What Is a Roots Blower? Structure, Working Principle and Key Specifications
A Roots blower consists of five major parts: the casing, side plates, impellers, oil tank and silencer.
Side plates: used mainly to connect the casing with the impellers, support impeller rotation and provide end-face sealing.
Impellers: the rotating part of the Roots blower. They come in two-lobe and three-lobe designs; because three-lobe impellers offer smaller discharge pulsation, lower noise and smoother operation, they have gradually replaced two-lobe designs.
Oil tank: stores the lubricating oil for the gears and bearings.
Silencer: reduces the noise generated by flow pulsation at the blower inlet and outlet.
Working Principle
The Roots blower is a type of positive displacement blower. Two three-lobe impellers rotate in opposite directions within a space sealed by the casing and side plates. Each impeller is profiled with an involute or epicycloid envelope curve; the three lobes of each impeller are identical, and the two impellers are identical as well, which greatly reduces machining difficulty. The impellers are machined on CNC equipment, ensuring that as the two impellers rotate at a fixed center distance, a constant minimal clearance is maintained at any position, keeping gas leakage within the allowed range.
The two impellers rotate toward each other. Because the clearances between impeller and impeller, impeller and casing, and impeller and side plate are extremely small, a vacuum is created at the inlet, and air enters the inlet chamber under atmospheric pressure. Then two lobes of each impeller, together with the side plate and casing, form a sealed chamber. As the impellers rotate, the air in the inlet chamber is continuously carried by the sealed chambers formed between lobes toward the discharge chamber; because the impellers mesh with each other at the discharge side, the air between the lobes is squeezed out. This continuous rotation delivers air steadily from the inlet to the outlet - the entire working process of a Roots blower.
Features
Thanks to the three-lobe rotor design and a rational inlet/outlet structure in the casing, the blower has low vibration and low noise.
The impeller and shaft are integral, and the impellers are wear-free, so blower performance remains unchanged over long-term continuous operation.
High volumetric utilization and volumetric efficiency, with a compact structure and flexible installation options.
A complete range of models meets the needs of different users and applications.
Operating Requirements
The inlet temperature of the conveyed medium should normally not exceed 40 degrees C.
Particulate content in the medium should not exceed 100 mg/m3, and the maximum particle size must not exceed half of the minimum working clearance.
In operation, bearing temperature must not exceed 95 degrees C and lubricating oil temperature must not exceed 65 degrees C.
Operating pressure must not exceed the pressure rise range specified on the nameplate.
The clearances between impeller and casing, impeller and side plate, and impeller and impeller are set at the factory; they must be preserved when reassembling.
During operation, the oil levels in the main and auxiliary oil tanks must stay between the two red lines of the oil level gauge.
Check that all inlet/outlet connections are tight and that pipe supports are complete. For blowers and vacuum pumps requiring cooling water, verify that the cooling water installation meets requirements.
Classification
By working arrangement, Roots blowers can be roughly divided into single-stage and two-stage types. A blower with only one compression stage is called a single-stage blower, while two single-stage blowers connected in series to compress the gas twice form a two-stage series blower.
By number of impeller lobes: two-lobe and three-lobe Roots blowers.
By application: shaft kiln blowers, gasification blowers, aeration blowers, etc.
By medium: air blowers, gas (coal gas) blowers, hydrogen blowers, sulfur dioxide blowers, etc.
By drive: direct-coupled and belt-coupled blowers.
By cooling method: air-cooled, water-cooled and counterflow-cooled blowers.
By structure: vertical, horizontal, vertical-shaft and compact grouped blowers.
By sealing type: labyrinth seal, piston-ring seal, packing seal and mechanical seal blowers.
Installation Precautions
Do not install the blower in areas where people frequently pass, to avoid injury and burns.
Do not install the blower where flammable, explosive or corrosive gases may be generated, to prevent fire and poisoning accidents.
When installing, check that the foundation is solid and level and whether it stands above ground level.
For outdoor installations, provide a rain shelter.
The blower can be used long term at ambient temperatures up to 40 degrees C; above 40 degrees C, install exhaust fans or other cooling measures to extend service life.
When conveying media such as air, biogas or natural gas, dust content should generally not exceed 100 mg/m3.
Operation and Maintenance
Check all fasteners and locating pins for looseness; there should be no oil leakage inside the blower body.
No scaling, rust or flaking should be present inside the blower body.
Watch lubrication and heat dissipation; check oil quality, listen regularly for abnormal noise, watch whether the unit is operating outside specified conditions, and grease bearings periodically.
Overload sometimes does not appear immediately, so watch inlet/discharge pressure, bearing temperature and motor current trends to judge whether the machine is running normally.
Before disassembly, measure and record all fitting dimensions and mark parts, so reassembly preserves the original fits.
For new or overhauled blowers, clean the oil tank and follow the startup procedure; it is recommended to replace all lubricating oil after 8 hours of operation.
Draw up a reasonable maintenance schedule based on actual service conditions, carry it out on time and keep records; one major overhaul per year, replacing bearings and wearing parts, is recommended.
Daily Checks
Check bolt and nut connections for tightness.
Check lubrication and keep the oil level at the center of the sight gauge.
Check belt tension and pulley alignment.
Check supply voltage and frequency.
Check that all instruments read normally; notify maintenance personnel promptly for replacement if abnormal.
Open the main valve on the pipeline and the outlet valve of the blower to be run; keep the outlet valves of idle blowers closed to avoid overload and machine damage.
Startup Modes
For line-frequency (50 Hz) operation, switch the corresponding blower breaker on the control cabinet to the line-frequency position and press Start. Keep the switches of other blowers and the inverter at Stop. Before pressing Start, always verify that the outlet valve of the blower to be run is open and that the selected switch matches the actual running blower.
For variable-frequency operation, switch the inverter to Start, then set the corresponding blower breaker to the variable-frequency position, keeping other blowers at Stop. Again verify the outlet valve is open and the switch matches the actual blower.
After the blower runs normally (usually about 1 minute), slowly close the discharge valve; closing it too quickly can cause a large inrush current and burn out the motor.
Operation Notes
Adjust the inlet valve of the contact oxidation tank for uniform aeration.
Early in operation, noise and high current may occur due to lubricating oil viscosity; they normally disappear after 10-20 minutes.
Flow must not be adjusted by opening/closing valves. This machine is a positive displacement compressor; change flow and shaft power by adjusting speed.
The pressure gauge switch is normally closed; open it when pressure measurement is needed.
Noise varies even for the same model, since the blower's position in the machine room and the piping arrangement affect noise levels.
Shutdown
Slowly open the discharge valve to fully open.
Press the Stop button on the control cabinet.
Commissioning a New or Overhauled Blower
To ensure safe operation, the blower must not carry external loads from pipelines, valves or frames.
Check the alignment of blower and motor.
Check that the base is fully shimmed all around and the anchor bolts are tight.
Fill the oil tank with the specified grade of machine oil up to the oil level line; the recommended lubricant is N220 medium-duty gear oil.
Check that the motor rotates in the indicated direction.
Install a belt guard (protective cover) at the belt wheel (coupling) for safe operation.
Fully open the inlet and outlet valves and turn the rotor by hand: it should rotate flexibly without impact or rubbing. Only after confirming everything is normal may the blower be started for trial operation.
No-Load Trial Run
All new or overhauled blowers should undergo a no-load trial run.
No-load means running with inlet and outlet valves fully open.
There should be no abnormal smell or smoke, no collision or rubbing sounds, and the radial vibration velocity at the bearings should not exceed 6.3 mm/s.
After about 30 minutes of no-load operation, if conditions are normal, loading may begin. If anything is abnormal, inspect, correct and repeat the no-load trial.
Loading and Normal Operation
Load up gradually and slowly to the rated load; never adjust to rated load in one step.
Rated load refers to the static pressure difference between inlet and outlet, per the nameplate value. With normal discharge pressure, watch inlet pressure changes to avoid overload.
During normal operation, never fully close the inlet and outlet valves. Watch pressure regularly and check whether the safety valve vents on overload; adjust the safety valve promptly and never run overloaded.
Because of the characteristics of Roots blowers, do not recirculate discharge gas directly back to the inlet for long periods (it changes the inlet temperature); if return-flow regulation is required, cooling measures must be used.
Do not stop the blower suddenly at full load; unload gradually before stopping to avoid damage. Users may draft their own detailed emergency shutdown rules.
Safe operation and service life depend on correct and regular maintenance. Beyond general repair procedures, pay particular attention to the following.
Troubleshooting: Gear and Bearing Issues
Gear wear causing excessive backlash.
Loose gear fastening failing to keep the impellers synchronized.
Bearing wear increasing clearance.
Remedies:
Remove contaminants and inspect internals for damage.
Adjust gear backlash; replace gears if side clearance exceeds the average by 30-50%.
Reassemble the gears, maintaining a tapered-fit contact area of 75%.
Corrosion Protection and Repair
Metal corrosion can be divided into two categories: general (uniform) corrosion and localized corrosion. The former occurs relatively evenly over the entire blower surface, the latter only locally - for example pitting, crevice corrosion, intergranular corrosion, stress corrosion cracking, corrosion fatigue, hydrogen attack, erosion corrosion and exfoliation corrosion. These are especially prominent in the petroleum and chemical industries and in marine atmospheres.
To deal with Roots blower corrosion, replacing equipment components is the usual approach, but affected by materials and manufacturing processes, such components are generally expensive - for example glass-lined equipment, PTFE parts and titanium components. Applying organic anti-corrosion coatings based on polymer composites is an effective measure. Surface bonded coatings can be widely used for repairing and pre-protecting areas subject to abrasion, cavitation and corrosion; with good chemical resistance and excellent mechanical and bonding properties, they offer simpler application, lower cost, better safety and better repair results than traditional welded pressure-vessel repairs.
Owing to the operating environment, shaft heads and keyways of Roots blowers often suffer wear. Traditionally such damage is repaired by build-up welding or brush plating followed by machining. However, the thermal stress from welding heat cannot be fully removed, leading easily to bending or fracture, and brush-plated coatings are limited in thickness and tend to peel off. Moreover, both methods repair metal with metal and cannot change the hard-on-hard fit relationship, so wear tends to recur under combined loads.
Polymer composite materials allow fast, effective repair of bearing housing wear without machining. There is no welding heat stress, repair thickness is unlimited, and the material's compliance - which metal lacks - absorbs shock and vibration while achieving 100% contact of the fitted surfaces, preventing renewed wear.
Cracks or fractures in Roots blower components often result from casting or machining defects, internal stress or overload. The conventional repair is welding, which frequently causes thermal deformation or thermal stress, especially in thin-walled parts; some components are made of hard-to-weld materials such as cast iron, aluminum alloys and titanium alloys. In explosion-hazard locations such as the petrochemical industry, welding repair is even less suitable.
Because Roots blowers often leak oil during operation, creating many safety hazards, the traditional remedy is a long shutdown to replace gaskets and recondition joint faces - impossible to achieve while the equipment is running. High-performance polymer sealants and lubricants with strong self-lubricating properties can reduce clearances at meshing locations and effectively mitigate noise and oil leakage at dynamic seals. Such lubricant additives do not contaminate or degrade the oil, removing safety hazards without stopping production and saving substantial repair or replacement costs.
Performance and Energy Saving
The discharge pressure of a Roots blower is not limited by speed: regardless of speed changes, the pressure can remain constant, while the air volume is proportional to speed (Q = KN).
A Roots blower is a constant-torque load, so its power-saving rate is proportional to the speed reduction. Although different from ordinary fans and pumps, its power is large and, as long as the furnace wall is intact, it works continuously around the clock with long running hours - hence considerable energy-saving potential and cost.
After technical retrofit, the Roots blower no longer relies on adjusting the outlet (inlet) valve opening to regulate pressure or flow. Labor intensity is reduced, response is timelier, product yield improves, and unit consumption drops significantly.
Long-Term Storage and Care
The safe operation and service life of a Roots blower depend on regular and correct maintenance and on watching for any early signs of trouble. If a three-lobe Roots blower is to be out of use for a long time, cut off all power to the unit and store it in a ventilated, dry place to effectively prevent rust during the idle period. In addition, oil the blower regularly during use to keep it operating smoothly.
Additional Maintenance Points
When disassembling the machine, measure and record all fitting dimensions and mark parts and their orientation, so reassembly preserves the original fits.
Overload sometimes does not show immediately, so watch changes in inlet/discharge pressure, bearing temperature and motor current to judge whether the machine is running normally.
Under normal conditions, replace the lubricating oil every 1,000 hours of operation. Check that oil cooling is normal, watch oil quality, listen regularly for abnormal noise, and make sure the unit is not operating outside specified conditions.
