Overspraying can reduce energy consumption by 30 per cent

Paul Boughton
Painting is one of the most energy intensive stages of the car manufacturing process. New technology has cut this by 30 per cent, along with demands for both process air and water. Sean Ottewell reports.

The newly-launched EcoDryScrubber system by Stuttgart, Germany-based Dürr works with recirculation of the booth process air and dry separation of the surplus paint particles during paint application. This leads to considerable improvements in energy consumption and emissions of both carbon dioxide and paint particles.

Ralf Dieter, chief executive officer, commented: "As a technology leader, Dürr addressed the issue of energy and carbon dioxide optimisation early on. We are therefore in a position today to offer our customers systems that allow significant reductions of energy and unit costs. Our dry separation system clearly underlines the results of our development efforts."

When applying wet paints, between 20 per cent (manual painting) and 85 per cent (automated exterior painting) of the material reaches the body, depending on the atomisers used. In the conventional process the rest of the paint, known as overspray, is carried into the lower section of the spray booth where it is mixed with water. The booth process air is drawn off as, through the contact with water, it absorbs too much moisture for it to be re-used, and it also contains paint particle residues. As a result, a fresh supply of air has to be treated and fed to the booth, which consumes a lot of energy.

By contrast, spray booths equipped with the EcoDryScrubber require far less fresh air. The reason: the process air does not pick up additional moisture and the paint particles are completely removed in the dry filtration system beneath the booth, so the process air can be re-used. Water demand and maintenance costs go down as well. Moreover, the cross section of the building is being reduced, as the booths are equipped with Dürr's new, space-saving EcoRP painting robots (Fig.1).

The air conditioning for the booths is the most energy-intensive part of the vehicle painting process. The optimisations achieved with the EcoDryScrubber therefore have a major impact on a paint system's total energy consumption. In a paint shop with an annual capacity of 158000 car bodies, use of the it lowers energy consumption by 16 million kWh/y. This is equivalent to a reduction of 5200tonnes in carbon dioxide emissions. Measured over a car model's total product life cycle of seven years Dürr's innovative solution can produce savings in the region of EUR8 million.

Key elements and processes

The DryScrubber system functions with an integrated air management system that only needs a fresh air re-supply of 5-20 per cent, giving significant energy savings. Process air containing the wet overspray particles is fed into the modular filter box and filter elements as a result of the constant air flow in the booth. The air is then filtered on the surface of the filter membrane and fed back into the supply as clean air.

At the bottom of the hopper the fresh dry pre-coat material is introduced and stored. Air nozzles blow up the dry material to join the air flow within the filter module, ensuring a complete layer of pre-coat material over the entire filter element surface and hopper walls. This layer prevents any direct contact of the wet overspray with the filter membrane or any other process equipment.

The process-optimised, automatic regenerated filters, which are covered by a membrane layer, provide particle filtration below 0.1mg/m3 (mass concentration of particles on the clean side of the filter). This filtration performance exceedsregulatory limits and allows the process air to be recirculated into the system without the need for any additional filtration. The filtration process itself takes place on the filter surface as neither pre-coat material nor paint particles can pass through the filter membrane or the filter base material.

The paint particles adhere to the filter surface and build up a filter cake. Once a certain drop of differential pressure is given, the automatic cleaning of the filter element is triggered. The filter element is back-flushed with compressed air for a quarter of a second and the paint particles are blown-off the filter surface. The average cleaning cycle of each filter element is about 25 minutes.

The contaminated pre-coat particles fall into the hopper and are re-used within the process until a certain saturation point is reached. At this point the used-up pre-coat material is removed and automatically replaced.

According to Dürr, this brings many cost per unit (CPU) savings when compared with the traditional wet separation process. EcoDryScrubber does not require process water, detackifier chemicals or the disposal of paint sludge. Combined with the significant savings achieved by the re-circulation of process air, the overall dry separation process provides annual energy savings outlined above.

The company also claims savings of up to 80 per cent on heating energy, 50 per cent on electricity and 80 per cent on water for air conditioning when compared with a standard wet separation process with heat recovery unit.

The new spray booth concept, with fully automated robots, can be built without floor grates and combines smaller booth dimensions with a reduced air volume flow. These elements alone result in EUR2 CPU savings. When combined with overall savings on energy, water, chemicals and heat recovery thanks to integrated recirculating systems, a saving of EUR7.20 is made per vehicle.

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