Why One Number Is Never Enough

Buyers often request a fan by airflow alone: "I need 10,000 CFM." That figure describes only half the job. A fan must deliver a volume flow rate at the pressure required to push that air through the system. Add a filter, extend a duct, or close a damper, and the airflow falls unless the fan has pressure to spare. Fan selection is therefore always a pair of numbers: airflow (CFM or m³/h) plus pressure (Pa or mmH₂O).

CFM and m³/h: How Much Air Moves

CFM (cubic feet per minute) is the imperial measure of volume flow; m³/h is the metric equivalent. The conversion is straightforward: 1 m³/h ≈ 0.5886 CFM, and 1 CFM ≈ 1.699 m³/h. This number tells you how much air crosses the fan every minute. On its own, it says nothing about whether that air will travel through the ductwork, filters, or heat exchanger you have in mind. A fan rated for high CFM at near-zero resistance can be a poor choice for a system with real resistance.

Pascals and mmH₂O: What the Air Must Push Against

Every system resists flow. Duct friction, filters, dampers, silencers, elbows, and vertical height all consume pressure, and the fan must generate at least that much to move the required volume. Pressure is stated in Pascals (Pa) or millimetres of water column (1 mmH₂O ≈ 9.81 Pa); imperial selections may use inches of water gauge. Total pressure is the sum of static pressure and velocity pressure. When a supplier asks for "static pressure," they mean the resistance the fan must overcome before the air leaves the system.

The Duty Point: Where Flow and Pressure Meet

A fan's own curve falls as pressure rises — the harder it must push, the less volume it moves. The system curve does the opposite: resistance rises with flow. Where the two curves cross is the duty point, the actual operating airflow and pressure. This is why a fan quoted at free delivery may sit far from its required flow once filters load with dust. Always select at a stated duty point, and ask where that point sits relative to the fan's peak efficiency.

Reading the Curve: AMCA 210 and ISO 5801

Reliable selection depends on tested performance curves, not catalogue impressions. SINDFAN fan performance is established by test to AMCA 210 / ISO 5801, so the published curve reflects measurable behaviour. Rotors are balanced to ISO 1940 G4.0 for smooth running across the operating range.

Matching Fan Type to the Duty Point

  • Axial fans suit high flow at low to medium pressure: general ventilation, cooling towers, tunnel supply, and dust collection. The SINDFAN SM axial series covers diameters from Ø250 mm to Ø3150 mm and reaches efficiencies above 80%.
  • Contra-rotating axial fans place two impellers in series, producing substantially more pressure at the same diameter. A contra-rotating unit built for mining duty delivers approximately 3400 Pa at 7.5 m³/s.
  • Jet fans are specified by thrust rather than static pressure, which is the correct metric for car parks and tunnels. SINDFAN jet fans generate 152–1691 N, measured per ISO 13350.

What to Send When Requesting a Selection

Provide both numbers and the context around them: required airflow in CFM or m³/h, required pressure in Pa or mmH₂O, gas temperature, air density or site altitude, and a short description of the system. Note whether the pressure figure is static or total, and whether it includes filters in a dirty condition. With those details, a fan can be selected on its curve at a realistic duty point rather than on airflow alone.

To discuss your duty, contact SINDFAN on WhatsApp at +852 9524 8434 or email info@sind-tech.com with your airflow, pressure, and system details, and our engineers will review the selection with you.