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OK, well, I was wrong about the 30% efficiency. I can’t find too much at the moment, but will try and find more if possible.
Here is what I got:
Multivane centrifugal 50-60 % efficiency. Advantages: Low peripheral speed, quite and compact. Disadvantages: Severely rising power characteristic requires large motor margin. Applications: All low and medium pressure atmospheric air and ventilation plants (air conditioners)
Backwards inclined centrifugal. 65 – 75% efficiency. Advantages: good efficiency. Non-overloading power characteristics. Disadvantages: High peripheral speed. Much louder than multivane centrifugal. Applications: Medium and high pressure applications.
Axial Flow (Porsche fan – 8 tip fan) 60 – 75 % efficiency. Advantages: Very compact, straight through flow, suitable for installation in any position. Disadvantages: High tip speed and sound level – slightly less severe for fan with guide vanes. Low pressure. Applications: Low pressure atmospheric air applications and large ventilation schemes such as tunnel ventilation.
A few things come to mind while I write this, especially the basic hydraulic theory of: high pressure = low volume and low pressure = high volume. Think of it this way. Hold a water hose pipe in your hand. Cover 90% of the open side, the water flies out at high speed (high pressure) and travels a long way, however it is a fine spray with very little quantity. Now open the hole again, the water lands almost by your feet (low pressure) but soon floods the local area to an extent much greater than the fine spray (high pressure).
The Flat 4 engine design does not provide much resistance to flow (many cooling vane openings), so in my opinion, a medium pressure fan is not ideal. The way that our engines cool is through high air flow over the fins. Low pressure = high volume.
If a fan, as with any pump, does not work within its design criteria, it will loose efficiency – it will still work, but not as well.
Also, the Porsche fan was used to cool a 3.6l 6 cylinder. If it didn’t work, they would have used the best fan for the application. Also, with the efficiency advantage, all things the same, you still get more air flow for the same amount of power used.
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