Roots Blower vs Centrifugal Fan

Roots blower vs centrifugal fan is a fundamental selection decision in industrial air moving applications. Both move air, but their operating principles are completely different – and the wrong choice costs energy and reliability. A roots blower is a positive displacement machine that delivers constant volume regardless of pressure. A centrifugal fan is a dynamic machine where flow decreases as pressure rises.

Based on field data from hundreds of installations, I have seen centrifugal fans selected for applications requiring constant flow – and fail. I have seen roots blowers selected for applications requiring high flow at low pressure – and waste energy. Understanding the performance characteristics of each is essential.

This guide provides a direct comparison: flow vs pressure, efficiency, maintenance, and application suitability.

Table of Contents

• What Is the Difference Between Roots Blower and Centrifugal Fan?

• Working Principle Comparison

• Performance Characteristics Comparison

• Performance Comparison Table

• Application Suitability

• Advantages – Each Technology

• Common Problems and Troubleshooting

• Selection Guide

• Performance and Engineering Calculations

• Cost Comparison

• Installation Considerations

• Maintenance Comparison

• Frequently Asked Questions

• Final Thoughts

What Is the Difference Between Roots Blower and Centrifugal Fan?

A roots blower and a centrifugal fan are fundamentally different machines with different operating principles.

Roots Blower:

• Positive displacement machine – traps fixed volume of air and pushes it out.

• Constant volume – delivers same ACFM regardless of pressure (within range).

• No internal compression – air is discharged at system pressure.

• Flow is determined by speed, not system resistance.

• Efficiency relatively constant across pressure range.

Centrifugal Fan:

• Dynamic machine – impeller accelerates air, converting velocity to pressure.

• Variable volume – flow decreases as system pressure increases (fan laws).

• Pressure is generated by impeller speed and design.

• Flow depends on system resistance curve.

• Efficiency peaks at design point, drops off-pressure.

The key difference: A roots blower is a constant volume machine. A centrifugal fan is a variable volume machine. In aeration applications where diffusers foul and pressure rises, a roots blower maintains flow – a centrifugal fan loses flow.

Based on plant data, this difference explains why roots blowers dominate wastewater aeration and centrifugal fans dominate ventilation.

Working Principle Comparison

Roots Blower:

1. Two rotors (lobes) rotate in opposite directions, synchronized by timing gears.

2. Rotors never contact each other or casing – tip clearance seals.

3. Air is trapped at inlet pressure and carried to discharge.

4. No internal compression – air is discharged at system pressure.

5. Backflow from discharge side creates pulsation and noise.

6. Flow is proportional to speed (flow ∝ RPM).

Centrifugal Fan:

1. Impeller rotates at high speed, accelerating air outward.

2. Air enters at eye of impeller, exits at periphery.

3. Velocity energy is converted to pressure in the scroll housing.

4. Flow follows fan laws: flow ∝ RPM, pressure ∝ RPM², power ∝ RPM³.

5. Flow decreases as system pressure increases.

6. Smooth, continuous flow – no pulsation.

Performance Characteristics Comparison

Roots Blower:

• Flow is constant regardless of pressure (2–15 psig range).

• At 8 psig, flow drops only 2–3% from 5 psig (slipback).

• Power increases linearly with pressure.

• Efficiency 72–78% across 5–10 psig range.

• No surge limit – can operate at any pressure within rating.

Centrifugal Fan:

• Flow decreases as pressure increases (fan law).

• At 8 psig, flow may be 30–40% less than at 5 psig.

• Power increases with flow and pressure.

• Efficiency peaks at design point – drops off-design.

• Surge limit – cannot operate below minimum flow.

The key difference in performance:

Performance Comparison Table

Application Suitability

Roots Blower Best Applications:

• Wastewater aeration (diffuser fouling tolerance)

• Pneumatic conveying (constant flow needed)

• Cement plant service (dusty)

• Vacuum systems (constant vacuum)

• Biogas handling (corrosive)

• Aquaculture (oil-free aeration)

• Dust collection (constant suction)

• Where flow must remain constant as pressure varies

Centrifugal Fan Best Applications:

• Ventilation (high flow, low pressure)

• Combustion air (steady pressure)

• HVAC systems (variable flow, low pressure)

• Cooling applications (high volume)

• Air handling (clean air)

• Where flow can vary with pressure

• Where efficiency at design point is critical

Decision criteria:

Advantages – Each Technology

Roots Blower Advantages:

• Constant flow regardless of pressure – critical for aeration

• Excellent VFD turndown (30–100%)

• High dust tolerance – handles dirty air

• No surge limit – stable operation

• Simple maintenance – in-house mechanics

• Handles liquids and debris

• Longer lifespan in dirty service

Roots Blower Disadvantages:

• Pulsation – requires silencers

• Higher noise level

• Lower efficiency at low pressure (<3 psig)

• Larger footprint for same capacity

• Higher first cost than centrifugal fans

Centrifugal Fan Advantages:

• Smooth, pulse-free flow – no silencers

• Quieter operation

• Higher efficiency at design point (75–80%)

• Smaller footprint

• Lower first cost

• Simple construction

Centrifugal Fan Disadvantages:

• Flow drops as pressure rises – critical limitation

• Poor turndown with VFD (70–100%)

• Surge limit – cannot operate below minimum flow

• Sensitive to system changes

• Dust damages impeller

• Efficiency drops off-design

Common Problems and Troubleshooting

Roots Blower Problems:

Centrifugal Fan Problems:

Selection Guide

Step 1 – Define pressure requirement.

• Above 5 psig: roots blower likely required

• Below 5 psig: centrifugal fan possible

• Aeration with diffuser fouling: roots required

Step 2 – Define flow requirement.

• Constant flow needed: roots blower

• Variable flow acceptable: centrifugal fan

Step 3 – Evaluate system stability.

• Pressure varies (fouling): roots blower

• Pressure stable: centrifugal fan

Step 4 – Define air quality.

• Dusty/dirty: roots required

• Clean: either possible

Step 5 – Calculate lifecycle cost.

• Include purchase, energy, maintenance

Decision matrix:

Performance and Engineering Calculations

Roots Blower Power:
BHP = (ACFM × psig) / (229 × ηmechanical × ηmotor)
ηmechanical = 0.85–0.90

Centrifugal Fan Power:
BHP = (ACFM × psig) / (229 × ηmechanical × ηmotor)
ηmechanical = 0.80–0.88 (depends on design and operating point)

Fan Laws:

• Flow ∝ RPM

• Pressure ∝ RPM²

• Power ∝ RPM³

Example – Aeration Application:
500 ACFM at 8 psig. Diffuser fouling increases pressure to 10 psig over 18 months.

Roots Blower:

• At 8 psig: flow 500 ACFM, power 85 HP

• At 10 psig: flow 485 ACFM (3% drop), power 106 HP (25% increase)

Centrifugal Fan:

• At 8 psig: flow 500 ACFM, power 80 HP

• At 10 psig: flow 350 ACFM (30% drop), power 65 HP (fan law: flow drops, power drops)

Observation: The centrifugal fan saves energy but loses flow – potentially starving the biology. The roots blower maintains flow but uses more power.

Cost Comparison

Purchase Cost (100 HP class, 2026 pricing):

Maintenance Cost (Annual):

10-Year Total Cost (500 ACFM at 8 psig, 8,000 hours/year, $0.10/kWh):

But this assumes clean air at steady pressure. In aeration with diffuser fouling:

• Centrifugal loses flow – biology may be compromised.

• To maintain flow, centrifugal must be oversized – increasing cost.

• Or diffusers must be cleaned more frequently – increasing maintenance.

Installation Considerations

Roots Blower:

• Foundation: rigid mass 3× blower weight

• Isolation: neoprene pads

• Piping: flexible connectors within 18 inches

• Silencers: required on inlet and discharge

• Filter: 10-micron minimum (2-micron for dusty)

Centrifugal Fan:

• Foundation: standard mounting

• Isolation: spring or rubber mounts

• Piping: flexible connectors recommended

• Silencers: not required (smooth flow)

• Filter: 10-micron typical

Maintenance Comparison

Roots Blower Maintenance:

• Monthly: check oil level, listen to bearings

• Quarterly: change oil (synthetic)

• Annually: measure tip clearance, replace seals

• Major overhaul: 40,000–50,000 hours (bearings)

• Rotor replacement: 60,000–100,000 hours

Centrifugal Fan Maintenance:

• Monthly: listen to bearings, check vibration

• Quarterly: check belt tension (belt drive), grease bearings

• Annually: inspect impeller for wear, balance check

• Major overhaul: 30,000–40,000 hours (bearings, shaft)

• Impeller replacement: 50,000–80,000 hours

Frequently Asked Questions

1. Which is better: roots blower or centrifugal fan?
Depends on application. For constant flow against varying pressure (aeration, conveying), roots blower is better. For high flow at low pressure with steady conditions (ventilation, HVAC), centrifugal fan is better. Roots maintains flow as pressure rises. Centrifugal fan loses flow as pressure rises – critical difference.

2. Why do roots blowers dominate wastewater aeration?
Because diffusers foul over time, increasing backpressure. A roots blower maintains constant airflow – the biology needs constant oxygen. A centrifugal fan loses flow as pressure rises – potentially starving the biology. In aeration, constant flow is more important than efficiency.

3. Which is more efficient: roots blower or centrifugal fan?
At design point, centrifugal fans are typically 2–5% more efficient. But off-design (variable pressure), roots blowers maintain efficiency while centrifugal fans drop. In aeration with fouling, roots blowers often have lower total energy cost because they maintain flow.

4. Can a centrifugal fan be used for pneumatic conveying?
Not recommended. Pneumatic conveying requires constant airflow to keep material suspended. A centrifugal fan loses flow as pressure rises – material drops out and plugs the line. Roots blowers are the standard for pneumatic conveying.

5. Which has better turndown with VFD?
Roots blower – excellent turndown from 30–100%. Centrifugal fan – poor turndown from 70–100%. Below 70% speed, centrifugal fan efficiency drops significantly. Roots maintains efficiency down to 30% speed.

6. What is surge in a centrifugal fan?
Surge occurs when flow drops below minimum – pressure fluctuates, fan vibrates, and can be damaged. Centrifugal fans require a minimum flow to operate stably. Roots blowers have no surge limit – they operate stably at any flow.

7. Which is quieter?
Centrifugal fan – typically 80–88 dBA vs 85–95 dBA for roots blowers. Centrifugal fans have smooth, pulse-free flow. Roots blowers have pulsation that creates noise. For noise-sensitive installations, centrifugal fans have an advantage.

8. Which has lower first cost?
Centrifugal fan – typically 30–50% lower first cost than roots blowers for the same capacity. But the total cost depends on energy and maintenance. For constant pressure applications, centrifugal fans may be lower total cost. For variable pressure, roots blowers may be lower.

9. Which handles dust better?
Roots blower – handles dust and debris much better than centrifugal fans. Centrifugal fans have high-speed impellers that can be damaged by dust erosion. In dusty applications, roots blowers are the standard.

10. Can I use VFD on both?
Yes. But turndown differs. Roots blower: 30–100% with good efficiency. Centrifugal fan: 70–100% – below 70%, efficiency drops significantly. For variable flow applications, roots blowers are preferred.

11. Which is better for high pressure?
Roots blower – operates efficiently at 5–15 psig. Centrifugal fans lose efficiency above 5 psig. Above 10 psig, centrifugal fans are in the stall region – very inefficient. For pressures above 5 psig, roots blowers are typically the better choice.

12. Which has lower maintenance?
Centrifugal fans have lower maintenance – bearings and belts. Roots blowers require oil changes, seal replacement, and tip clearance measurement. But roots blowers last longer in dirty environments. In clean environments, centrifugal fans have lower maintenance.

13. Can both be oil-free?
Roots blowers can be oil-free with lip seals or labyrinth seals. Centrifugal fans are oil-free by design – no lubricant in air stream. For food and pharmaceutical applications, centrifugal fans may be preferred for this reason.

14. Which is more reliable?
In dirty environments, roots blowers are more reliable. In clean environments, both are reliable. Roots blowers have fewer failure modes (no high-speed impeller). Centrifugal fans have high-speed impeller that can fail from fatigue or imbalance.

15. Which should I choose for my application?
Choose roots blower for: aeration, conveying, vacuum, dusty air, variable pressure, constant flow required. Choose centrifugal fan for: ventilation, HVAC, combustion air, clean air, steady pressure, high flow at low pressure, smooth flow required.

Final Thoughts

After decades of specifying both technologies, here is my practical advice:

Selection logic. Roots blower for constant flow against variable pressure (aeration, conveying, vacuum). Centrifugal fan for high flow at low pressure with steady conditions (ventilation, HVAC, combustion air). The flow characteristic difference is the key selection criterion.

Pressure is the deciding factor. Above 5 psig, roots blowers are typically the better choice. Below 3 psig with steady conditions, centrifugal fans are more efficient. In the 3–5 psig range, evaluate based on pressure stability.

Fouling changes everything. If pressure varies over time (diffuser fouling, filter loading), choose roots blower. Centrifugal fans lose flow as pressure rises. The flow loss can be 30% or more – potentially compromising the process.

Consider turndown. If your flow varies significantly, roots blower has better VFD turndown (30–100% vs 70–100%). Variable flow applications favor roots blowers.

The bottom line. Roots blower vs centrifugal fan is not a simple efficiency comparison. Flow characteristic, pressure stability, and turndown are more important than efficiency at a single point. Zhanggu and other manufacturers offer both technologies. Choose based on application characteristics, not just first cost. The wrong choice costs performance – and that is often more expensive than energy.