Positive displacement blowers (PDB) are devices responsible for moving air or gas using reciprocating pistons or rotary lobes for different applications. These are essential parts of PCS for efficient performance. This article will take a closer look at things people need to know about pneumatic conveying systems.
Pneumatic conveying systems
These things are the transport of bulk solids in pipes by using negative or positive streams. It requires:
Transferring of kinetic energy, from gas to solid
Pressure differential that causes movement
Air mass at a certain velocity to move mass at speeds that overcome frictional forces
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How do positive displacement blowers work?
Movers are the heart of pneumatic conveying systems, and their specifications involve volumetric flow rates, as well as the pressure it needs to deliver. Choosing the mover is very important as there’s a variety of moving equipment, although all movers are not fitted for pneumatic conveying.
These include fans like Axial and Centrifugal, which produce high volumetric flows at low pressure, as well as positive displacement compressors that can deliver volumes of wind at high pressures. The main characteristic of the PDB is that these things can be deployed on both negative pressure or vacuum or positive pressure pneumatic conveying apps.
For pneumatic conveying systems, minimum conveying velocities need to be maintained (usually 3,500 feet per minute). If it drops below the critical speed, pipelines may block. A PDB is a constant flow mover that will maintain a constant velocity in pipes against different discharge pressures, which helps prevent various blockages.
Kinds of positive displacement blowers
Rotary or roots-type
In the 1800s, Roots invented original rotary displacement blowers. These things generate a constant volumetric flow by displacement of rotating two or three lobes at discharge pressures. A twin-rotor is mounted on parallel shafts within casings and turns in different directions. Because of this, the pressure is drawn into the spaces between the container and rotors; upon reaching the outermost part of the port, it gets compressed.
Check out https://www.popularmechanics.com/home/how-to/a151/how-air-compressors-work to know more about how compressors work.
In some instances, twisted rotors and three lobes are also used to minimize noise and pulsation levels. The maximum compression ratio is usually two is to one. Because of the compression, the discharge air will get heated, and at fifteen pound-force per square inch, it reaches above 350 degrees Fahrenheit.
It is why in PCS, positive displacement blower discharge air should be cooled down in air coolers to help avoid damage to heat-sensitive products. Cool wind enters blowers transferred around cylinders (where it significantly warms) before being cooled, as well as pushed out through discharge ports. PDBs are now widely used in conveying apps where discharge pressures don’t exceed fifteen pound-force per square inch.
For negative applications, discharge pressures are usually much lower (more or less 15 inches of mercury). The reason for this phenomenon is that high temperatures in negative applications will need constant cooling of seals above fifteen inches of mercury (where vacuum or negative pumps are used)
Consistent speed operating features provide a basis on which durable and heavy-duty systems can be pretty soundly designed. Pressure surges in the system will result in minimal reductions inflows, which can be within design margins.
A particular characteristic of positive displacement blowers is that adequate clearance is maintained adequately between their moving parts. That is why care is needed to avoid fine dust entering the device by installing secondary dust filters or blower protection filters at the inlet to prevent unnecessary damage to moving parts inside.
What are regenerative or side channel blowers?
These things are also used in a couple of PC apps as these are better compared to conventional fans. These devices provide variable volumetric flows, but with higher pressures than centrifugal fans but lower compared to PDBs. The primary issue is that these devices operate in extreme design conditions, as well as expecting more wear and tear than PDBs. Impeller blades passing inlet ports draw air into its blowers.
The blades then accelerate the air forward and outward using centrifugal actions. The regenerative principles take their effect as annular-shape housings turn back the air. Every regeneration provides more pressure to the atmosphere. The vacuums and pressures are generated by one or two spinning, oil-free and non-contacting impellers.
In some cases where high discharge pressures are needed (less than fifteen pound-force per square inch), PDCs are used. These devices operate on the same concept as PDBs but operate in multistage, either screw or rotary compressors.
When PDBs are used in creating vacuums in this industry, these are sometimes referred to as vacuum blowers or pumps. The maximum vacuum developed is still fifteen inches of mercury. But unique design pumps are also used in negative …