What Is the Difference Between Roots Blower and Compressor

The difference between a roots blower and a compressor comes down to internal compression. A roots blower has no internal compression – it traps a fixed volume of air and moves it from inlet to discharge. A compressor has internal compression – it reduces the volume of air, increasing pressure before discharge. This fundamental difference affects pressure capability, efficiency, cost, and application suitability.

Based on commissioning experience across hundreds of installations, understanding this distinction is essential for proper equipment selection. Roots blowers are for low-pressure (2–15 psig) applications where constant flow is critical. Compressors are for high-pressure (15+ psig) applications where efficiency matters.

This guide explains the engineering differences, performance characteristics, and selection criteria. Use it to choose the right equipment.

Table of Contents

• What Is the Difference Between Roots Blower and Compressor?

• How Roots Blowers Work

• How Compressors Work

• Internal Compression – The Key Difference

• Pressure Capability Comparison

• Efficiency Comparison

• Application Suitability

• Cost Comparison

• Maintenance Comparison

• Selection Guide

• Frequently Asked Questions

• Final Thoughts

What Is the Difference Between Roots Blower and Compressor?

The primary difference is internal compression.

Roots blower:

• No internal compression – volume is constant

• Traps air at inlet pressure, carries it to discharge

• Backflow at discharge creates pressure equalization

• Pressure is created by system resistance

• Best for low pressure (2–15 psig)

• Constant flow characteristic

Compressor:

• Has internal compression – volume decreases

• Reduces volume, increasing pressure before discharge

• Smooth discharge – no backflow

• Pressure is created by compression

• Best for high pressure (15+ psig)

• Higher efficiency at high pressure

Based on field data, roots blowers are used for 80% of wastewater aeration applications. Compressors are used for high-pressure air systems, gas compression, and industrial air supply.

How Roots Blowers Work

Step 1 – Air intake. Motor turns drive shaft. Timing gears force both rotors to spin at identical speed in opposite directions. As a lobe passes the inlet port, the cavity opens to atmosphere. Air fills this space.

Step 2 – Trapping and transport. The rotor continues turning, sealing the cavity against the casing wall. The trapped air is carried toward the discharge port at inlet pressure.

Step 3 – Discharge and backflow. When the cavity reaches the discharge port, it opens to higher pressure. The rotor does not compress the air. Higher-pressure air from the discharge side backflows into the lobe cavity until pressures equalize. This takes milliseconds.

Step 4 – Pushing the volume. The rotor finishes rotation and pushes the volume out. The cycle repeats.

Key characteristic: The volume of the trapped air does not change during the cycle. No internal compression.

How Compressors Work

Screw compressor example:

Step 1 – Air intake. Air enters the inlet of the screw compressor as the male and female rotors rotate.

Step 2 – Trapping. Air is trapped between the rotors and the casing as the rotors mesh.

Step 3 – Compression. As the rotors continue to rotate, the trapped volume decreases – the air is compressed internally. Pressure rises as the volume decreases.

Step 4 – Discharge. When the trapped volume reaches the discharge port, the compressed air is discharged at system pressure.

Key characteristic: The volume of the trapped air decreases during the cycle. Internal compression occurs.

Comparison:

Internal Compression – The Key Difference

What is internal compression?
Internal compression means the volume of trapped air is reduced before it is discharged. This increases pressure. The compression ratio is determined by the rotor geometry and discharge port position.

Why internal compression matters:

For roots blowers (no internal compression):

• Air is discharged at system pressure

• Backflow occurs – higher-pressure air enters the cavity

• Backflow creates pulsation and energy loss

• Efficiency drops at high pressure

For compressors (internal compression):

• Air is compressed before discharge

• No backflow – smooth discharge

• Higher efficiency at high pressure

• Compression ratio is fixed by design

The efficiency difference:
At 8 psig, roots blowers are 72–78% efficient. Screw compressors are 68–72% (oil-free) – roots is slightly better. At 15 psig, roots drops to 65–72%, screw is 75–80% – screw is better. At 20 psig, roots is 60–68%, screw is 76–82% – screw is significantly better.

Pressure Capability Comparison

Roots blower pressure capability:

• Standard three-lobe: 2–15 psig continuous

• High-pressure design: 10–25 psig

• Above 15 psig: efficiency drops, temperature rises

• Above 25 psig: screw compressor is better

Compressor pressure capability:

• Screw: 15–150 psig (oil-free), 15–200+ psig (oil-flooded)

• Centrifugal: 5–150 psig (multistage)

• Reciprocating: 15–1,000+ psig

Efficiency Comparison

Crossover point: 10–12 psig.

• Below 10 psig: roots is more efficient

• 10–12 psig: similar efficiency

• Above 12 psig: screw is more efficient

Why roots wins at low pressure:
No internal compression means no fixed compression ratio. Roots operates efficiently across a wide pressure range. Screw has a fixed compression ratio – if operating below design pressure, it over-compresses and wastes energy.

Why screw wins at high pressure:
Internal compression means less backflow loss. At high pressure, roots backflow loss is significant. Screw internal compression becomes more efficient.

Application Suitability

Roots blower applications:

• Wastewater aeration (6–10 psig)

• Pneumatic conveying (8–15 psig)

• Biogas handling (3–10 psig)

• Aquaculture (2–5 psig)

• Vacuum systems (5–18 inches Hg)

• Dust collection (vacuum)

• Cement plant (10–15 psig)

• Where constant flow is critical

Compressor applications:

• Industrial compressed air (100–150 psig)

• Nitrogen generation

• High-pressure pneumatic conveying (>15 psig)

• Gas compression

• Refrigeration

• Pipeline gas

• Where high pressure is required

Decision factors:

Cost Comparison

Purchase cost (100 HP class, 2026 pricing):

10-year total cost (500 ACFM, 8,000 hours/year, $0.10/kWh):

At 8 psig:

• Roots: $20,000 + $155,200 + $30,000 = $205,200

• Screw: $45,000 + $168,800 + $75,000 = $288,800

• Roots saves $83,600

At 15 psig:

• Roots: $20,000 + $316,800 + $30,000 = $366,800

• Screw: $45,000 + $284,000 + $75,000 = $404,000

• Roots saves $37,200

At 20 psig:

• Roots: $20,000 + $461,600 + $30,000 = $511,600

• Screw: $45,000 + $368,800 + $75,000 = $488,800

• Screw saves $22,800

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

• Maintenance cost: $2,000–4,000/year

Screw compressor maintenance:

• Monthly: check oil level, inspect filters, log temperatures

• Quarterly: change oil, air/oil separator, filters

• Annually: bearing inspection, vibration analysis

• Major overhaul: 20,000–30,000 hours (rotors, bearings)

• Requires specialized technicians

• Maintenance cost: $5,000–10,000/year

Key difference: Roots blowers have lower maintenance frequency and cost. Screw compressors require more frequent maintenance and specialized service.

Selection Guide

Step 1 – Define pressure requirement.

• Below 10 psig: roots blower likely best

• 10–12 psig: compare both

• Above 12 psig: compressor likely best

• Above 15 psig: compressor required

Step 2 – Define flow requirement.

• Constant flow needed: roots blower

• Variable flow: both can work with VFD

Step 3 – Define air quality.

• Dusty/dirty: roots blower

• Clean: both possible

Step 4 – Calculate lifecycle cost.
Include purchase, energy, maintenance over 10 years.

Decision matrix:

Frequently Asked Questions

1. What is the main difference between a roots blower and a compressor?
The main difference is internal compression. A roots blower has no internal compression – it traps a fixed volume of air and moves it. A compressor has internal compression – it reduces the trapped volume, increasing pressure. This affects pressure capability, efficiency, and application suitability.

2. Is a roots blower a compressor?
No – a roots blower is a blower, not a compressor. Compressors have internal compression. Roots blowers do not. Roots blowers are positive displacement blowers – they move volume without compression. They are sometimes called "lobe blowers" or "Roots-type blowers."

3. Why are roots blowers more efficient at low pressure?
At low pressure, the backflow loss in roots is small. Screw compressors have a fixed compression ratio – if operating below design pressure, they over-compress and waste energy. Roots has no fixed compression ratio – efficiency is consistent across a wide pressure range.

4. Why are compressors more efficient at high pressure?
Compressors have internal compression – they reduce volume before discharge. At high pressure, roots backflow loss is significant. Compressor internal compression becomes more efficient. At 15 psig, screw compressors are 8–10% more efficient than roots blowers.

5. What pressure can a roots blower achieve?
Standard three-lobe roots blowers: 2–15 psig continuous. High-pressure designs: 10–25 psig. Above 15 psig, efficiency drops and temperature rises. Above 20 psig, screw compressors are more efficient. For continuous duty above 15 psig, consider a compressor.

6. What pressure can a compressor achieve?
Screw compressors: 15–150 psig (oil-free), 15–200+ psig (oil-flooded). Centrifugal compressors: 5–150 psig (multistage). Reciprocating compressors: 15–1,000+ psig. Compressors are used for high-pressure applications.

7. Which is more expensive – roots blower or compressor?
Compressors are 2–3× more expensive than roots blowers. Example: 100 HP roots blower $15,000–25,000. 100 HP screw compressor $35,000–60,000. But compressors are more efficient at high pressure – the higher first cost can be justified by energy savings.

8. Which has lower maintenance – roots blower or compressor?
Roots blower – lower maintenance frequency and cost. Screw compressors require more frequent maintenance (oil changes, separator replacement) and specialized technicians. Over 10 years, screw compressor maintenance is typically 2–3× higher.

9. Can a roots blower be used for high-pressure applications?
Not efficiently. Above 15 psig, efficiency drops significantly. Discharge temperature rises – 210–240°F at 15 psig, 250–280°F at 20 psig. For continuous duty above 15 psig, a compressor is usually the better choice.

10. Can a compressor be used for low-pressure applications?
Yes – but not efficiently. Screw compressors have a fixed compression ratio – if operating below design pressure, they over-compress and waste energy. At 5–8 psig, screw compressors are 3–5% less efficient than roots blowers.

11. What is internal compression?
Internal compression means the volume of trapped air is reduced before it is discharged. This increases pressure. The compression ratio is determined by rotor geometry and discharge port position. Screw compressors have internal compression. Roots blowers do not.

12. What is the crossover point for efficiency?
10–12 psig. Below 10 psig, roots is more efficient. Above 12 psig, screw is more efficient. At 10 psig, efficiency is similar – consider other factors. At 15 psig, screw advantage is 8–10%.

13. Which handles dust better – roots blower or compressor?
Roots blower. Compressors (especially screw) are sensitive to dust – dust damages rotors and bearings. Roots blowers tolerate dust – small particles pass through without damage. In dusty applications (cement, mining), roots blowers are the standard.

14. Which is quieter – roots blower or compressor?
Screw compressor – typically 82–90 dBA vs 85–95 dBA for roots blowers. Screw compressors have smooth, pulse-free flow. Roots blowers have pulsation (even with 3-lobe) that creates noise. For noise-sensitive installations, compressors have an advantage.

15. How do I choose between roots blower and compressor?
Choose roots blower for: below 10 psig, dusty air, constant flow required, simple maintenance, lower first cost. Choose compressor for: above 15 psig, clean air, efficiency priority, higher pressure capability. At 10–12 psig, compare lifecycle cost.

Final Thoughts

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

The difference is internal compression. Roots blowers have no internal compression – they trap a fixed volume and move it. Compressors have internal compression – they reduce volume, increasing pressure. This is the fundamental engineering difference.

Pressure determines the choice. Below 10 psig, choose roots. Above 15 psig, choose compressor. At 10–12 psig, compare lifecycle cost. The crossover point is where efficiency and cost meet.

Dust determines the choice. If your air is dusty, choose roots. Compressors cannot tolerate dust. In dusty applications, roots blowers outlast compressors by 2–3×.

Calculate lifecycle cost. Don't just compare first cost. Calculate 10-year total cost including purchase, energy, and maintenance. At 8 psig, roots wins. At 15 psig, roots still wins for many applications due to lower purchase and maintenance. At 20 psig, compressor wins after 3–5 years.

The bottom line. The difference between a roots blower and a compressor is internal compression. This affects pressure capability, efficiency, and application suitability. Zhanggu and other manufacturers offer both technologies. Choose based on pressure, air quality, and lifecycle cost. The wrong choice costs money every year.