“Pneumatic conveying involves the moving of powdered, granulated or pelleted goods through a closed piping system. It is commonly applied in cement plants, flour mills and the food processing industry to move material from one side of the plant to another, to mix products or to fill into cyclone feeders, silo or bins,” says Aerzen Airgas marketing co-ordinator, Andreas Stubel.
Air is used to lift a product, provided that it moves faster than the terminal velocity of the material. The process air can be supplied by fans, roots blowers or screw compressors. The type of machine used depends on the density of the product, the length of the piping system and the pressure required to convey the material. A product’s resistance to air also determines the speed at which air needs to move to overcome the resistance, therefore air volume flow is of utmost importance.
Resistance within pneumatic conveying pipelines is made up of two factors, air flow and material flow. Air flow resistance is comprised of the force needed to accelerate the air, friction between the air and the pipe wall, and the dynamic losses due to changes in direction of the piping system.
Material flow resistance includes the force needed to accelerate the material, the energy needed to lift the material, and the loss of energy caused by particles impacting each other and the pipeline wall. The total resistance calculated in a pneumatic conveying system also includes the resistance from other machinery attached to the system, such as receivers, filters, rotary feeders and cyclones.
Pneumatic conveying systems come in two forms, positive pressure and negative pressure. The major difference between the two is the static pressure available to the systems. As a consequence of the greater static pressure available to overcome resistance, a positive pressure system is used where long horizontal pipeline runs with many bends are present.
Air is blown in from the feed end and pushes the product forward. In a negative pressure system, air is sucked from the receiving end, effectively pulling the product along. Negative pressure conveying is limited to systems where the conveying is vertical with few bends and short horizontal runs.
In the majority of positive pressure systems, the pressure differences required is below 1 bar. In this case, an Aerzen Delta Blower Generation 5 unit is commonly used to deliver air to the blowline. In very long systems, where the pressure differences are larger than 1 bar, an oil-free Aerzen Delta Screw Generation 5 compressor is ideally-suited.
For the energy-conscious market, the highly-efficient Aerzen Delta Hybrid rotary lobe compressor can be applied, with energy savings of up to 14 percent when compared to standard equipment. The Delta Hybrid bridges the gap between a roots blower and screw compressor, providing positive pressures up to 1.5 bar. All three machine types are also available as vacuum units. All Aerzen machines come standard with non-return valves on the discharge side. If a motor fails, the non-return valve shuts and prevents any product from entering the machine due to the back-pressure from the system. In the case of a blockage in the piping system, a safety relief valve on the discharge silencer relieves excess pressure build-up and protects the unit from over-pressurising.