How does a Roots blower work?
Two (or three) lobed rotors turn in opposite directions inside an oval casing. Timing gears at one end keep them in step so the lobes never touch each other or the casing. Small running clearances, rather than oil or seals, separate inlet from outlet, so the air path is oil-free when the shaft seals are in good condition.
As each lobe passes the inlet, it traps a fixed pocket of air against the casing and carries it round to the outlet. The pocket is not squeezed on the way. When it opens to the discharge port, the higher-pressure air in the outlet flows back into the pocket and the pressure equalises suddenly. That is why a Roots blower is often described as an external-compression machine, and it explains three of its main characteristics:
- Pulsation and noise. The sudden back-flow at each opening creates pressure pulses at lobe-pass frequency, which need silencers and careful piping.
- Heat. Compression against full back-pressure is less efficient than internal compression, so discharge temperature rises quickly with differential pressure.
- Efficiency falls as pressure rises. At very low differentials the penalty is small. As the differential approaches the machine’s limit, a screw blower with internal compression usually uses less power.
Roots blower or air compressor: what is the difference?
The difference is not size but duty. A plant-air compressor raises air to 7–10 bar (101.5–145 psig) with internal compression, usually in an oil-injected screw. A Roots blower moves a large volume across a small pressure difference, often a few hundred millibar. Using a compressor and a regulator to do a blower’s job wastes most of the energy, because air is compressed to a high pressure only to be throttled down. Our page on blowers vs compressors explains the boundary in more detail.
| Feature | Roots blower | Screw blower | Centrifugal blower | Side-channel blower |
|---|---|---|---|---|
| Principle | Positive displacement, no internal compression | Positive displacement, internal compression | Dynamic (impeller and diffuser) | Dynamic, regenerative |
| Flow behaviour | Near-fixed volume per revolution | Near-fixed volume per revolution | Flow varies strongly with pressure | Flow falls steeply with pressure |
| Typical pressure class | Low, typically up to about 1 bar single stage | Low, often somewhat higher than Roots | Low, large flows | Very low, small to medium flows |
| Strengths | Simple, robust, tolerant of dust and pressure swings | Better efficiency at higher differentials | Efficient at large steady flows | Compact, quiet, little maintenance |
| Watch points | Pulsation, noise, heat at high differential | Higher purchase cost | Surge, narrow operating envelope | Limited pressure and flow |
Ranges are typical and vary by manufacturer and model. Always select from the maker’s current performance data for your actual duty.
Where are Roots blowers used in Indian industry?
- Effluent and sewage treatment. Aeration of activated-sludge tanks and equalisation basins, air scouring of filters, and agitation. Pharmaceutical, chemical and food plants in Telangana and Andhra Pradesh run ETPs where the blower often runs round the clock.
- Pneumatic conveying. Dilute-phase transport of cement, fly ash, flour, sugar, plastic granules and chemicals.
- Vacuum duty. Vacuum conveying, suction lifting and dewatering, within the limits of the vacuum-rated variant.
- Combustion and process air. Burner air, oxidation air and agitation in process tanks.
- Fish ponds and aquaculture along the Andhra Pradesh coast, where low-pressure aeration runs continuously.
- Textiles, paper and packaging. Blowing, drying and handling air.
How to size a Roots blower
A Roots blower delivers nearly the same volume per revolution regardless of pressure, so its flow is set mainly by speed. Pressure is set by the system: water depth over diffusers, conveying-line resistance, filter and silencer losses. Size it in this order:
- Define the process duty. Medium (air or another gas), average and peak flow, and whether duty is continuous or intermittent.
- Fix inlet conditions. Inlet pressure (atmospheric, or below it for vacuum), inlet temperature and altitude. Hot summer intake air in the Deccan, often 40–45 °C, is less dense, so the blower delivers less mass of air at the same speed.
- Build the pressure budget. For aeration, add submergence (about 0.1 bar per metre of water), diffuser loss, pipe and valve losses and a margin for diffuser fouling. For conveying, use the conveying system designer’s line pressure.
- Calculate the differential pressure. Outlet pressure minus inlet pressure. Vacuum duty and pressure duty have different limits, even within the same frame.
- Allow for slip. Some air leaks back through the clearances, and slip rises with differential pressure and wear. Manufacturers’ curves include this.
- Select speed and frame. Choose a frame and speed that deliver the required inlet flow at the required differential, within the maker’s limits for speed, differential and discharge temperature.
- Check power. Shaft power rises almost in proportion to flow multiplied by differential pressure. Add drive losses and select the motor with a margin.
A blower sized only from a “cfm” figure, without stating inlet conditions and differential, will either run hot at a higher pressure than expected or deliver less air than the process needs.
What does a complete Roots blower package include?
| Component | Purpose | What to check |
|---|---|---|
| Inlet filter and silencer | Protects clearances from dust, reduces intake noise | Element condition, pressure loss, dusty sites need heavier filtration |
| Discharge silencer | Absorbs pulsation and noise | Internal packing intact, no rattling |
| Pressure relief valve | Protects against blocked discharge and overload | Set point below the machine’s limit, test regularly |
| Non-return valve | Prevents back-flow and reverse rotation when stopped | Seating, especially on parallel blowers |
| Flexible connectors | Isolate vibration from piping | Cracks, temperature rating |
| Belt or direct drive, motor | Sets speed and delivers power | Belt tension and alignment, motor current |
| Acoustic hood (optional) | Reduces noise at site boundary | Ventilation, so the package does not overheat |
| Gauges and temperature switch | Shows differential and protects against high discharge temperature | Working, readable, calibrated |
Never start a Roots blower against a closed discharge valve. Because it is a positive-displacement machine, pressure will rise until something gives: the relief valve, the belt, the motor or the rotors.
Common Roots blower faults and causes
| Symptom | Likely causes |
|---|---|
| High discharge temperature | Differential higher than design (fouled diffusers, blocked filter, choked line), high inlet temperature, worn clearances causing recirculation |
| Knocking or metallic noise | Rotor contact from worn timing gears or bearings, thermal growth at excessive differential, debris ingestion |
| Low air delivery | Slipping belts, wrong speed, blocked inlet filter, worn clearances, relief valve leaking |
| Oil in the air or on the casing | Failed shaft seals, overfilled gear or bearing oil, blocked vents |
| High motor current | Excessive differential, bearing failure, misalignment |
| Excessive vibration | Misalignment, loose foundation, unbalanced pulley, piping strain, bearing wear |
In Indian conditions, dust and summer heat are the two biggest enemies. A choked inlet filter raises differential and temperature at the same time, so filter inspection is the cheapest maintenance task with the largest effect.
Routine maintenance for Roots blowers
- Check gear and bearing oil level and condition regularly, and change it at the interval in the manufacturer’s manual, sooner in hot and dusty duty.
- Inspect and clean or replace inlet filter elements; record their pressure loss.
- Check belt tension, pulley alignment and wear.
- Log differential pressure, discharge temperature and motor current; a slow rise in any of them is an early warning.
- Test the relief valve and check the non-return valve.
- Listen for changes in noise and check vibration at the bearing housings.
- On aeration duty, track diffuser fouling, because it raises pressure on the blower over months.
Controlling flow and saving energy
Because flow is proportional to speed, the efficient way to match a Roots blower to varying demand is speed control, usually a VSD, within the maker’s minimum speed limit. Throttling the inlet or blowing excess air off through a valve wastes energy. On aeration, linking blower speed to dissolved-oxygen readings often matches air to biological demand far better than fixed-speed running. For larger, steadier duties at the upper end of the pressure range, compare a Roots machine against a screw or centrifugal blower on a whole-life energy basis before replacing like for like.
How Reddy Compressors helps
We are an independent multi-brand compressed-air specialist based at Tarnaka, Secunderabad, with service engineers and technicians covering Telangana and Andhra Pradesh. Blowers often sit beside compressors in the same plant, and we look at the low-pressure duty as part of the whole air system.
- We help define the duty properly: flow, inlet conditions, differential pressure and duty cycle, so the right technology is chosen.
- We check whether a duty is better served by a blower than by compressed air throttled down, which is a common cause of waste.
- We sell, rent and service blowers – new and used/refurbished Roots, screw, centrifugal and side-channel units – and supply their spare parts. Send us the nameplate and duty and we will confirm the right machine or part.
- Where plant air and blower air share a site, we audit both and recommend how to split the duties.
Call +91 99590 60718 or +91 92468 88812, or email reddycompressors@gmail.com.
What to send us
- The application: aeration, conveying, vacuum, combustion air or other.
- Required flow, with units and whether it is at inlet or standard conditions.
- Inlet and outlet pressure, or vacuum level, and the inlet air temperature.
- Duty pattern: continuous, intermittent, number of duty and standby units.
- For an existing blower, a nameplate photo, motor rating, speed and pulley sizes.
- Any symptom: heat, noise, low flow, oil leaks.
Frequently asked questions
What pressure can a Roots blower produce?
Single-stage Roots blowers typically work up to a differential of about 1 bar (around 15 psi), with many applications running well below that, such as aeration at a few hundred millibar. The exact limit depends on the frame, speed, inlet temperature and whether it is configured for pressure or vacuum. Always check the manufacturer’s curve for your duty, because discharge temperature usually sets the practical limit.
Why is my Roots blower running hot?
The usual cause is a higher differential pressure than the blower was selected for. Fouled aeration diffusers, a choked inlet filter, a partly closed valve or a blocked conveying line all raise pressure, and a Roots blower’s discharge temperature climbs quickly with pressure. Hot summer intake air, wrong oil level and worn clearances make it worse. Measure differential and inlet temperature first.
Is a Roots blower oil-free?
The air path in a Roots blower is normally oil-free, because the rotors run with clearances and no oil is injected into the compression space. Oil lubricates only the timing gears and bearings, separated from the air by shaft seals. If those seals fail or the oil is overfilled, oil can reach the air stream, so seal condition matters for sensitive processes.
Roots blower or screw blower: which is better?
Neither is better for every duty. A Roots blower is simple, robust and economical to buy, and works well at low differentials and in dusty or variable service. A screw blower uses internal compression and generally uses less power at higher differentials and on long running hours. Compare both at your actual flow, pressure and annual running hours before deciding.
Can I use a compressor instead of a blower for aeration?
You can, but it is usually very wasteful. A plant-air compressor raises air to about 7 bar, and aeration may need only a few hundred millibar, so most of the compression energy is thrown away at the regulator. For continuous aeration, a correctly sized blower almost always uses far less power. A compressor makes sense only for very small or temporary aeration needs.
Why does a Roots blower need a relief valve?
A Roots blower is a positive-displacement machine, so it keeps pushing the same volume of air whatever the resistance. If the discharge is blocked or a valve is closed, pressure rises until the belt, motor, rotors or casing are overloaded. The relief valve opens below the machine’s limit and protects it. It should be set correctly and tested regularly.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Roots blower duty: Differential pressure and power
For an aeration duty, build the differential pressure a Roots blower must deliver from water depth, diffuser, pipe, filter and silencer losses and a fouling margin, then estimate the shaft power from flow × differential.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Water depth above the deepest diffuser or sparger (m; at the highest level): 4
- Liquid density relative to water (fresh water 1.000, sea water about 1.025): 1
- Vessel head-space back-pressure (mbar g; 0 for an open basin, pond or tank): 0
- Diffuser or sparger loss (mbar; fouled or end-of-life figure): 50
- Pipe, header, valve, meter and non-return valve losses (mbar; worst branch at peak flow): 30
- Air treatment losses (mbar; filters, cooler and dryer at end of life; 0 if none): 0
- Inlet filter and silencer loss (mbar): 20
- Agreed margin (mbar): 30
- Site atmospheric pressure (bar a; about 1.013 at sea level, about 0.95 at Hyderabad): 1.013
- Flow basis: Actual m³/min at the blower inlet
- Required airflow (in the basis chosen above): 10
- Working broth volume per vessel (m³; used only for vvm): 50
- Number of vessels at peak together (used only for vvm): 1
- Inlet air temperature (°C; hottest expected): 40
- Machine type for the power estimate: Roots blower (no internal compression)
- Overall blower and drive efficiency (%): 60
Result
- Static head of the water column: 392 mbar = 0.392 bar (5.69 psi)
- Blower discharge pressure required: 0.502 bar g = 502 mbar g = 7.29 psig = 50.2 kPa g
- Share of the discharge pressure from static head: 78 % (the rest is losses, back-pressure and margin)
- Inlet absolute pressure after the filter and silencer: 0.993 bar a
- Blower differential pressure (outlet minus inlet): 522 mbar = 0.522 bar (7.57 psi)
- Pressure ratio across the blower: 1.526 (absolute outlet ÷ absolute inlet)
- Flow on the normal basis: 513 Nm³/h (0 °C, 1.013 bar a)
- Actual flow at the blower inlet: 600 m³/h at 40 °C and 0.993 bar a = 10 m³/min = 353 cfm
- Air mass flow: 663 kg/h (dry air)
- Hot-intake effect: 6.8 % more inlet volume per kg of air than at a 20 °C intake
- Ideal discharge temperature: 87 °C (before machine losses; the real figure is higher, read the maker’s curve)
- Ideal air power: 8.71 kW (inlet flow × differential)
- Estimated shaft or input power: 14.51 kW at 60% overall efficiency; select the motor with a margin
Method. Static head = depth × density × g = about 0.0981 bar per metre of fresh water (× relative density for sea water or broth). Discharge pressure (gauge) = static head + vessel back-pressure + diffuser or sparger loss + pipe, valve and meter losses + treatment losses + margin. The inlet filter and silencer loss lowers the inlet absolute pressure, so blower differential = discharge pressure + inlet loss and pressure ratio = (atmospheric + discharge) ÷ (atmospheric − inlet loss). Flow: actual inlet volume = normal volume × (T_inlet ÷ 273.15 K) × (1.01325 ÷ p_inlet absolute), dry air; vvm × working volume × vessels is taken on the normal basis (0 °C, 1.013 bar a). Power: a Roots blower has no internal compression, so its ideal air power is inlet flow × differential; a screw blower, centrifugal or compressor is taken as isentropic, P = Q₁ × p₁ × k/(k−1) × (r^((k−1)/k) − 1) with k = 1.4. Shaft or input power = ideal power ÷ overall efficiency. Ideal discharge temperature: Roots T₁ × (1 + (r − 1)(k − 1)/k); internal compression T₁ × r^((k−1)/k); real machines run hotter. Screening values; select from the maker’s curve at the hottest inlet and the fouled-diffuser pressure.
What to take from it. Size on the deepest diffuser at the highest water level, the fouled or end-of-life losses, peak flow and the hottest intake. Hot air is thinner, so the same normal flow needs more inlet volume in an Indian summer.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.