Screw blowers: internal compression for low-pressure air

How does a screw blower work?

A screw blower uses a pair of helical rotors, a male and a female, meshing inside a casing. Air enters at one end and is trapped in the space between the rotor lobes and the casing. As the rotors turn, that space moves along the rotors and gets smaller, so the air is compressed before the discharge port opens. The point at which the port opens sets the machine’s built-in pressure ratio.

In blower duty the rotors usually run dry, with timing gears keeping them apart, so the air path is oil-free. Oil lubricates only the bearings and gears, sealed from the air. This is the same family as the rotary screw compressor, but the package is designed for a very different operating envelope: a pressure difference of a fraction of a bar up to around one bar or a little more, at large volume flows.

Screw blower vs Roots blower: why internal compression matters

A Roots blower carries air to the outlet without compressing it. When each pocket opens, outlet air rushes back in and pressure equalises suddenly. That external compression wastes energy and creates strong pulsation, and the penalty grows as differential pressure rises.

A screw blower compresses the air progressively. When the built-in pressure ratio is close to the actual system pressure ratio, the back-flow loss almost disappears. In practice this means:

• Lower specific power at medium to higher blower differentials, typically the upper part of the Roots range.
• Less pulsation, so smaller silencers and quieter piping.
• Lower discharge temperature for the same duty, which helps in high Indian summer ambient conditions.

The advantage shrinks when the actual pressure is far from the built-in ratio. If a machine designed for a higher ratio runs at a very low differential, it over-compresses and the air expands back at the outlet, wasting energy. That is why the operating range of your process, not the brochure, decides whether a screw blower pays back.

Screw blower vs screw compressor: same family, different machine

A plant-air screw compressor is oil-injected and works at 7–10 bar (101.5–145 psig) or higher. A screw blower runs dry at a much lower pressure ratio, at higher rotor tip speeds and with very different cooling. Do not treat one as a substitute for the other. Running a plant-air compressor and throttling the air down to blower pressure wastes most of the compression energy. Our page on blowers vs compressors sets out where the boundary lies.

Where do screw blowers fit best?

• Wastewater aeration with deep tanks and long running hours, where energy dominates whole-life cost. Municipal sewage plants and industrial ETPs across Telangana and Andhra Pradesh run aeration blowers round the clock.
• Pneumatic conveying of cement, fly ash, lime, flour, sugar and polymer granules at the upper end of the blower pressure range.
• Process air for fermentation, oxidation, agitation and flotation.
• Replacing large or multiple Roots blowers where logged duty shows steady operation at higher differentials.

For very low differentials or short running hours, a Roots blower may still be the economic choice. For very large, steady flows a centrifugal or turbo blower can compete. The decision is made on your duty data.

How to select a screw blower

1. Define the process. Minimum, average and peak flow, duty pattern, temperature, contaminants and whether any oil contact is acceptable.
2. Fix inlet conditions. Inlet pressure, temperature and site altitude. Hot intake air of 40–45 °C in summer reduces air density and mass flow.
3. Build the pressure budget. Aeration: submergence (about 0.1 bar per metre of water), diffuser loss with allowance for fouling, pipe and valve losses. Conveying: the line pressure from the conveying design, including peak and blockage conditions.
4. Use differential pressure, not a “bar” label. State inlet pressure, outlet pressure and the difference together.
5. Map the operating range. Minimum, normal and maximum flow and pressure, and how many hours a year are spent at each. This decides turndown and whether one machine, or a duty/assist arrangement, is right.
6. Compare specific power across the range. Ask for performance at several operating points, measured at the package boundary, including drive, VSD and cooling losses, not only bare-rotor figures.
7. Check controls and redundancy. VSD speed range, integration with dissolved-oxygen or pressure control, and the standby arrangement for critical duty.

Turndown and VSD control

A screw blower’s flow is roughly proportional to speed, so most packages use a variable-speed drive. The useful turndown depends on the minimum speed the manufacturer allows; below it, efficiency falls and temperatures rise. Ask what flow range the package covers at your pressure and what the specific power is at the bottom of that range. If your demand often falls below the minimum, a smaller duty machine with a larger assist unit, or a blower plus storage, may give better results than one large unit.

What does a screw blower package include?

Enclosure ventilation deserves attention in Indian plant rooms. If hot air from the package recirculates in a closed blower house, the inlet temperature rises and both capacity and margin to the temperature trip fall.

Common screw blower problems

Maintenance priorities

• Inspect inlet filters often in dusty seasons and log their pressure loss.
• Check bearing and gear oil, and change oil and filters at the manufacturer’s intervals.
• Trend differential pressure, discharge temperature, speed and power; drift tells you about diffuser fouling or wear long before a trip.
• Keep the enclosure fan, cooling paths and VSD ventilation clean.
• Test protection devices: relief valve, temperature switches, non-return valve.
• Follow the maker’s recommendations for bearing replacement and airend inspection at the specified running hours.

Screw blower vs tri-lobe vs centrifugal (turbo) blower

All three technologies cover the same low-pressure band, but they respond very differently to differential pressure, part load and hot inlet air. The figures below are typical for single-stage industrial units. Efficiency is wire-to-air, on an isentropic basis, at about 0.6 bar differential. Always compare actual quotations at your own duty points.

Most of the gap between tri-lobe and screw blowers is thermodynamic. A tri-lobe blower compresses by back-flow at constant volume, so its ideal power is inlet volume × Δp. A screw blower approaches isentropic compression. With an inlet at 0.93 bar(a), the theoretical tri-lobe penalty over isentropic work is 10.8% at 0.3 bar differential, 20.6% at 0.6 bar and 32.5% at 1.0 bar (inlet volume × Δp ÷ isentropic work − 1). That is why the screw blower’s advantage grows with pressure, as the comparison earlier on this page notes.

Sizing an aeration blower: worked example

This is an illustrative sewage treatment plant near Hyderabad, treating 5,000 m³/day with fine-bubble diffusers at 4.5 m submergence. Every assumption is labelled. Replace each one with your process designer’s figures. For the wider system, see aeration blower system integration.

1. Oxygen demand (AOR). BOD removed is 5,000 × (250 − 10) ÷ 1,000 = 1,200 kg/day. At an assumed 1.2 kg O₂ per kg BOD, that needs 1,440 kg/day. Nitrifying an assumed 35 mg/l of TKN needs 5,000 × 0.035 × 4.57 = 800 kg/day. Taking no denitrification credit, AOR = 2,240 kg/day, or 93.3 kg O₂/h.
2. Field to standard oxygen (SOR). The formula is SOR = AOR × C_s20 ÷ [α × (β × C_s,T,H − C) × 1.024^{(T − 20)}]. The assumptions are α 0.55, β 0.95, working DO 2.0 mg/l and water at 30 °C, with C_s 7.56 mg/l corrected for altitude by 0.950 ÷ 1.013 to 7.09 mg/l. C_s20 is 9.09 mg/l. So SOR = 93.3 × 9.09 ÷ [0.55 × (0.95 × 7.09 − 2.0) × 1.268] = 257 kg O₂/h, which is 2.75 times AOR. Depth enrichment of saturation is ignored, which is conservative.
3. Air flow. At an assumed standard oxygen transfer efficiency of 25% (about 5.5% per metre × 4.5 m), and with 0.279 kg O₂ per m³ of standard air (20 °C, 1.013 bar(a)): 257 ÷ (0.25 × 0.279) = 3,685 m³/h, or 61.4 m³/min standard.
4. Blower inlet volume. On a 40 °C summer day at 0.950 bar(a): 61.4 × (1.013 ÷ 0.950) × (313.15 ÷ 293.15) = 70.0 m³/min (2,471 cfm) on average. Taking a diurnal peak factor of 1.5 and a minimum of 0.5 gives a peak of 105 m³/min and a minimum of 35 m³/min.
5. Pressure. Submergence is 4.5 × 0.0981 = 0.441 bar. Add 0.04 bar for diffusers, 0.05 bar for pipework and valves and 0.03 bar for diffuser fouling, giving a discharge of 0.56 bar(g). With a 0.02 bar inlet filter loss, the differential is 0.58 bar, and the ratio is (0.950 + 0.561) ÷ (0.950 − 0.020) = 1.625.
6. Arrangement. Use two duty units plus one standby, each good for 52.5 m³/min at 0.58 bar. One unit at 67% speed covers the night minimum. Two units at 67% cover the average, and both at full speed cover the peak. Every operating point stays inside the VSD range.

Energy worked example: tri-lobe vs screw blower savings

The isentropic air power at the average duty above is 70.0 ÷ 60 × 93,000 Pa × 3.5 × (1.625^0.286 − 1) = 56.5 kW. The comparison assumes wire-to-air efficiencies of 56% for a tri-lobe blower and 70% for a screw blower, with an illustrative tariff of ₹8.50/kWh.

Three cautions apply before you bank that saving.

• The saving scales with differential pressure. At 0.3 bar the gap narrows sharply.
• Measure the existing tri-lobe blowers’ actual power and flow first, because worn rotors and slipping belts often make the baseline worse than the datasheet.
• The biggest aeration saving usually comes from DO-based flow control. Aerating to 2.0 mg/l instead of an uncontrolled 4–5 mg/l can cut air demand more than any change of blower.

Pneumatic conveying at 0.8–1.0 bar sits where the theoretical tri-lobe penalty is 27–32%, so a screw blower’s advantage there is at least as large.

Screw blower airend reliability and maintenance

A dry screw blower airend has no oil between its rotors. Its life therefore depends on bearings, timing gears, shaft seals and the rotor coating, and above all on how far it runs from its rated differential and temperature. Trend these readings against the commissioning baseline, at the same speed and pressure.

Most premature airend failures in Indian plants come back to three causes: hot recirculated inlet air, inlet filters left loaded through the dusty season, and a differential pushed beyond the rating because diffusers fouled or a conveying line was extended. Fix the cause before replacing parts. See airend bearings and seals and oil analysis.

How Reddy Compressors helps

We are an independent multi-brand compressed-air specialist working from Tarnaka, Secunderabad, with engineers and technicians covering Telangana and Andhra Pradesh. We are not tied to any manufacturer, so we can compare blower technologies neutrally against your duty.

• We help gather the duty data that decides the choice: flow, inlet conditions, differential pressure and annual operating profile.
• We can log an existing Roots blower’s power, pressure and running hours to show whether a screw blower would pay back.
• We check whether plant compressed air is being throttled for blower duties and recommend the right split.
• We sell, rent and service blowers – new and used/refurbished Roots, screw, centrifugal and side-channel units – and supply their spare parts, as an independent multi-brand specialist. Send us the nameplate and duty and we will confirm the right machine or part.

Call +91 99590 60718 or +91 92468 88812, or email reddycompressors@gmail.com.

What to send us

• Application and process description.
• Flow required, stating inlet or standard conditions, with minimum, normal and maximum values.
• Inlet and outlet pressure, inlet temperature and site altitude.
• Annual running hours and how demand varies.
• Details of existing blowers: nameplates, motor ratings, speeds and any logged power data.
• Space, noise and power-supply constraints at the blower house.

Frequently asked questions

Is a screw blower more efficient than a Roots blower?

Often, but not always. A screw blower compresses air internally, so it avoids the back-flow loss of a Roots blower and usually uses less power at medium to higher differentials. At very low differentials the difference can be small. The real answer comes from comparing package power at your actual flow, pressure and running hours, measured at the package boundary.

What pressure does a screw blower work at?

Screw blowers are low-pressure machines, generally used from a few hundred millibar up to around one bar or somewhat more, depending on the model. They overlap with the upper part of the Roots blower range. Select from the manufacturer’s current performance data at your inlet pressure, outlet pressure and temperature rather than from a general range.

Is a screw blower oil-free?

Most screw blowers run with dry rotors held apart by timing gears, so no oil is injected into the compression space and the air path is oil-free. Oil lubricates bearings and gears behind seals. For sensitive processes, confirm the design with the manufacturer and keep the seals and oil vents in good condition, because seal failure can let oil reach the air.

Should I replace my Roots blowers with screw blowers?

Consider it when the blowers run long hours at differentials toward the upper end of the Roots range, because that is where internal compression saves the most energy. Log the existing blowers’ power, pressure and running hours first, then compare package power at those duty points. For short running hours or very low pressure, the Roots blower may remain the better choice.

Why does my screw blower trip on high temperature?

The common causes are a higher differential than designed, often from fouled diffusers or a choked line, hot inlet air in summer, and poor enclosure or blower-house ventilation that lets hot air recirculate. A blocked inlet filter or wrong oil level can add to it. Check differential pressure and inlet temperature first, then the cooling path.

Can one screw blower cover all my demand with a VSD?

Only if your demand stays within the speed range the manufacturer allows. Below the minimum speed, efficiency falls and temperatures rise. Plants with very low night-time or seasonal demand often do better with a smaller duty machine and a larger assist unit, or a mix of technologies. Map your hours at each flow before deciding.

What is the difference between a rotary screw blower and a turbo blower?

A rotary screw blower is a positive-displacement machine. Its flow stays almost constant as back-pressure changes, and with a VSD it typically turns down to 25–30% of maximum speed. A high-speed turbo blower is a dynamic machine. It can be slightly more efficient at its design point, but its flow falls steeply if discharge pressure rises, and surge limits turndown to about 45–50% of rated flow. Screw blowers suit variable flow or changing pressure. Turbos suit large, steady duties at a stable differential.

How do I calculate the blower size for an aeration tank?

Start from oxygen demand (AOR), from BOD removal and nitrification. Convert it to standard oxygen (SOR) using α, β, temperature, altitude and the working DO, then divide by diffuser transfer efficiency × 0.279 kg O₂ per m³ to get standard air flow. Correct that to blower inlet conditions, which add about 14% at 40 °C and Hyderabad altitude. Pressure is submergence × 0.0981 bar per metre, plus diffuser, pipework and fouling losses. The worked example on this page goes from 93 kg O₂/h to 70 m³/min at 0.58 bar differential.

How reliable is a screw blower air end?

Reliable when it runs within its rated differential and temperature, because the dry rotors never touch. Life is set by the bearings, timing gears and shaft seals. These fail early mainly through hot inlet air, a loaded inlet filter, degraded oil or a differential pushed above the rating. Trend vibration, discharge temperature and flow at fixed speed against the commissioning baseline. A doubling of vibration, or a 10 °C temperature rise at the same duty, is the signal to investigate before damage spreads.

How much energy does a low-pressure screw blower save over a tri-lobe blower?

It depends on the differential. The theoretical tri-lobe penalty is about 11% at 0.3 bar, 21% at 0.6 bar and 32% at 1.0 bar. In the illustrative aeration example on this page, at 0.58 bar and 70 m³/min, a screw blower draws about 80.7 kW against 100.9 kW for a tri-lobe blower. That saves about 176,800 kWh a year, or about ₹15 lakh at an illustrative ₹8.50/kWh. Measure your existing blower’s real power and flow before relying on any figure.

Related pages

• Blower suppliers in Hyderabad
• Roots blowers
• Centrifugal blowers
• Side-channel blowers
• Blowers vs compressors
• Low-pressure aeration blower system integration
• Pneumatic conveying blower packages