The Sirocco Fan: Many Shallow Forward-Curved Blades

US 662,395, 'Centrifugal Fan or Pump', filed 21 September 1898. The rotor is *"constructed with numerous thin elongated blades arranged in substantially drum form"*, enclosing *"a relatively large and practically unobstructed intake-chamber"*. The numbers are in the text: each blade's length should be *"at the least three times their depth"* and *"preferably as much as nine or more times"*; in the preferred form the intake chamber's diameter is *"ten times the depth of the individual blades"*. Curved blades are mounted so that *"their concave faces are toward the direction of rotation"* with the outer edges *"tilted forward slightly"*. Those three ratios are the design brief for step 3.

**Sirocco wheel:** 100 mm outside diameter, 36 shallow blades each 8 mm deep radially (an intake about ten times that across, per Davidson), 75 mm long (over nine times the depth), each blade curved with its concave face forward. A solid back disc carries the hub; the front is an open ring. **Backward-curved wheel:** same diameter and width, 8 thicker blades curving the other way. **Scroll casing:** a spiral whose gap to the wheel grows round to a rectangular outlet, with a round inlet on the axis. Mount the hobby motor behind it, wheel on its shaft.

Run each wheel from the bench supply at the same voltage. Hold the anemometer across the outlet and read the air speed; flow is air speed × outlet area. Read the motor current on the supply. Then throttle the outlet with a card, in steps from open to nearly closed, and read both again at each step. The Sirocco wheel moves more air at the same speed. Watch its current: it is highest with the outlet wide open and falls as you close it — the notebook's rising power curve, measured. The backward-curved wheel's current changes far less. That is why a Sirocco blower must never be run without the duct it was sized for.

**US 662,395, 'Centrifugal Fan or Pump', Samuel Cleland Davidson of Belfast, Ireland; filed 21 September 1898, patented 27 November 1900.** Davidson's Sirocco Engineering Works had begun with tea-drying machinery — hot air blown through leaves — and the fan grew out of the need to move large volumes of warm air. It became standard for ventilation, mine and ship fans, and it is still the small blower in countless appliances. **Honest limits.** Forward-curved fans are less efficient than good backward-curved ones, and their rising power curve can overload the motor when the system resistance falls. Their many small blades collect dust and grease. They suit low pressures and large flows, not high pressures.