Here is something we see almost every day. An inquiry arrives that says only:"Need one centrifugal fan, model 4-72, 5.5 kW."No airflow. No pressure. No gas temperature. Nine times out of ten, the fan that gets shipped will either be oversized and waste power, or undersized and never reach the airflow the customer actually needs.

A centrifugal fan is not bought by model number — it is bought by a duty point. This guide walks through the five steps our application engineers use, and finishes with a worked example you can copy for your own project. At the end you can also download our sizing data sheet: fill it in and send it back, and you will have a firm selection and a price within 24 hours.

The 6 parameters you must have before you ask for a quote

Step 1 — Define the airflow

Airflow is usually fixed by the process, not by the fan. Three common ways to get it:

• Room ventilation: Q = room volume × air changes per hour. A 600 m³ paint booth at 20 ACH needs 12,000 m³/h.
• Local extraction: Q = capture area × face velocity × 3600. A 3.0 m × 1.2 m hood at 0.5 m/s needs about 6,480 m³/h.
• Process gas: taken directly from the burner, kiln or dryer heat balance.

Always add a margin of 5–10% for duct leakage and filter ageing — but resist the temptation to add 30%. Oversizing pushes the duty point to the right on the curve, into a region of lower efficiency and higher noise.

Step 2 — Estimate the system resistance

This is where  projects go wrong, because pressure is invisible. Add up every element the air has to pass through:

Figures are typical planning values for estimating an inquiry. Final values must come from your duct design or from measurements on the existing line.

Step 3 — Correct for temperature and altitude

Fan catalogues publish performance in standard air (density 1.2 kg/m³, 20 °C, sea level). Your gas is rarely standard air. Two corrections apply:

Practical shortcut: every 1,000 m of altitude costs you about 8% of pressure; gas at 200 °C costs you about 40%. Ignore this and the fan that looked nice on paper will arrive on site and move too little air.

Step 4 — Read the performance curve

Plot your required airflow on the horizontal axis and your corrected pressure on the vertical axis; the crossing is your duty point. Three rules:

1. The duty point should sit on the rising-to-peak part of the curve, not on the far right where the curve flattens or turns unstable.
2. Keep it inside the high-efficiency band (usually 70–90% of the fan's max flow).
3. Leave 10% headroom on pressure, because your resistance estimate is never exact.

As a rough family guide: 4-72 covers general ventilation with high airflow at low to medium pressure; 9-19 is a high-pressure, lower-flow design for forced draft and material conveying; 9-26 delivers high pressure at higher flow than 9-19. Always confirm against the actual curve for the model and speed.

Quick model guide for ZIFENG standard ranges

Step 5 — Size the motor

For belt-driven sets also add belt and bearing losses (about 3–5%). If the fan starts against a closed damper the starting torque is low; if it starts against full system resistance, specify a higher service factor or a soft starter / VFD.

Worked example: a paint booth extraction fan

Given: required airflow 6,000 m³/h; ductwork with 8 elbows, one cartridge filter (loaded), booth inlet; gas temperature 80 °C; plant altitude 500 m; clean-ish air with paint mist; power 380 V / 50 Hz.

1. Resistance: duct 40 m ≈ 40 Pa · elbows 8 × 20 = 160 Pa · filter loaded 900 Pa · hood/inlet 100 Pa · total ≈ 1,200 Pa
2. Air density: 1.2 × (293 / 353) × 0.95 ≈ 0.95 kg/m³ → correction factor 0.95 / 1.2 = 0.79
3. Select on standard air: required catalogue pressure = 1,200 / 0.79 ≈ 1,520 Pa at 6,000 m³/h
4. Model: this duty falls inside the 4-72 range (roughly a No.5, direct drive). Exact model and rpm come from the curve; we would also flag paint mist and specify a drain port plus easy-opening housing for cleaning.
5. Motor: Q = 6,000 / 3,600 = 1.67 m³/s → P_shaft = 1.67 × 1,200 / (1000 × 0.70) = 2.9 kW → × 1.15 = 3.3 kW → specify 4 kW

Example values are illustrative; every project is confirmed against the actual fan curve and test data before we quote.

5 mistakes we see again and again

• Quoting only kW. Power is a result of the duty point, not an input — it cannot be used to select a fan.
• Sizing the filter clean. Six months later the filter loads up, resistance doubles, and airflow collapses.
• Ignoring hot air. A fan selected for 20 °C will deliver roughly 40% less pressure at 200 °C.
• Using a radial-blade fan in a clean-air HVAC duty (or a backward-curved fan in heavy dust) — blade choice is a duty decision, not a preference.
• No inlet protection. Missing inlet guards, screens or rain hoods are the  common cause of early impeller damage.

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