How to Size a Centrifugal Blower: Airflow, Pressure, Margins and a Worked Example

Sizing a blower means turning a process requirement into three numbers: the airflow, the pressure, and the power. Get them right, correct them for the real gas and site, and check the operating point on a curve, and the blower will perform. Skip the corrections or pile on margin without thinking, and it will underperform or waste energy for years. This guide walks through the method with a worked example whose figures you can reuse.

Step 1: Establish the Airflow

Airflow comes from the process. Typical routes:

• Combustion: from the heat input and fuel, with excess air.
• Drying: from the water to evaporate and the moisture pickup of the air.
• Cooling: from the heat load, air density, specific heat, and the allowed temperature rise.
• Extraction: from capture velocity at hoods, or from conveying velocity in ducts.

State whether the airflow is actual (at the gas conditions) or standard (normalised), because they can differ by a large factor at high temperature or altitude. To convert normal cubic metres per hour to actual, multiply by (101.325 ÷ absolute pressure in kPa) × (temperature in K ÷ 273.15).

Step 2: Add Up the Resistance

List every element in the air path, estimate its pressure drop at the design flow, and add them: hoods, ducts, bends, dampers, filters, coils, beds, burners, and outlets. Resistance rises roughly with the square of flow. Add the worst-case condition: loaded filters, fouled coils, or a compacted bed. Use the equipment suppliers’ resistance figures where you have them.

Step 3: Choose the Pressure Basis

Static pressure is what the system’s resistance consumes. Total pressure adds the velocity pressure of the air leaving the blower. Compare quotations on the same basis, and state it in the specification. Where the blower discharges into a duct, compare total pressure at the blower. Where it discharges freely, the velocity pressure is lost, and should be counted in what the system needs.

Step 4: Correct for Gas Density

Blower ratings are quoted at standard air, about 1.204 kg/m³. At temperature or altitude, the blower develops less pressure at the same speed, so convert the required pressure to a standard-air rating:

Standard-air rating = required pressure × (1.204 ÷ actual gas density)

Worked Example

A process needs 15 m³/s. The system resistance, at the site’s air conditions, is 2,600 Pa static pressure. The site is 1,800 m above sea level, with air at 35 °C, so the air density is 0.921 kg/m³.

• Air power = 15 × 2,600 = 39.0 kW.
• Standard-air pressure rating = 2,600 × 1.204 ÷ 0.921 = 3,398 Pa, about 346 mm WC.
• At 70% total efficiency, shaft power = 39.0 ÷ 0.70 = 55.7 kW.
• With a 10% margin, about 61.3 kW.

A duty of 346 mm WC at standard air is inside our 500 mm WC standard maximum, and it sits in the high pressure class. The motor would be the next standard size above 61.3 kW, for example 75 kW.

Step 5: Check the Cold-Start Power

Power is highest when the air is densest. If the same site has a cold morning at 15 °C, the air density rises to about 0.985 kg/m³, about 7% above the design value. The blower absorbs about 7% more power at the same speed, so 61.3 kW becomes about 65.6 kW. The 75 kW motor still has room. Always check the motor at the coldest, densest condition, including start-up. See gas temperature, altitude and density correction.

Step 6: Select from the Curve

Plot the system curve and read the operating point on the blower’s curve. Check that flow and pressure meet the duty at both clean and worst-case resistance, and that the point is in the blower’s efficient range. See blower curves, system curves and operating point drift.

Step 7: Margins, and the Debate About Them

Designers commonly add margin for leakage, fouling, tolerances, and future change. Some practitioners advise around 10 to 15% on airflow and more on pressure, and traditional practice on some heavy process fans has used about 15% on volume. Others argue that margin is often overdone. Too little leaves the system short as it ages. Too much moves the operating point away from the best-efficiency region, wastes energy, and can overload the motor if the real resistance is lower than calculated.

A sound approach:

1. Calculate the worst-case resistance directly, so margin is not hiding a poor estimate.
2. Add a modest margin only for things you cannot calculate.
3. Use speed control, so the blower can be trimmed to the real duty after commissioning.

Step 8: Choose Wheel, Inlet, and Drive

Match the wheel type to the application, pressure, and gas, choose single or double inlet for the airflow and space, and decide on direct or belt drive and motor position. See backward-curved versus forward-curved blower wheels and single-inlet versus double-inlet blowers.

Common Sizing Mistakes

1. Mixing static and total pressure.
2. Skipping the density correction at altitude or temperature.
3. Using only clean-filter resistance.
4. Stacking margins, so airflow, pressure, and motor each carry a separate cushion.
5. Sizing the motor for the design point only, and forgetting cold start.
6. Quoting airflow without saying whether it is actual or standard.

Where Our Range Fits

NextAir Systems manufactures centrifugal blowers up to 800 HP (about 597 kW) and up to 500 mm WC (about 4.9 kPa) static pressure. If the corrected standard-air pressure lands above 500 mm WC, the duty is outside our standard range, and we will say so early. Airflow and drive arrangement are confirmed for each selection.

Have Us Size Your Blower

Send your airflow, resistance, and site conditions, and we’ll size the blower and check the operating point. Contact us for a free quote.

Frequently Asked Questions