A rotary lobe pump carries liquid around the casing in pockets between its lobes and chamber wall. Explore that flow path and compare pump displacement, rotational speed and an entered volumetric efficiency.
Rotary Lobe Pump Displacement-Flow Calculator
Enter pump displacement per 100 revolutions, speed and a volumetric efficiency taken from an applicable curve or test. The calculator performs arithmetic only; it does not predict efficiency or select a pump.
Equation Used
- q is entered in US gallons per 100 revolutions and n in revolutions per minute.
- The entered volumetric efficiency applies to the exact operating point being considered.
- Pressure, power, NPSH, product rheology, clearances, pulsation and mechanical limits are outside the arithmetic.
Displacement multiplied by rotational speed gives theoretical flow. The entered volumetric efficiency adjusts that result for the operating point you specify; the calculator does not determine efficiency or select the pump.
Externally timed, noncontact rotors
Each rotor is mounted on its own shaft and external timing gears synchronize the shafts so the rotors do not contact. As the lobes separate at the inlet, cavity volume increases. Liquid is trapped between lobes and the close-fitting casing, carried around the chamber wall, and expelled as cavity volume decreases at the outlet. It does not travel between the rotor pair.
The animation shows that topology schematically. Clearances, lobe profile and chamber dimensions are not product geometry.
Use manufacturer data for the duty
Rotary lobe pumps are used where positive displacement, noncontact rotors and cleanable construction are useful. Actual suitability depends on the exact product, viscosity or rheology, solids, temperature, pressure, seals, materials, speed, NPSH and cleaning requirements.
Manufacturer handbook flow arithmetic
With q in US gallons per 100 revolutions and n in rpm, qn/100 is theoretical flow in gpm. Multiplying by entered volumetric efficiency gives an arithmetic comparison. Efficiency must come from the applicable pump curve or test; this calculator does not predict it.
Arithmetic example
For q=10 US gal/100 rev, n=300 rpm and entered efficiency 90%, theoretical flow is 30.0 gpm, entered-efficiency flow is 27.0 gpm and the arithmetic difference is 3.0 gpm.
Model boundary and safety
Use the displacement data for the exact rotor and pump. Rotor count alone does not establish displacement or pulsation. Slip and volumetric efficiency depend on the specific pump, clearances, liquid and differential pressure. Positive-displacement pumps require the manufacturer's pressure-protection provisions and must not be operated against a closed discharge.
Rotary lobe pump questions
Does the calculator predict slip?
No. Volumetric efficiency is entered from applicable data.
How do I account for the rotor type?
Use the displacement published for the exact pump and rotor, together with volumetric efficiency for the relevant operating point.
Can this select speed or confirm NPSH?
No. Those require the manufacturer's performance data and the complete system.
Manufacturer references
- Alfa Laval Pump Handbook: rotary-lobe displacement, speed, flow and volumetric-efficiency formulas and model-specific limits.
- Viking Pump FAQ, Lobe Pumps: externally timed noncontact rotors and the casing-wall flow path.
- Alfa Laval SRU Rotary Lobe Pump Instruction Manual: model-specific installation, operation and service requirements.
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