Subject: Fan power (was: Cube of the wind speed??)

From: [email-masked] (Nick Pine)
Date: 8 Mar 1995 08:58:52 -0500

>  The power required to operate a fan varies with the cube of the amount of 
>air moved (usually in CFM, cubic feet per minute). 

I think we might add, "for a given duct size." If we make the air ducts large,
and the airspeed low, we can make the dynamic air pressure and duct friction
and fan power as low as we want, practically-speaking. For instance, 100' of
a 2' x 2' duct with a cross sectional area of 4 ft^2, carrying 2,000 CFM and
500 linear feet per minute (lfm) of air, has a back pressure of only about
0.01" of water, according to my 1993 ASHRAE HOF.

This goes well with the performance of the 16" Grainger 4C861 1600 rpm (max),
high-temp (311 F), high-priced ($359), very reliable (3 year guarantee),
low-power (240 watts, max, with no speed control) fan, which is rated to
deliver 2765 CFM in free air, and 2190 CFM at 0.20 inches of water.

And this pressure and fan power is quite low compared to many hot air heating
systems using more powerful blowers with tiny cheap ducts. One way to make big
ducts is to blow air between multiple studs or joists or rafters... So if
we want to minimize fan power, and use fans instead of more energy-consuming
blowers, we should make the ducts big and the air velocities and rpm's low.
The last two things also minimize acoustical noise.

Norman Saunders recommends that new houses have a 2' x 2' airshaft from
top to bottom, so solar heat can be stored upstairs in the winter, and
easily transported downstairs, and cool air from the basement floor can
easily be transported upstairs in the summer. He says this allows easy
air movement of 1 m^3/sec, which transfers 1 kW/degree Kelvin, or was that
10 degrees Kelvin? Let's see. One Btu can heat 55 ft^3 of air 1 degree F...

Nick