In a message dated 3/14/99 2:38:11 PM Eastern Standard Time,
105671.471@compuserve.com writes:
> Dan Masters writes:
> >3) Power required to spin the fan blades increases exponentially (I think
> that
> >is right - correct me if I'm wrong) as fan speed increase.
>
> Close. The power required is a cube function of the speed. This is
> because
> the pressure drop through the radiator is a square function of the rate of
> air flow. So to calculate power you multiply the air flow X the pressure
> differential. If you will, the air flow is analogous to current and the
> pressure differential is analogous to voltage.
Yes, but I hedged my bets! I didn't know if it was the square or the cube, so
I just said "exponentially," which covers them both. Slick, huh?
> OK. Using this "feinstance" if the mechanical fan uses 0.09 HP at, say,
> 1000
> RPM then at 5000 RPM (in a competitive situation) the fan uses:
> 0.09 HP X (5000/1000)^^3 or 0.09 X 125 = 11.25 HP!
Yep, using the cube changes my statement below to "will draw much more,"
rather than "may draw more." I don't think there is any question but that an
electrical fan is the way to go for competition, but I wonder how many of the
folks on this list have EVER had their engine up to 5000RPM? Show of hands,
please! I wonder how many even routinely run the engines over 3500 RPM (app
75 mph in 4th)? 0.09 HP X (3500/1000)^^3 or 0.09 X 43 = 3.87 HP
> >2) At very high speed (and corresponding high engine speed), the
> mechanical
> >fan may draw more power than an electrical fan; however, because neither
> is
> >required, the electrical fan will be off, so the mechanical fan will draw
> >significantly more power.
> A very good disertation, Dan. Please accept my modifications in the spirit
> they were meant. (That is from a busybody know it all! ;-) )
Thanks, and I do.
> Another point is that the electric fan draws power from the engine mostly
> when it is idling and we don't miss the HP anyway!
Good point.
> The reason most cars of the era had belt driven fans is because that type
> of fan is cheap. The reason most modern cars have electric fans today is
> because the motors are mounded sideways and there are no reasonable
> alternatives.
I agree, but there is another advantage to using a mechanical fan - if the
engine is running, the fan is running. With an electrical fan, there are
several failure modes that can leave you with a functioning engine, but not
enough cooling to get you out of the traffic jam and back home. If you can
make to the open road, you'll be OK, but if you don't get moving soon, you'll
be in big trouble.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://members.aol.com/danmas/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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