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Roots Blower
Brief description
A Roots blower (or rotary lobe blower) is a positive displacement rotary pump that moves fluids or gases using a pair of meshing lobed rotors. Operating without internal compression, it delivers a constant volume of air per revolution, making it an essential air booster for metallurgical furnaces, gasifiers, and forced ventilation systems.
Use / Function
- Blast Furnace & Forge Blower: Providing high-volume forced air combustion draft to Blast Furnace and Cupola Furnace smelting setups.
- Engine Supercharging: Supplying forced air charge to internal combustion engines to increase volumetric efficiency and power output.
- Pneumatic Conveying & Aeration: Pushing granular bulk materials through pipes and aerating wastewater treatment tanks or fermentation vessels.
- Gas Circulation: Transporting synthesis gas in Wood Gasifier systems and chemical processing lines.
Operating principle
- Dual Rotor Setup: Two figure-eight or figure-three shaped lobed rotors are mounted on parallel shafts inside an oval housing.
- Synchronized Rotation: External timing gears spin the rotors in opposite directions with precise, non-contact clearance between lobes and casing.
- Trapping Volume: As each lobe passes the inlet port, air is trapped in the pocket between the rotor lobe and outer housing wall.
- Positive Displacement: The rotating lobes carry the trapped air pocket around the perimeter to the outlet port, forcing it out into the discharge pipe under pressure.
How to create it
- Cast & Machine the Housing: Cast a heavy Cast Iron oval casing with flat end plates and precision-bored inlet and outlet ports.
- Fabricate the Lobed Rotors: Machine two identical two-lobe or three-lobe rotors from Cast Iron or Steel with accurate epicycloid or involute profiles.
- Mount Bearings & Shafts: Fit high-precision shafts into side end plates lined with Bronze sleeve bearings or roller bearings.
- Install Timing Gears: Mount a pair of identical helical spur gears made of Steel or Brass on the external rotor shaft ends to lock the rotors at a strict $90^\circ$ phase shift.
- Drive Connection: Connect a drive pulley or coupling to one shaft to power the blower via steam engine, motor, or hand crank.
Materials needed
- Housing & Rotors: Rigid Cast Iron or machined structural Steel.
- Timing Gears: Steel or Brass precision gears.
- Bearings & Seals: Bronze bushings, oil seals, and gasket sheets.
- Drive System: V-belt pulley or direct shaft coupling.
Variants and improvements
- Two-Lobe vs. Three-Lobe: Three-lobe rotors with twisted helical lobes reduce pressure pulsation, air turbulence, and mechanical noise compared to straight two-lobe designs.
- Liquid Ring Blower: Incorporates a liquid sealant film to achieve higher pressure differentials and handle wet gas mixtures.
- Multistage Blowers: Stacking two blowers in series allows higher pressure ratios for industrial vacuum systems or deep aeration tanks.
Limits and risks
- No Internal Compression: Because air is compressed only upon entering the discharge pipe against backpressure, thermal efficiency drops at high pressure ratios ($>1\text{ bar}$).
- Overheating Under Load: Continuous operation against high resistance generates significant heat, which can cause rotor expansion and binding.
- Precision Clearance Required: Excessive rotor clearance allows air backflow (slip), drastically lowering output efficiency; insufficient clearance causes catastrophic rotor contact.