Stationary compressors deliver conveying air for bulk materials

On average, five large trucks deliver silica sand, cement and other raw materials to the Detmold plant of construction material manufacturer Schomburg every day. Each day, some 125 t are fed to the silos at the plant, in addition to sacks and big bags which are transferred to the smaller hall silos. The company specializes in system construction materials and processes aggregates, mainly producing dry mortars such as mineral tile adhesives.

To unload the silo trucks, the drivers feed compressed air to the tank chambers of their trucks and supply additional air to the conveyor line to make the product flow. For this purpose, the trucks are equipped with mobile compressor units. Until quite recently, these compressor units were also used to unload the trucks at the Schomburg plant. This is normal practice at plants where bulk materials are handled but the procedure is quite noisy. The noise emissions of the compressors, driven by the truck engines, at the two unloading stations, were a considerable nuisance for employees and neighbours. “Each unloading operation took about an hour and the compressor and the diesel engine of the truck were running all the time,” says Dietmar Bobe, Head of Plant Systems at the plant. This illustrates the duration of the noise involved – in principle, the noise continued all day.

One oil-free low-pressure compressor

per unloading station

Schomburg decided to solve the problem using stationary compressors. In 2017, the company bought two Atlas Copco oil-free screw compressors. Two type ZE 2 G275 5E compressors with external coolers and cyclone separators now generate dry, pure conveying air (according to ISO 8573-1, Class 0) for truck unloading. One compressor is installed for each unloading station, which means that two silo trucks can be unloaded at the same time. The external coolers cool the compressed air delivered by the compressors from about 160°C to about 90°C which means that the conveying air is dry, but not too hot.

The noise problem was solved immediately by the two compressors. The compact ZE 2 units operate very silently and are installed at a central compressed air station in one of the buildings together with three GA oil-injected screw compressors from Atlas Copco, which have also been newly purchased. The GA compressors supply compressed air for production. In addition to the employees at the plant, residents of the local housing estate can also breathe a sigh of relief.

Higher product quality thanks

to new unloading method

By purchasing the two ZE compressors, Schomburg not only solved the noise problem. The higher performance of these compressors means that the product is discharged to the silo with an absolutely even flow of air. A ZE 2 delivers 500 m³ of conveying air per hour at a gauge pressure of 2.5 bar. The ZE series includes compressors capable of generating gauge pressures of between 2 and 4 bar. These compressors are designed to avoid pressure drops in the air cycle where possible. In addition, the Atlas Copco compressors are genuine plug-and-play systems. Following delivery to the site, the units only need to be connected to the power and compressed air net.

Schomburg also benefits from the fact that the compressed air supplied by the ZEs is free from oil and condensate. In contrast to the mobile compressor units on the trucks, the ZE units are equipped with cyclone condensate separators, ensuring that the compressed air remains clean. According to Schomburg, the mobile truck compressors had sucked in droplets from the diesel fumes from the ambient air, contaminating the compressed air. Thanks to the simultaneous air injection and the dry air, the bulk goods also remain clean and are pumped in a more homogeneous way.

Piping maintenance only required

at intervals of three months

Another benefit of the new unloading method concerns the piping on the top of the outdoor silos. Thanks to the improvement of the conveyor process, the piping is not worn so fast and it has been possible to double the maintenance intervals. In logistics terms, Schomburg and its suppliers also benefit as unloading times have been reduced by between 10 and 15 minutes. The drivers only need about one hour to unload a truck, representing a time saving of about 20 %.

Schomburg opted for Atlas Copco as the sizing of the low-pressure compressors ensured the best match with the company’s requirements. The cost/benefit analysis and advice provided by Atlas Copco ahead of the purchasing decision also convinced the customer.

Variable speed drive screw compressors

to control air improvement efficiency

Together with the ZE compressors, Schomburg ordered three oil-lubricated GA screw compressors from Atlas Copco for its Detmold plant. As these units are also very compact and take up little space, it was possible to install them in the same compressor room as the ZE units. In production, Schomburg needs compressed air as control air for cylinders and pumps. The main units connected are four vacuum conveyors which suck materials through the piping and are operated when required. Large quantities of air are also required on a sporadic basis for major orders received from construction sites. The new Atlas Copco compressors meet all these requirements reliably.

To compensate for fluctuating consumption within the compressed air network, Schomburg opted for the new generation of variable speed drive GA screw compressors: two GA 37 VSD⁺ FF and one GA 30 VSD⁺ FF supply the compressed air required for production. “VSD” in the product name stands for “variable speed drive”. This means that the compressed air supply is adapted to actual demand. With VSD⁺ technology, the energy consumption can even be reduced by about 50 % compared with conventional compressors with full load/idle control and low capacity deployment.

“FF” (Full Feature) means that a latest-generation refrigerant dryer is also integrated in the compressor. For Schomburg, this was an additional argument in favour of the compact, silent screw compressors. Previously, the company had used compressors from another manufacturer with an external dryer. Frequent malfunctions had impaired the product quality. Plant technician Dietmar Bode mentions one example: powder adhesive had been filled into containers using moist compressed air. This had resulted in lumping and was of course undesirable as it meant that tilers could not work efficiently on site. With the Atlas Copco compressors equipped with integrated dryers, this problem no longer arises.

System pressure reduced by 0.5 bar

To compensate for fluctuations within the compressed air network, Schomburg purchased a buffer in the form of a 3000-liter compressed air vessel and a higher-level ES6 controller. This starts up the compressors when compressed air is required and switches between the compressors to ensure even loading and to extend their service lives. The two GA units with an installed power of 37 kW supply the base load for production on an alternating basis. According to Schomburg, the third GA unit, with 30 kW, is adequate to maintain the pressure required in the sprinkler system and for the high-bay shelving when the plant is not in operation. It is also started up together with the other two units when necessary to meet peak demand. This means that the company normally has one 37-kW compressor in reserve and can also meet high air demand at any time.

The network pressure also shows that the new system is considerably more efficient than the old system, which had reached a certain age. Previously, a system pressure of 7.5 bar was used at the plant; now, 7 bar is adequate. This represents a significant efficiency saving. A further improvement in terms of efficiency and sustainability is the heat recovery system installed on the compressors. This supports the heating system and also heats process water for bitumen processing.

To date, scarcely any maintenance has been needed on the brand-new station. However, Schomburg is well-prepared in this respect. Maintenance can be carried out during production because the compressors for the two compressed air networks are redundant.

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