Roots Blower Catalog

A roots blower catalog is the essential engineering document for selecting the right blower. It contains performance curves, dimensional drawings, specification tables, and application data. But reading a catalog correctly requires understanding the difference between SCFM and ACFM, how to interpret capacity charts, and what specifications actually mean.

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

This guide explains how to read a roots blower catalog, interpret performance curves, and select the right blower. Use it to navigate manufacturer catalogs and avoid common selection errors.

Table of Contents

• What Is a Roots Blower Catalog?

• Working Principle

• How to Read a Roots Blower Catalog

• Performance Curves – Capacity Charts

• Dimensional Data

• Specification Tables

• SCFM vs ACFM – Critical Distinction

• Altitude and Temperature Correction

• Selection Guide

• Performance and Engineering Calculations

• Comparison With Alternatives

• Frequently Asked Questions

• Final Thoughts

What Is a Roots Blower Catalog?

A roots blower catalog is a technical document that provides:

• Performance curves (flow vs pressure at various speeds)

• Dimensional drawings (mounting dimensions, connections)

• Specification tables (model numbers, motor sizes, weights)

• Application data (pressure ranges, temperature limits)

• Accessory information (silencers, filters, VFDs)

• Selection guidelines

Based on hundreds of catalog reviews, the most important section is the performance curves (capacity charts). These curves show airflow (ACFM) vs pressure (psig) at different speeds (RPM) and power consumption (BHP).

A roots blower catalog 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.

Working Principle

Step 1 – Air intake. Motor turns drive shaft. Timing gears synchronize rotors. Air enters through inlet port.

Step 2 – Trapping and transport. Rotor cavities seal against casing. Air is carried to discharge.

Step 3 – Discharge and backflow. When cavity reaches discharge port, air is pushed out. Backflow occurs briefly.

Step 4 – Cycle repeats. The blower continuously moves air through the system.

Why the catalog matters: The catalog translates this operating principle into engineering data you can use for selection. It shows how much air the blower moves at different pressures and speeds.

How to Read a Roots Blower Catalog

Step 1 – Identify the blower model.
Each catalog section corresponds to a specific blower frame size. Look for the model number and rotor size.

Step 2 – Find the performance curves (capacity charts).
This is the most important section. Performance curves show:

• Flow (ACFM) on the horizontal axis

• Pressure (psig) on the vertical axis

• Speed (RPM) curves

• Power (BHP) curves

Step 3 – Check units.
Ensure the flow is in ACFM (actual cubic feet per minute), not SCFM. Most catalogs use ACFM or ICFM. If the catalog uses SCFM, you must correct for altitude and temperature.

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

Step 5 – Determine required speed.
Read the RPM curve at your intersection point. This is the speed required to deliver your flow at your pressure.

Step 6 – Determine required power.
Read the BHP 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.

Step 8 – Verify dimensional data.
Check that the blower fits your space. Verify inlet and discharge connection sizes.

Step 9 – Check accessories.
Verify silencer, filter, and baseplate compatibility.

Performance Curves – Capacity Charts

Capacity chart example:

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).

Dimensional Data

Typical dimensional data in a catalog:

• Overall length, width, height

• Inlet and discharge flange sizes

• Baseplate mounting dimensions

• Shaft centerline height

• Weight

• Connection details

Why dimensional data matters:

• Verify fit in your space

• Confirm pipe connections

• Check foundation requirements

• Plan for maintenance access

Specification Tables

Typical specifications in a catalog:

• Model number and frame size

• Rotor diameter and length

• Lobe count (2 or 3)

• Pressure rating

• Speed range

• Motor horsepower range

• Maximum discharge temperature

• Weight

Why specification tables matter:

• Confirm model matches requirements

• Check pressure and temperature limits

• Verify motor compatibility

SCFM vs ACFM – Critical Distinction

SCFM (Standard Cubic Feet per Minute):

• Defined at standard conditions: 14.7 psia, 60°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

• Most catalogs use 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 from a catalog 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.

Selection Guide Using the Catalog

Step 1 – Define flow requirement (ACFM).
Calculate required flow at operating conditions. If you have SCFM, correct to ACFM using altitude and temperature.

Step 2 – Define pressure requirement (psig).
Determine system pressure at blower discharge. Add 15–20% margin for fouling.

Step 3 – Find your operating point on the capacity chart.
Locate your ACFM on the horizontal axis and pressure on the vertical axis.

Step 4 – Determine required speed (RPM).
Read the RPM curve at your operating point.

Step 5 – Determine required power (BHP).
Read the BHP curve at your operating point.

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

Step 7 – Verify dimensional data.
Check fit and connections.

Step 8 – Confirm with manufacturer.
Have the manufacturer verify your selection. Zhanggu and other reputable manufacturers offer selection assistance.

Performance and Engineering Calculations

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

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

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

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

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

Roots Blower Catalog vs Other Technologies

Frequently Asked Questions

1. What is a roots blower catalog?
A roots blower catalog is a technical document containing performance curves, dimensional drawings, specification tables, and application data. It is used to select the right blower for an application. The most important section is the performance curves (capacity charts) showing flow vs pressure at various speeds and power consumption.

2. What units does a roots blower catalog use?
Flow is typically in ACFM (actual cubic feet per minute) or ICFM (inlet cubic feet per minute). Pressure is in psig. Speed is in RPM. Power is in BHP. Always check the catalog for units – some use SCFM but most use ACFM. Zhanggu and other reputable manufacturers clearly specify units.

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 in a catalog?
ACFM is actual volume at site conditions (altitude, temperature). SCFM is volume at standard conditions (14.7 psia, 60°F). Most catalogs 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 catalog data?
Altitude reduces atmospheric pressure. The catalog 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. Use the correction formula: ACFM = SCFM × (14.7/Patm) × (T/520).

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

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

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

10. What does the power curve in the catalog 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. Can I use a catalog 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.

12. How does VFD affect catalog 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.

13. What if my required point is not on the catalog 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.

14. What dimensional data is in a catalog?
Overall dimensions, inlet and discharge flange sizes, baseplate mounting dimensions, shaft centerline height, weight, and connection details. Use this to verify fit in your space and plan piping connections.

15. Where can I get catalogs for different blower models?
Manufacturers provide catalogs for each blower model. Zhanggu and other established manufacturers include catalogs on their websites and provide printed copies. Request catalogs for the specific models you are considering.

Final Thoughts

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

The catalog is a tool. It provides verified performance data. But it's only as good as the inputs. Use the correct units (ACFM, not SCFM). Correct for site conditions (altitude, temperature). Add margin (15–20% for pressure, power). Select in the middle of the chart range – not at the extremes.

Check units first. The most common mistake is using SCFM instead of ACFM. Always check the catalog units. If the catalog uses SCFM, correct to ACFM using altitude and temperature. At 5,000 ft and 100°F, the correction is 30% – significant.

Verify with the manufacturer. Zhanggu and other reputable manufacturers provide catalogs and selection assistance. Use their expertise. A phone call can save thousands in wrong selection. The catalog is the starting point – not the final answer.

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