The historical Fabry rotary blower described by Hiscox uses two three-toothed wheels inside a casing. Their alternating engagement separates the inlet and discharge passages. The two-lobed illustration below is a general rotary-blower comparison, not the Fabry tooth geometry. Use the calculator to compare a low-pressure power approximation for entered flow, pressure rise and efficiency.
Fabry Rotary Blower Interactive Calculator
Vary inlet flow, pressure rise and an assumed efficiency to compare approximate power. The pressure-plus-margin result is arithmetic only, not a relief-valve setting. The two-lobed diagram is a general comparison rather than the historical Fabry rotor shape.
Equation Used
The power approximation increases with flow and pressure rise and decreases with the entered efficiency. The kW tile is the same power converted from horsepower; it is not a separate motor-selection result. The pressure-plus-margin tile does not establish a permissible relief setting, which depends on the actual equipment and installation.
- Low-pressure positive displacement air blower approximation.
- Flow is inlet volumetric flow in CFM.
- Efficiency is an assumed ratio of useful air power to the input power being estimated. Separate electrical-motor and drive losses are not calculated.
- Pressure plus margin is an arithmetic comparison only; it must not be used to set a relief valve.
The historical Fabry arrangement
Hiscox identifies the Fabry blower as movement 629. Two wheels, each with three teeth, rotate in a two-part cylindrical casing. Their teeth engage alternately near the line between the shaft axes, closing the passage between the wheels as they turn.
The accompanying two-lobed comparison diagram illustrates a different rotor outline. It explains carrying pockets of air around the casing, but does not reproduce Fabry tooth contact or provide manufacturing dimensions.
What determines a working blower
Rotor shape, timing, casing clearance, seals and bearings must suit the machine and its temperature and pressure range. The calculator does not establish those dimensions, the delivered-flow curve, discharge temperature or a suitable operating speed.
Comparing air-power requirements
Modern positive-displacement blowers are used for tasks such as pneumatic bulk-material transport and wastewater aeration. AERZEN lists these uses for its current blower families. Their product-specific ratings should not be applied to the historical Fabry mechanism.
Use the calculation to compare how assumed flow, pressure rise and efficiency affect estimated power. Selecting equipment requires its performance curve and the inlet conditions and pressure losses of the actual system.
Low-pressure power approximation
For flow Q in cubic feet per minute, pressure rise Δp in psi and assumed efficiency η as a fraction, the calculator uses P = QΔp/(229η) horsepower. The constant converts psi·CFM to horsepower, rounded for this approximation. The corresponding power in kW is 0.7457P.
This treats volume flow as approximately constant through a small pressure rise. Gas compression and temperature changes limit its accuracy as the pressure ratio increases. The calculation does not separate blower, drive and motor efficiencies.
The load-rate tile is horsepower per 100 CFM at the entered pressure and efficiency. The pressure-plus-margin tile simply adds two entered numbers; it is not a valve-selection or commissioning calculation.
Example: changing flow and efficiency
Enter 100 CFM, a 1 psi pressure rise and 70% efficiency. The approximate power is 100 × 1/(229 × 0.70) = 0.624 hp, or 0.465 kW.
Doubling the flow to 200 CFM at the same pressure and efficiency doubles those values to 1.248 hp and 0.930 kW. At 100 CFM and 50% efficiency, the result is 0.873 hp, or 0.651 kW.
These are comparisons within the stated approximation, not ratings for a Fabry blower or a recommendation for a particular motor. The pressure-margin control does not change the power calculation.
What the comparison leaves out
Flow and pressure alone cannot establish machine size, allowable speed, temperature, noise, service life or power consumption across an operating range. Those quantities depend on rotor geometry, leakage, gas properties and the drive system.
The historical Fabry arrangement, a modern rotary-lobe blower and a centrifugal blower are distinct designs. Compare actual manufacturer curves at the required inlet condition and duty cycle rather than assuming one common efficiency or maintenance interval.
Do not use the pressure-plus-margin tile as an installation instruction. Pressure protection and startup procedures must follow the equipment documentation and the limits of the connected system.
Frequently asked questions
Does the two-lobed drawing show the historical Fabry rotor shape?
No. Hiscox describes the Fabry blower with two three-toothed wheels. The drawing here is a general two-lobed comparison and does not reproduce that tooth geometry.
Does the calculator predict delivered flow?
No. Flow is an input. The calculator estimates power for that entered flow, pressure rise and assumed efficiency; it does not calculate displacement or leakage.
Why do the horsepower and kW tiles move together?
They represent the same estimated power in two units. Electrical-motor efficiency and drive losses are not calculated separately.
Is pressure plus margin a recommended relief setting?
No. It is only the sum of two inputs. It does not check the equipment pressure limit or select a protective device.
Can these results establish a safe running speed or temperature?
No. The model contains no speed, heat-balance, rotor-clearance or material-strength calculation.
References
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