbH & Co. KG, Kreuztal
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Extracting fumes in rolling mills

Aluminium foils and sheets pass through a cold roll stand during the last processing step of their production. Rolling oils are sprayed on to cool and lubricate the rolls. Some of these kerosene-like hydrocarbons evaporate and vaporise in the ambient air. Large extraction hoods above the roll stand collect the contaminated air and feed it to a recycling plant. Their fans are the largest individual loads besides the actual rolling operation. However, using newly developed extraction hoods plant engineers can almost halve the exhaust air volume and thus the required fan output.

Cast-poured, heavy-weight aluminium bars undergo a whole series of rolling processes before they are transformed into sheets or foils. The initial reshaping is carried out at a temperature of more than 500 °C on a hot roll stand. Two superimposed rotating rolls, whose spacing is successively reduced, draw the material through the nip. The resulting aluminium strip is wound into coils.
The coiled aluminium achieves the desired material thickness in a cold roll stand at room temperature. Depending on the system, millimetre-thick sheets or even aluminium foils just a few micrometres thick are produced for beverage packaging.

The low foil thickness and narrow tolerances in the production of foils create considerable expenditure in terms of the process engineering and energy used. Depending on the material characteristics, the plant operator selects the machine settings and uses rolling oil with specially adapted additives. Flat jet nozzles spray the liquid for cooling and lubrication onto the work rolls.
The rolling oil improves the surface quality of the material and prevents the thin foils from adhering to or tearing on the rolls. At the same time, the rolling oil flushes ultra-fine particles away from the roll and thereby prevents deposits ("roll coating"). Last but not least, the lubricant reduces the roll wear.

About one per cent of the volatile organic compounds (VOCs) escape into the ambient air through atomisation, vaporisation and evaporation. For some rolling mills, the exhaust air is polluted with up to one gram of VOCs per cubic metre. Large extraction hoods above the roll stand suck up the contaminated air and feed it to a recycling plant. However, until now extraction hoods installed in the input and outlet area of the roll stand have only achieved non-directional extraction. In order to capture as much of the oil vapours as possible, this concept requires very large volume flows. Another reason relates to safety issues: rolling oil is a combustible, hazardous substance. It is therefore essential to prevent a build-up of vapours above the explosion limit, such as in stationary air vortices. Depending on the plant, 60,000 to 120,000 m3/h of air are extracted from the stand area and passed into scrubbers that wash out the rolling oil from the air.


Large savings potential in the rolling oil circuit

Although plant construction companies cannot considerably influence the energy consumption of the actual rolling process, considerable savings potential is still provided by the rolling oil circuit. In large rolling mills, up to 100 kg of the volatile VOCs are filtered out each hour from the exhaust air and recycled as a valuable raw material. The rolling oil circuit consists of stocking, conveying, extraction, filtering and cleaning. Its share of the total energy consumption of a roll stand can amount to more than 20% in foil production. After the actual rolling, it is therefore the main energy load in the plant.

The largest individual loads in the circuit are the fans in the extraction hoods for the oil vapour. For large strip rolling plants, their drive power can amount to 200 kW.

Scientists from the Achenbach Buschhütten plant construction company have set themselves the goal of halving this energy consumption without sacrificing the quality of the exhaust air purification. According to the researchers' calculations, a typical rolling mill could save up to 330,000 kWh of electrical energy annually.

Push and pull for the oil vapour

In the research project, the engineers are investigating for the first time how oil vapour is generated in the stand area. In addition to laboratory tests on vapour formation and extensive simulation calculations, they carried out measurements on rolling mills with various construction and performance sizes. Based on this, the scientists developed a directional extraction system according to the push and pull principle. Here an exhaust air gap, which circulates in a U-shaped manner inside the hood, is supplemented by a corresponding supply air gap below the band run. This blows directly onto the exhaust air gap. This results in an air curtain similar to a shop entrance. It directs the vapours to the exhaust air gap and also prevents them spreading in the factory.


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