What is a Positive Displacement Blower?
A positive displacement blower is a machine responsible for moving either gas or air for various applications using rotary lobes or reciprocating pistons. These are critical components in pneumatic conveying systems for efficient performance, and in this blog post, were going to cover everything you need to know about them:
- Overview of Pneumatic Conveying Systems
- How Does a Positive Displacement Blower Work?
- Types of Positive Displacement Blowers
- What is a Regenerative Blower?
- Positive Displacement Compressors
- Vacuum Pumps
Overview of Pneumatic Conveying Systems
Pneumatic Conveying is the transport of bulk solids in a pipe by a positive or negative air stream.
Pneumatic Conveying Requires:
- Pressure differential to cause air movement.
- Air mass at velocity to move material mass at a velocity that overcomes frictional forces.
- Kinetic energy transfers from gas to solid.
How Does a Positive Displacement Blower work?
The air mover is the heart of any pneumatic conveying system, and the specification involves the volumetric flow rate of free air and the pressure it must be delivered.
The choice of air mover is critical as there is a variety of air moving equipment, though all air movers are not suitable for pneumatic conveying.
These include fans (centrifugal Fans or Axial Fans) that produce high volumetric flow at relatively low pressure and positive displacement compressors that can deliver air volumes at high pressures.
The main feature of Positive Displacement Blowers (PD Blowers) is that these can be deployed for both positive pressure and negative pressure (vacuum) pneumatic conveying applications.
For a pneumatic conveying system, a minimum air conveying velocity must be maintained (typically 3500 FPM). If this drops below, the pipeline may block.
A positive displacement blower is a constant flow air mover that will always maintain constant air velocity in the pipe against various discharge pressures, which helps prevent blockages.
Blocker and Wallace demonstrate how the air flows through the system in the diagram below.
Types of Positive Displacement Blowers
Roots-Type (Rotary) Blowers
In 1854, Roots invented the original rotary displacement blower.
These generate constant volumetric flow (by displacement of rotating 2 or 3 lobes) at discharge pressure. Twin rotors are mounted on a parallel shaft within the casing and rotate in the opposite direction.
Watch the rotary blower in action:
As a result, the air is drawn in the space between the rotors & casing and, upon reaching the outer port, gets compressed. See the diagrams below to see how exactly the air moves into the blower and is transferred around the cylinder before being pushed out of the blower.
Twisted Blowers
In some cases, three lobes and twisted rotors are also used to reduce pulsation level and noise. The maximum compression ratio is typically 2:1. Due to the compression, the discharge air gets heated, and at 15 PSIG, it reaches above 350 deg. F.
This is the reason in pressure conveying systems, PD blower discharge air needs to be cooled down in an air cooler to avoid damage to the heat-sensitive products.
As seen in the diagram above, cool air enters the blower, is transferred around the cylinder (where it warms significantly) before being cooled and pushed out through the discharge port.
The PD blower is now widely used in pneumatic conveying applications where the discharge pressure does not exceed 15 PSIG. For Vacuum application, the discharge pressure is generally much lower (approximately 15” HG mercury).
One reason for this is the high temperatures in vacuum applications will need cooling of the seals for above 15” HG (where vacuum pumps are used – see blog end).
Blower Protection Filters
Constant speed operating characteristics provide a basis on which heavy-duty conveying systems can be reliably designed. A pressure surge in the conveying system will result in a minimal reduction in the airflow, which can be within the design margin.
A specific feature of a positive displacement blower is that a very adequate clearance is maintained between the moving parts.
Hence care should be taken to avoid any fine dust entering the PD blower by installing secondary dust filters (Blower Protection Filters) at the blower inlet to prevent damage to the moving parts.
What is a Regenerative Blower? (or Side Channel Blowers)
Regenerative Blowers are also used in some pneumatic conveying applications as these are better than fans. These provide variable volumetric airflow but at higher pressures compared to centrifugal fans, but lower than PD blowers.
The main issue is that these operate at extreme design conditions and expect more wear and tear compared to PD blowers.
The impeller blades passing the inlet port draw air into the blower.
The impeller blades then, by centrifugal action, accelerate the air outward and forward.
Here the “regenerative” principle takes effect as the air is turned back by the annular-shaped housing.
Each “regeneration” imparts more pressure to the air. The pressures or vacuums are generated by the one or two spinnings, non-contacting, oil-free impellers.
Positive Displacement Compressors
In some instances where high discharge pressures are required (>15 PSIG), positive displacement compressors are used. These operate on the same principle as PD blowers but operate in multistage either rotary or screw compressors.
Vacuum Pumps
In the pneumatic conveying industry when positive displacement blowers are used in creating a vacuum, these are sometimes referred to as vacuum pumps (or vacuum blowers).
But the maximum vacuum developed is still 15” HG.
But special design vacuum pumps are also used in vacuum conveying application to develop high vacuum (22” to 28” HG) where the rotating parts/seals are cooled by air (in Dry lobe pumps_ or oil (in oil-cooled pumps) or water (in liquid ring pumps). Learn more about how vacuum pumps are used.
Next Steps
There are many elements that make up positive displacement blowers. They are a critical element to pneumatic conveying systems and ensuring the process flows as smoothly as possible.
If you’re wanting to learn more about positive displacement blowers and looking for a reliable product, view our positive displacement blowers or contact us today to start building your pneumatic conveying system. Here at Pneu-Con, our blowers are low-maintenance and highly dependable.
