A Wedding Rotary Blower is a positive-displacement twin-lobe air blower built by Wedding & Associates and similar lobe-blower manufacturers, in which two figure-8 rotors mesh inside a cast housing to trap and discharge fixed slugs of air on every revolution. You'll find this style of blower on wastewater aeration basins, pneumatic conveying lines, and PM10 high-volume air samplers. It exists to deliver clean, oil-free, low-pressure air at a constant volumetric flow regardless of downstream pressure swings — typically 0.3 to 15 psig and 50 to 5,000 CFM, with the air sampler variant pulling a steady 1.13 m³/min for 24-hour particulate runs.
Wedding Rotary Blower Interactive Calculator
Vary blower flow and run time to see total delivered air while the canvas shows the twin-lobe blower moving air pockets from inlet to discharge.
Equation Used
The worked article example describes a Wedding rotary blower pulling a steady 1.13 m3/min for a 24-hour particulate run. This calculator treats the blower as a constant-flow positive-displacement source and multiplies flow by run time to estimate total air moved.
- Blower maintains the entered volumetric flow for the full run.
- Flow is treated at actual line conditions, not corrected to standard air.
- Internal slip and filter loading are included only if they are already reflected in the entered flow.
How the Wedding Rotary Blower Actually Works
Two intermeshing lobed rotors spin in opposite directions inside a precisely machined housing. As each rotor turns, it scoops a fixed volume of air from the inlet, carries it around the housing wall, and delivers it to the discharge port. Nothing compresses inside the blower itself — that's the key concept. The air only gets compressed when it meets the back-pressure of whatever you're pushing it into, which is why we call it a positive-displacement blower rather than a compressor. The lobes never touch each other, never touch the housing, and never need lubrication in the air path. That's how you get oil-free air with no contamination.
The rotors stay synchronised through a pair of timing gears on the drive end, and the clearances are tight — typically 0.10 to 0.20 mm rotor-to-rotor and 0.15 to 0.25 mm rotor-to-housing on a 4-inch blower. If those clearances open up because of bearing wear or thermal growth, the blower's volumetric efficiency drops fast. You would be amazed how much air slips back through a 0.4 mm gap at 5 psig — easily 15-20% of rated flow. If you notice discharge temperature climbing above 120°C, that's usually the first warning that internal slip is rising and the rotor pack needs attention.
Common failure modes are predictable. Timing-gear backlash above 0.05 mm lets the lobes kiss and gall the cast iron. Bearing failure on the non-drive end allows the rotor to drift axially and chew the end plate. Ingested debris — a loose nut, a chunk of pipe scale — will instantly destroy a rotor pack because there is zero clearance to absorb it. Always run an inlet filter rated 10 µm or finer, and always run a discharge silencer with a relief valve set 1 psig above your operating pressure.
Key Components
- Twin Lobe Rotors: Two figure-8 cast or ductile iron rotors, machined to within 0.05 mm profile tolerance, that mesh without contact. Each rotor displaces a fixed volume per revolution — typically 0.5 to 50 litres per rev depending on frame size.
- Timing Gears: A matched gear pair on the drive shaft end that keeps the rotors phased exactly 90° apart. Backlash must stay below 0.05 mm; once it opens up, the lobes contact and you'll hear a metallic chirp at every revolution.
- Cast Iron Housing: The pressure-bearing cylinder and end plates that contain the rotors. Bore roundness is held to 0.025 mm, and rotor-to-bore clearance is set at 0.15 to 0.25 mm cold to allow for thermal expansion at running temperature.
- Sealed Bearings: Tapered roller or deep-groove ball bearings on each rotor end, oil-bath lubricated on the gear side and grease-packed on the drive side. L10 life typically 40,000 hours at rated load and speed.
- Inlet Filter and Discharge Silencer: A 10 µm pleated inlet filter protects the rotors from debris ingestion, and a chambered reactive silencer on the discharge knocks 25-30 dB off the pulsation noise these blowers naturally produce.
- Pressure Relief Valve: A spring-loaded poppet valve set 1 psig above operating pressure. Without it, a blocked discharge will overload the motor and twist a shaft within seconds — these blowers cannot regulate their own pressure.
Real-World Applications of the Wedding Rotary Blower
Rotary lobe blowers like the Wedding-style design show up wherever you need a steady, oil-free, low-pressure air stream that doesn't care about downstream pressure variation. The flow stays constant whether you're pushing 0.5 psig or 12 psig — only the motor amperage changes. That predictability is exactly what process engineers want for aeration, conveying, and sampling duties.
- Environmental Air Quality Monitoring: The Wedding & Associates IP10 PM10 high-volume air sampler uses a brushless rotary blower to pull a regulated 1.13 m³/min through a quartz filter for 24-hour ambient particulate sampling at EPA reference-method sites.
- Municipal Wastewater Treatment: Aeration basin diffuser supply at plants like the Stickney Water Reclamation Plant in Cicero, Illinois — twin-lobe blowers from manufacturers like Aerzen and Howden push 8,000+ CFM into fine-bubble diffusers at 7-9 psig.
- Pneumatic Conveying: Bulk cement and flour transport — a typical truck-mounted setup like a Fruehauf pneumatic tanker uses a Roots-style rotary blower at 12 psig to convey 30 tons of cement through a 4-inch line in under 45 minutes.
- Aquaculture: Trout raceway oxygenation at hatcheries like the Riverdale National Fish Hatchery — rotary blowers feed sub-surface diffuser grids to maintain dissolved oxygen above 6 ppm in 30,000-gallon raceways.
- Cement and Lime Plants: Kiln combustion air boost on rotary cement kilns — Lhoist and Lafarge installations run twin-lobe blowers at 6-10 psig to support secondary combustion air injection.
- Vacuum Service: Run in reverse, the same blower geometry creates 50-80% vacuum for paper-mill sheet pickup or food-industry packaging machines like the Multivac R 245 thermoformer.
The Formula Behind the Wedding Rotary Blower
The volumetric flow a rotary lobe blower actually delivers depends on its swept volume per revolution, its rotational speed, and its volumetric efficiency — which itself drops as differential pressure climbs. At the low end of typical operating pressure (1-2 psig), volumetric efficiency runs 95% and the blower delivers nearly its theoretical CFM. At the nominal mid-range (5-7 psig), efficiency drops to about 88% as more air slips back through the rotor clearances. Push it to the high end of its rating (12-15 psig) and efficiency falls below 80%, the discharge gets hot, and you start losing flow fast. The sweet spot for most plant duties sits around 5-8 psig where the unit runs cool and efficient.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Qactual | Actual delivered volumetric flow at inlet conditions | m³/min | CFM |
| Vd | Swept displacement per revolution (geometric, fixed by rotor size) | m³/rev | ft³/rev |
| N | Rotor shaft speed | rev/min | RPM |
| ηv | Volumetric efficiency (slip loss factor, varies with ΔP) | dimensionless | dimensionless |
Worked Example: Wedding Rotary Blower in a craft malt-house pneumatic grain conveyor
A 4,000-tonne-per-year craft malt house in Hamilton, Ontario is sizing a twin-lobe rotary blower to pneumatically convey malted barley from a receiving hopper through a 100 mm diameter line to a 40-tonne storage silo 35 m away. The blower frame is an Aerzen GM 4S equivalent with a swept displacement of 0.0042 m³/rev. Target conveying pressure is 5 psig at the blower discharge, and they want 12 m³/min of free air delivery to maintain a 18 m/s line velocity with a 4:1 solids-to-air mass ratio.
Given
- Vd = 0.0042 m³/rev
- ΔPnominal = 5 psig
- Qtarget = 12 m³/min
- ηv at 5 psig = 0.88 —
Solution
Step 1 — solve the displacement equation for required shaft speed at the nominal 5 psig operating point, where volumetric efficiency runs about 88%:
That speed sits comfortably inside the GM 4S frame's 2,000-4,500 RPM operating window. The motor size at this duty works out to about 18.5 kW, and the blower will run with a discharge temperature near 95°C — well below the 130°C trip threshold.
Step 2 — check the low end of the typical operating range. At 2 psig (a lighter-load conveying scenario, say a partial-line transfer), volumetric efficiency climbs to 95%:
You get roughly 8% more flow at the same shaft speed because less air slips back through the rotor clearances. In practical terms the line velocity creeps up to 19.5 m/s, which is fine for barley but would start abrading softer products like cocoa nibs.
Step 3 — check the high end. Push the blower to 12 psig (a plugged filter or partially blocked diverter valve scenario) and ηv drops to about 0.78:
You've lost 11% of your flow, line velocity drops to 16 m/s, and the conveyed barley starts to settle and saltate along the bottom of the pipe — exactly the condition that leads to a line plug and a service call at 2 AM. Discharge temperature also climbs past 130°C at this duty, which will cook the bearing grease inside 200 hours.
Result
At 3,247 RPM and 5 psig nominal operating pressure, the blower delivers the required 12 m³/min of free air. That flow holds the 18 m/s line velocity needed to keep barley fully entrained without abrading the kernels. Across the operating range, you'll see roughly 13 m³/min at the 2 psig low end and 10.6 m³/min if pressure climbs to 12 psig — meaning the blower self-throttles its useful output by about 18% from best to worst case. If your measured flow at 5 psig comes in 10%+ below the predicted 12 m³/min, the three most common causes are: (1) timing-gear backlash above 0.05 mm letting the lobes drift out of phase and dump air back through the inlet, (2) a clogged inlet filter pulling the inlet pressure below atmospheric and shrinking the actual mass flow, or (3) a slipping V-belt drive turning the rotors at 3,000 RPM instead of the calculated 3,247 RPM. Check the belt tension first — it's the cheapest fault to confirm.
Choosing the Wedding Rotary Blower: Pros and Cons
Rotary lobe blowers compete against centrifugal blowers and screw compressors for low-pressure air duty. Each technology owns a different region of the pressure-flow map, and picking the wrong one will cost you either capital cost, energy efficiency, or oil-contamination headaches. Here's how they stack up on the dimensions that actually matter when you're sizing a job.
| Property | Twin-Lobe Rotary Blower | Multistage Centrifugal Blower | Oil-Free Screw Compressor |
|---|---|---|---|
| Pressure range | 0.3-15 psig | 1-25 psig | 30-150 psig |
| Flow range (typical) | 50-5,000 CFM | 500-50,000 CFM | 100-3,000 CFM |
| Volumetric efficiency at design point | 85-92% | 70-78% | 88-95% |
| Capital cost (per CFM at 7 psig) | $30-50 USD | $80-120 USD | $150-220 USD |
| Behaviour with pressure swings | Constant flow, motor amps rise | Flow drops sharply (surge risk) | Constant flow, holds well |
| Bearing/seal service interval | 20,000-40,000 hr | 30,000-60,000 hr | 8,000-16,000 hr |
| Discharge pulsation | High (needs silencer) | Smooth | Smooth |
| Best application fit | Aeration, conveying, PM samplers | Large WWTP aeration >5,000 CFM | Process air >30 psig |
Frequently Asked Questions About Wedding Rotary Blower
Discharge temperature on a lobe blower is dominated by inlet temperature plus the heat-of-compression rise across the unit, which scales with pressure ratio. If pressure and flow look right but temperature is high, the most likely cause is a hot recirculation loop — air leaking from discharge back to inlet through a worn check valve or an undersized inlet filter pulling vacuum and dropping inlet density. Check inlet pressure with a manometer; if it reads more than 0.3 psi below atmospheric, replace the filter element.
Second cause is ambient air being pulled from a hot location. Mounting the blower inside a sealed acoustic enclosure with no ventilation will recirculate its own waste heat and raise inlet air to 50-60°C, which then exits the discharge at 140°C+.
Always size for actual delivered flow at your operating pressure, not nameplate CFM. Manufacturer rated CFM is the swept geometric displacement, which assumes 100% volumetric efficiency. Real volumetric efficiency at 5 psig sits around 88%, and at 12 psig it drops to 78-80%. If you specify a 1,000 CFM nameplate blower for a 12 psig duty, you'll only get about 800 CFM into the process line.
Rule of thumb: take nameplate CFM and multiply by 0.85 for duty up to 7 psig, or 0.78 for duty between 7 and 14 psig. Build that derate into your sizing or you'll be undersized on day one.
Tri-lobe rotors produce smaller, more frequent air pulses — six per revolution versus four — which cuts pulsation amplitude by roughly 50% and discharge noise by 5-7 dB. Pick a tri-lobe when noise is a hard constraint (urban WWTP near residences, indoor process plants) or when downstream piping has resonance issues that a twin-lobe pulsation frequency excites.
Stick with twin-lobe when capital cost matters more than noise — twin-lobe units run 15-20% cheaper for the same flow and pressure, and the timing gears are simpler to service. For pneumatic conveying or aeration where a discharge silencer is already mandatory, twin-lobe is almost always the right call.
The most common explanation is that the data sheet quotes flow at standard inlet conditions (20°C, 1 atm, 36% RH) and your actual inlet is hotter, more humid, or at higher altitude. A blower at 1,500 m elevation pulls 17% less mass flow than the same unit at sea level because inlet air density is lower — even though volumetric flow at the inlet stays identical.
Convert the data sheet number to your actual site conditions before comparing. If the discrepancy stays after that correction, suspect inlet filter restriction or a leaking discharge non-return valve before assuming the blower itself is faulty. A 5 kPa pressure drop across the inlet filter alone will cost you 5% of mass flow.
Yes, and it works cleanly because lobe blowers are positive-displacement machines — flow is set by rotor speed, not by a pressure-flow curve, so they don't fight each other the way two centrifugals can. Two identical blowers in parallel will deliver almost exactly twice the single-unit flow at the same discharge pressure.
The catch: each blower needs its own check valve on the discharge to prevent reverse rotation when one is offline, and each needs its own pressure relief valve. Skip the check valves and the running blower will spin the idle one backward, which destroys the timing gears within minutes because they were never designed for reverse torque.
Because positive-displacement blowers cannot self-regulate pressure. When the discharge plugs, the rotors keep displacing air into a closed volume and pressure rises until something gives — usually the motor overload at 1.5× full-load amps, sometimes the pressure relief valve, occasionally a twisted shaft if both safeties fail.
This is why a properly sized pressure relief valve set 1 psig above operating pressure is non-negotiable on every lobe-blower installation. The relief valve should also be sized for full blower flow, not a token 10% bypass — a half-sized relief will let pressure climb past the trip point anyway. We've seen field installations where someone fitted a 1-inch relief on a 4-inch discharge line and wondered why the motor still tripped.
References & Further Reading
- Wikipedia contributors. Roots-type supercharger. Wikipedia
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