Roots Blower Capacity Chart

A roots blower capacity chart shows the relationship between airflow, pressure, speed, and power for a specific blower model. It is the essential tool for selecting the right blower for your application. But reading a capacity chart correctly requires understanding the difference between SCFM and ACFM, the effect of altitude, and how pressure affects flow.

Based on field experience, I have seen engineers select blowers incorrectly because they misread capacity charts. They used SCFM instead of ACFM, ignored altitude correction, or selected at the wrong pressure. Each mistake resulted in an undersized or oversized blower.

This guide explains how to read roots blower capacity charts, how to correct for site conditions, and how to use the data for proper selection. Includes example charts and step-by-step interpretation.

Table of Contents

• What Is a Roots Blower Capacity Chart?

• How to Read a Roots Blower Capacity Chart

• SCFM vs ACFM – Critical Distinction

• Altitude and Temperature Correction

• How to Use the Chart for Selection

• Example Capacity Charts

• Common Chart Reading Mistakes

• Performance and Engineering Calculations

• Comparison With Alternatives

• Frequently Asked Questions

• Final Thoughts

What Is a Roots Blower Capacity Chart?

A roots blower capacity chart is a performance curve that shows how much air a blower can deliver at different pressures, speeds, and power levels. The chart typically displays:

• Flow rate (ACFM or ICFM) on the horizontal or vertical axis

• Pressure (psig) on the vertical or horizontal axis

• Speed (RPM) as a series of curves

• Power (BHP) as a series of curves

Based on hundreds of sizing reviews, the capacity chart is the fundamental tool for matching a blower to your application. The blower model is selected by finding the intersection of required flow and pressure on the chart – then reading the required speed and power.

A roots blower capacity chart from reputable manufacturers like Zhanggu includes factory-tested data for each blower frame size. The data is verified on an ISO 1217 test stand – not calculated estimates.

How to Read a Roots Blower Capacity Chart

Step 1 – Identify the blower model and frame size.
Each chart corresponds to a specific blower model – defined by rotor diameter, length, and lobe profile. Different frame sizes have different flow ranges.

Step 2 – Locate the flow axis.
Flow is typically shown on the horizontal axis. Units are ACFM (actual cubic feet per minute) or ICFM (inlet cubic feet per minute). The chart will specify which units are used.

Step 3 – Locate the pressure axis.
Pressure is typically on the vertical axis. Units are psig (pounds per square inch gauge). Some charts show vacuum in inches Hg.

Step 4 – Find the intersection of your flow and pressure.
Locate your required ACFM on the horizontal axis. Locate your required pressure on the vertical axis. Find the intersection point.

Step 5 – Determine the required speed (RPM).
The diagonal curves on the chart represent blower speed (RPM). Read the RPM curve at your intersection point. This is the speed required to deliver your flow at your pressure.

Step 6 – Determine the required power (BHP).
The dashed or dotted curves represent power consumption (BHP). Read the power curve at your intersection point. This is the required brake horsepower.

Step 7 – Select motor size.
Add 15–20% safety factor to BHP. Select the next standard motor size (e.g., 5, 7.5, 10, 15, 20, 25, 30, 40, 50 HP).

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

ACFM (Actual Cubic Feet per Minute):

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

• Used for blower sizing

• The capacity chart uses ACFM (or ICFM)

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

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

If you use SCFM on a roots blower capacity chart that requires ACFM, you will select an undersized blower.

Altitude and Temperature Correction

Atmospheric pressure at altitude:

Temperature correction:

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

Example: 500 SCFM at 5,000 ft, 100°F.
ACFM = 500 × (14.7/12.23) × (560/520) = 500 × 1.20 × 1.08 = 648 ACFM.
The blower must deliver 648 ACFM – 30% more than SCFM.

How to Use the Chart for Selection

Example: Wastewater Aeration Selection

• Required flow: 500 ACFM at 8 psig

• Site: sea level, 70°F

Step 1 – Find the capacity chart for the blower frame size.
Assume we are considering a 10-inch rotor blower.

Step 2 – Locate 8 psig on the vertical axis.
Move horizontally to the 500 ACFM line.

Step 3 – Determine speed.
At the intersection, the RPM curve is approximately 1,800 RPM.

Step 4 – Determine power.
At the intersection, the power curve is approximately 85 BHP.

Step 5 – Select motor.
85 BHP × 1.15 safety factor = 97.8 HP → 100 HP motor.

Step 6 – Verify on chart.
Check that operating point is within the chart range – not at the extreme of the curve.

Example Capacity Charts

Three-Lobe Roots Blower – Typical Chart Data

Model: 100 HP class, 10-inch rotor, 3-lobe

Interpretation:

• At 8 psig, 1,800 RPM delivers 630 ACFM at 81 BHP.

• Flow decreases slightly as pressure increases (slipback effect).

• Power increases linearly with pressure (for same flow, power ∝ pressure).

Note: These are example values. Actual manufacturer charts vary by rotor size, lobe profile, and design.

Common Chart Reading Mistakes

1. Using SCFM instead of ACFM
The capacity chart uses ACFM (or ICFM). Using SCFM undersizes the blower. At 5,000 ft, the error is 20%.

2. Not correcting for altitude
Altitude reduces atmospheric pressure. The chart assumes sea level (14.7 psia) unless otherwise noted. At altitude, the blower must work harder to maintain the same pressure ratio.

3. Ignoring temperature
Temperature affects volume. Higher temperature = more ACFM for same SCFM. The chart assumes 60°F unless otherwise noted. At 100°F, correction is 8%.

4. Selecting at the wrong pressure
The capacity chart shows flow vs pressure. If you select at the wrong pressure, the speed and power will be wrong.

5. Not adding safety factor
The chart shows BHP at operating point. Add 15–20% safety factor for motor selection. Motors lose capacity at altitude and from heat.

6. Operating at the extreme of the curve
Selecting at the edge of the chart means no room for variance. Select in the middle 70% of the curve range.

7. Forgetting slipback
At higher pressures, slipback reduces flow. The capacity chart accounts for this – but the effect is more significant at high pressure.

8. Using generic charts
Each blower model has its own capacity chart. Using a generic chart for a different frame size or lobe type gives wrong results.

Performance and Engineering Calculations

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

Pressure correction for altitude:
The pressure ratio changes with altitude. At sea level, 8 psig = 22.7 psia / 14.7 psia = 1.54 pressure ratio. At 5,000 ft, 8 psig = 20.2 psia / 12.2 psia = 1.66 pressure ratio – higher ratio for same gauge pressure.

Power calculation from chart:
BHP = (ACFM × psig) / (229 × ηmechanical × ηmotor)
This formula matches the chart data. Use the chart for accuracy – the formula is an approximation.

Speed vs flow relationship:
Flow ∝ RPM (approximately). Doubling speed doubles flow. The relationship is linear for roots blowers.

Pressure vs power relationship:
Power ∝ Pressure (for constant flow). Doubling pressure doubles power.

Efficiency from chart:
η = (ACFM × psig) / (229 × BHP)
This is the mechanical efficiency at the operating point. Typical: 85–92%.

Comparison With Alternatives

Roots Blower Capacity Chart vs Centrifugal Blower Performance Curve

Why roots charts are easier to use:
For roots blowers, flow is primarily a function of speed. Pressure has a small effect. For centrifugals, flow changes significantly with pressure – requiring more complex charts.

Roots Blower Capacity Chart vs Screw Compressor Performance

Frequently Asked Questions

1. What is a roots blower capacity chart?
A roots blower capacity chart is a performance curve that shows airflow (ACFM) vs pressure (psig) at various speeds (RPM). The chart also includes power consumption (BHP) curves. It is used to select the correct blower model and motor size for an application.

2. What units does the capacity chart use?
Flow is typically in ACFM (actual cubic feet per minute) or ICFM (inlet cubic feet per minute). Pressure is in psig (pounds per square inch gauge). Speed is in RPM. Power is in BHP (brake horsepower). Always check the chart for units – some charts use SCFM but most use ACFM.

3. How do I read a roots blower capacity chart?
Locate your required ACFM on the horizontal axis and required pressure on the vertical axis. Find the intersection. Read the RPM curve to determine required speed. Read the BHP curve to determine power. Add 15–20% safety factor for motor selection. Confirm the operating point is not at the extreme of the chart.

4. What is the difference between ACFM and SCFM on a capacity chart?
ACFM is actual volume at site conditions (altitude, temperature). SCFM is volume at standard conditions (14.7 psia, 60°F). Most capacity charts use ACFM. If you use SCFM, you will undersize the blower. Correct SCFM to ACFM using the formula: ACFM = SCFM × (14.7/Patm) × (T/520).

5. How does altitude affect the capacity chart?
Altitude reduces atmospheric pressure. The chart assumes sea level (14.7 psia) unless otherwise noted. At altitude, you need to correct flow and check that the blower can achieve the required pressure ratio. The chart may need correction for altitude.

6. How do I size a blower using the capacity chart?
Find your required ACFM and pressure on the chart. Determine the RPM required. Determine the BHP required. Add 15–20% safety factor. Select the next standard motor size. Verify that the operating point is in the middle of the chart range – not at the extreme.

7. Why does flow decrease at higher pressure?
Flow decreases at higher pressure due to slipback – air leakage through the rotor tip clearance. The higher the pressure, the more leakage. The capacity chart accounts for this effect. At high pressure (15+ psig), slipback becomes significant.

8. What is the effect of temperature on the capacity chart?
Higher temperature increases air volume. ACFM = SCFM × (T/520). At 100°F, the correction is 8%. The chart assumes 60°F unless otherwise noted. Correct flow for actual temperature before using the chart.

9. How accurate are manufacturer capacity charts?
Reputable manufacturers (Zhanggu, etc.) test blowers on ISO 1217 test stands. The chart data is verified – typically within ±3–5% of actual performance. Field conditions (inlet filter, piping, altitude) affect actual performance.

10. What does the power curve on the capacity chart show?
The power curve shows BHP (brake horsepower) required to deliver the flow at that pressure and speed. BHP increases with pressure (for constant flow) and with flow (for constant pressure). Use the power curve for motor sizing.

11. How do I use the capacity chart for vacuum applications?
Vacuum charts show flow vs vacuum (inches Hg) at various speeds. The same principles apply. Flow is on the horizontal axis, vacuum on the vertical. Power curves show BHP. Vacuum charts often have different efficiency curves – check units.

12. Can I use a capacity chart to estimate energy cost?
Yes. BHP × 0.746 × hours × $/kWh = annual energy cost. Example: 100 BHP × 0.746 × 8,000 hr × $0.10 = $59,680. The capacity chart gives BHP at your operating point – use it for energy cost estimation.

13. How does VFD affect capacity chart reading?
VFD changes the speed (RPM) of the blower. The capacity chart shows flow at different speeds. For VFD operation, choose the speed that delivers your required flow. The power curve gives BHP at that speed.

14. What if my required point is not on the chart?
If your point is between chart curves, interpolate. If your point is above the highest speed curve, the blower is too small – select a larger frame. If your point is below the lowest speed curve, the blower is too large – select a smaller frame or use VFD.

15. Where can I get capacity charts for different blower models?
Manufacturers provide capacity charts for each blower model. Zhanggu and other established manufacturers include charts in their catalogs and selection software. Request charts for the specific model you are considering.

Final Thoughts

After decades of using roots blower capacity charts for selection, here is my practical advice:

The chart is a map. It shows you where the blower can operate. But it doesn't tell you where you should operate. Select in the middle 70% of the chart range – not at the extremes. Operating at the edge means no room for variance.

Correct for site conditions. The chart assumes sea level, 60°F, and clean air. Your site is different. Correct SCFM to ACFM using altitude and temperature. If your site is at 5,000 ft and 100°F, the correction is 30% – significant.

Add margin. The chart shows BHP at the operating point. Add 15–20% for motor selection. Motors lose capacity at altitude and from heat. Undersized motors trip. Oversized motors cost more but prevent nuisance trips.

Verify with the manufacturer. Zhanggu and other reputable manufacturers provide charts for their blowers. They also have selection software. Use it. A phone call to the manufacturer can save thousands in wrong selection.

The reality. A roots blower capacity chart is a powerful tool. It provides verified performance data for selection. But it's only as good as the inputs. Use the correct units. Correct for site conditions. Add margin. Select in the middle of the range. Do these things and the chart will serve you well. Ignore them and you will select the wrong blower.