Quote:
Originally Posted by trader4
Then someone should be able to explain the physics of why this
little 15" fan can require so much power to operate where all
similar size fans, doing similar work, don't require anywhere near
that much power. What is so special?
For example, an attic fan is fractional hp, 1/10hp. A central
AC condenser fan for a house that moves a huge amount of air
is 1/4 to 1/3. It's a very similar application, it's pulling air
through a radiator scheme. A furnace blower fan that moves
2000 cfm is 1/3 or 1/2hp. And I also showed where a 10hp
fan moves 66,000 cfm with a 5 ft blade. How do you reconcile
that with an auto fan?
Also note that a dyno test is without the car moving. With
the car moving air is being rammed into the fan, which would
substantially reduce the power needed. At highway speed there
is enough air that the fan probably isn't needed at all. And
was the test done with the electric fan running or off?
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It boils down to aerodynamic drag. As with increasing a vehicle's speed, it takes four times the power to double the speed. So if the engine fan is consuming 1 horsepower at 750 rpm, it requires 4 horsepower to spin it at 1500 rpm, and 16 horsepower at 3000 rpm. Moving air through the louvered fins of the AC condensor and radiator which are designed to create turbulence in the core for improved heat transfer, then push that air out of the engine compartment, requires a lot more power than moving air with a room fan or attic fan.
Electric fans are able to meet cooling requirements becaue they tend to move more air at lower vehicle speeds than the engine fan and because radiators now use aluminum cores for better heat transfer. As the vehicle's speed increases, enough air is forced through the radiator to cool the engine generally, although the electric fan will kick in for additional support under high engine operating temperatures or when the AC compressor is running.
Attic fans, room fans and ceiling fans move high volumes of air because of their blade area. As a result, they don't need to run at high rpms and thus don't have the same power requirements. And I don't think any of us would want a 50 inch tall radiator core.
As a side note, my 1981 Bonneville diesel was equipped with the heavy duty police cooling option which consisted of a steeper pitched fan, heavy duty locking fan clutch, and double belt drive pulley. When the fan clutch engaged, it felt like it could suck a small car right through the grille, but it would knock 20 degrees off the temperature guage in less than two minutes. When it comes to automobile cooling, it's all about air velocity vs radiator frontal area.
2002 X5 3.0 281,400 miles
2004 325i 120,700 miles