Lobe Blowers – Working Principle

Twin Lobe Blowers, also commonly known as Positive Displacement Blowers (or PD Blowers) are available for flow rates ranging from 14-cfm to 5,900-cfm in single stage and up to any capacity in parallel configurations, for working pressures up to 14.2-psi.

As previously mentioned, TWIN Lobe Rotary Blowers belong to the category of Positive Displacement Blowers. They consist of a pair of involute profiled (shape of 8) lobes/rotors rotating inside an oval shaped casing, closed at ends by side plates. One lobe is the driving lobe, which is driven by the external power while the driven lobe is driven by a pair of equal ratio gears. Both the lobes thus, rotate at same speed but in opposite direction.

Operation of Positive Displacement Blowers

As the rotors rotate, air is drawn into the inlet side of the cylinder and forced out from the outlet side against the system pressure. With each revolution, four such volumes are displaced.

The air, which is forced out, is not allowed to come back due to the small internal clearance within the internals of the machine except a very small amount called ‘SLIP’. There is no change in the volume of the air within the machine but it merely displaces the air from the suction end to the discharge end, against the discharge system resistance i.e. no compression takes place in the machine.

Since the lobes run within the casing with finite clearances, no internal lubrication is required. The air, thus, delivered is 100% Oil Free. These blowers deliver, practically, a constant flow rate independent of the discharge pressure conditions. The flow rate is dependent, largely on the operating speed.

Due to these constructional features it has the following distinct characteristics.

  • The flow is largely dependent on the operating speed
  • The input Power is largely dependent on the total pressure across the machine
  • The Suction and Discharge pressures are determined by the system conditions
  • The temperature rise of the discharge air and machine is largely dependent on the differential pressures across it.

System Pressure / Back Pressure on the Blower

There is no compression or change in volume within the machine but the Blower works under system back pressure conditions. To illustrate further, let us consider a case when the discharge of a Blower is connected to the bottom of a tank, having water to a depth of ‘H’ mm. The air-discharged accumulates in the discharge line until sufficient pressure is built (slightly over ‘H’ mm of WG), when it starts to escape out. The system resistance or the static load on the Blower is thus ‘H’ mm WG. The power consumed by the blower depends upon the flow rate and the total pressure head on the Blower.

The total pressure across the Blower is taken as the pressure across the inlet and the discharge port of the Blower. The pressure drop through inlet accessories and discharge accessories are a part of the system drop. The figure above indicates:

  • Pa as the ambient pressure
  • Ps is the pressure at the suction port which is slightly below the ambient due to suction filter and silencer drop.
  • Pressure Pd is the pressure at the discharge port of the Blower, and
  • Ps’ is the actual system back pressure. As seen from the curve the total work done by the Blower is to raise the pressure of inlet volume from Ps to Pd.

Ideally, the blower is capable of resisting high pressures but the mechanical limitations, increased power intake, temperature rise, and increase in ‘SLIP’ restrict the working pressure head to about 9.6-psi for Air-Cooled Blowers and 14.2-psi for Water Cooled Blowers in single stage operation.

It is therefore, important to ensure that the drop between Pa and Ps (Inlet drop) and Pd and Ps (outlet drop) should be as low as possible. This can be achieved by using adequate size piping and large radius bends wherever possible.

The Blowers are generally selected for the maximum system pressure, which they may encounter during operation and the prime mover is selected accordingly. When in operation, the Blower offers a considerable power saving since the power consumed by it depends upon the actual working pressure under which it operates and not the rated pressure.

Rotary Air Blowers are widely used in applications demanding medium pressures and relatively large flow rates.

  • Water Treatment Plants: For back-washing of filter beds.
  • Effluent Treatment Plants: For diffused aeration and agitation of effluent.
  • Cement Plants: For Blending, Aeration, Fluidisation, and Conveying.
  • Slurry Agitation: For maintaining the B.O.D. / C.O.D
  • Aquaculture: For Maintaining the dissolved oxygen level.
  • Biogas Boosting: Transferring of Biogas from gasholder to boiler.
  • Flocculation: To increase the removal of suspended solids in primary setting facility.
  • Chemical Plants: For supplying of process air.
  • Electroplating Plants: For Oil free air agitation of electrolyte to maintain uniform density.
  • Paper Plants: For Coating of paper/Knife edge
  • Yarn Drying: Vacuum/Pressure Drying of yarn.
  • Polyester Chip Conveying & Drying: For transfer of polyester Chips
  • Reverse Jet Filters: For reverse cleaning of Filter bags.
  • Pneumatic Conveying: Vacuum, Pressure and Combination Conveying of cereals, cement, husk, baggage, granules, powders and other similar material.

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