A Fabry rotary blower is a positive displacement low-pressure air machine that uses two intermeshing lobed rotors turning in opposite directions inside a close-tolerance casing to trap and transfer fixed pockets of air from inlet to discharge. The design traces back to French engineer Charles Fabry's early 20th century rotary lobe work, refined for industrial duty by makers like Aerzen and Howden. It delivers near-constant volumetric flow regardless of back-pressure, typically 100 to 25,000 CFM at 3 to 15 psig. You see it driving wastewater aeration basins, pneumatic grain conveying, and vacuum sewer collection across plants worldwide.
Fabry Rotary Blower Interactive Calculator
Vary blower flow, discharge pressure, efficiency, and relief margin to see shaft power, motor power, relief setting, and loading change on an animated lobe-blower diagram.
Equation Used
The calculator uses the common low-pressure blower power approximation. Higher flow or pressure increases shaft horsepower directly, while better total efficiency reduces required power. The relief setting is the normal discharge pressure plus the selected safety margin, matching the article guidance that relief valves are set 1 to 2 psi above the design point.
- Low-pressure positive displacement air blower approximation.
- Flow is inlet volumetric flow in CFM.
- Efficiency is total wire-to-shaft/blower efficiency entered as a percent.
- Relief valve is set above the normal operating pressure by the entered margin.
Operating Principle of the Fabry Rotary Blower
Two figure-8 lobed rotors spin in opposite directions inside a cast-iron housing. As each rotor passes the inlet port it scoops a fixed volume of air into the cavity between the lobe and the casing wall, carries it around the bore, and discharges it on the other side when the trailing lobe sweeps past the outlet port. The rotors never touch each other and never touch the casing — they're held in precise mesh by a pair of timing gears on the drive end, with running clearances usually held to 0.10 to 0.20 mm on the lobe tips and 0.15 to 0.25 mm between the rotors themselves. That oil-free air path is the whole reason this machine exists: no internal lubrication ever contacts the process gas.
Because the trapped pocket has no internal compression, all the pressure rise happens at the discharge port the instant that pocket opens to the downstream piping. If the system back-pressure spikes — say a downstream diffuser fouls in an aeration basin — the rotors still pump the same volume per revolution but the motor draws more current and the discharge air heats up fast. That's why a Fabry rotary blower always runs with a discharge relief valve set 1 to 2 psi above the design point, and why discharge temperature is the single best diagnostic for trouble.
Get the clearances wrong and the machine fails in predictable ways. Tip clearance below 0.08 mm and thermal growth at full load will cause the lobes to rub the casing — you'll hear it as a rising metallic squeal within minutes. Tip clearance above 0.25 mm and slip becomes the problem: real delivered CFM falls 15 to 30 percent below the displacement-curve prediction because high-pressure air leaks back across the lobe tips into the suction side. Timing gear backlash above 0.05 mm lets the rotors clash on shock loads and chip a lobe edge, and that's a rebuild not a field fix.
Key Components
- Lobed Rotors: Two ductile-iron or aluminium rotors with two or three lobes each, machined to a profile that maintains constant clearance through the full mesh cycle. Tip-to-casing clearance held at 0.10 to 0.20 mm. Three-lobe rotors give smoother flow and lower pulsation than two-lobe at the cost of slightly more cost per CFM.
- Timing Gears: A matched pair of helical or spur gears on the drive end keeps the rotors phased exactly 90° apart with no contact between the lobes themselves. Backlash must stay under 0.05 mm — beyond that the rotors clash on pressure pulses and chip the tips.
- Cast-Iron Casing: Houses the rotors in a figure-8 bore machined to ±0.025 mm cylindricity. The casing also forms the inlet and discharge ports — port geometry sets how abruptly the trapped pocket sees the back-pressure, which directly drives noise and pulsation.
- Shaft Seals: Labyrinth or piston-ring seals separate the oil-flooded gear and bearing chambers from the dry process air path. A small purge of process air bleeds outward through the seal so no oil ever migrates into the airstream — critical for aeration and food-grade duty.
- Discharge Silencer: An absorptive or chamber-type silencer mounted directly on the outlet knocks down the strong blade-pass pulsation. Without it a 2,000 CFM unit puts out 105 to 110 dBA at one metre.
- Discharge Relief Valve: Spring-loaded valve set 1 to 2 psig above operating pressure. Because a positive-displacement blower will keep pumping into a closed line until something breaks, this valve is non-optional — it's the only thing protecting the casing and motor from a blocked-discharge event.
Who Uses the Fabry Rotary Blower
You find Fabry rotary blowers wherever a process needs a steady, oil-free, moderate-pressure air flow that doesn't change much when downstream conditions wobble. The combination of constant volumetric delivery and dry air path makes it the default choice for biological, food, and bulk-handling duty.
- Municipal Wastewater: Diffused aeration at the Stickney Water Reclamation Plant in Chicago — banks of Aerzen GM series rotary blowers feed fine-bubble diffusers in the activated-sludge basins at 8 to 10 psig.
- Pneumatic Conveying: Dilute-phase flour transfer at General Mills bakery flour silos, using Howden Roots URAI blowers to push flour from rail tankers into storage at 6 to 12 psig.
- Vacuum Sewer Systems: Iseki and Roediger vacuum collection stations in low-lying coastal towns use Fabry-type blowers in vacuum service to pull 6 to 8 psig vacuum on the collection main.
- Cement Production: Fluidising bulk cement powder in tanker trucks and silo unloaders at Lafarge terminal yards — 80 to 200 CFM units mounted directly on the truck PTO.
- Aquaculture: Oxygen transfer in indoor recirculating aquaculture systems at AquaBounty's Indiana salmon farm — low-pressure blowers feeding diffuser grids in 30,000-litre grow-out tanks.
- Petrochemical: Carbon-black pellet conveying lines at Cabot Corporation plants, where the dry oil-free air path prevents pellet contamination.
The Formula Behind the Fabry Rotary Blower
Sizing a Fabry rotary blower comes down to picking a displacement and a speed that give the actual delivered flow you need at the working back-pressure, after you take slip into account. At the low end of the typical 30 to 60 percent of max-RPM operating window the machine runs cool and slip is minimal but you're carrying oversized iron. At the high end it delivers more CFM per dollar of capital but discharge temperature climbs fast and bearing life shortens. The sweet spot for most plant duty sits around 70 to 80 percent of the rated maximum speed where volumetric efficiency stays above 90 percent and discharge temperature holds below 140 °C.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Qactual | Actual delivered air flow at suction conditions | m³/min | CFM (ft³/min) |
| Vd | Displacement volume per revolution (sum of both rotors) | m³/rev | ft³/rev |
| N | Rotor shaft speed | rev/min | RPM |
| Qslip | Back-leakage flow across lobe tip and end-plate clearances | m³/min | CFM |
Worked Example: Fabry Rotary Blower in a brewery wort aeration system
A craft brewery in Asheville North Carolina runs a 60 hL fermentation cellar and needs to aerate cooled wort on its way from the heat exchanger to the fermenter. The process calls for 8 ppm dissolved oxygen at a wort flow of 25 hL/h, which works out to a sterile-air demand of roughly 12 CFM at the diffuser stone, with a system back-pressure of 7 psig from the sterile filter and the diffuser depth. You're sizing an Aerzen Delta Blower GM 3S unit with a displacement of 0.018 ft³/rev per lobe pair (0.036 ft³/rev total) and rated for 1,200 to 3,600 RPM.
Given
- Vd = 0.036 ft³/rev
- Nnominal = 2,400 RPM
- Qslip at 7 psig = 8 CFM
- Required Qactual = 12 CFM (with margin to ~50 CFM for purge and surge)
Solution
Step 1 — at the nominal speed of 2,400 RPM, compute the theoretical displacement flow:
Step 2 — subtract the slip at 7 psig back-pressure to get actual delivered flow at the nominal point:
That's well above the 50 CFM target, so the unit is correctly oversized for turn-down. Now check the operating range. At the low end of the typical window — 1,200 RPM — slip stays close to 8 CFM because slip is driven by pressure not speed:
At 35 CFM the blower can just about meet the 50 CFM peak demand if you open up the purge — too tight. You'd want to run at 1,500 RPM minimum for comfort. At the high end, 3,600 RPM:
That delivers 121 CFM but discharge temperature at 7 psig and 3,600 RPM climbs to roughly 145 °C, right at the seal-material limit. The sweet spot sits at 2,000 to 2,500 RPM where you get 64 to 82 CFM, discharge stays below 120 °C, and the unit isn't being thrashed.
Result
At the nominal 2,400 RPM the blower delivers 78. 4 CFM at 7 psig — comfortably more than the 50 CFM peak the brewery needs and giving you turn-down room when only one fermenter is filling. The range tells the real story: 35 CFM at 1,200 RPM is marginal and 121 CFM at 3,600 RPM cooks the seals, so size the VFD to hold the unit between 2,000 and 2,500 RPM most of the time. If you measure 60 CFM instead of the predicted 78 CFM at 2,400 RPM, the three usual culprits are: (1) a partially fouled inlet filter pulling 0.5 psi vacuum on suction which inflates apparent slip, (2) lobe tip clearance opened up past 0.25 mm from prior thermal events, easy to confirm with feeler gauges through the inspection port, or (3) a leaking discharge check valve letting compressed air recirculate back through the casing on shutdown.
Fabry Rotary Blower vs Alternatives
A Fabry rotary blower competes with two main alternatives in the low-pressure air space: the Roots-type lobe blower (a close cousin with a different rotor profile) and the high-speed centrifugal turbo blower that's taken over a lot of large aeration duty in the last 15 years. The right choice depends on flow rate, turn-down requirement, oil-free criticality, and whether you're paying for the iron once or paying for the electricity for 20 years.
| Property | Fabry Rotary Blower | Roots Lobe Blower | Centrifugal Turbo Blower |
|---|---|---|---|
| Typical flow range | 100 to 25,000 CFM | 50 to 50,000 CFM | 2,000 to 80,000 CFM |
| Discharge pressure range | 3 to 15 psig | 1 to 15 psig | 5 to 25 psig |
| Volumetric efficiency at design point | 88 to 95% | 85 to 92% | 70 to 82% |
| Turn-down ratio with VFD | 3:1 to 4:1 | 2.5:1 to 3:1 | 1.5:1 to 2:1 |
| Specific power (kW per 100 CFM at 8 psig) | 3.8 to 4.5 kW | 4.0 to 4.8 kW | 2.8 to 3.5 kW |
| Capital cost per CFM (relative) | 1.0× | 0.9× | 2.5 to 3.5× |
| Bearing/timing-gear rebuild interval | 40,000 to 60,000 h | 30,000 to 50,000 h | 80,000 to 100,000 h (air-foil) |
| Sensitivity to back-pressure swing | Very low — flow stays constant | Very low — flow stays constant | High — flow falls off the curve |
| Best application fit | Mid-size aeration, conveying, brewery | Cement, bulk handling, vacuum sewer | Large WWTP aeration > 10,000 CFM |
Frequently Asked Questions About Fabry Rotary Blower
The motor amperage is set by discharge pressure not by flow — a positive displacement machine pumps the same theoretical volume per revolution whether it's actually delivering it downstream or recirculating it internally as slip. So if amps are at nameplate but flow is low, the air is going somewhere inside the casing.
Check three things in order: lobe tip clearance with a feeler gauge through the inspection port (anything past 0.25 mm and slip jumps fast), end-plate clearance which opens up after thermal events, and the inlet filter pressure drop. A filter pulling more than 0.3 psi vacuum on suction reduces inlet density and shows up as a flow shortfall on a mass-flow basis even though the volumetric reading at suction looks fine.
Three-lobe for aeration, almost always. The pulsation amplitude of a two-lobe rotor at blade-pass frequency is roughly 2.5× higher than a three-lobe at the same flow, and that pulsation is what fatigues diffuser membranes and rattles the discharge piping. EPDM fine-bubble diffusers in particular crack at the boss within 18 to 24 months on heavy two-lobe pulsation versus 5+ years on three-lobe.
Two-lobe still wins for short-duty conveying duty where the noise and pulsation don't matter and the slightly lower cost per CFM matters more.
Measure dBA at one metre off the discharge flange both upstream and downstream of the silencer. A healthy absorptive silencer drops the blade-pass tone by 18 to 25 dB. If you're seeing less than 12 dB attenuation the rockwool fill has either washed out (common when condensate finds its way back into the silencer body) or the perforated liner has corroded through and the sound is bypassing the absorptive media.
A chamber-type reactive silencer doesn't have media to fail but it's tuned to a narrow frequency band — if the blower has been re-VFD'd to a different operating speed the tuning is now off and the silencer is doing maybe 30% of its rated job.
Slip energy. Every cubic foot of air that leaks back across the lobe tips from discharge to suction gets compressed again on the next revolution, and that re-compression work shows up as heat in the airstream. At rated speed slip might be 8% of throughput; at maximum RPM with the same back-pressure slip can stay similar in absolute CFM but the re-compressed fraction's heat dump is now concentrated in a faster-moving airstream that has less residence time to shed heat into the casing.
If you're more than 15 °C above the manufacturer's predicted discharge temperature curve, suspect either internal clearances opened up beyond spec or a partially blocked discharge that's raised real back-pressure above what your gauge is showing — gauges plumbed downstream of a fouled silencer will lie to you.
You can, and many vacuum sewer systems do exactly that, but the de-rating is significant. In vacuum service the pressure ratio is reversed — atmospheric pressure becomes the discharge — and you're typically limited to about 50% of the pressure-rise capability of the same unit run in pressure mode. So a blower rated for 15 psig pressure duty will only pull about 7 to 8 psig vacuum reliably.
Two practical changes: you must add an inlet check valve to prevent reverse rotation when the unit shuts down under vacuum, and you need to oversize the cooling because heat-of-compression is now dumped into a much lower-density air stream that carries heat away poorly. Don't skip the cooling sizing — vacuum-service blowers fail from heat far more often than from wear.
The crossover sits around 8,000 to 12,000 CFM total plant demand at 8 to 10 psig. Below that, the capital premium for a turbo (typically 2.5 to 3.5× per CFM versus a positive displacement unit) doesn't pay back within 10 years even with the turbo's better specific power. Above 12,000 CFM the electricity savings are large enough — roughly 25% lower kWh per pound of oxygen transferred — that the payback drops to 4 to 6 years.
The other factor is load profile. Turbos hate turn-down: their efficient operating window is 70 to 100% of rated flow. If your plant runs at 40% load most nights, a turbo will either surge or run inefficiently at the bottom of its curve, and the positive-displacement bank with a VFD wins on annual energy even at higher flow.
Almost always cold-system back-pressure. Diffuser membranes, especially EPDM and silicone fine-bubble units, are stiff when new and stiff when cold — they can show 2 to 3 psi higher head loss on the first few hours of operation than they will after they've broken in and warmed up. Stack that on top of a relief valve set only 1 psi above design point and you'll lift the relief on the first ramp-up.
Standard practice is to start with the discharge throttle valve cracked open about 30%, bring the blower up to speed, then slowly open the throttle over 5 to 10 minutes while watching discharge pressure. After 24 hours of operation the diffusers will have wetted out and softened, and you can re-check the relief setpoint against actual steady-state discharge pressure.
References & Further Reading
- Wikipedia contributors. Roots-type supercharger. Wikipedia
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