How Do You Specify a Screw Blower So the Quoted Flow Is Actually Achievable?

Short answer: A screw blower compresses air internally between two rotors and reaches 0.3–1.0 bar (30–100 kPa) with far less slip loss than a Roots blower. The number that decides whether a quotation is usable is FAD quoted at a stated pressure. Worked at 0.65 overall efficiency, 40 m³/min against 0.6 bar already costs about 62 kW of shaft power — so a 55 kW frame can never deliver that duty.

Written by Johnny Wayne, Managing Director, AirSpace Machinery Co., Ltd. — a 4,000 m² compressor and blower manufacturing facility in Shanghai, ISO 9001:2015 certified and CE marked. Reviewed by Richard Moore, Field Technical Manager.

Most screw blower quotations are not wrong on purpose. They are unusable because the flow figure and the pressure figure were measured at two different operating points and then printed side by side, as though they described one machine at one condition. We have watched that formatting cost a buyer a unit that could not hold its rated duty after commissioning, so this guide sets out how to read a quotation, how to size a blower from physics rather than from a catalogue, and what the running cost actually looks like across the aeration, conveying, FGD and fermentation duties that make up most of the low-pressure installed base.

What is a screw blower, and how does it compress air differently?

A screw blower is a positive-displacement machine that traps a volume of air between two precisely machined rotors, seals it, and shrinks that volume before the discharge port opens. That is internal compression. A Roots blower works the other way round: its lobes push air into the discharge line, and the line pushes it straight back on every lobe cycle. That backflow is pure loss, and it is why the two designs diverge so sharply in specific power above roughly 0.4 bar.

Every low-pressure air machine is ultimately judged on one relationship, and it is fixed by physics rather than by marketing:

Shaft power (kW) = Airflow (m³/s) × Pressure rise (kPa) ÷ Overall efficiency

Two consequences follow from that identity, and they drive every decision on this page.

First, airflow and pressure are not independent. Double the pressure at the same flow and you double the power. A flow rating therefore means nothing until the pressure at which it was measured is attached to it.

Second, efficiency is what actually separates two machines. Two blowers at the same kW and the same pressure do the same amount of physical work; the one with better rotor profiles, better sealing and a better-matched motor simply needs fewer kilowatts to do it. That is the honest basis for comparing two quotations.

How do screw blowers compare with Roots blowers on running cost?

The gap is a shape-of-curve difference, and it grows as pressure rises. At the low end of the band the two designs are within roughly 10% of each other; by 1.0 bar the backflow loss of a Roots machine typically accounts for a materially larger share of its input power. In continuous duty, where a blower runs 8,000 hours a year, that difference is paid every hour for the life of the machine.

Our own published data on the WindForce screw air blower series states up to 20–30% lower power consumption than a conventional Roots blower at matched duty, and that figure follows directly from the internal-compression principle rather than from a test on one site.

When a Roots blower is still the right answer. Very small flows, short daily duty cycles, or a site where the unit is genuinely a standby machine rather than the base load. We say this because the wrong answer costs money either way — but for a plant running an aeration blower 8,000 hours a year, the specific-power gap typically repays the purchase-price difference inside the first year.

Why must FAD always be quoted at a stated pressure?

Because a flow figure without its pressure is not a specification. A number like “10–25 m³/min” tells you nothing about what the machine will deliver at your duty point, and the three things that have to be attached to a flow figure are always the same: the pressure rise at which it is delivered, the reference intake condition for the volume (which is what FAD means), and the test standard the figure was obtained under.

Here is the pattern that causes most of the damage. A supplier publishes a flow range against a pressure range — for example “65–100 m³/min at 0.3–1.0 bar” — and the buyer reads it as one envelope. In reality those two numbers come from two different operating points: the high flow at the lowest pressure, and the highest pressure at the lowest flow. Size a plant from that envelope and you will specify a machine for 65 m³/min at 1.0 bar, when the physical work alone for that point is (65 ÷ 60) × 100 = 108.3 kW — before any loss at all. A 75 kW package was never capable of it, whoever built it.

The one format you can size from reads like this:

“At 0.7 bar (70 kPa), this model delivers 41.8 m³/min of FAD, drawing 75 kW.”

Two sentences, three numbers, one operating point. That is a specification. Everything else is a range that has been averaged into ambiguity. Some of the products on our own low-pressure range had to be re-tabulated for exactly this reason, and the current planning tables now read across a row at a stated pressure rather than down a column of ranges.

What does a real duty point look like when you work it out?

Take an aeration design that calls for 40 m³/min at 0.6 bar (60 kPa). This takes under a minute with a calculator, and it will tell you more than the whole quotation document.

1. Convert the flow: 40 m³/min ÷ 60 = 0.667 m³/s
2. Compute the ideal work: 0.667 m³/s × 60 kPa = 40.0 kW
3. Apply a realistic overall efficiency of 0.60–0.70 for a screw machine: 40.0 ÷ 0.65 ≈ 61.5 kW of shaft demand
4. Add frame margin for ambient and altitude: a 75 kW frame, not a 55 kW frame

Notice what step 2 already proves. No amount of engineering can move 40 m³/min against 60 kPa for less than 40 kW, so any quotation offering that duty on a smaller motor is arithmetically impossible rather than merely optimistic. That check costs you nothing and you can run it on every quotation you receive, at any pressure, in your head.

Efficiency in step 3 is the honest variable. We use 0.60–0.70 as the range for a screw package because it covers the spread between a well-matched rotor set at its design point and the same machine pushed off-design; if you have a declared specific power figure in kW/m³, use that instead, because it is a measurement rather than an estimate.

What is the WindForce screw blower series and its planning range?

AirSpace Machinery builds the WindForce series for continuous low-pressure duty, with PMV/VSD control and oil-free air delivery, in five frame sizes from 22 kW to 110 kW. The table below is arranged so that you read across a row, at the pressure that matches your process, instead of reading a flow range against a pressure range.

Planning values derived from FAD = motor kW × 0.65 ÷ pressure rise (kPa) × 60, at an overall efficiency of 0.65, quoted as FAD at standard intake conditions. These are indicative figures for quotation layout and preliminary selection, not factory-verified performance data. Final selection, dimensions, weight, voltage, frequency and configuration are confirmed by our engineering team against your duty point, and verified performance is issued with the factory test report at commissioning.

Why we publish it in this shape. Every quotation in this market that states a flow range over a pressure range is quietly asking you to accept two operating points as if they were one. That format cannot be selected from, and the buyer carries the risk at commissioning. Read a row in the table above, pick the pressure column that matches your process, and you have a number you can defend to your own engineering manager.

How is a screw blower sized for wastewater aeration?

Municipal and industrial wastewater plants are the largest installed base for low-pressure blowers, because biological treatment is an aerobic process: the bacteria that consume organic load need dissolved oxygen, and dissolved oxygen comes from air pumped to the bottom of the basin. Typical aeration duty sits between 0.5 and 0.8 bar, driven mostly by diffuser submergence depth plus pipework losses, and it runs 24 hours a day.

Four things decide the selection:

• The load is never constant. Inflow, and therefore biological oxygen demand, varies through the day and across seasons. A fixed-speed blower sized for peak flow runs at full power around the clock and blows off the surplus.
• VSD matches air to actual oxygen demand. With PMV/VSD control, the machine follows the basin’s dissolved-oxygen setpoint instead of the clock.
• Energy is the whole cost story. Aeration typically accounts for the majority of a treatment plant’s electricity bill, so every point of specific power saved is multiplied by 8,760 hours.
• Oil-free matters for compliance. A Class 0 oil-free air stream keeps lubricant out of the basin and out of the effluent obligation — see our note on how ISO 8573-1 Class 0 air protects biological processes.

What we need from you to size it: basin volume and depth, target dissolved oxygen, diffuser type and submergence, design flow, and the pressure rise across diffusers plus pipework.

What does the field record show about pressure stability?

This is the part of a blower project that a data sheet cannot tell you, so here is what our engineers log on site. In a 2025 project in Ho Chi Minh City, we replaced an ageing fixed-speed machine at an iron fabrication plant running 15,000 m² of shop floor with more than 200 employees on 16-hour production days. The complaint was not capacity — it was that system pressure moved by ±0.5 bar across the shift, which disrupted assembly work and stressed downstream equipment.

Our commissioning procedure is the same for a blower as it was there: survey the existing system before quoting, measure the actual duty point rather than the nameplate, install pressure and flow instrumentation, and re-measure after commissioning so the customer has a before-and-after record instead of a promise. Performance was verified against the acceptance-test method of ISO 1217, and air quality against ISO 8573-1. The full write-up, including the pre-installation measurements, is in this Vietnam PMV installation case study.

The lesson we carry into every blower enquiry is narrow and practical: we will not quote a flow figure without the pressure it was measured at, and we will not size a machine from a range. If your current supplier cannot produce that pair of numbers for your duty point, the risk of a commissioning shortfall is yours.

How is a screw blower sized for pneumatic conveying of powder and granules?

Dense-phase and dilute-phase conveying both need a constant air stream at stable low pressure, and instability is the enemy: surge and pulsation drop particles out of suspension and block the line. A blocked conveying line on a cement, ceramic or grain line is a stopped process, not a delayed one. Conveying air typically runs between 0.4 and 1.0 bar depending on distance and material, and the duty is usually intermittent.

What a screw design gives you here:

• Continuous, pulsation-free delivery keeps material moving at a stable velocity, with no lobe-frequency shock to drop particles out of the air stream.
• Variable speed tracks batch demand. Conveying runs in batches, so a VSD machine follows the cycle instead of running flat out and blowing off the difference.
• Oil-free delivery matters wherever the material is food, feed, pharmaceutical or any product that must not carry lubricant residue — which is most of the powder-handling world.

What we need from you: material type and bulk density, conveying distance and vertical lift, tonnage per hour, and the air velocity the material needs to stay airborne.

What about plants that need a blower and a low-pressure air compressor?

This is where buyers usually get two product categories confused, so it is worth separating them clearly. A low-pressure air compressor for the textile / ceramic / glass / cement industry is a screw package engineered to deliver its rated FAD between 3 and 5 bar, whereas a blower is built for the 0.3–1.0 bar band. They are different machines doing different jobs in the same plant, and a plant rarely needs only one of them.

Our low-pressure range covers 15–300 HP for the 3–5 bar duty, and the blower range covers the sub-1 bar duty; both are listed in our download centre as separate series. In the field we see most of the selection errors in these four industries come from sizing a 7–13 bar general-purpose compressor for a 3–5 bar application, which is expensive in both capital and running cost.

What does an FGD oxidation air blower specification actually require?

Wet flue gas desulfurisation scrubbers use oxidation air to convert calcium sulfite to calcium sulfate, and the oxidation blower runs in a wet, mildly corrosive, high-availability environment, typically at 0.6–1.0 bar. Blower failure here takes the scrubber down and with it the emission limit, so the specification priorities invert: availability comes before specific power. Most FGD installations run duty plus standby, and the standby machine is not a luxury — it is the compliance margin.

What matters in this duty is bearing selection and sealing for the wet end, service access without dismantling the header, and pressure stability as the scrubber load moves. What we need from you: scrubber type and throughput, required oxidation air flow, header pressure, and whether the unit is duty or duty-plus-standby.

Why does fermentation air supply need oil-free delivery?

In fermentation the air going into the vessel is a process input rather than a utility, so any hydrocarbon carry-over contaminates the batch. That makes oil-free delivery a specification rather than a preference, from pilot scale through multi-tonne production. Typical fermentation air demand sits between 0.5 and 1.5 bar at the sparger depending on vessel geometry, and it follows a growth curve rather than a schedule.

The fit is straightforward: oil-free air protects batch integrity and downstream validation; VSD follows the oxygen uptake curve, which rises steeply through the growth phase and flattens in production; and steady pressure supports dissolved-oxygen control, which is the parameter the process is actually run on. What we need from you: vessel volume and headspace pressure, required air flow and oxygen transfer rate, sterile filtration arrangement, and the batch cycle profile.

What about fluidized bed drying and aquaculture duty?

Both are large, steady, low-pressure air duties, and both are sized from one dominant figure. Fluidized bed and industrial drying processes need a large air stream to suspend and dry material at typically 0.3–0.6 bar, and the air is often acting as the heat carrier as well, so flow stability determines product quality as much as throughput. For those duties we need bed area and particle size, required superficial velocity, operating air temperature and the product moisture target.

Intensive aquaculture lives or dies on dissolved oxygen at typically 0.3–0.5 bar, and the aeration must be continuous: an hour of interrupted oxygenation is an inventory loss event, not an inconvenience. For those systems we need pond or tank volume, stocking density, required dissolved-oxygen level and the redundancy plan for the blower station — because the redundancy plan is usually what decides the number of machines, not the flow calculation.

Why does a 50°C full rating change the machine you need?

Because a blower’s airflow figure is only valid at the intake condition it was measured at. A package rated at 20°C delivers less mass flow in a plant room that sits above 40°C, and it runs hotter at exactly the time of year when a treatment plant or a conveying line cannot stop. In a pump house in Manila, Jakarta, Ho Chi Minh City or Chennai, that is the normal operating condition rather than a design corner case.

The WindForce series is rated for full output at 50°C ambient. That matters commercially as much as it does technically, in three ways:

• It changes the model you need. A unit that has to be de-rated at 40°C must be bought one frame larger to deliver the same mass flow — and it then burns the power of that larger frame for its whole service life.
• It changes uptime. Oversized cooling duty is what prevents nuisance high-temperature trips during the hottest week of the year, and that week is when a basin or a conveying line is least able to stop.
• It is checkable. Ask any supplier what ambient temperature their quoted flow figure is valid at, and ask for it in writing. If the figure only holds at 20°C, your real duty point is unfunded.

What we need from you to size it: your site’s maximum ambient temperature and altitude, and whether the blower sits indoors or in a shaded outdoor enclosure. Both change the selection.

Why does variable speed matter more on a blower than on a compressor?

Because blower load moves constantly by design — the oxygen demand of a basin, the batch rhythm of a conveying line, the growth curve of a fermentation — whereas a compressor’s load often sits within a narrow band. At constant pressure, a screw blower’s power follows its flow almost directly, so a machine delivering 70% of design flow needs roughly 70% of design power.

A fixed-speed machine asked for 70% of design flow typically holds 85–95% of full-load power and blows off the remainder. Between those two behaviours sits the entire savings case, and it scales directly with hours per year: at 8,000 hours a year the difference is large enough to dominate the purchase price, while at 1,500 hours a year it may not justify the VSD premium at all. That hourly figure, not the model number, is what should decide whether you buy variable speed.

How do you calculate the real energy cost before you buy?

Run five steps before you compare a single quotation; they convert a specification sheet into a number your finance team already recognises.

1. Establish one duty point — flow in m³/min and pressure rise in kPa, plus the expected turn-down. Not a range.
2. Compute the ideal-work floor — Ideal kW = (m³/min ÷ 60) × kPa. Any quotation below this is wrong, whoever sent it.
3. Add efficiency — divide by 0.60–0.70, or use the supplier’s declared specific power in kW/m³ at your duty point.
4. Annualise — Annual kWh = kW × hours per year, then multiply by your electricity tariff.
5. Compare against the alternative — the difference in kWh between two options, times hours, times tariff. Purchase price is paid once; specific power is paid every year.

A worked figure makes the scale plain: a 15 kW difference in shaft power between two options, running 8,000 hours a year at 0.08 USD/kWh, is 120,000 kWh and 9,600 USD per year of electricity — on a machine whose purchase-price difference is usually a fraction of that. If you want the compressor-side version of the same arithmetic on your own operating hours, our free compressed air kWh forecast system runs it for you.

What should you ask every screw blower supplier before you order?

Ten questions, in this order. The first two are the ones that decide whether the rest of the document means anything.

1. At what pressure was that FAD figure measured? If the answer is a range, keep asking.
2. What is the declared specific power, in kWh/m³, at my duty point?
3. Is the air oil-free, and to which ISO 8573-1 class?
4. What happens at 60% flow — VSD turn-down or blow-off?
5. What is the discharge air temperature at my duty point?
6. Which parts are wear parts, and what is the service interval?
7. What is the documented lead time for this exact configuration?
8. Is the unit factory-tested before shipment, and is the test report included?
9. What does the warranty cover, and what does it exclude?
10. Who services it in my country, and what is the spare-parts route?

A supplier who cannot answer questions 1 and 2 in writing is asking you to buy a number they cannot defend, and the risk lands on you at commissioning rather than on them at the quotation stage.

Key takeaways

• FAD without a stated pressure is not a specification. A flow range printed against a pressure range describes two different operating points, and sizing from it is how commissioning shortfalls happen.
• Physics sets a floor you can check in your head: kW = (m³/min ÷ 60) × kPa ÷ efficiency. At 0.65 efficiency, 40 m³/min against 0.6 bar needs about 62 kW, so a 55 kW frame cannot do it.
• Screw blowers win on continuous duty, not on low duty. The specific-power advantage over a Roots machine compounds across 8,000 hours a year and is marginal at 1,500 hours.
• Variable speed is the whole savings case for a blower, because blower load moves by design: a fixed-speed machine at 70% flow can still draw 85–95% of full-load power.
• A 50°C full-output rating is a selection input, not a decoration. A package that must be de-rated at 40°C has to be bought one frame larger and then runs at that larger frame’s power for its service life.
• Ask for the ambient temperature at which the quoted flow is valid, in writing, before you sign — that single question catches most over-stated flow ratings.

Frequently asked questions

How do I choose between a screw blower and a Roots blower?

Start with annual running hours. Below roughly 2,000 hours a year on a light intermittent duty, a Roots blower can be the pragmatic choice on purchase price. Above that, and certainly at continuous 24/7 aeration or conveying duty, the specific-power gap typically repays the price difference quickly, and screw designs also give better turn-down, lower noise and lower discharge temperature.

What pressure range do screw blowers operate in?

Low-pressure duty, typically 0.3–1.0 bar (30–100 kPa). That is the band where aeration, pneumatic conveying and oxidation air sit. Above roughly 1.5 bar you are in compressor territory and should be specifying against 7 bar or 10 bar compressor performance instead, which is a different machine and a different cost base.

Can a screw blower run continuously, 24 hours a day?

Yes — continuous duty is its design point, and it is the reason screw designs displaced Roots blowers in municipal aeration. What decides whether a machine survives the hottest week of the year is its ambient and cooling rating: the WindForce series is rated for full output at 50°C ambient. Ask for the ambient temperature at which the quoted flow is valid, and get it in writing during the RFQ rather than assuming it from a catalogue.

Is the air oil-free?

The WindForce series delivers oil-free air, which is why it suits biological aeration, food and pharmaceutical conveying and fermentation air supply. Ask for the oil-content classification against the ISO 8573-1 class that applies to your process, and have it stated in the quotation rather than inferred from a brochure claim.

How do I size a blower for a wastewater aeration basin?

Take basin volume and depth, target dissolved oxygen, diffuser type and submergence, then calculate the pressure rise from submergence plus pipework losses — typical results land between 0.5 and 0.8 bar. Size the flow for peak demand, use VSD to track the actual oxygen requirement, and send us those figures so we can return a sized selection instead of a catalogue page.

What is the delivery lead time?

Factory-direct delivery is normally planned within 7–35 days depending on configuration, testing, documentation and production scheduling. Lead time is quoted against a fixed configuration, so confirm the model, pressure, voltage and frequency before you schedule installation.

What if the flow figure I was quoted turns out to be unachievable?

It usually means the figure was measured at a different pressure than the one you are operating at. Require every quotation to state pressure, FAD and kW together at your duty point; if a supplier cannot, the commissioning risk sits with you, and the cheapest remedy is to ask the question before the order rather than after the delivery.

Sources and standards

• ISO 1217 — Displacement compressors: acceptance tests. The method behind the performance verification work described in the field section above.
• ISO 8573-1:2010 — Compressed air, Part 1: contaminants and purity classes, including Class 0 for oil-free air.
• ISO 9001:2015 — Quality management systems, the certification held by our 4,000 m² Shanghai manufacturing facility, alongside CE marking for machinery safety.
• Company and product references — planning data for the WindForce screw air blower series, the low-pressure compressor range for 3–5 bar duty, and downloadable specification sheets in the download centre.
• Field record — pre- and post-installation measurements from our 75 kW PMV installation case study in Vietnam, including the ±0.5 bar pressure-variation finding described above.

About the author

Johnny Wayne is Managing Director of AirSpace Machinery Co., Ltd., a compressor and blower manufacturer operating from a 4,000 m² production facility in Shanghai and exporting through distributors into more than twenty markets. The company holds ISO 9001:2015 certification and CE marking for its machinery, and builds the WindForce screw blower series for oil-free low-pressure duty. This article was technically reviewed by Richard Moore, Field Technical Manager, whose commissioning records supplied the pressure-stability findings reported above.

Send your duty point — flow, pressure rise, hours per day and the process behind it — to our engineering team and we will come back with a sized selection, the specific power at that duty point, and a delivery schedule.

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