Reliable Pneumatic Conveying

Atlas Food Processing Systems has over 60 years of experience in the bulk handling industry and has supplied over 1,000 pneumatic conveying systems to bakery and food processing companies.
Pneumatic cylinder conveying of bulk solids is an established and accepted method of both transferring and accurately feeding materials into downstream processes such as mixers and blenders, and for packaging and distribution systems.
It must be said, that the most important consideration in providing a reliable conveying system, must be given to the specific characteristics of the conveyed materials, such as its bulk density, particle size, moisture content, abrasiveness, fragility and sensitivity to temperature.
Consideration of the materials specific characteristics allows the selection for the most appropriate method for conveying the material. The reliability of the conveying system is also dependant on the design and installation of the conveying system.
Once the specific characteristics of the material to be conveyed are identified and the appropriate conveying method selected. Consideration of the batch sizes, batches per hour and the number of delivery points allows calculation of the conveying rate, which will satisfy the production requirements.
Having established the conveying rate, the next step would be to select the ideal conveying pipe diameter and feeder, which will guarantee throughput tonnage, and ensure reduced power consumption.
When establishing a pipe route, it is good practice to consider using horizontal and vertical pipe runs. This will ensure a consistent and stable flow of material during conveying.
It is best to avoid inclined pipe runs. Inclined pipe runs affect the efficiency of the conveying system because the material will tend to drop out of the air flow and descend back down the pipe. At best the material will be re-conveyed back up the pipe and at worst could develop pneumatic valve into line blockages at the bottom of the inclined pipe section.
Always use straight conveying pipe lengths between bends to allow the conveyed material to accelerate to its desired velocity after each bend, and thereby reduce the overall pressure drop in the system.
Some times overlooked is pressure drop in the supply line from the blower, particularly important if the blower is located some distance away from the pick-up point, and also the increase in temperature due to the compression of the conveying air.
The use of long radius bends will reduce wear and pressure loss within a system although tee bends can be sometimes used on fragile or very abrasive products.
In a conveying line of constant diameter the conveying pressure drop is directly proportional to line length. As the pressure drops the air volume increases, which in turn increases in air velocity. High material velocities will lead to increased wear on bends and material degradation.
The simplest way to avoid the excessive pressure drops and increased conveying velocities is to ensure that conveying lines are installed taking the shortest route possible to the discharge point and using as least bends as possible.
Apart from improved reliability, the other benefit of using shorter pipe runs is reduce the burden on the conveying blower, which means cost savings on power consumption.
Alternatively, if it is not possible to have short runs to compensate for the increase in air volume and relative air velocity a technique called “line stepping” can be used to increase the pipe diameter at selected points along the pipe run.
Line stepping is an effective technique to reduce conveying line pressure and ensure constant conveying velocity in the pipe line. Promoting a more efficient and stable conveying system, reducing wear on bends and product degradation

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Never step a conveying pipe before a bend and it is always good practice to step pipes six metres after a bend.
Rotary valve feeders can also have an affect on the reliability of the conveying system. The rotary valve’s pockets are full until the material is discharged into the conveying line. The empty pockets are then pressurised by the conveying air. If not vented the pressurised air in the empty pockets will pass into the silo and the material above the rotary valve leading to bridging.
One of the simplest ways to avoid this problem is to use a rotary valve that incorporates an outlet for venting the empty pockets. The air can then be vented to a air/dust separator and any carry over can be automatically re-introduced into the conveying system.