Roots Blower CFM

Roots blower CFM is the most important specification for selecting a blower – but it's also the most misunderstood. CFM (cubic feet per minute) 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.

This guide explains the difference between SCFM and ACFM, how to calculate required flow, how to correct for altitude and temperature, and how to select the right CFM for your application. Use it to size blowers correctly the first time.

Table of Contents

• What Is Roots Blower CFM?

• SCFM vs ACFM – Critical Distinction

• How to Calculate Required CFM

• Altitude and Temperature Correction

• Flow vs Pressure – The Effect of Slipback

• Selection Guide

• Performance and Engineering Calculations

• Common Sizing Mistakes

• Frequently Asked Questions

• Final Thoughts

What Is Roots Blower CFM?

CFM (cubic feet per minute) is the measure of airflow a roots blower delivers. It is the most important specification for blower selection. But CFM is not a single value – it depends on pressure, temperature, and altitude.

Roots blower CFM characteristics:

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

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

• Flow decreases slightly as pressure increases (slipback effect)

• 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.

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 CFM

Step 1 – Determine application requirement.
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.

Flow vs Pressure – The Effect of Slipback

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

Typical flow loss:

• 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

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

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)

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

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

Pressure ratio effect:
At altitude, the pressure ratio for the same gauge pressure is higher.
Sea level: 8 psig = 22.7 psia / 14.7 psia = 1.54
5,000 ft: 8 psig = 20.2 psia / 12.2 psia = 1.66

Common Sizing 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
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 airflow requirements.

Frequently Asked Questions

1. What is roots blower CFM?
CFM (cubic feet per minute) is the measure of airflow a roots blower delivers. Roots blowers are constant volume machines – they deliver the same ACFM regardless of pressure (within range). But CFM must be expressed at actual operating conditions (ACFM), not standard conditions (SCFM).

2. What is the difference between SCFM and ACFM?
SCFM is flow at standard conditions (14.7 psia, 60°F). ACFM is 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 CFM matter for blower selection?
CFM determines the blower size and motor power. An undersized blower cannot deliver required flow – processes fail. An oversized blower wastes energy and short-cycles. Correct CFM selection is essential for reliable operation and energy efficiency.

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

8. How do I select the right CFM 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 CFM 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. This rule works for quick estimates but always use detailed calculations for final sizing.

10. Can I increase CFM by increasing speed?
Yes – flow is proportional to speed (RPM). Doubling speed doubles 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. What is the difference between ICFM and ACFM?
ICFM (inlet cubic feet per minute) is flow measured at the blower inlet. ACFM is actual flow at operating conditions. They are essentially the same for roots blowers – the capacity chart may use either. Check the catalog units.

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

13. Should I add margin to CFM?
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.

14. How do I calculate CFM 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.

15. How do I calculate CFM 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.

Final Thoughts

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

CFM 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 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 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 CFM 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 flow reliably.