Why do Roots and screw blowers pulsate?
A Roots (twin- or tri-lobe) blower traps a fixed volume of gas between each lobe and the casing and carries it from inlet to discharge without internal compression. When that pocket opens to the discharge port, higher-pressure gas from the outlet flows back into it until the pressures equalise. This backflow, repeated every time a lobe passes the port, is the main source of pressure pulsation and of the characteristic blower noise.
The fundamental excitation frequency is the lobe-passing frequency: shaft speed in revolutions per second multiplied by the number of lobes on both rotors that open to the port each revolution. A two-lobe Roots blower produces four discharge pulses per revolution; a three-lobe machine produces six smaller pulses. Harmonics at two, three and four times that frequency also carry energy. Because most packages run on variable-frequency drives or belt drives with several pulley options, the excitation frequency is not one number but a band across the operating speed range.
Screw blowers compress internally before the discharge port opens, so the backflow pulse is smaller when the built-in pressure ratio matches the system pressure. When the system pressure is well above or below the design ratio, over- or under-compression returns and pulsation rises. Tri-lobe rotors, pre-inlet channels (sometimes called jet-flow or pulsation-damping slots) and helical lobes reduce pulse amplitude in Roots designs, but none of them remove the need for proper silencing and piping.
Three outcomes that must not be confused
A successful blower installation satisfies three separate requirements. They are related, but meeting one does not prove the others.
| Outcome | What it means | How it is checked | Typical mistake |
|---|---|---|---|
| Mean flow at differential pressure | The process receives the required inlet volume flow at its working pressure (for example 0.5–1.0 bar(g) / 7–15 psig for aeration and conveying) | Flow measurement or motor-power and speed check against the blower curve at actual inlet temperature | Accepting nameplate flow without correcting for hot, thin summer inlet air |
| Pulsation and mechanical response | Pressure ripple and pipe vibration stay acceptable at every speed in the operating range | Dynamic pressure transducer and vibration readings during a slow speed sweep | Testing only at one fixed speed and missing a resonance elsewhere in the VFD range |
| Airborne noise | Sound pressure levels at agreed points (operator position, boundary, neighbour) are acceptable | Sound level meter with octave or one-third-octave analysis, background recorded | A single dB(A) figure that hides a strong low-frequency tone |
A silencer can reduce radiated noise while a troublesome pressure mode remains in the discharge header. A flexible connector can cut transmitted vibration without curing an acoustic resonance. Treat each outcome as its own acceptance item.
How inlet and discharge silencers work
Blower silencers use two principles, often in the same housing.
- Reactive (chamber) silencing: expansion chambers, internal tubes and Helmholtz-type volumes reflect pressure waves back towards the source. They are effective at low frequencies, which is where the lobe-passing tone sits, and they are the core of most discharge silencers.
- Absorptive silencing: perforated tubes backed by mineral wool or similar fill convert acoustic energy into heat. They work best at mid and high frequencies and are common on inlet silencers and combined filter-silencers.
- Combination units: a reactive section to tame the fundamental tone plus an absorptive section for the broadband noise above it.
When reviewing a silencer offer, ask for insertion loss across the frequency range, not just a single dB(A) reduction, together with pressure drop at the actual flow and temperature. Every millibar of silencer loss on the discharge side is pressure the blower must generate and power the motor must supply. On the inlet side, extra loss lowers the inlet absolute pressure, reduces mass flow and raises the pressure ratio and discharge temperature.
Absorptive fill must be retained properly. Packing that breaks loose from a discharge silencer can travel downstream into diffusers, conveying lines or process vessels, and a collapsed packing also stops silencing. Hot discharge air, typically well above ambient after a blower, shortens fill life if the material is not rated for the temperature.
Piping, resonance and why pipe lengths matter
A pipe run behaves like an organ pipe. When the lobe-passing frequency, or one of its harmonics, coincides with an acoustic natural frequency of the pipe length between two reflecting points (the blower, a silencer, a tee, a closed valve, a vessel), the pressure ripple can grow far beyond the source amplitude. The quarter-wave and half-wave lengths depend on the speed of sound in the hot discharge air, which rises with temperature.
Mechanical resonance is a separate risk: a pipe span, a small-bore branch or an instrument tapping with a natural frequency near the excitation band can vibrate strongly even when the acoustic level is moderate. Small-bore connections such as gauge lines, drain points and thermowell bosses are classic fatigue locations.
Practical layout rules used on blower packages:
- Calculate the excitation band from minimum to maximum running speed, including harmonics up to at least the fourth.
- Estimate the acoustic natural frequencies of the main pipe lengths between reflection points at the expected discharge temperature, and avoid lengths that place a natural frequency inside the band.
- Fit flexible connectors on inlet and discharge so that package vibration does not drive the pipework, and anchor the pipe immediately downstream so the connector is not used as a pipe support.
- Support pipe spans so their mechanical natural frequencies sit clear of the excitation band; brace small-bore branches back to the main pipe rather than to a separate structure.
- Keep the non-return valve and safety valve close to the blower but outside the zone of strongest pulsation where the maker allows; a chattering check valve is often a pulsation symptom.
- Where a VFD must cross a resonance, use skip-frequency settings in the drive to pass through quickly rather than dwelling on it.
Symptoms, likely causes and what we check
| Symptom | Likely cause | What we check |
|---|---|---|
| Loud low-frequency hum heard far from the blower house | Lobe-passing tone escaping through the discharge pipe wall, an open inlet or a building wall | Octave-band readings near pipe and inlet, silencer condition, acoustic lagging of the discharge pipe |
| Noise rises sharply at one speed | Acoustic or mechanical resonance inside the VFD range | Speed sweep with vibration and pressure readings; pipe length and support review |
| Cracked gauge line, drain nipple or thermowell | Small-bore branch resonating with pulsation | Branch length and bracing, use of snubbers and remote-mounted gauges |
| Check valve chattering or wearing quickly | Valve sitting in strong pulsation, or oversized for the flow | Valve type and location, flow at minimum speed |
| Rising motor current with no extra process demand | Blocked inlet filter-silencer, damaged discharge silencer, fouled diffusers raising backpressure | Differential pressure across filter and silencer, discharge pressure trend |
| Fibrous material at diffusers or in conveying line | Absorptive packing breaking out of a silencer | Silencer internals, retaining mesh and temperature rating |
| High bearing vibration on the blower itself | Pipe loads on the casing, misalignment, belt problems, rotor contact | Nozzle loads, flexible connector condition, alignment, belt tension, timing-gear and bearing condition |
Acoustic enclosures and the blower house
Many modern blower packages come inside an acoustic hood with integral inlet filter-silencer and discharge silencer. The hood reduces radiated casing and motor noise, but it creates two new duties: ventilation and access. The cooling air flow through the hood must remove heat from the motor, the blower casing and the discharge pipe inside it. In Telangana and Andhra Pradesh, where summer ambient air commonly reaches 40–45 °C, and above 45 °C on open sites, a hood that was adequate in a cooler climate can overheat, push the discharge temperature towards its trip setting and shorten belt, bearing and oil life.
- Draw inlet air from the coolest, cleanest point available, away from hot exhaust and dust sources.
- Duct hot hood exhaust out of the room so it is not re-ingested.
- Check that hood louvres and the room’s openings are not themselves the dominant noise path.
- Leave access to filters, belts, oil sight glasses and the safety valve without dismantling acoustic panels.
- Clean inlet filter elements on a dust-driven schedule rather than by the calendar; local dust loads in dry months are high.
A practical method for a new or problem installation
- Define the operating envelope: minimum, normal and maximum flow; differential pressure range; inlet temperature range; speed range and drive type.
- Obtain the excitation data: lobe count, speed range and the maker’s guidance on pulsation and allowable nozzle loads.
- Specify silencers on duty, not on pipe size: actual flow, pressure, temperature, required insertion loss by frequency band, maximum pressure drop, orientation and drain points.
- Review the piping: lengths between reflection points, branch connections, supports, flexible connectors, check and relief valve positions.
- Agree measurement points: sound at operator and boundary positions, pressure ripple near the blower and at the far end of the header, vibration on the package and on pipe spans.
- Measure a baseline with the blower stopped (background noise) and then loaded across the speed range, including start, stop and unload events where safe.
- Correct and re-test: silencer change, extra support, skip frequencies, lagging, relocation of a branch. Record the before and after readings.
The full acoustic and pipe-stress design for a large or sensitive installation stays with the blower manufacturer, the silencer supplier and the plant’s piping engineer. Our part is the practical site work: getting correct data, spotting the common errors and fixing the machine and its immediate connections.
How Reddy Compressors helps
Reddy Compressors, based at Tarnaka in Secunderabad since 2002, sells new and used/refurbished blowers, rents them, services them and supplies their spares. We work with Roots, screw, centrifugal and side-channel blowers of many makes as an independent multi-brand specialist, not an authorised dealer of any brand. We have tie-ups with all major and reputed brands. Our team of more than 20 service engineers and technicians covers the whole of Telangana and Andhra Pradesh, and we supply blowers and spares across India.
- Site inspection of blower packages, silencers, flexible connectors, supports and drive arrangements.
- Diagnosis of noise, pulsation and vibration complaints, including speed sweeps on VFD-driven units.
- Replacement filter elements, silencer parts, flexible connectors, belts, bearings, seals, timing gears and safety valves, identified from the nameplate and the old part before quoting.
- Overhaul of Roots blowers and package refurbishment.
- Supply of new or refurbished blower packages sized for your actual duty, and rental blowers for shutdowns or while a unit is repaired.
- Service plans through our AMC for blower stations that run continuously.
What to send us
- Blower make, model, serial number and a clear nameplate photo.
- Duty: application, flow, discharge pressure, inlet temperature, speed or VFD range.
- Photos of the blower, silencers, flexible connectors and the first few metres of piping.
- A short description of the complaint: when it happens, at what speed, and where it is heard or felt.
- Any readings you have: discharge pressure, motor current, temperature, sound or vibration.
- Site location in Telangana, Andhra Pradesh or elsewhere in India.
Frequently asked questions
Why is my Roots blower so noisy even with a silencer fitted?
Often the silencer is working but noise escapes another way: through a thin, unlagged discharge pipe, through the inlet when the filter-silencer is damaged, or through the package casing and motor. Low-frequency lobe-passing tones also pass easily through walls. An octave-band reading at several points shows which path dominates, so the fix targets the real source rather than adding another silencer that does not help.
What is the lobe-passing frequency of a blower?
It is the rate at which rotor pockets open to the discharge port: shaft speed in revolutions per second multiplied by the number of discharge pulses per revolution. A two-lobe Roots blower gives four pulses per revolution and a three-lobe blower six. This frequency and its harmonics drive pulsation, piping vibration and the tonal noise, so it should be calculated across the whole speed range before piping is laid out.
Does a tri-lobe blower need a discharge silencer?
Yes, in almost every installation. Tri-lobe rotors produce smaller and more frequent pressure pulses than two-lobe rotors, which lowers pulsation and noise, but the discharge still carries a strong tonal component. Most makers supply or specify a discharge silencer as part of the package. Removing it to save pressure drop usually raises noise and pipe vibration to unacceptable levels.
Why does my blower pipework vibrate only at certain speeds?
At those speeds the lobe-passing frequency or one of its harmonics matches an acoustic natural frequency of a pipe length, or the mechanical natural frequency of a pipe span. Away from that speed the response falls. Solutions include extra pipe supports, bracing small branches, changing a pipe length, or programming skip frequencies in the VFD so the blower passes quickly through the problem speed band.
How much pressure drop should a blower silencer have?
There is no single figure. Silencer loss should be stated by the supplier at your actual flow, temperature and pressure, and judged against the total differential pressure of the blower. On a low-pressure duty such as aeration, a few tens of millibars is a meaningful share of the total, so it costs power continuously. Compare silencer options on insertion loss and pressure drop together.
Can a flexible connector cure blower pulsation?
No. A flexible connector isolates the pipework from package vibration and accommodates thermal movement, but it does not change the pressure pulses travelling inside the pipe. It must also be anchored on the pipe side so it is not stretched or used as a support. Pulsation is controlled by the silencer, by pipe geometry and by avoiding resonant lengths.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Blower pulsation frequency and pipe resonance check
Work out the lobe-passing frequency band over the blower’s speed range, with harmonics, and the acoustic natural frequencies of the pipe lengths between reflection points at the discharge temperature. Lengths with a mode inside a band are flagged with the shaft speed that excites them.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Discharge pulses per revolution (2-lobe Roots 4, 3-lobe 6): 4
- Minimum shaft speed (rpm): 1500
- Maximum shaft speed (rpm): 3000
- Highest harmonic to check: 4
- Gas temperature in the pipe (°C): 100
- Ratio of specific heats k (air 1.4): 1.4
- Gas constant R (J/kg·K; air 287): 287
- Pipe lengths between reflection points (m): 1.0, 2.5, 4.0
- End condition: Half-wave: both ends open or both closed
- Separation margin around each band (%): 10
- Drive frequency at maximum speed (Hz; 0 if not on a VFD): 50
Result
- Speed of sound: 387.2 m/s at 100 °C
- Fundamental (lobe-passing) band: 100–200 Hz
- Harmonic 2 band: 200–400 Hz
- Harmonic 3 band: 300–600 Hz
- Harmonic 4 band: 400–800 Hz
- Pipe length 1 m: Resonance in band first modes 193.6, 387.2, 580.8, 774.4 Hz; excited at about 1,500, 1,940, 2,180, 2,900 rpm (drive 25, 32.3, 36.3, 48.4 Hz)
- Pipe length 2.5 m: Resonance in band first modes 77.4, 154.9, 232.3, 309.8 Hz; excited at about 1,500, 1,550, 1,740, 1,940, 2,030, 2,320, 2,610, 2,710 and 2 more rpm (drive 25, 25.8, 29, 32.3, 33.9, 38.7, 43.6, 45.2 and 2 more Hz)
- Pipe length 4 m: Resonance in band first modes 48.4, 96.8, 145.2, 193.6 Hz; excited at about 1,500, 1,630, 1,690, 1,820, 1,940, 2,000, 2,180, 2,360 and 6 more rpm (drive 25, 27.2, 28.2, 30.3, 32.3, 33.3, 36.3, 39.3 and 6 more Hz)
Method. Excitation f = shaft rpm ÷ 60 × discharge pulses per revolution × harmonic number (2-lobe Roots 4 pulses, 3-lobe 6, or as the maker states), taken from minimum to maximum speed. Speed of sound c = √(k × R × T), T in kelvin. Half-wave (both ends open, or both closed): f_m = m × c ÷ (2L). Quarter-wave (one end open, one closed): f_m = (2m − 1) × c ÷ (4L). A mode within a band (widened by the separation margin) is flagged with the shaft speed that would excite it; with a VFD, the drive frequency is pro-rata to speed from the frequency at maximum speed. Screening only: the acoustic and pipe-stress design for a large installation stays with the blower maker, the silencer supplier and the piping engineer.
What to take from it. Check the whole speed band and harmonics up to at least the fourth, at the hot discharge temperature, before laying out the pipe. Where a VFD must cross a resonance, set skip frequencies so it passes through quickly.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.