>> Degreasing, painting, heat treatment (baking) and rewinding: Those are the steps involved in the continuous coating process in which the cold-rolled thin sheet is “finished” from coils at Salzgitter Flachstahl AG. The 600 to 1,870 mm wide and 0.35 to 2.0 mm thick steel sheet is run through the system at a speed of up to 120 meters per minute.

At the beginning of the system, the sheet is fed through a reservoir, which retains “reserves” in huge loops to allow a new roll (coil) of steel sheet to be tack-welded on to existing sheets. The reservoir capacity is required to ensure continuous operation of the system and its volume depends on the processing speeds for the individual system groups. The quantity (in meters) of steel sheet available in the reservoir is indicated in the system. This value is then used to calculate how quickly a new coil must be prepared.

The sheet is then degreased and cleaned. Activation (chemical treatment) and application of the primer and top coats are carried out in so-called coaters. The sheet is run through an oven system after each coating is applied – for baking and for evaporating the paint solvents that are present. Depending on the order being processed, the non-painted side of the sheet can be sprayed with a special preserving oil for rust protection.

At the end of the system the sheet is run through a second reservoir up to the separating station. There, the steel sheet is cleanly cut off and removed on a continuous basis. In the meantime, the second reservoir takes up the painted sheet from the production line, with sample pieces being taken for quality control. At the end of the process the finished coils are packed, weighed and prepared for shipping.

Major purchasers of coated steel materials include the automotive industry, the construction industry (wall and ceiling profiles) and manufacturers of so-called “white goods” – mass-produced merchandise such as refrigerators, washing machines, lamps, etc. Manufacturers of entertainment electronics products also purchase these materials.

The advantages of sheet coating done at the steel manufacturer’s are obvious: Cost-intensive piece-by-piece painting is not required, quality and color are uniform, and the process can be configured to be energy saving, meaning it is environmentally friendly and economical at the same time. The main factor for this is a nearly closed oven concept: Ambient air is routed through the coating units, through a heat exchanger and later to the oven units. Each oven unit consists of several zones and is equipped with burners that heat the air even higher and with circulation fans. The common exhaust air is routed into the burner chamber of a thermal reactor, where the evaporated solvents are nearly totally burned off.

The winning concept: The more combustible solvents that are present in the air circulating in the oven units, the less natural gas that is required for fueling the thermal reactor, making the system more economical. The same applies for each single zone of the oven units with burners. The less fresh (and cold) air that is supplied, the greater the temperature of the circulating air, likewise resulting in less natural gas being required for the burners.

There is one dangerous catch however: If a critical concentration is reached, the so-called lower explosion limit (LEL), the open flames in the burner could cause an explosion, resulting in disastrous damage. This is why the critical zones in the oven units, and the common exhaust air that is routed to the thermal reactor, must be monitored by extremely rapid gas warning devices. Concentration meters are also used for regulating both the circulating air flow rates in the individual zones for the ovens and the exhaust air flow rates – resulting in enhanced enrichment and, hence, cost-effective operation.

Salzgitter Flachstahl AG uses one BERNATH ATOMIC EuroFID measuring instrument for this in the first three zones and in the common exhaust air path for the oven units. A leading manufacturer of instrumentation forsheet coating systems, BERNATH ATOMIC has been supplying concentration meters for this type of application since 1977. The operator control units for the up to eight (8) concentration meters that are used were all installed in one instrument control cabinet at the request of Salzgitter Flachstahl AG.

Mr. Volkmar Weiß, from Salzgitter Flachstahl AG had this to say: “The products from BERNATH ATOMIC were specifically recommended to us by our oven supplier, GATV from Leverkusen, Germany. The system was commissioned in 2000 and, up to now, I have yet to hear anything negative about it from the Service department that is responsible for servicing and maintaining of the system. This has given us a reassuring feeling about overall plant safety and dependability.”

Praise was also bestowed upon the after-sales service of BERNATH ATOMIC. “Their after-sales service was so good that we also retrofitted our older systems with their instruments – and this despite the fact that such a monitoring system is not at all legally required for an existing system such as the one we have, which is operated only to a maximum of up to 25% LEL.”