Positive Displacement Blower vs Centrifugal

Positive displacement blower vs centrifugal is a fundamental selection decision in industrial air moving. Positive displacement blowers (like roots blowers) trap fixed volumes of air and deliver constant flow regardless of pressure. Centrifugal blowers accelerate air with an impeller – flow decreases as pressure rises. The choice determines reliability, energy consumption, and process stability.

Based on field data from hundreds of installations, positive displacement blowers dominate applications where constant flow is critical – aeration, conveying, and vacuum. Centrifugal blowers dominate high-flow, low-pressure applications like ventilation. Understanding the performance characteristics of each is essential for proper selection.

This guide provides a direct comparison: operating principles, flow characteristics, efficiency, maintenance, and application suitability.

Table of Contents

• What Is the Difference Between Positive Displacement and Centrifugal Blowers?

• Working Principle Comparison

• Flow Characteristic Comparison

• Pressure Capability 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 Positive Displacement and Centrifugal Blowers?

The primary difference is operating principle and flow characteristic.

Positive Displacement Blower (Roots Blower):

• Traps fixed volume of air and moves it from inlet to discharge

• 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

• Best for: aeration, conveying, vacuum

Centrifugal Blower:

• Impeller accelerates air, converting velocity to pressure

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

• Internal compression in the impeller/diffuser

• Flow depends on system resistance curve

• Pressure: 1–12 psig (typical)

• Best for: ventilation, HVAC, combustion air

Based on field data, positive displacement blowers are used for 80% of wastewater aeration applications. Centrifugal blowers are used for ventilation and high-flow, low-pressure applications.

Working Principle Comparison

Positive Displacement 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).

Centrifugal Blower:

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. Internal compression occurs in the impeller/diffuser.

5. Smooth, continuous flow – no pulsation.

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

Comparison:

Flow Characteristic Comparison

Positive Displacement Blower:

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

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

• Flow is determined by speed, not system resistance

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

• Turndown with VFD: 30–100%

Centrifugal Blower:

• Flow decreases as pressure increases (fan laws)

• At 8 psig, flow may be 20–30% less than at 5 psig

• Flow depends on system resistance curve

• Surge limit – cannot operate below minimum flow

• Turndown with VFD: 70–100% (limited)

The key performance difference:

Pressure Capability Comparison

Positive displacement pressure capability:

• Standard three-lobe: 2–15 psig continuous

• High-pressure design: 10–25 psig

• Best efficiency: 5–10 psig

Centrifugal pressure capability:

• Single-stage: 1–10 psig

• Multistage: 5–15 psig

• Efficiency peaks at design point

The key difference: Positive displacement blowers maintain flow at higher pressure. Centrifugal blowers lose flow as pressure rises.

Efficiency Comparison

Centrifugal wins at low pressure: At 3–5 psig, centrifugal is 3–5% more efficient.

Positive displacement wins at higher pressure: Above 8 psig, positive displacement maintains efficiency while centrifugal drops.

Why centrifugal efficiency drops at high pressure: Centrifugal blowers are designed for a specific operating point. Off-design, efficiency drops. Positive displacement has flat efficiency across its pressure range.

Application Suitability

Positive Displacement Blower Best Applications:

• Wastewater aeration (diffuser fouling tolerance)

• Pneumatic conveying (constant flow needed)

• Cement plant service (dusty)

• Biogas handling (corrosive)

• Aquaculture (oil-free aeration)

• Dust collection (constant suction)

• Vacuum systems

• Where pressure varies, flow must remain constant

• Where air quality is poor (dusty)

Centrifugal Blower Best Applications:

• Ventilation (high flow, low pressure)

• HVAC systems (variable flow, low pressure)

• Combustion air (steady 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

Positive Displacement Advantages:

• Constant flow regardless of pressure

• 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

Positive Displacement Disadvantages:

• Pulsation – requires silencers

• Higher noise level

• Lower efficiency at low pressure (<3 psig)

• Larger footprint

• Higher first cost than centrifugal fans

Centrifugal Advantages:

• Smooth, pulse-free flow – no silencers

• Quieter operation

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

• Smaller footprint

• Lower first cost

• Simple construction

Centrifugal 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

Positive Displacement Problems:

Centrifugal Problems:

Selection Guide

Step 1 – Define pressure requirement.

• Above 8 psig: positive displacement likely required

• Below 5 psig: centrifugal fan possible

• Aeration with diffuser fouling: positive displacement required

Step 2 – Define flow requirement.

• Constant flow needed: positive displacement

• Variable flow acceptable: centrifugal

Step 3 – Evaluate system stability.

• Pressure varies (fouling): positive displacement

• Pressure stable: centrifugal

Step 4 – Define air quality.

• Dusty/dirty: positive displacement required

• Clean: either possible

Step 5 – Calculate lifecycle cost.

• Include purchase, energy, maintenance

Decision matrix:

Performance and Engineering Calculations

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

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

Fan Laws (Centrifugal):

• Flow ∝ RPM

• Pressure ∝ RPM²

• Power ∝ RPM³

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

Positive Displacement:

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

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

Centrifugal:

• 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)

The critical difference: The centrifugal blower saves energy but loses flow – potentially starving the biology. The positive displacement blower maintains flow but uses more power. Constant flow is more important than small efficiency differences.

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):

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.

Maintenance Comparison

Positive Displacement 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

Centrifugal 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

• Maintenance cost: $1,500–3,000/year

Frequently Asked Questions

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

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

3. Which is more efficient?
At design point, centrifugal fans are typically 2–5% more efficient. But off-design (variable pressure), positive displacement maintains efficiency while centrifugal drops. In aeration with fouling, positive displacement often has lower total energy cost because it maintains flow.

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

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

6. What is surge in a centrifugal blower?
Surge occurs when flow drops below minimum – pressure fluctuates, blower vibrates, and can be damaged. Centrifugal blowers require a minimum flow to operate stably. Positive displacement has no surge limit – it operates stably at any flow.

7. Which is quieter?
Centrifugal – typically 80–88 dBA vs 85–95 dBA for positive displacement. Centrifugal has smooth, pulse-free flow. Positive displacement has pulsation that creates noise.

8. Which has lower first cost?
Centrifugal – typically 30–50% lower first cost than positive displacement for the same capacity. But the total cost depends on energy and maintenance.

9. Which handles dust better?
Positive displacement – handles dust and debris much better than centrifugal. Centrifugal impellers can be damaged by dust erosion. In dusty applications, positive displacement is the standard.

10. Can I use VFD on both?
Yes. But turndown differs. Positive displacement: 30–100% with good efficiency. Centrifugal: 70–100% – below 70%, efficiency drops significantly. For variable flow applications, positive displacement is preferred.

11. Which is better for high pressure?
Positive displacement – operates efficiently at 5–15 psig. Centrifugal loses efficiency above 5 psig. Above 8–10 psig, centrifugal is in the stall region – very inefficient.

12. Which has lower maintenance?
Centrifugal fans have lower maintenance – bearings and belts. Positive displacement requires oil changes, seal replacement, and tip clearance measurement. But positive displacement lasts longer in dirty environments.

13. Can both be oil-free?
Positive displacement can be oil-free with lip seals or labyrinth seals. Centrifugal is oil-free by design – no lubricant in air stream.

14. Which is more reliable?
In dirty environments, positive displacement is more reliable. In clean environments, both are reliable. Positive displacement has fewer failure modes (no high-speed impeller). Centrifugal has a high-speed impeller that can fail from fatigue or imbalance.

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

Final Thoughts

After decades of specifying both positive displacement and centrifugal blowers, here is my practical advice:

Flow characteristic is the deciding factor. Positive displacement for constant flow against variable pressure (aeration, conveying, vacuum). Centrifugal for high flow at low pressure with steady conditions (ventilation, HVAC, combustion air).

Pressure is the deciding factor. Above 8 psig, positive displacement is typically the better choice. Below 3 psig with steady conditions, centrifugal is more efficient. In the 3–8 psig range, evaluate based on pressure stability.

Fouling changes everything. If pressure varies over time (diffuser fouling, filter loading), choose positive displacement. Centrifugal loses flow as pressure rises – 20–30% or more – compromising the process.

The bottom line. Positive displacement blower vs centrifugal 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.