Roots Blower Air Flow

Roots blower air flow is the most important specification for selecting a blower – but it's also the most misunderstood. Air flow is measured in CFM (cubic feet per minute), but CFM comes in two forms: SCFM (standard) and ACFM (actual). Using the wrong one leads to undersized or oversized blowers.

Based on decades of sizing experience, the most common mistake is using SCFM instead of ACFM – which can undersize a blower by 20–30% at altitude. Roots blowers are constant volume machines: they deliver the same ACFM regardless of pressure (within their operating range). Air flow is proportional to speed – doubling RPM doubles flow.

This guide explains the difference between SCFM and ACFM, how to calculate required air flow, how to correct for altitude and temperature, and how to select the right air flow for your application.

Table of Contents

• What Is Roots Blower Air Flow?

• SCFM vs ACFM – Critical Distinction

• How to Calculate Required Air Flow

• Altitude and Temperature Correction

• Air Flow vs Pressure – The Effect of Slipback

• Air Flow vs Speed – How RPM Affects Flow

• How to Measure Air Flow

• Selection Guide

• Performance and Engineering Calculations

• Application Air Flow Examples

• Common Mistakes

• Frequently Asked Questions

• Final Thoughts

What Is Roots Blower Air Flow?

Roots blower air flow is the volume of air or gas the blower delivers per unit of time. It is measured in cubic feet per minute (CFM) – the most important specification for blower selection.

Key characteristics of roots blower air flow:

• Constant volume machine – delivers same ACFM regardless of pressure (within range)

• Air flow is proportional to speed (RPM) – doubling speed doubles flow

• Air flow decreases slightly as pressure increases (slipback effect)

• Air flow must be expressed at actual operating conditions (ACFM)

Based on field data, the most common sizing error is using SCFM instead of ACFM. At 5,000 ft elevation, the correction is 20% – a significant error that undersizes the blower.

Air flow units:

• CFM = Cubic feet per minute

• SCFM = Standard cubic feet per minute (at 14.7 psia, 60°F)

• ACFM = Actual cubic feet per minute (at site conditions)

• ICFM = Inlet cubic feet per minute (similar to ACFM)

SCFM vs ACFM – Critical Distinction

SCFM (Standard Cubic Feet per Minute):

• Defined at standard conditions: 14.7 psia, 60°F (some use 68°F)

• Does not change with altitude or temperature

• Used for material balance calculations

• Cannot be used directly for blower sizing

ACFM (Actual Cubic Feet per Minute):

• Actual volume at site conditions (altitude, temperature, pressure)

• Used for blower sizing

• The blower capacity chart uses ACFM (or ICFM)

The problem with SCFM:
SCFM is a reference condition – it does not reflect actual volume at your site. If you size a blower using SCFM, you will undersize it at altitude or high temperature.

Example:
500 SCFM at 5,000 ft (12.2 psia), 100°F (560°R).
ACFM = 500 × (14.7/12.2) × (560/520) = 500 × 1.20 × 1.08 = 648 ACFM.
The blower must deliver 648 ACFM – 30% more than SCFM.

How to Calculate Required Air Flow

Step 1 – Determine application requirement.
Air flow depends on the application:

• Wastewater aeration: Calculate from oxygen demand. Typical: 0.5–1.5 SCFM per 1,000 cubic feet of basin volume.

• Pneumatic conveying: Calculate from material flow rate and solids loading ratio.

• Vacuum system: Calculate from system air removal requirement.

• Industrial ventilation: Calculate from hood capture velocity and duct area.

Step 2 – Calculate required SCFM.
Use process engineering calculations to determine SCFM required.

Step 3 – Correct SCFM to ACFM.
ACFM = SCFM × (14.7 / Patm) × (T / 520)

Step 4 – Add margin.
Add 15–20% margin for:

• Future expansion

• Filter/diffuser fouling

• System changes

Altitude and Temperature Correction

Atmospheric pressure at altitude:

Temperature correction:

Correction formula:
ACFM = SCFM × (14.7 / Patm) × (T / 520)

Example:
500 SCFM at 5,000 ft (12.2 psia), 100°F (560°R).
ACFM = 500 × (14.7/12.2) × (560/520) = 500 × 1.20 × 1.08 = 648 ACFM.

Air Flow vs Pressure – The Effect of Slipback

How pressure affects air flow:
Air flow decreases slightly as pressure increases due to slipback – air leakage through the rotor tip clearance.

Typical air flow loss at different pressures:

• At 5 psig: air flow = 100% of theoretical

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

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

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

Why this matters:
For aeration applications, as diffusers foul and pressure rises, a roots blower maintains air flow much better than a centrifugal fan. The air flow drop is only 2–10% – not 20–40%.

Slipback factors:

• Tip clearance – tighter = less slipback

• Pressure ratio – higher = more slipback

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

• Rotor condition – worn rotors = more slipback

Air Flow vs Speed – How RPM Affects Flow

Air flow is proportional to speed:
Air flow ∝ RPM (approximately). Doubling speed doubles air flow.

Speed ranges:

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

• Maximum speed: depends on blower size (2,000–4,000 RPM)

• Minimum speed for VFD: 30% of rated (some designs)

Speed limitations:

• Maximum speed limited by bearing capacity and rotor stress

• Minimum speed limited by oil system and efficiency

• VFD turndown: roots blowers achieve 30–100% of rated flow

Speed selection:

• Select speed to achieve required air flow

• Use the capacity chart to find speed for your flow and pressure

• Consider VFD for variable air flow applications

How to Measure Air Flow

Measurement methods:

1. Flow meter.

• In-line flow meter (thermal, differential pressure)

• Most accurate method

• Requires installation in piping

2. Pitot tube traverse.

• Measures velocity at multiple points

• Calculate average velocity × area

• Good for field measurements

3. Pressure drop method.

• Measure pressure drop across known restriction (orifice, nozzle)

• Use flow vs pressure drop correlation

4. Blower capacity chart.

• Measure pressure and RPM

• Read air flow from capacity chart

• Less accurate than direct measurement

Measurement locations:

• At blower discharge (for pressure)

• At blower inlet (for vacuum)

• In stable flow section (straight pipe, 5–10 diameters from bends)

Selection Guide

Step 1 – Define required SCFM.
Calculate process requirement.

Step 2 – Correct to ACFM.
Use altitude and temperature correction.

Step 3 – Add margin.
Add 15–20% for fouling and expansion.

Step 4 – Define pressure.
Determine system pressure at blower discharge.

Step 5 – Select from capacity chart.
Find ACFM and pressure on the capacity chart. Read RPM and BHP.

Step 6 – Select motor.
Add 15–20% safety factor to BHP.

Step 7 – Verify.
Confirm with manufacturer.

Example sizing:

Performance and Engineering Calculations

SCFM to ACFM:
ACFM = SCFM × (14.7 / Patm) × (T / 520)

Power calculation:
BHP = (ACFM × psig) / (229 × ηmechanical × ηmotor)

Air flow vs speed:
Air flow ∝ RPM (approximately). Doubling speed doubles air flow.

Air flow vs pressure:
Air flow = Theoretical flow × (1 – slipback factor)
Slipback increases with pressure and clearance.

Application Air Flow Examples

Example 1: Wastewater Aeration

• Basin: 500,000 gallons (66,800 cu ft)

• Required: 1.0 SCFM per 1,000 cu ft

• SCFM = 66.8 SCFM

• Site: 3,000 ft, 90°F

• ACFM = 66.8 × 1.114 × 1.058 = 78.8 ACFM

• Pressure: 15 ft depth = 6.5 psig + 2.0 psig losses + 1.5 psig margin = 10.0 psig

• Select: 5 HP three-lobe blower delivering 80 ACFM at 10 psig

Example 2: Pneumatic Conveying

• Material: Cement, 10 tons/hr

• Solids loading ratio: 10

• Air required: 10 tons/hr × 2,000 lb/ton / (10 × 60 × 0.08 lb/ACF) = 416 ACFM

• Pressure: 12 psig + 2 psig margin = 14 psig

• Select: 40 HP three-lobe blower delivering 420 ACFM at 14 psig

Example 3: Vacuum System

• Required: 200 ACFM at 10 inches Hg

• Select: 7.5 HP three-lobe vacuum blower delivering 200 ACFM at 10 inches Hg

Common Mistakes

1. Using SCFM instead of ACFM
Most common mistake. At 5,000 ft, SCFM undersizes blower by 20%. Always correct for altitude and temperature.

2. No altitude correction
Many plants at elevation. Atmospheric pressure at 5,000 ft is 12.2 psia vs 14.7 at sea level. This is a 17% difference.

3. No margin for fouling
Systems clog. Sizing exactly at clean conditions guarantees overload. Add 15–20% margin.

4. Forgetting pressure effect
Air flow drops at higher pressure due to slipback. The capacity chart accounts for this – but the effect is more significant at high pressure.

5. Using the wrong temperature
The correction formula uses absolute temperature (°R = °F + 460). Using °F directly gives wrong results.

6. Not adding motor safety factor
Use 15–20% safety factor for motor sizing. Motors lose capacity at altitude and from heat.

7. Ignoring future expansion
Plants grow. Add margin for future air flow requirements.

Frequently Asked Questions

1. What is roots blower air flow?
Air flow is the volume of air or gas the blower delivers per unit of time, measured in CFM (cubic feet per minute). Roots blowers are constant volume machines – they deliver the same ACFM regardless of pressure (within range). Air flow is proportional to speed – doubling RPM doubles flow.

2. What is the difference between SCFM and ACFM?
SCFM is air flow at standard conditions (14.7 psia, 60°F). ACFM is air flow at actual site conditions (altitude, temperature, pressure). SCFM does not change with altitude or temperature. ACFM changes with altitude and temperature. Blowers are sized in ACFM, not SCFM.

3. How do I convert SCFM to ACFM?
ACFM = SCFM × (14.7 / Patm) × (T / 520). Patm = local atmospheric pressure (psia). T = local absolute temperature (°R = °F + 460). At 5,000 ft, the correction is 1.20. At 100°F, the correction is 1.08. Combined correction is 1.30 – 30% more ACFM than SCFM.

4. Why does air flow matter for blower selection?
Air flow determines the blower size and motor power. An undersized blower cannot deliver required air flow – processes fail. An oversized blower wastes energy and short-cycles. Correct air flow selection is essential for reliable operation and energy efficiency.

5. How does altitude affect air flow?
Altitude reduces air density. For the same mass flow, you need more volume flow. ACFM = SCFM × 14.7 / Patm. At 5,000 ft (12.2 psia), the correction is 1.20 – you need 20% more ACFM. Sizing with SCFM undersizes the blower at altitude.

6. How does temperature affect air flow?
Higher temperature increases air volume. ACFM = SCFM × (T/520). At 100°F (560°R), correction is 1.08 – 8% more volume. At 120°F, correction is 1.12 – 12% more volume. Always correct for actual temperature.

7. What is slipback and how does it affect air flow?
Slipback is air leakage through the rotor tip clearance. As pressure increases, more air leaks back from discharge to inlet. Air flow decreases slightly at higher pressure. At 8 psig, air flow is 97–98% of theoretical. At 15 psig, air flow is 90–93%. The capacity chart accounts for this effect.

8. How do I select the right air flow for my application?
Calculate required SCFM from process requirements. Correct SCFM to ACFM using altitude and temperature. Add 15–20% margin for fouling and expansion. Find ACFM on the capacity chart at your operating pressure. Select the blower that delivers the required ACFM.

9. What is the rule of thumb for air flow and motor size?
At 8 psig, three-lobe blower requires approximately 18–20 HP per 100 ACFM. Example: 500 ACFM at 8 psig → 90–100 HP. Add 15–20% safety factor → 105–120 HP → select 125 HP motor.

10. Can I increase air flow by increasing speed?
Yes – air flow is proportional to speed (RPM). Doubling speed doubles air flow. But increasing speed increases power and wear. Stay within the manufacturer's speed range. Maximum speed is typically 2,000–3,000 RPM depending on blower size.

11. How does pressure affect air flow?
Air flow decreases slightly as pressure increases due to slipback. At 8 psig, air flow drops 2–3% from 5 psig. At 15 psig, air flow drops 7–10%. The capacity chart shows this relationship. For most applications, the effect is small.

12. Should I add margin to air flow?
Yes – add 15–20% margin for filter/diffuser fouling and future expansion. Systems clog over time. A blower sized exactly at clean conditions will lose capacity as filters load. Margin is not waste – it is reliability.

13. How do I calculate air flow for wastewater aeration?
Calculate oxygen demand from BOD loading (1.0–1.5 lb O2/lb BOD). Convert to SCFM using standard oxygen transfer efficiency (15–25%). Correct to ACFM using altitude and temperature. Add 30% margin for diffuser fouling and peak loading.

14. How do I calculate air flow for pneumatic conveying?
For dilute phase: ACFM = (material flow rate lb/hr) / (solids loading ratio × air density lb/ACF × 60). Typical SLR = 5–15. Air density at 12 psig, 100°F = 0.12 lb/ACF. Add 20–30% margin – under-sizing causes plugging.

15. How do I measure air flow in the field?
Use a flow meter, pitot tube traverse, or measure pressure and RPM then read from capacity chart. For accurate measurement, install a flow meter in straight pipe section (5–10 diameters from bends). For field checks, the capacity chart method is acceptable.

Final Thoughts

After decades of sizing roots blowers, here is my practical advice:

Air flow is critical – but only if you use the right units. The most common mistake is using SCFM instead of ACFM. At 5,000 ft and 100°F, the correction is 30% – a significant error. Always correct SCFM to ACFM using altitude and temperature.

Add margin. The second most common mistake is no margin. Add 15–20% to air flow for fouling and expansion. A blower sized exactly at clean conditions will lose capacity as filters load. Margin is reliability.

Check the capacity chart. The capacity chart shows air flow vs pressure at different speeds. Find your ACFM and pressure on the chart. Read RPM and BHP. Use the chart for accurate selection – not just rules of thumb.

The bottom line. Roots blower air flow is about understanding the difference between SCFM and ACFM, correcting for site conditions, and adding margin. Zhanggu and other established manufacturers provide capacity charts and selection assistance. Use the correct units. Correct for site conditions. Add margin. Select in the middle of the chart range. Do these things and the blower will deliver the required air flow reliably.