Roots Blower vs Turbo Blower

Roots blower vs turbo blower is a critical selection decision for industrial air applications – especially in wastewater aeration. Roots blowers are positive displacement machines that deliver constant volume regardless of pressure (2–15 psig). Turbo blowers are dynamic machines (high-speed centrifugal) that deliver high efficiency (80–85%) but lose flow as pressure rises – and require clean inlet air.

Based on field data from hundreds of wastewater plants, roots blowers remain the standard for plants under 10 MGD. Turbo blowers are gaining share in larger plants where energy savings justify higher first cost. The choice depends on plant size, air quality, maintenance capability, and budget.

This guide provides a direct comparison: efficiency, flow characteristic, maintenance, cost, and application suitability.

Table of Contents

• What Is the Difference Between Roots Blower and Turbo Blower?

• Working Principle Comparison

• Performance Characteristics Comparison

• Efficiency Comparison

• Application Suitability

• Advantages – Each Technology

• Common Problems and Troubleshooting

• Selection Guide

• Performance and Engineering Calculations

• Cost Comparison

• Maintenance Comparison

• Frequently Asked Questions

• Final Thoughts

What Is the Difference Between Roots Blower and Turbo Blower?

The primary difference is operating principle and flow characteristic.

Roots Blower:

• Positive displacement – traps fixed volume of air and moves it

• 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

• Pressure: 2–15 psig

• Efficiency: 72–78% at 8 psig

Turbo Blower (High-Speed Centrifugal):

• Dynamic machine – impeller accelerates air, converts velocity to pressure

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

• Internal compression – impeller creates pressure

• Flow depends on system resistance curve

• Pressure: 2–15 psig

• Efficiency: 80–85% at design point

Based on field data, roots blowers dominate wastewater aeration for plants under 10 MGD. Turbo blowers are more common in larger plants where energy savings justify higher first cost.

Working Principle Comparison

Roots Blower:

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

2. Rotors never contact – 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.

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

Turbo Blower:

1. High-speed impeller (10,000–30,000+ RPM) rotates.

2. Air enters at eye of impeller, is accelerated outward.

3. Velocity energy is converted to pressure in diffuser.

4. Internal compression occurs in the impeller/diffuser.

5. Smooth, continuous flow – no pulsation.

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

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

• Turndown with VFD: 30–100%

Turbo Blower:

• Flow decreases as pressure increases (fan law)

• At 8 psig, flow may be 20–30% 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

• Turndown with VFD: 50–100% (limited)

The key performance difference:

Efficiency Comparison

Roots wins at low pressure: Below 8 psig, roots and turbo are closer. Above 10 psig, turbo has efficiency advantage.

The crossover point: At 8–10 psig, turbo is 5–8% more efficient. But roots maintains flow as pressure varies – critical for aeration with diffuser fouling.

Why turbo efficiency drops off-design: Turbo blowers are designed for a specific operating point. Off-design, efficiency drops. Roots blowers have flat efficiency across their pressure range.

Application Suitability

Roots Blower Best Applications:

• Wastewater aeration (diffuser fouling tolerance) – standard for <10 MGD

• Pneumatic conveying (constant flow needed)

• Cement plant service (dusty)

• Biogas handling (corrosive)

• Aquaculture (oil-free aeration)

• Dust collection (constant suction)

• Where pressure varies, flow must remain constant

• Where air quality is poor (dusty)

Turbo Blower Best Applications:

• Wastewater aeration – large plants (>10 MGD) where energy savings matter

• Clean air applications (1-micron filtration required)

• Steady operating point

• Where efficiency is the primary criterion

• Where maintenance contract available

• New plants with clean inlet air

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

• Lower first cost

Roots Blower Disadvantages:

• Lower efficiency (72–78% vs 80–85%)

• Pulsation – requires silencers

• Higher noise level

• Larger footprint

Turbo Blower Advantages:

• Higher efficiency (80–85%)

• Smooth, pulse-free flow

• Quieter operation

• Smaller footprint

• No oil in air stream

• Lower maintenance (fewer wearing parts)

• Higher efficiency at design point

Turbo Blower Disadvantages:

• Flow drops as pressure rises – critical limitation in aeration

• Poor turndown with VFD (50–100%)

• Surge limit – minimum flow required

• Sensitive to system changes

• Clean inlet air required (1-micron + moisture removal)

• Higher first cost

• Specialized maintenance required

Common Problems and Troubleshooting

Roots Blower Problems:

Turbo Blower Problems:

Selection Guide

Step 1 – Define pressure requirement.

• 5–10 psig: both work – compare lifecycle cost

• Above 10 psig: turbo efficiency advantage

• Variable pressure: roots (constant flow)

Step 2 – Define flow requirement.

• Constant flow needed: roots

• Variable flow acceptable: turbo

Step 3 – Evaluate system stability.

• Pressure varies (fouling): roots

• Pressure stable: turbo

Step 4 – Define air quality.

• Dusty/dirty: roots required

• Clean: either possible

Step 5 – Define maintenance capability.

• In-house mechanics: roots

• Specialized service: turbo

Step 6 – Calculate lifecycle cost.

• Include purchase, energy, maintenance over 10 years

Decision matrix:

Performance and Engineering Calculations

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

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

Fan Laws (Turbo Blower):

• 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)

Turbo Blower:

• At 8 psig: flow 500 ACFM, power 80 HP (75% efficiency)

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

The critical difference: The turbo blower saves energy but loses flow – potentially starving the biology. The roots blower maintains flow but uses more power. Constant flow is more important than small efficiency differences in aeration.

Cost Comparison

Purchase Cost (100 HP class, 2026 pricing):

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

Observation: Roots total cost is lower despite lower efficiency due to lower purchase price and simpler maintenance. Turbo energy savings are offset by higher first cost and specialized maintenance.

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

• Roots maintains flow – biology is protected.

• Turbo loses flow – biology may be compromised.

• To maintain flow, turbo 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)

Turbo Blower:

• Foundation: standard mounting

• Isolation: spring or rubber mounts

• Piping: flexible connectors recommended

• Silencers: not required (smooth flow)

• Filter: 1-micron + moisture removal (critical)

• Cooling: often water-cooled or air-cooled

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

• In-house maintenance

• Maintenance cost: $2,000–4,000/year

Turbo Blower Maintenance:

• Monthly: check filters, log temperatures, vibration

• Quarterly: change filters, inspect bearings

• Annually: bearing inspection, vibration analysis

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

• Specialized technicians required

• Maintenance cost: $3,000–6,000/year

Key difference: Turbo blowers have fewer wearing parts but require specialized maintenance. Roots blowers have more wearing parts but can be maintained by in-house mechanics.

Frequently Asked Questions

1. Which is better: roots blower or turbo blower?
Depends on application. Roots blower is better for constant flow against varying pressure (aeration with diffuser fouling). Turbo blower is better for clean air at steady pressure where efficiency is the top priority. For most municipal wastewater plants under 10 MGD, roots remains the standard.

2. Which is more efficient: roots blower or turbo blower?
Turbo blower – typically 80–85% vs 72–78% for roots at 8 psig. But turbo efficiency drops off-design – roots maintains efficiency across its pressure range. At 10 psig, turbo advantage is 5–8%. At 15 psig, turbo advantage is 8–10%.

3. Which has better turndown?
Roots blower – excellent turndown from 30–100%. Turbo blower – fair turndown from 50–100%. Below 50% speed, turbo efficiency drops significantly. Roots maintains efficiency down to 30% speed.

4. What is surge in a turbo blower?
Surge occurs when flow drops below minimum – pressure fluctuates, blower vibrates, and can be damaged. Turbo blowers require a minimum flow to operate stably. Roots blowers have no surge limit – they operate stably at any flow.

5. Which handles diffuser fouling better?
Roots blower – maintains constant airflow as pressure rises. Turbo blower loses flow as pressure rises – potentially starving the biology. This is the single most important advantage of roots blowers in aeration.

6. Which has lower first cost?
Roots blower – typically $15,000–25,000 for 100 HP vs $40,000–70,000 for turbo. The first cost difference is significant – 2–3×. This is why roots blowers dominate smaller plants.

7. Which has lower maintenance?
Roots blower has lower maintenance cost ($2,000–4,000/year) and can be maintained by in-house mechanics. Turbo blower has higher maintenance ($3,000–6,000/year) and requires specialized technicians.

8. Which is quieter?
Turbo blower – typically 75–85 dBA vs 85–95 dBA for roots. Turbo blowers have smooth, pulse-free flow. Roots blowers have pulsation that creates noise.

9. Which is more reliable in dirty environments?
Roots blower – handles dust and debris much better than turbo blowers. Turbo blowers require clean inlet air (1-micron + moisture removal). In dusty applications, roots blowers are the standard.

10. What is the payback for upgrading from roots to turbo at 8 psig?
At 8 psig, turbo is 5–8% more efficient – saves $4,000–6,000/year on 100 HP. Turbo costs $25,000–45,000 more than roots. Simple payback: 5–10 years. For intermittent duty (<4,000 hours/year), payback exceeds 10 years – roots is better.

11. Can I use VFD on both?
Yes. Roots blower: excellent turndown (30–100%). Turbo blower: fair turndown (50–100%). Below 50% speed, turbo efficiency drops. For variable flow applications, roots is preferred.

12. Which has longer lifespan?
Roots blower – 60,000–100,000 hours (7–12 years). Turbo blower – 40,000–60,000 hours (5–7 years). Roots blowers last longer in dirty environments.

13. What is the ideal operating point for each?
Roots: 5–10 psig – efficiency is highest and constant. Turbo: design point – efficiency peaks at design pressure and flow. Off-design efficiency drops.

14. Can roots blower be used in large plants?
Yes – multiple roots blowers can be used in parallel. But turbo blowers are often preferred in large plants (>20 MGD) where energy savings justify higher first cost.

15. Which should I choose for my application?
Choose roots for: aeration with diffuser fouling, dirty air, variable pressure, in-house maintenance, plants under 10 MGD. Choose turbo for: clean air, steady pressure, efficiency priority, specialized maintenance available, plants over 20 MGD.

Final Thoughts

After decades of specifying both roots and turbo blowers, here is my practical advice:

Flow characteristic is the deciding factor. Roots blowers maintain constant flow as pressure rises – critical for aeration with diffuser fouling. Turbo blowers lose flow as pressure rises – potentially compromising the biology. In wastewater aeration, constant flow is more important than small efficiency differences.

Efficiency is not the only consideration. Turbo blowers are 5–8% more efficient at 8 psig. But they cost 2–3× more, require clean air, and need specialized maintenance. For most municipal plants under 10 MGD, roots blowers have lower total cost of ownership.

Air quality matters. Turbo blowers require clean inlet air – 1-micron filtration plus moisture removal. In dirty environments, roots blowers are the only option. Dust destroys turbo impellers.

The bottom line. Roots blower vs turbo blower is not just about efficiency. Flow characteristic, air quality, maintenance capability, and total cost of ownership all matter. Zhanggu and other manufacturers offer both technologies. Choose based on application, not just efficiency. The wrong choice costs money and performance.