Propeller fans

These may be regarded as a special type of axial fan designed to operate without a casing, the impeller being situated in a hole in a wall or partition. The fans are simple low cost units with broad bladed impellers usually formed from sheet metal. The blades are shaped to operate with an orifice flow pattern, de­flecting the air with the minimum flow separation or vortex for­mation. Design techniques make use of flow visualization with stroboscopically viewed smoke trails.

The blade form is usually optimised for pressure differences across the partition from zero to about 100 Pa. Above the de­signed pressure the flow pattern changes drastically. The outlet jet assumes an expanding conical form with reverse circulation at its core as sketched. Towards zero volume flow, discharge is radially outwards, and the centrifugal mechanism is now re­sponsible for pressure development.

Propeller fans are quiet and effective for ventilation purposes, both supply and exhaust. They are also used for unit heaters and similar applications where some resistance is encoun­tered. For these an experimental matching of the fan and the unit is important since the pressure development and the flow pattern over the heat exchanger are very dependent on the blade and orifice plate positions.

As more of the impeller projects on the outlet side of the orifice, the free flow volume falls, because the inlet orifice flow no lon­ger covers all the blade. At the same time the pressure at low flow rises because more blade is exposed on the outlet side for centrifugal action. The free flow can be substantially increased by rounding the orifice edge or fitting a rounded inlet ring. (See Figure 1.94 for the variants).

This is because the vena contracta is expanded and less veloc­ity pressure is required for a given volume flow. Moderate pres­sure performance is also helped, but high pressure develop­ment is impaired. If the rounding is enlarged into a true bellmouth and a short tunnel formed around the impeller, the fan becomes in effect an axial fan, and is better served by an aerofoil section impeller.

The impellers of propeller fans are almost invariably mounted on the shaft of the driving motor. The air flow cools the motor, which can be totally enclosed to keep out dust. The impeller power rises rather sharply if the volume flow is drastically re­stricted, and the motor could be over-heated, particularly if on the downstream side, where centrifugal flow starves it of cool­ing air. However, propeller fans are not often used in systems where such excessive resistance could arise. Typical perfor­mance curves are shown in Figures 1.95 and 1.96.

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