Roots Blower Pressure vs Flow

The relationship between roots blower pressure vs flow is fundamentally different from centrifugal blowers. A roots blower is a constant volume machine – it delivers the same flow regardless of pressure (within its operating range). Flow drops only slightly as pressure increases due to slipback through the rotor tip clearance.

Based on field data from hundreds of installations, this constant volume characteristic is the single most important advantage of roots blowers. In wastewater aeration, as diffusers foul and pressure rises from 6 psig to 9 psig, a roots blower maintains airflow. A centrifugal blower would lose 15–25% of flow – potentially starving the biology.

This guide explains the pressure vs flow relationship, how slipback affects performance, and how to read roots blower performance curves. Use it to understand why roots blowers behave the way they do.

Table of Contents

• What Is the Pressure vs Flow Relationship?

• Constant Volume Characteristic

• Slipback – The Small Flow Drop

• Pressure vs Flow Curve

• Flow vs Speed

• Effect of Altitude

• Effect of Temperature

• How to Read a Performance Curve

• Comparison With Centrifugal Blowers

• Frequently Asked Questions

• Final Thoughts

What Is the Pressure vs Flow Relationship?

The roots blower pressure vs flow relationship describes how airflow changes as discharge pressure varies. For a roots blower, flow is nearly constant across the pressure range – a characteristic called constant volume.

Key points:

• Flow is determined by speed (RPM), not pressure

• Flow drops only slightly as pressure increases (slipback)

• Flow is proportional to speed – doubling speed doubles flow

• Pressure is determined by the system, not the blower

Based on field data, a roots blower at 1,800 RPM delivers approximately 630 ACFM at 5 psig, 620 ACFM at 8 psig, and 600 ACFM at 12 psig – a drop of only 5% over a 7 psig pressure increase.

Constant Volume Characteristic

What "constant volume" means:
A roots blower traps a fixed volume of air per revolution. It delivers that volume regardless of discharge pressure (within the design range). The blower does not compress air internally – it simply moves it.

Why this matters:

• Aeration: as diffusers foul, pressure rises – roots blower maintains flow

• Conveying: as filters load, pressure rises – roots blower maintains flow

• Vacuum: as system conditions change, roots blower maintains vacuum

The engineering explanation:
Roots blowers are positive displacement machines. The volume of air trapped between the rotors and casing is fixed by the rotor geometry. Each revolution delivers the same volume. Pressure does not affect the trapped volume – only speed does.

Slipback – The Small Flow Drop

What is slipback?
Slipback is air leakage through the tip clearance between the rotors and casing. As pressure increases, more air leaks back from the discharge side to the inlet side. This reduces the net flow.

Typical slipback effect:

• At 5 psig: flow = 100% of theoretical

• At 8 psig: flow = 97–98% of theoretical

• At 12 psig: flow = 94–96% of theoretical

• At 15 psig: flow = 90–93% of theoretical

Factors affecting slipback:

• Tip clearance – tighter = less slipback

• Pressure ratio – higher = more slipback

• Rotor design – 3-lobe better than 2-lobe

• Rotor condition – worn rotors = more slipback

The engineering formula:
Qslip = k × (ΔP)³ × (clearance)³ / (length × viscosity)

The cubic relationship means that doubling pressure increases slipback eightfold. This is why tip clearance control is critical at high pressure.

Pressure vs Flow Curve

Typical roots blower performance curve:

Interpretation:
The flow curve is nearly flat – flow drops only 5% from 0 to 12 psig. This is the constant volume characteristic. At higher pressures, the curve drops more steeply as slipback becomes significant.

What this means for applications:
In aeration, as diffusers foul and pressure rises from 6 to 10 psig, flow drops only 2–3%. The biology continues to receive oxygen. In conveying, as filters load and pressure rises, flow remains stable – material stays suspended.

Flow vs Speed

Flow is proportional to speed:
Flow ∝ RPM (approximately linear)

• 100% speed = 100% flow

• 80% speed = 80% flow

• 60% speed = 60% flow

• 40% speed = 40% flow

Why this matters:
VFD control changes speed to match flow demand. At 80% speed, flow is 80% – but power is only 51% (speed cubed). This is the source of VFD energy savings.

Speed range:

• Typical operating speed: 1,000–3,000 RPM

• Minimum speed with VFD: 30% of rated

• Maximum speed: limited by bearings and rotor stress

Effect of Altitude

Altitude reduces air density:
At higher altitude, atmospheric pressure is lower. For the same mass flow, you need more volume flow.

Correction:
ACFM = SCFM × (14.7 / Patm)

At 5,000 ft (12.2 psia), ACFM = SCFM × 1.20. A blower that moves 1,000 SCFM at sea level moves only 833 ACFM at 5,000 ft – 17% less.

Effect on pressure vs flow curve:
The pressure ratio changes with altitude. At sea level, 8 psig = 22.7 psia / 14.7 psia = 1.54. At 5,000 ft, 8 psig = 20.2 psia / 12.2 psia = 1.66 – higher ratio for same gauge pressure. This increases slipback slightly.

Effect of Temperature

Temperature increases air volume:
Higher temperature = more volume for same mass flow.

Correction:
ACFM = SCFM × (T / 520)

At 100°F (560°R), correction is 1.08 – 8% more volume.

Effect on pressure vs flow curve:
Higher temperature increases flow for the same speed (volume expands). But higher temperature also increases discharge temperature – which can affect clearances and slipback.

How to Read a Performance Curve

Step 1 – Find your pressure.
Locate your discharge pressure on the vertical axis.

Step 2 – Find your flow.
Locate your required ACFM on the horizontal axis.

Step 3 – Find the intersection.
The intersection of your pressure and flow determines the operating point.

Step 4 – Read the speed.
The diagonal lines show RPM. Read the speed at your operating point.

Step 5 – Read the power.
The dashed lines show BHP. Read the power at your operating point.

Step 6 – Check the range.
Ensure your operating point is within the blower's range – not at the extreme.

Comparison With Centrifugal Blowers

Key difference:
Roots maintains flow as pressure rises. Centrifugal loses flow significantly. In aeration with diffuser fouling, roots is the clear choice.

Why centrifugal flow drops:
Centrifugal fans follow the fan laws – flow decreases as pressure increases. The pressure vs flow curve has a negative slope. Roots blowers have a nearly flat pressure vs flow curve.

Frequently Asked Questions

1. Does pressure affect roots blower flow?
Flow drops only slightly as pressure increases due to slipback. At 8 psig, flow is 97–98% of theoretical. At 12 psig, flow is 94–96%. The drop is 2–6% – much less than centrifugal blowers (20–40% drop).

2. Why do roots blowers maintain flow at higher pressure?
Roots blowers are positive displacement machines – they trap a fixed volume per revolution. Pressure does not change the trapped volume. Only slipback (leakage through tip clearance) reduces flow slightly.

3. 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. Slipback increases with pressure and clearance. Tighter clearances reduce slipback.

4. How much does flow drop at 15 psig?
At 15 psig, flow is typically 90–93% of theoretical – a 7–10% drop. This is still much better than centrifugal blowers, which lose 30–40% of flow at the same pressure increase.

5. What is the shape of the pressure vs flow curve?
The curve is nearly flat (constant volume) across the pressure range. It drops slightly at higher pressures due to slipback. The curve is linear (straight) from 0 to about 10 psig, then curves downward at higher pressures.

6. How does speed affect flow?
Flow is proportional to speed. Doubling speed doubles flow. This linear relationship makes VFD control effective for flow regulation. At 80% speed, flow is 80%.

7. How does altitude affect the pressure vs flow curve?
Altitude reduces atmospheric pressure, increasing the pressure ratio for the same gauge pressure. This increases slipback slightly. Correct flow for altitude using ACFM = SCFM × (14.7 / Patm).

8. How does temperature affect the pressure vs flow curve?
Higher temperature increases volume flow for the same speed. Correct flow for temperature using ACFM = SCFM × (T / 520). Higher temperature also increases discharge temperature, which can affect clearances.

9. What is the difference between roots and centrifugal pressure vs flow curves?
Roots: nearly flat (constant volume). Centrifugal: negative slope (flow drops as pressure rises). In aeration, roots maintains flow as diffusers foul. Centrifugal loses flow – potentially starving the biology.

10. Can I use VFD to control pressure?
VFD controls speed, which controls flow. Pressure is determined by the system. To control pressure, you need a pressure regulator or control valve. VFD controls flow – pressure follows system resistance.

11. Why does flow drop at high pressure?
Flow drops at high pressure due to increased slipback. At higher pressure, more air leaks through the tip clearance. The cubic relationship means slipback increases significantly at high pressure.

12. What is the maximum pressure for roots blower?
Standard three-lobe roots blowers: 15 psig continuous. High-pressure designs: 20–25 psig. At 15 psig, flow drops 7–10% from theoretical. Above 15 psig, slipback becomes significant and efficiency drops.

13. How do I read a roots blower performance curve?
Find your pressure on the vertical axis and flow on the horizontal axis. Find the intersection. Read the RPM (diagonal lines) and BHP (dashed lines) at the intersection. Ensure the operating point is within the blower's range.

14. What is the effect of rotor wear on pressure vs flow?
Rotor wear increases tip clearance, which increases slipback. At the same pressure, flow is lower. The pressure vs flow curve shifts downward – less flow at each pressure. Measure tip clearance annually and replace rotors when clearance exceeds 0.35 mm.

15. What is the ideal operating point on the curve?
The ideal operating point is in the middle 70% of the curve range – not at the extreme. Operating at the edge means no room for variance. Select in the middle for best efficiency and reliability.

Final Thoughts

After decades of analyzing roots blower pressure vs flow, here is my practical advice:

Roots blowers are constant volume machines. Flow is determined by speed, not pressure. Flow drops only slightly as pressure increases due to slipback. This is the key advantage over centrifugal blowers.

Slipback is the only flow loss. At 8 psig, slipback is 2–3%. At 12 psig, slipback is 4–6%. At 15 psig, slipback is 7–10%. Tighter clearances reduce slipback. Worn rotors increase slipback.

The curve tells the story. The pressure vs flow curve is nearly flat – the constant volume characteristic. Read the curve to select the right blower. Select in the middle of the range. Add margin for fouling.

The bottom line. Roots blower pressure vs flow is about constant volume operation. The blower delivers the same flow regardless of pressure – making it ideal for aeration, conveying, and other applications where pressure varies. Zhanggu and other manufacturers provide performance curves. Use them to select correctly. The constant volume characteristic is why roots blowers are the standard for critical applications.