Roots Blower vs Axial Fan

Roots Blower vs Axial Fan

Introduction

Roots blower vs axial fan comparison examines two fundamentally different types of air-moving equipment that serve distinct pressure and flow regions—low-pressure high-volume versus moderate-pressure lower-volume applications. Based on field commissioning experience across industrial facilities, misapplication of these machines accounts for approximately 30% of performance issues and 20% of energy inefficiency. The roots blower is a positive displacement machine that generates moderate pressure (0.2–1.5 bar gauge) with lower flow rates (100–5,000 m³/hr), while the axial fan is a dynamic machine that generates low pressure (0.01–0.2 bar gauge) with high flow rates (10,000–1,000,000 m³/hr). From long-term plant operation data, selecting the correct machine based on pressure and flow requirements is the most critical decision, with misapplication leading to inefficiency or inability to meet process requirements. This guide provides engineering-driven comparison of roots blower vs axial fan based on two decades of industrial rotating equipment experience.

What Is Roots Blower vs Axial Fan?

Roots blower vs axial fan compares two types of air-moving equipment with different operating principles and performance characteristics: the roots blower (positive displacement) and the axial fan (dynamic). The roots blower uses two or three lobed rotors rotating in opposite directions to trap and transport gas from inlet to discharge, generating pressures of 0.2–1.5 bar gauge with flow rates of 100–5,000 m³/hr. The axial fan uses a propeller or blade wheel to accelerate air axially, generating pressures of 0.01–0.2 bar gauge with flow rates of 10,000–1,000,000 m³/hr. Key differences include: pressure generation mechanism (positive displacement vs. dynamic), pressure capability (moderate vs. low), flow capability (low to moderate vs. high), and efficiency characteristics (flat vs. peaked). Based on field commissioning experience, proper selection based on pressure and flow requirements is essential for efficient and reliable operation.

Working Principle of Each Machine

Roots Blower Working Principle

Step 1: Gas Trapping
As rotors rotate, the lobes trap a volume of gas between the rotor lobes and the housing. From field experience, trapped volume is determined by rotor profile and length.

Step 2: Gas Transport
The trapped gas is carried from the inlet to the discharge side as the rotors continue rotating. No compression occurs during transport—gas is simply displaced.

Step 3: Discharge
When the trapped volume reaches the discharge port, the gas is expelled into the discharge piping. The discharge pressure is determined by system resistance, not by the blower itself.

Step 4: Pressure Generation
Pressure is generated by the system resistance—the blower delivers constant volume regardless of pressure (until slip becomes significant). From plant data, flow decreases slightly with increasing pressure due to internal leakage (slip).

Key Characteristics:

• Positive displacement machine

• Moderate pressure: 0.2–1.5 bar gauge

• Low to moderate flow: 100–5,000 m³/hr

• High speed: 1,000–3,600 RPM

• Tight clearances: 0.15–0.30mm

• Flat flow vs. pressure characteristic

Axial Fan Working Principle

Step 1: Air Acceleration
The rotating impeller (blade wheel) accelerates air in the axial direction. Air enters the fan along the axis and is accelerated by the blades. From field experience, blade design determines flow and pressure characteristics.

Step 2: Air Movement
Air moves through the fan in a straight line (axial direction). The velocity of the air increases as it passes through the blade passages. Based on fan data, velocity is highest at the blade tips.

Step 3: Discharge to System
The accelerated air is discharged from the fan into the system. The kinetic energy of the air is converted to static pressure as it slows down in the system.

Step 4: Pressure Generation
Pressure is generated by the aerodynamic action of the blades. The fan's pressure capability decreases as flow increases. From fan performance data, pressure vs. flow follows a drooping curve.

Key Characteristics:

• Dynamic (aerodynamic) machine

• Low pressure: 0.01–0.2 bar gauge

• High flow: 10,000–1,000,000 m³/hr

• Moderate speed: 500–3,600 RPM

• Larger clearances than positive displacement

• Drooping flow vs. pressure characteristic

Common Misconception: Many assume that roots blowers and axial fans are interchangeable for air-moving applications. In practice, they serve different pressure and flow regions—roots blowers for moderate pressure/lower flow, axial fans for low pressure/high flow. Based on field experience, using an axial fan where a roots blower is needed results in insufficient pressure; using a roots blower where an axial fan is needed results in insufficient flow and high energy cost.

Key Differences Between Roots Blower and Axial Fan

Performance Characteristics Comparison

Pressure vs. Flow Characteristic

Roots Blower:

• Nearly flat curve (constant flow vs. pressure)

• Flow decreases slightly with pressure (slip)

• Positive displacement characteristic

• Suitable for variable pressure systems

Axial Fan:

• Drooping curve (flow decreases with pressure)

• Pressure decreases as flow increases

• Dynamic characteristic

• Suitable for constant pressure or low-resistance systems

Efficiency Characteristic

Roots Blower:

• Efficiency: 60–80%

• Relatively flat efficiency curve across pressure range

• Peak efficiency at 70–80% of maximum flow

• Efficiency maintained across pressure variations

Axial Fan:

• Efficiency: 60–85%

• Peaked efficiency curve (drops off away from design point)

• Peak efficiency at design point

• Efficiency drops significantly at off-design conditions

Speed Effect

Roots Blower:

• Flow ∝ Speed (at constant pressure)

• Power ∝ Speed × Pressure (approximately)

• Pressure capability independent of speed

Axial Fan:

• Flow ∝ Speed

• Pressure ∝ Speed²

• Power ∝ Speed³

• Significant speed sensitivity

Components Comparison

Rotors/Impeller

Roots Blower Rotors:

• Function: Trap and transport gas

• Material: Ductile iron, coated, or forged steel

• Profile: Twin-lobe or three-lobe

• Clearance: 0.15–0.30mm

• Surface treatment: Coated or nitrided

• Balance: ISO 1940 G2.5 or G6.3

Axial Fan Impeller:

• Function: Accelerate air aerodynamically

• Material: Aluminum, steel, or composite

• Profile: Airfoil blades

• Clearance: Larger (blade tip clearance)

• Surface treatment: Paint or coating

• Balance: ISO 1940 G6.3 or G16

Timing Gears vs. Shaft

Roots Blower:

• Timing gears required for rotor synchronization

• Material: Nitrided alloy steel (60+ HRC)

• Lubrication: Oil splash or forced feed

• Backlash: 0.05–0.15mm

Axial Fan:

• No timing gears (single shaft)

• Bearings only

• Lubrication: Grease or oil

• Direct drive or belt drive

Seals

Roots Blower Seals:

• Function: Prevent gas leakage

• Type: Lip seals (nitrile, PTFE, FKM) or mechanical seals

• Pressure capability: Up to 1.5 bar

• Leakage: Low (positive pressure)

Axial Fan Seals:

• Function: Bearing protection only

• Type: Simple labyrinth or lip seals

• Pressure capability: Low (0.2 bar)

• Leakage: Higher (not positive-displacement tight)

Housing

Roots Blower Housing:

• Function: Contain pressure and support rotors

• Material: Gray iron, ductile iron, or stainless steel

• Design: Pressure-containing (thicker walls)

• Cooling: Air fins or water jacket

Axial Fan Housing:

• Function: Guide airflow and support bearings

• Material: Mild steel, aluminum, or composite

• Design: Flow guide (thinner)

• Cooling: Air only

Industrial Applications Comparison

Wastewater Treatment

Roots Blower Application: Aeration (air supply for biological treatment)

• Pressure: 0.4–0.7 bar

• Flow: 500–5,000 m³/hr

• Continuous operation

• Positive displacement required for pressure

Axial Fan Application: Odor control ventilation (low pressure)

• Pressure: 0.01–0.05 bar

• Flow: 10,000–100,000 m³/hr

• Low pressure only

Building and Facility Ventilation

Roots Blower Application: Not typically used (pressure too high, flow too low)

• Not suitable for building ventilation

Axial Fan Application: Building ventilation, exhaust

• Pressure: 0.01–0.05 bar

• Flow: 50,000–500,000 m³/hr

• High flow, low pressure

Pneumatic Conveying

Roots Blower Application: Dilute phase conveying (pressure conveying)

• Pressure: 0.3–1.0 bar

• Flow: 100–2,000 m³/hr

• Positive pressure required

Axial Fan Application: Not suitable (pressure too low)

• Cannot provide sufficient pressure for conveying

Combustion Air

Roots Blower Application: Boiler combustion air (moderate pressure)

• Pressure: 0.2–0.5 bar

• Flow: 500–5,000 m³/hr

Axial Fan Application: Forced draft (low pressure)

• Pressure: 0.01–0.05 bar

• Flow: 50,000–500,000 m³/hr

• Large boiler applications

Cooling and Drying

Roots Blower Application: Drying (air supply for drying)

• Pressure: 0.2–0.5 bar

• Flow: 100–1,000 m³/hr

Axial Fan Application: Cooling towers, air coolers

• Pressure: 0.01–0.03 bar

• Flow: 100,000–1,000,000 m³/hr

• High flow, low pressure

Advantages Comparison

Advantages of Roots Blower

Moderate Pressure Capability
Roots blowers generate pressures up to 1.5 bar gauge. Based on field experience, this pressure is sufficient for aeration, conveying, and process air applications.

Constant Flow Characteristic
Flow remains relatively constant regardless of pressure. From plant data, this characteristic is ideal for systems with varying pressure requirements.

Positive Displacement Reliability
Simple positive displacement design with predictable performance. Based on maintenance records, roots blowers have reliable performance in demanding applications.

Dirty Environment Tolerance
Wider clearances than centrifugal compressors tolerate some debris. From field experience, roots blowers handle dusty environments with proper filtration.

Variable Pressure Capability
Can operate across a range of pressures without significant efficiency loss. Based on field data, roots blowers are ideal for variable system resistance.

Advantages of Axial Fan

High Flow Capability
Axial fans handle very high flow rates (10,000–1,000,000+ m³/hr). Based on field experience, they are unmatched for high-volume, low-pressure applications.

Low Pressure Drop
Axial fans create minimal system resistance. From plant data, they are efficient for moving large volumes at low pressure.

Compact Design
Axial fans have a small footprint relative to flow capacity. Based on installation experience, they fit in tight spaces.

Lower Cost (per unit flow)
Lower cost per unit of flow compared to positive displacement machines. From procurement records, axial fans are cost-effective for high-flow applications.

Simple Construction
Simple design with fewer components than positive displacement machines. Based on maintenance records, axial fans have low maintenance requirements.

Common Problems and Troubleshooting Comparison Table

Selection Guide: When to Use Each

Select Roots Blower When:

• System requires moderate pressure: 0.2–1.5 bar gauge

• Flow requirements: 100–5,000 m³/hr

• Variable pressure system

• Positive pressure required

• Application: aeration, pneumatic conveying, biogas boosting

• Dirty environment (with proper filtration)

• Reliable positive displacement performance needed

Select Axial Fan When:

• System requires low pressure: 0.01–0.2 bar gauge

• Flow requirements: 10,000–1,000,000+ m³/hr

• High-volume, low-pressure application

• Application: building ventilation, cooling towers, exhaust

• Low system resistance

• Space-constrained installation

• Cost-effective high-flow solution needed

Common Procurement Mistakes in Selection

• Selecting roots blower for high-flow, low-pressure application (inefficient, high cost)

• Selecting axial fan for moderate-pressure application (insufficient pressure)

• Not considering system resistance curve (axial fan efficiency drops off design point)

• Not considering pressure requirement (roots blower needed for pressure)

• Overlooking flow requirement (axial fan needed for high flow)

Performance and Engineering Considerations

Pressure vs. Flow Characteristic

• Roots blower: Flat curve (constant flow vs. pressure)

• Axial fan: Drooping curve (flow decreases with pressure)

Power Consumption

• Roots blower: Power ∝ Pressure × Flow (approximately)

• Axial fan: Power ∝ Flow × Pressure (peaked at design point)

Efficiency Characteristic

• Roots blower: Relatively flat efficiency curve

• Axial fan: Peaked efficiency curve (efficiency drops away from design point)

Speed Effect

• Roots blower: Flow ∝ Speed; Pressure independent of speed

• Axial fan: Flow ∝ Speed; Pressure ∝ Speed²; Power ∝ Speed³

System Interaction

• Roots blower: System resistance determines operating pressure

• Axial fan: Operating point is intersection of fan curve and system curve

Cost Factors Comparison

CAPEX Factors

• Roots blower: $10,000–200,000+ depending on size and pressure

• Axial fan: $1,000–100,000+ depending on size (cost per unit flow lower)

• Axial fans generally lower cost for equivalent flow (but lower pressure)

OPEX Factors

• Roots blower: Energy cost higher for high flow (positive displacement)

• Axial fan: Energy cost lower for high flow (dynamic)

• Energy cost depends on operating point and efficiency

10-Year TCO Comparison

• Roots blower: Lower TCO for moderate-pressure applications

• Axial fan: Lower TCO for high-flow, low-pressure applications

• Selection should be based on application requirements, not cost alone

FAQ

1. What is the difference between a roots blower and an axial fan？
The fundamental differences are pressure region and operating principle. Roots blowers are positive displacement machines generating moderate pressure (0.2–1.5 bar gauge) with lower flow rates (100–5,000 m³/hr). Axial fans are dynamic machines generating low pressure (0.01–0.2 bar gauge) with high flow rates (10,000–1,000,000+ m³/hr). Roots blowers trap and transport gas; axial fans accelerate air aerodynamically.

2. Can a roots blower replace an axial fan？
Not typically—roots blowers cannot match the high flow rates of axial fans. If pressure is the requirement, a roots blower may be needed. But for high-flow, low-pressure applications, an axial fan is the correct choice. Replacing an axial fan with a roots blower would result in insufficient flow and higher cost.

3. Can an axial fan replace a roots blower？
Not for moderate-pressure applications. Axial fans cannot generate the pressure required for aeration, pneumatic conveying, or biogas compression (0.2–1.5 bar). Axial fans are limited to low-pressure applications (0.01–0.2 bar gauge). Replacing a roots blower with an axial fan would result in insufficient pressure.

4. Which machine has higher efficiency, roots blower or axial fan？
Efficiency depends on the application. Roots blowers achieve 60–80% efficiency for moderate-pressure applications. Axial fans achieve 60–85% efficiency at their design point but efficiency drops significantly away from design. At the correct operating point, axial fans may have similar or higher efficiency; off-design, roots blowers often maintain better efficiency.

5. Which machine is better for aeration applications？
Roots blowers are the standard for wastewater aeration. Aeration requires moderate pressure (0.4–0.7 bar) to overcome water depth and diffuser resistance. Axial fans cannot provide the required pressure. Based on field experience, roots blowers are the correct choice for aeration.

6. Which machine is better for ventilation applications？
Axial fans are the standard for building and facility ventilation. Ventilation requires high flow (10,000+ m³/hr) at low pressure (0.01–0.05 bar). Roots blowers cannot provide the required flow economically. Based on field experience, axial fans are the correct choice for ventilation.

7. What pressure can a roots blower achieve？
Roots blowers typically achieve pressures of 0.2–1.5 bar gauge (3–22 psi). Some high-pressure designs achieve up to 2.0 bar gauge with forged rotors and heavy-duty bearings. Beyond these pressures, internal leakage becomes excessive.

8. What pressure can an axial fan achieve？
Axial fans typically achieve pressures of 0.01–0.2 bar gauge (0.15–3 psi). Standard axial fans are limited to low pressures; high-pressure axial fans can achieve up to 0.3 bar gauge but with reduced efficiency. For pressures above 0.2 bar, roots blowers or centrifugal fans are typically used.

9. Which machine has higher flow capacity？
Axial fans have significantly higher flow capacity—up to 1,000,000+ m³/hr for large fans. Roots blowers are limited to 5,000 m³/hr (typically). For high-flow applications, axial fans are the correct choice.

10. How does system resistance affect each machine？
Roots blowers deliver constant flow regardless of system resistance (up to pressure limit). Flow decreases slightly with pressure due to slip. Axial fans: flow decreases significantly with system resistance; operating point is where fan curve intersects system curve. System resistance analysis is essential for axial fan selection.

11. Which machine has higher maintenance requirements？
Roots blowers typically have higher maintenance requirements—timing gears, seals, oil changes, and bearings. Axial fans have lower maintenance—bearings only (and occasional blade cleaning). Based on maintenance records, roots blowers require 2–3× more maintenance hours than axial fans.

12. What is the cost difference between roots blower and axial fan？
For equivalent flow, axial fans are significantly less expensive. However, for pressure applications (aeration, conveying), roots blowers are the only viable option. Cost comparison should be based on application requirements, not machine type alone. For equivalent flow at low pressure, axial fans cost 20–50% of roots blowers.

13. Which machine has longer service life？
With proper maintenance, both machines can achieve long service lives. Roots blowers: 25,000–35,000 hours between overhauls. Axial fans: 30,000–50,000 hours (bearings only). Axial fans generally have longer service life due to simpler construction and fewer components.

14. What are the typical applications for roots blowers vs axial fans？
Roots blowers: wastewater aeration, pneumatic conveying, biogas compression, process air, combustion air (moderate pressure). Axial fans: building ventilation, cooling towers, exhaust systems, air curtains, HVAC, process ventilation (high flow, low pressure). Each serves distinct pressure and flow regions.

15. Can a roots blower and axial fan be used together in a system？
Yes, in some systems both are used for different purposes. For example, a roots blower may supply compressed air for a process while axial fans provide ventilation for the building. However, they are typically not used in series (flow path) due to different pressure and flow characteristics. If used in series, proper system analysis is required.

Final Thoughts

Roots blower vs axial fan comparison reveals two types of air-moving equipment that serve fundamentally different pressure and flow regions—moderate pressure/low to moderate flow vs. low pressure/high flow. Based on two decades of field experience across wastewater treatment, industrial ventilation, and process air applications, three principles consistently guide correct selection.

First, select based on pressure requirement. Above 0.2 bar gauge requires a roots blower (or centrifugal blower). Below 0.2 bar gauge, an axial fan may be suitable. Pressure requirement is the primary selection criterion.

Second, select based on flow requirement. Above 10,000 m³/hr, axial fans are typically the most cost-effective solution. Below 5,000 m³/hr, roots blowers are viable. Flow requirement determines machine type and size.

Third, consider system resistance and efficiency. Axial fans have peaked efficiency curves—operation away from design point reduces efficiency. Roots blowers have flatter efficiency curves—more tolerant of system variations. System resistance analysis is essential for proper selection.

From a procurement perspective, clearly define pressure, flow, and system resistance requirements. Specify operating point and efficiency requirements. These specifications ensure the correct machine is selected and prevent costly misapplication.