Lobe Blowers (Roots Blowers)
Definition
A lobe blower, commonly referred to as a Roots blower, is a positive displacement machine that moves air or gas using two or more meshing lobed rotors rotating in opposite directions without internal compression.
Physical Mechanism
Gas is trapped in pockets between the lobes and casing and carried from inlet to outlet. Compression occurs externally when the discharged gas meets system backpressure.
Because there is no internal compression, flow is inherently pulsating, and vibration is strongly influenced by:
- Rotor passing events
- Clearance between lobes and casing
- Discharge pressure fluctuations
- Mechanical timing between rotors
Signal Signature
Lobe blowers typically exhibit distinct vibration characteristics:
- Strong lobe pass frequency (LPF) component
- Harmonics of rotor passing frequency
- High levels of low-frequency pulsation energy
- Modulation of running speed harmonics
- Possible sidebands around lobe-related frequencies
f_{LPF} = Z \cdot f_r
Where:
- ( Z ) = number of lobes per rotor
- ( f_r ) = rotational frequency
Diagnostic Relevance
Vibration in Roots blowers is primarily influenced by airflow pulsation and rotor interaction, making them sensitive to:
- Rotor timing errors (gear synchronization issues)
- Excessive clearance between lobes and casing
- Bearing wear or misalignment
- Outlet pressure restrictions or system backpressure
- Pulsation resonance in connected piping
Interpretation Notes
- Elevated lobe pass frequency is normal but should remain stable under steady operating conditions.
- Increasing harmonic content often indicates wear, clearance changes, or timing issues.
- Pressure system interaction can significantly amplify vibration even if mechanical condition is healthy.
- Do not confuse lobe pass frequency with imbalance; imbalance will present as a separate 1× running speed component.
Summary
Lobe (Roots) blowers generate vibration primarily from rotor passing events and gas pulsation effects. Their vibration signature is dominated by lobe pass frequency and its harmonics, and condition assessment relies heavily on distinguishing mechanical rotor issues from system-induced pulsation effects.