How to Specify a Centrifugal Blower

Everything starts at the duty point

A duty point is two numbers that belong together: the airflow you need, and the static pressure the blower has to produce at that airflow. Airflow alone is not a duty point. Pressure alone is not a duty point. And the two are never multiplied into a third number sent instead.

The reason sits in how catalogue figures are produced. Maximum airflow is measured at zero static pressure, with nothing in the way; maximum static pressure is measured at zero airflow, with the outlet blanked. They are the two ends of one curve and never occur together. On one of our 146 mm EC double inlet blowers — 230 V, 115 W, 1,450 rpm — the curve at a 10 V control input runs from about 670 Pa at the blanked end to about 1,009 m³/h at the open end. Yours sits on the line between them.

Multiplying the two is a separate trap. Airflow × pressure is air power, worth sixty seconds as a sanity check: 1,000 m³/h at 400 Pa is 1,000 ÷ 3,600 × 400 = 111 W, and no blower reaches that point on less input power. A datasheet claiming otherwise can be put down. What air power is not is a duty.

Estimating system resistance when you have no curve

Most OEMs have never measured a system curve. Resistance rises roughly with the square of airflow, so one honest pair of numbers is enough to start. Three routes that work:

• Measure the machine you already have. A static pressure reading at the blower outlet at a known airflow beats any estimated table.
• Add up the components at design flow. Filter, coil, heat exchanger, grille, silencer, duct and bends each publish a pressure drop at a stated flow. Sum them at your design airflow.
• Send us the assembly. A drawing, a STEP file or the cabinet itself; we work from the free area, the inlet approach and the outlet route.

Then state three things about the number, because each moves it further than it looks: static or total pressure; clean or dirty filter — the honest figure is the final resistance the machine still has to run at; and the air state. Catalogue and selector curves are standard air, 20 °C and 1.2 kg/m³; above roughly 1,000 m, or in hot gas, say so and the correction gets applied.

Reading a P-Q curve, and where your point should sit

A P-Q curve plots static pressure up the side against airflow along the bottom. A speed-controlled blower has a family of them: the same 146 mm EC wheel reaches about 670 Pa on a 10 V input, roughly 380 Pa at 6.5 V and roughly 160 Pa at 4.5 V. Your system curve crosses one of those lines in exactly one place, and that crossing is what the machine does in the field. Where it belongs is easiest to state as two places it does not.

• Not at free delivery, the open right-hand end. A forward-curved wheel has a rising power curve: the less resistance it sees, the more air it moves and the more shaft power it pulls. Sit out there and the motor is at maximum load every time a damper opens — the classic commissioning trip. Ask for the maximum shaft power at the lowest resistance your system can reach, and size the supply for that.
• Not at shut-off, the left-hand end. Airflow is unstable there, noise climbs, and a slightly dirtier filter drops you further into it.
• Aim for a 10–25% pressure margin at your airflow — the static pressure still left above your point. Below about 10%, a loaded filter pushes the machine into the unstable region. Above 25%, you have bought capacity you will throttle away for the life of the product; our selection tool scores a 139% margin down, not up.

Curves, drawings and CAD for individual models are on our selection site. Family range limits are on the centrifugal blower hub, and the forward-curved power curve on the forward curved squirrel cage blower page.

The ten lines that decide the model

Copy this into your RFQ. Anything you do not know, write “unknown” rather than guessing: a guessed figure is worse than a blank one, because it gets designed in.

Name the certification target before the drawing is frozen

Approvals are scope-limited, and the scope is the part that matters. Our KTS CE set of 11 July 2022 — LVD KB2207084L01, EMC KB2207085E01, RoHS KB2207086R01 — covers AC forward-curved blowers LWFA108 to LWFA450 only. Part of the EC and DC range holds ETL Recognized Component status under Intertek control number 5022563, inside that report’s model rule and power ceiling (DC: 600 W at 48 V, 500 W at 24 V, 200 W at 12 V; around 1,100 W on the EC side). Air performance is tested to the AMCA 210 method, which is how the curves you receive are produced; noise is measured in a semi-anechoic room calibrated under report SSD202500326. IP figures are design ratings — we hold no third-party IP certificate. Send the exact model code and the scope is confirmed in writing; full list on the quality and certifications page.

What comes back, and how it is proved

The first reply is normally the nearest catalogue model — the fastest route to something you can fit and test — with a modified version quoted beside it. Selection is confirmed against your duty point — airflow, static pressure, supply voltage, control mode — and validated on your own unit before the drawing is frozen. Samples of catalogue models ship in 1–3 days, production runs 15–30 days, warranty is 12 months. The route from enquiry to first article is at OEM process.

Send the ten lines, or just the first one, to our engineering desk and you will have candidate models, their curves with your point marked and a sample date — usually by the next working day.