Choosing the wheel: FC, backward-curved, radial and radial-tip

Put four different impellers in the same scroll casing and you get four different machines — different efficiency, different power behaviour, different tolerance for dust. The blade form is the fan. Get it wrong and no amount of casing, coating or motor margin rescues the selection. Here is the one-page map: what each wheel is for, and how to match it to the air you actually move.

Forward-curved: the compact clean-air wheel

Many shallow blades leaning into rotation. Compact and quiet, which is why it dominates small HVAC boxes — but two things push serious industrial duty past it. Efficiency is modest (around 55–65%), and the power curve is overloading: shaft power keeps rising with flow, so a duct change or a filter that clears can pull the motor past its rating. That rising-power behaviour is the reason we do not build forward-curved wheels for process duty — the risk sits on the wrong side of the curve.

Backward family: efficiency, and a motor that cannot be overloaded

Blades leaning away from rotation — flat (backward-inclined), curved, or a true aerofoil section. This family gives the highest efficiencies of any centrifugal wheel (roughly 78–90%, aerofoil at the top) and a non-overloading power curve: brake power peaks and then falls, so the motor is safe across the whole operating range. It is the default for clean and lightly dusty process air, and where most efficient plant duty should land.

One honest caution: a hollow aerofoil blade erodes if you feed it particulate — the thin drawn section is the first casualty. In dust, step down to the backward plate versions, which trade a little efficiency for a solid blade that survives the airstream. That plate wheel is the everyday workhorse of dusty-but-not-abrasive process air.

Radial and radial-tip: built for the air that punishes other wheels

Heavy radial (paddle) blades with wide spacing are self-cleaning and shrug off abrasive, sticky, high-pressure duty — the coal dust, fly ash and material-handling streams that would clog or grind away a backward wheel. You pay for it in efficiency (around 50–65%), and you buy survival and high static in return. This is the wheel for the ugly end of dust extraction and pneumatic conveying.

The radial-tip wheel is the bridge between the two worlds: radial where the dust and gas strike, backward-leaning in overall behaviour. It handles erosive, particulate-laden gas at meaningfully better efficiency than a pure paddle — the middle ground when the air is too dirty for a backward plate but you still want the power bill under control.

The selection map

The rule, and the question to ask your vendor

Match the wheel to the airstream first, the duty point second. Clean air → backward or aerofoil for efficiency. Light dust → backward plate. Heavy, abrasive or sticky → radial-tip, then radial. High pressure at modest flow → radial or the pressure-blower end of the range. Only once the blade form is right do you pin the diameter and speed to hit the duty point — and the efficiency you chose the wheel for is exactly what the cube law then rewards, hour after hour.

The one test that sorts a real fan engineer from a catalogue seller: ask them to put the wheel rationale in writing. A single line — “why this wheel for my air?” — tells you more than any brochure. If the airstream is genuinely abrasive, that same conversation should cover blade thickness and wear protection, not just the curve.

The Jitamitra answer

We build the full span of the map — aerofoil, backward-curved, backward plate, backward-flat, radial-tip, radial and large-backward wheels — and select against your stated duty with our own engineering software, with the wheel rationale written into the quotation. One family per airstream, not one wheel forced across all of them.

Further reading. Browse the fan family pages to see each wheel type in build; then Specifying the duty point for the flow-and-pressure side of the selection, Dust-extraction fan anatomy for the dirty-air end, and The cube law for what wheel efficiency is worth over a running year. Blade-form classification and the standard performance curves follow the US Department of Energy fan sourcebook, Improving Fan System Performance (2003, public domain).

Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.

What that efficiency actually costs you in kW

Blade form is an efficiency decision, and efficiency is a power bill. Here is the sum, so you can run it before you ask anyone for a quotation.

Take 100,000 m³/h at 200 mmWC and put the two wheels above through it:

Thirty-one kilowatts, running. At 6,000 hours a year that is roughly 186,000 kWh — which is why the wheel is a running-cost decision and not only a first-cost one. It is also why we will not put an efficient aerofoil in dirty gas to win a comparison: it would not survive, and a wheel that fails is not efficient at any number.

The boundary that causes most arguments

That figure is fan shaft power. It is not what your energy meter reads. Motor input adds the motor's own losses, and a belt drive adds two to five per cent more. So if you measured input power at the panel and compared it with a catalogue efficiency, you compared two different quantities and the fan will look worse than it is. Compare shaft-to-shaft, or add the losses to both sides.

η here is fan total efficiency. A static-efficiency figure is a smaller number for the same machine, so check which one a competitor's datasheet is quoting before you compare two fans on it.