Roots Blowers

Roots blower — the core principle

The uploaded compressor engineering manual defines a Roots blower as:

A valve-less displacement compressor without internal compression

It describes two generally symmetrical counter-rotating rotors operating inside a housing and synchronised through gears.

The authoritative Reddy Compressors blower master places rotary-lobe machines within:

Positive-displacement blowers

and says they deliver approximately fixed displacement per revolution for:

• Aeration
• Pneumatic Conveying

and other:

Low-Pressure Air / Gas Duties.

Roots blower ≠ conventional high-pressure air compressor

The useful distinction is not:

“SMALL COMPRESSOR vs BIG COMPRESSOR.”

It is:

Roots blowers belong primarily to the blower side of the air/gas system architecture.

The engineering equation

01 — roots is a positive-displacement technology

Positive-displacement blowers deliver approximately:

FIXED DISPLACEMENT PER REVOLUTION.

That is fundamentally different from a centrifugal/turbo blower, where the operating point is governed by aerodynamic performance curves.

02 — two counter-rotating rotors

The uploaded manual describes the Roots principle using:

Two counter-rotating rotors

inside a housing.

Their motion transfers pockets of air or gas from inlet to outlet.

03 — the rotors are synchronised

The rotors are synchronised through:

A SET OF GEARS.

Those remain model-specific.

04 — roots blowers do not use internal compression in the same way as screw blowers

This is one of the most important technical distinctions.

The manual describes Roots blowers as having:

NO INTERNAL COMPRESSION.

05 — pressure equalisation occurs when the trapped volume opens to the outlet

The uploaded manual explains that when the compression chamber reaches the outlet, pressure-side air flows back into the housing and the trapped gas is exposed to the discharge-side pressure.

This is why Roots machines are often referred to as:

External-compression positive-displacement blowers

06 — do not say the roots blower “compresses internally like a screw”

That would erase the defining technology difference.

Roots / rotary lobe

Transfers trapped volume and develops pressure against system backpressure.

Screw blower

Uses internal compression before discharge.

07 — flow should be defined first

Before selecting a Roots blower:

How much air or gas must move?

The answer should normally include:

• Average Flow
• Peak Flow
• Minimum Flow
• where variability matters

08 — flow reference conditions matter

Flow should be stated as:

• Actual
• Normal

or:

• Standard
• with the applicable reference temperature and pressure

Therefore:

“2000 m³/h”

by itself may be incomplete.

09 — differential pressure is a primary selection variable

In practice:

USE DIFFERENTIAL PRESSURE & INLET CONDITIONS.

This is more useful than selecting from:

• Motor kW
• or a vague discharge-pressure value alone

10 — define inlet pressure

Roots blower performance depends on the incoming condition.

Blower duty should include:

INLET PRESSURE.

11 — define outlet pressure

Note:

OUTLET PRESSURE.

Only then can the required differential pressure be correctly understood.

12 — define differential pressure

Conceptually:

Δp = outlet pressure − inlet pressure

using consistent units and conditions.

For blower selection, this is often more meaningful than discharge pressure alone.

13 — use absolute pressure for compression-ratio calculations

In practice:

USE ABSOLUTE PRESSURE FOR COMPRESSION RATIO.

This becomes especially important for vacuum service and elevation effects.

14 — gas density matters

In practice:

POWER RISES WITH PRESSURE DIFFERENTIAL & GAS DENSITY.

Therefore the actual medium and inlet conditions belong in the selection.

15 — inlet temperature matters

Performance should be corrected for:

INLET TEMPERATURE / PRESSURE.

Do not assume one catalogue flow applies unchanged at every site.

16 — altitude can matter

Changes in inlet pressure or altitude can materially affect:

Density

and:

• Mass Flow
• This is especially relevant in low-pressure applications

17 — process gas requires an additional compatibility review

Performance may need correction for:

PROCESS GAS.

If the medium is not ordinary air, additional review may be required for:

• Composition
• Materials
• Seals
• Temperature
• Hazards

18 — roots blower selection is system-resistance driven

The blower hub says the actual operating point is influenced by:

• Piping
• Diffusers
• Filters
• Valves

and:

Process Backpressure.

Therefore the blower should never be selected independently of the system it serves.

19 — system resistance can change over time

Examples can include:

• Dirty Filters
• Changing Diffuser Condition
• Longer Piping
• Valve Position
• Process Level / Depth

or:

• Product Build-Up
• depending on the application

20 — positive-displacement behaviour makes overpressure protection critical

Be aware:

Pd blowers can continue to displace flow against rising resistance

Therefore:

RELIEF & CONTROL ARE ESSENTIAL.

21 — blocked discharge is not a normal operating condition

A positive-displacement blower should not be allowed to keep building against an uncontrolled restriction.

Protection must follow the actual package/manufacturer design.

Protect the blower & system against excessive differential pressure

But exact:

• Relief Set Pressure
• Valve Size
• Discharge Route
• must come from the actual design and applicable standards

23 — do not invent a universal relief setpoint

No current source supports:

“RELIEF AT X bar.”

Keep all values model/project-specific.

24 — roots blowers can be used for aeration

Key points:

Aeration

as a common positive-displacement blower duty.

Later industry/application pages can address wastewater and aquaculture in greater detail.

25 — AERATION SELECTION SHOULD BE BASED ON THE PROCESS—not THE POND OR TANK NAME

Useful engineering inputs can include:

• Required Airflow
• Water / Process Depth
• Diffuser Pressure Loss
• Piping Pressure Loss
• Operating Variability
• Redundancy

26 — pneumatic conveying is a source-supported roots application

The uploaded compressor manual specifically identifies Roots blowers for:

PNEUMATIC CONVEYANCE.

27 — pneumatic conveying airflow must follow material & pipeline requirements

Do not select from blower name alone.

Actual conveying design can depend on:

• Material
• Throughput
• Pipe Size
• Pipeline Length
• Vertical Lift
• Bends
• Pickup Velocity
• Pressure Loss
• and other project parameters
• Where source data are absent, keep them project-specific

28 — roots blowers can also operate in vacuum duty

The uploaded manual says Roots blowers are frequently used as:

VACUUM PUMPS.

But pressure service and vacuum service are not interchangeable specifications.

29 — vacuum duty needs its own operating review

A machine configured for suction/vacuum operation may have different:

Cooling

and:

• Operating Limits
• than positive-pressure service

30 — do not use pressure-duty ratings for vacuum duty without OEM confirmation

Same machine family:

Does not automatically mean same limits

31 — pulsation is a real design consideration

In practice:

CONTROL PULSATION & NOISE.

This is central to Roots installations.

32 — silencers can form part of the acoustic system

Key points:

SILENCERS.

But their size, pressure drop and placement should be matched to the actual blower.

33 — piping layout can affect acoustic / vibration performance

Key points:

Piping layout

as a factor.

Therefore acoustic control is not only about an enclosure.

34 — flexible connections can be part of installation engineering

This includes:

Flexible connections

among factors that influence acoustic/vibration performance.

Exact connection specification remains manufacturer/project-specific.

35 — foundation design can matter

Key points:

Foundations

as part of pulsation/vibration control.

36 — do not claim roots blowers are “quiet”

The older compressed-air manual explicitly associates traditional Roots-blower operation with relatively high noise because of the external-compression principle.

Actual package noise, however, must come from current model data.

37 — do not claim one universal noise level

Noise depends on:

• Blower
• Speed
• Differential Pressure
• Silencing
• Enclosure
• Installation
• Measurement Distance / Method
• Verify supplier sound data

38 — speed control can match flow to demand

In practice:

SPEED CONTROL CAN MATCH FLOW TO DEMAND.

This makes VSD/speed control relevant for variable-flow applications.

39 — VSD is not an unlimited operating range

In practice:

• Minimum Speed
• Maximum Speed
• Cooling

and:

• System Stability
• are model-specific

Therefore:

VSD ≠ 0–100% universal turndown

40 — do not assume speed can be reduced indefinitely

Minimum speed can be constrained by:

• Cooling
• Lubrication
• Motor
• Blower
• Process Stability
• or other manufacturer limits
• Use the actual model documentation

41 — do not assume more speed is always better

Higher speed can affect:

• Flow
• Power
• Temperature
• Noise

and:

• Mechanical Limits
• Maximum operating speed is an OEM limit

DIFFERENTIAL PRESSURE + GAS DENSITY.

Therefore a motor rating without the actual blower operating condition is not enough for selection.

43 — DO NOT SELECT FROM MOTOR kW ALONE

A larger motor does not automatically mean:

• More Flow
• More Pressure

or:

• Better Efficiency
• for the required duty
• Use the blower curve and operating conditions

44 — efficiency must be compared at the real duty

In practice:

NO GENERIC EFFICIENCY CLAIM vs SCREW / TURBO BLOWERS

Use the actual:

SYSTEM DUTY + CURRENT SUPPLIER PERFORMANCE DATA.

• 45 — ROOTS vs SCREW BLOWER
• Roots / rotary lobe

Positive displacement without the same internal-compression mechanism as screw blowers.

Screw blower

Internal compression.

Screw blower benefits are application-dependent and must be compared at actual:

Differential Pressure

and:

• Volume Flow
• No universal winner

46 — ROOTS vs CENTRIFUGAL / TURBO BLOWER

ROOTS

Positive displacement, approximately fixed displacement per revolution.

Turbo / centrifugal

Dynamic compression with aerodynamic performance curves.

Compare:

• Flow
• Differential Pressure
• Turndown
• System Curve
• Energy
• Noise
• Maintenance
• Reliability

47 — ROOTS vs SIDE-CHANNEL BLOWER

Both can serve low-pressure or vacuum duties in different operating envelopes.

No generic efficiency or pressure winner should be declared here.

48 — ROOTS BLOWER vs ROTARY SCREW COMPRESSOR

These should not be selected from:

Motor size

Roots blower

Typically low differential-pressure/high-volume duty.

Rotary screw compressor

Conventional compressed-air pressure duty.

The system pressure requirement should determine which class belongs in the shortlist.

49 — roots blower ≠ “air compressor substitute” for every low-pressure application

It can be excellent where its operating envelope matches the duty.

But it should not be forced into an application because:

“The pressure is lower.”

Check:

• Flow
• Backpressure
• Temperature
• Noise
• Turndown
• Process Reliability

50 — air-cooled / oil-free language needs care

The uploaded manual says Roots blowers are:

USUALLY AIR-COOLED & OIL-FREE.

Many rotary-lobe blower architectures keep lubricant outside the conveyed-air chamber, but the exact:

• Air-Path Oil Status
• Gearbox Lubrication
• Seal Arrangement

and:

• Cooling Method
• must be verified for the actual model

51 — “oil-free air path” ≠ no oil in the package

Timing gears and bearings may require lubrication depending on the machine.

Do not write:

“Zero oil blower”

without exact supplier evidence.

52 — oil-free compression path ≠ complete air-quality specification

Even where lubricant is kept out of the air chamber, final air quality can still depend on:

• Inlet Air
• Pipework
• Process Contamination

and:

• Downstream Components
• Do not infer product-contact suitability

53 — roots blower temperature must be controlled to model limits

Differential pressure and inlet condition can affect discharge condition.

Cooling limits are model-specific.

No universal discharge-temperature limit should be published.

54 — intake condition should be protected

Where the process or environment contains:

• Dust
• Solids
• or other contamination, inlet protection can influence reliability
• Use actual supplier/application guidance

55 — process gas needs a different safety gate

If a Roots blower handles anything other than ordinary air, define:

• Gas Composition
• Temperature
• Pressure
• Hazard
• Materials
• Seals

and:

Leakage Consequence.

56 — do not assume roots blowers are suitable for flammable or toxic gas

No such universal claim is supported.

Exact manufacturer approval and process-safety review are required.

57 — reliability should include the complete system

A reliable blower package can still fail the process if:

• Diffusers Block
• Filters Restrict
• Piping Leaks
• Relief Opens
• Control Logic Fails

or:

• Power Is Lost
• The complete application should be engineered

58 — redundancy should follow process consequence

For aeration or production-critical conveying, ask:

What happens if one blower stops?

The answer determines whether the project needs:

• Standby Blower
• Shared Capacity
• Backup Power
• Emergency Air
• or another continuity strategy
• No universal N+1 rule is supported

59 — maintenance should follow model & duty

Seal Replacement Hours.

Use OEM documentation.

60 — inspection should be condition-led where practical

Useful observations can include:

• Noise Change
• Vibration
• Temperature
• Pressure
• Flow
• Power
• Oil / Gearbox Condition
• where the actual package allows monitoring
• Exact limits remain supplier-specific

61 — current reddy commercial capability remains evidence-gated

The controlled master classifies positive-displacement blowers as:

SOURCED ON REQUEST / SUPPLIER CONFIRMATION REQUIRED.

without evidence.

62 — roots blower model data required before product publication

Verify:

Manufacturer

Exact Model

Rotary-Lobe / Roots Architecture

Number / Form of Lobes if Published

Flow Basis

Rated Flow

Inlet Pressure

Outlet Pressure

Maximum Differential Pressure

Vacuum Rating if Applicable

Inlet Temperature Limits

Discharge Temperature Limits

Speed Range

Motor Rating

Input Power

VSD Compatibility

Cooling Arrangement

Lubrication Boundary

Relief Requirement

Silencer Arrangement

Sound Data

Connection Sizes

Dimensions / Weight

Maintenance Requirements

Current Availability

Reddy Capability / Relationship State

No family value should be inferred from another model.

63 — customer data to request

For a Roots blower enquiry:

Application

Air or Process Gas?

Average Flow

Peak Flow

Minimum Flow

Actual / Normal / Standard Flow Basis

Inlet Pressure

Outlet Pressure

Differential Pressure

Positive Pressure or Vacuum?

Inlet Temperature

Altitude if Relevant

System Resistance / Backpressure

Piping Details

Diffuser / Filter / Process Losses

Operating Hours

Required Turndown

Noise Requirement

Oil-Contact Requirement

Contamination / Dust

Existing Blower

Redundancy Requirement

Future Expansion

64 — roots blower selection gate

What flow?

Define conditions.

What inlet pressure?

Absolute where required.

What outlet pressure?

Define it.

What differential pressure?

Primary selection variable.

What medium?

Air or process gas?

What system resistance?

Build the real duty point.

Pressure or vacuum?

Do not interchange ratings.

What speed range?

OEM-specific.

What turndown?

Verify approved range.

How is overpressure protected?

Relief/control essential.

How will pulsation & noise be managed?

Silencers + installation engineering.

What cooling is required?

Model-specific.

What happens if the blower fails?

Define continuity.

Only then select the blower.

Controlled roots blower map

Roots blowers in one line

A roots blower is a positive-displacement rotary-lobe machine that transfers approximately fixed volume per revolution without the same internal-compression process used by screw blowers

And it should be selected from flow, inlet condition, differential pressure, system resistance, speed range, overpressure protection, noise / pulsation and real process duty

Closing

• Roots blower selection is not
• “How many HP?”

or:

“How many CFM?”

It is:

How much flow

• Through what system
• Against what differential pressure
• At what inlet condition
• For what process

Across what operating range?

The uploaded compressed-air engineering manual gives the core technology definition: a Roots blower is a valve-less displacement compressor without internal compression; two synchronised counter-rotating rotors operate in a housing, and these machines are used principally in low-pressure service including vacuum and pneumatic conveying.

The practical selection discipline: rotary-lobe positive-displacement blowers deliver approximately fixed displacement per revolution, and selection must account for differential pressure, inlet conditions and gas density; overpressure protection is essential because the machine can continue displacing flow against rising system resistance.

And the wider blower framework requires the machine to be selected against the full system: piping, diffusers, filters, valves and process backpressure determine the operating point, while whole-lifecycle evaluation should include efficiency, controls, maintenance, noise, heat and process reliability.

Reddy compressors

• Compressed air & process air. engineered for the actual duty
• Define the flow
• Define the inlet condition
• Define the differential pressure
• Build the system resistance
• Verify pressure or vacuum duty
• Verify the speed range
• Protect against overpressure
• Control pulsation & noise
• Verify cooling
• Verify the medium & contamination
• Plan redundancy
• Compare actual duty performance
• Select from verified model data
• Primary CTA

Request a Roots Blower Application Review

Secondary CTAs

Submit My Flow & Differential Pressure

Review Pressure vs Vacuum Duty

Compare Roots vs Screw Blower

Review Aeration / Pneumatic Conveying Duty

Verify Roots Blower Sourcing & Availability