Why Roots Blower is Called Positive Displacement
Why Roots Blower is Called Positive Displacement
A roots blower is called positive displacement because it traps a fixed volume of air between the rotors and casing and positively moves that volume from inlet to discharge with each revolution. Regardless of discharge pressure, the blower delivers the same volume per revolution – this is the defining characteristic of positive displacement machines.
Based on field data from hundreds of installations, this fixed volume characteristic is what separates roots blowers from dynamic compressors like centrifugal fans. The blower doesn't compress air internally – it simply moves a trapped volume. This is why roots blowers maintain constant flow as pressure varies.
This guide explains what positive displacement means, how roots blowers achieve it, and why it matters for industrial applications.
Table of Contents
What Is Positive Displacement?
How Roots Blowers Achieve Positive Displacement
The Fixed Volume Per Revolution
Positive Displacement vs Dynamic Compressors
Why No Internal Compression?
Constant Volume Characteristic
Applications That Benefit
Common Misconceptions
Frequently Asked Questions
Final Thoughts
What Is Positive Displacement?
Positive displacement definition:
A positive displacement machine traps a fixed volume of fluid (gas or liquid) and physically moves it from the inlet to the discharge. Each cycle delivers the same volume regardless of discharge pressure (within limits).
Key characteristics of positive displacement:
Fixed volume per revolution or cycle
Flow is determined by speed, not pressure
Flow remains constant as pressure varies
No internal compression (in roots blowers)
Pressure is created by system resistance
Examples of positive displacement machines:
Roots blowers
Rotary screw compressors
Rotary vane pumps
Reciprocating pumps
Gear pumps
Why "positive":
The machine positively displaces the fluid – it traps a volume and forces it to move. The fluid cannot slip back (except for leakage through clearances). The machine controls the volume, not the pressure.
How Roots Blowers Achieve Positive Displacement
The mechanism:
Two synchronized rotors (lobes) rotate in opposite directions. As they turn, cavities form between the rotors and the casing. These cavities trap a fixed volume of air at the inlet port.
The trapping process:
A lobe passes the inlet port – the cavity opens to the inlet.
Air fills the cavity at inlet pressure.
The rotor continues turning – the cavity closes, trapping the air.
The trapped volume is carried to the discharge port.
The cavity opens to discharge – the air is pushed out.
Why it's positive displacement:
The volume of the cavity is fixed by the rotor geometry. Each revolution traps the same volume. The blower cannot vary the trapped volume – it is physically fixed. This is the definition of positive displacement.
The engineering detail:
The volume per revolution is determined by:
Rotor lobe geometry (2-lobe, 3-lobe, helical)
Rotor diameter
Rotor length
Casing geometry
This volume is fixed at the time of manufacture. It does not change with pressure.
The Fixed Volume Per Revolution
The trapped volume:
Each rotor lobe creates a cavity with the casing. The volume of this cavity is fixed. As the rotors turn, each lobe traps and moves a fixed volume.
Volume per revolution:
For a typical 3-lobe rotor, each revolution delivers 6 air pulses (3 lobes × 2 rotors). Each pulse has the same volume. The total volume per revolution is fixed.
The equation:
Flow = (trapped volume per revolution) × RPM
Flow is proportional to speed. Doubling speed doubles flow. Pressure does not appear in the equation – it does not affect the trapped volume.
Why this matters:
This fixed volume per revolution is the defining characteristic of positive displacement. It is why roots blowers are called positive displacement machines.
Positive Displacement vs Dynamic Compressors
| Parameter | Positive Displacement (Roots) | Dynamic (Centrifugal) |
|---|---|---|
| Volume trapping | Fixed volume trapped | No trapping – continuous flow |
| Flow vs pressure | Constant (slight slipback) | Drops as pressure rises |
| Control | Speed controls flow | Speed and system control flow |
| Internal compression | No (roots) or Yes (screw) | Yes |
| Pressure generation | System resistance | Impeller energy |
The key difference:
Positive displacement machines trap a fixed volume. Dynamic machines accelerate fluid and convert velocity to pressure. Roots blowers are positive displacement because they trap a fixed volume.
Why this matters for applications:
In aeration, as diffusers foul, pressure rises. A positive displacement roots blower maintains flow. A dynamic centrifugal blower loses flow. This is why roots blowers are preferred for aeration.
Why No Internal Compression?
Roots blowers have no internal compression:
Unlike screw compressors, roots blowers do not reduce the trapped volume. The volume trapped at the inlet is the same volume discharged.
What happens at discharge:
When the trapped volume reaches the discharge port, it opens to higher pressure. The rotor does not compress the air. Instead, higher-pressure air from the discharge side backflows into the cavity. The pressures equalize. Then the rotor pushes the volume out.
This is still positive displacement:
Even without internal compression, the blower traps a fixed volume and moves it. It is still positive displacement – it just doesn't compress internally.
Positive displacement ≠ compression:
Positive displacement means fixed volume trapping and moving. Compression is a separate process. Some positive displacement machines compress (screw compressors). Others do not (roots blowers). Both are positive displacement.
Constant Volume Characteristic
What constant volume means:
The blower delivers the same ACFM regardless of pressure (within range). Flow is determined by speed, not pressure.
The performance curve:
Flow is nearly flat across the pressure range
Flow drops only slightly at higher pressure (slipback)
The curve is almost horizontal
Slipback:
Even roots blowers have some flow drop at high pressure due to leakage through the tip clearance. This is called slipback. But the drop is small – 2–6% at 8–12 psig.
Why this is called positive displacement:
The constant volume characteristic is the hallmark of positive displacement machines. It is what separates them from dynamic machines.
Applications That Benefit
Wastewater aeration:
Diffusers foul over time – pressure rises from 6 to 9 psig. A roots blower maintains airflow. The biology continues to receive oxygen. This is the most important application for positive displacement roots blowers.
Pneumatic conveying:
Filters load – pressure rises. Roots blower maintains flow. Material stays suspended. Line plugging is prevented.
Vacuum systems:
System conditions change – vacuum varies. Roots blower maintains constant vacuum. Process stays stable.
Biogas handling:
Digester pressure fluctuates – roots blower maintains flow. Upgrading systems stay stable.
Why positive displacement matters:
In all these applications, constant flow is critical. Positive displacement delivers constant flow. Dynamic machines do not.
Common Misconceptions
Misconception 1: "Roots blowers are not positive displacement because they have no internal compression."
False. Positive displacement means trapping a fixed volume and moving it. Internal compression is a separate feature. Roots blowers trap a fixed volume – they are positive displacement.
Misconception 2: "Positive displacement means constant pressure."
False. Positive displacement means constant volume. Pressure is determined by the system. The blower delivers constant volume – the system determines pressure.
Misconception 3: "All positive displacement machines have internal compression."
False. Some positive displacement machines compress internally (screw compressors). Others do not (roots blowers). Both are positive displacement.
Misconception 4: "Roots blowers are compressors."
False. Roots blowers are blowers – they move volume, they do not compress. They are positive displacement blowers, not compressors.
Frequently Asked Questions
1. Why is a roots blower called positive displacement?
A roots blower is called positive displacement because it traps a fixed volume of air between the rotors and casing and positively moves that volume from inlet to discharge with each revolution. The trapped volume is fixed – it does not change with pressure. This is the defining characteristic of positive displacement machines.
2. What does positive displacement mean?
Positive displacement means trapping a fixed volume of fluid and physically moving it from inlet to discharge. Each cycle delivers the same volume regardless of discharge pressure. The machine controls the volume – the system determines pressure.
3. Is a roots blower positive displacement even without internal compression?
Yes. Positive displacement means trapping a fixed volume and moving it. Internal compression is a separate feature. Roots blowers trap a fixed volume – they are positive displacement. Some positive displacement machines compress (screw compressors). Others do not (roots blowers).
4. What is the difference between positive displacement and dynamic compressors?
Positive displacement machines trap a fixed volume and move it – flow is constant regardless of pressure. Dynamic machines accelerate fluid and convert velocity to pressure – flow drops as pressure rises. Roots blowers are positive displacement. Centrifugal blowers are dynamic.
5. Does a roots blower maintain constant flow at different pressures?
Yes – within its operating range. Flow drops only slightly at higher pressure due to slipback (leakage through tip clearance). At 8 psig, flow is 97–98% of theoretical. At 12 psig, flow is 94–96%. This is the constant volume characteristic of positive displacement.
6. Why is constant flow important in aeration?
As diffusers foul, pressure rises from 6 to 9 psig. A positive displacement roots blower maintains airflow – the biology continues to receive oxygen. A dynamic centrifugal blower would lose flow – potentially starving the biology. Constant flow is why roots blowers are preferred for aeration.
7. What is slipback?
Slipback is air leakage through the rotor tip clearance. As pressure increases, more air leaks from discharge back to inlet. This reduces net flow slightly. Slipback is small (2–6%) – the blower is still positive displacement.
8. How does a roots blower trap a fixed volume?
Two synchronized rotors (lobes) rotate in opposite directions. Cavities form between the rotors and casing. These cavities trap a fixed volume of air at the inlet. The volume is determined by rotor geometry – it is fixed at manufacture. Each revolution delivers the same volume.
9. Why don't roots blowers compress internally?
Roots blowers do not reduce the trapped volume. The volume trapped at the inlet is the same volume discharged. When the cavity reaches the discharge port, higher-pressure air backflows to equalize pressure – no compression occurs. The blower simply moves volume.
10. What is the difference between positive displacement and constant volume?
Positive displacement is the mechanism – trapping a fixed volume and moving it. Constant volume is the result – flow is constant regardless of pressure. Roots blowers are positive displacement machines that deliver constant volume.
11. Are screw compressors positive displacement?
Yes – screw compressors trap a fixed volume and reduce it (internal compression). They are positive displacement compressors. Roots blowers are positive displacement blowers (no internal compression). Both are positive displacement machines.
12. What applications benefit from positive displacement?
Aeration, pneumatic conveying, vacuum systems, biogas handling – any application where constant flow is critical. Positive displacement delivers constant flow. Dynamic machines do not.
13. How does speed affect a positive displacement blower?
Flow is proportional to speed – doubling speed doubles flow. This linear relationship makes VFD control effective for flow regulation. Pressure does not affect flow – only speed does.
14. Is a roots blower a compressor or a blower?
Roots blowers are blowers – they move volume without internal compression. They are positive displacement blowers. They are not compressors (which have internal compression). The term "blower" is correct.
15. Why is positive displacement important for industrial applications?
Positive displacement provides constant flow regardless of pressure changes. In aeration, diffuser fouling raises pressure – flow remains constant. In conveying, filter loading raises pressure – flow remains constant. This reliability is why positive displacement roots blowers are the standard for critical applications.
Final Thoughts
After decades of explaining why roots blowers are called positive displacement, here is my practical advice:
Positive displacement means fixed volume trapping. Roots blowers trap a fixed volume of air between the rotors and casing and move it from inlet to discharge. The volume is fixed by rotor geometry – it does not change with pressure. This is the definition of positive displacement.
Internal compression is not required. Some positive displacement machines compress internally (screw compressors). Others do not (roots blowers). Both are positive displacement. The term refers to volume trapping – not compression.
Constant flow is the result. The fixed volume per revolution delivers constant flow regardless of pressure. This is the key advantage over dynamic machines. In aeration, conveying, and vacuum applications, constant flow is critical.
The bottom line. A roots blower is called positive displacement because it traps a fixed volume and moves it – the definition of positive displacement. The fixed volume per revolution is determined by rotor geometry. Pressure does not affect the trapped volume. This is why roots blowers maintain constant flow. Zhanggu and other manufacturers build positive displacement roots blowers for critical applications where constant flow matters.
