Pneumatic Conveying Pneumatic Conveying at 1.5 Bar Maximal Pressure to a Height of 65 m

Editor: Dr. Jörg Kempf

The new Delta Hybrid series of rotary lobe compressors from Aerzener Maschinenfabrik is a world first in creating a symbiosis between rotary lobe blowers (for a lower pressure range) and screw compressors (for a higher pressure range), for the first time ever permitting maximum pressure levels of up to 1.5 bar in pneumatic conveyance. In field trials at a milling operation it was rated “very good”.

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The rotary lobe compressors were successfully tested in a large-scale field trial conducted in a wide variety of industries including milling operations. (Picture: Aerzener Maschinenfabrik/Fotolia; [M]-Albrecht)
The rotary lobe compressors were successfully tested in a large-scale field trial conducted in a wide variety of industries including milling operations. (Picture: Aerzener Maschinenfabrik/Fotolia; [M]-Albrecht)

The pneumatic transport of powders is an indispensable component in proven material flow systems in many branches of industry. Until now, the required feed air with pressures of up to 1 bar was generated in many cases by means of conventional, oil-free compressing rotary lobe blowers. At higher pressures of up to 1.5 bar, oil-free compressing screw compressors have to be used. However, these units are configured in a single-stage design for considerably higher pressures of 2 or 3.5 bar while incurring disproportionately high investment costs for a pressure range of “only” 1.5 bar. The Delta Hybrid series of rotary lobe compressors from Aerzener Maschinenfabrik resolves this problem — as the Saale mill in Alsleben in the German state of Saxony-Anhalt found out.

At first, the compressed air required for the production process for the internal pneumatic transport of the various milled products (e.g. small grain cereals, cracked wheat, flour, animal feed pellets) was produced using approx. 20 non-soundproofed, air-cooled Aerzener rotary lobe blowers, which achieve maximum pressures of 0.4 bar and in some cases 0.95 bar, depending on the requirement. Most of the compressors installed at the company were consolidated at a central station with a number also installed at decentralized points, and some of them are now approx. 20 years old.

Conveying 20 Meters Higher

The various milled products are fed into pneumatically charged pipeline systems by means of scales and bucket wheel feeders. At first, the downstream pneumatic conveyor operated absolutely perfectly in all three milling plants at the Saale mill with a delivery pressure of 0.90 to 0.95 bar — as long as the products had to be conveyed pneumatically into a silo with a height differential of only 45 meters. Transport problems only arose when, from 2007 onwards, the products from mill 1, after an initially horizontal conveyor section of approx. 50 m, had to be pneumatically transported to a new flour silo at the considerable height of 65 meters — 20 meters higher than the previous feed height of “only” 45 meters. The old feed height had presented no problems to the Aerzener GM35S blower used in this conveyor line because it was configured with sufficient scope to accommodate this feed height. With the new feed height of 65 meters, however, the quantities of flour that needed to be conveyed at a rate of approx. 25 tons per hour had to be transported more than 40 percent higher. The existing GM35S blower, with its existing pressure differential of 0.95 bar, could no longer cope with this increase.

Pneumatic Blower Passes the Test

“Despite that, we tried charging the 65-meter feed height using the existing Aerzener blower by simply increasing the maximum pressure from 0.95 bar upward and thus deliberately overloading the blower. It could only be a matter of time before problems developed. And, sure enough, the expected malfunctions and faults on the blower soon occurred. However, these problems arose solely as a result of our actions. That made it vital for us to look for a solution to this problem so that we could convey the flour into the new, now 65 m high silo unit without any problems,” Holger Mattner points out.

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