
re: fan air, don't pump water heat exchanger
19 oct 2000
toby wrote:
> suppose we gather heat in some sort of tank (like 55gal, waterfilled
> drums). ..we might coil 5/8" dia, black irrigation tubing ($25
> per 500ft) inside your drums and blow air thru tubing.
>the data/calc's below, show that a $11, 12vdc, 2w, computercase fan
>can blow 4cfm of air through a static pressure of 3"h2o (incurred by
>the air/tubesurface friction) in a 100' long, 1/2" diameter pipe.
was that a 1/2" or a 5/8" diameter pipe?
>the air will be travelling at 1000 fpm (11 mph).
but 4ft^3/min/(pi(1/2/12)^2ft^2) = 2,934 ft/min (33 mph)...
>by extending the "duct pressure loss" chart from the 1965 ashrae hof
>book to show cfm's below 10cfm, and for pipes with diameter of 1/2
>inch, i found that if air was moving at 1500 ft/min (17 mph) for a 1/2"
>diameter pipe, you would move 7 cfm against a pressure, due to friction
>in the "pipe" (duct) of 10 inches of h20/100 ft of pipe.
didn't you say this air was moving at 1000 vs 1500 fpm?
say it's 2934 ft/min. page 32.5 of the 1993 ashrae hof says the reynolds
number is 8.56dhv = 8.56(1/2)2934 = 12.5k, and table 1 says pvc pipe might
have an absolute minimum roughness eps = 0.00003 ft on a good day (0.00015
on a bad day.) the altshul approximation (21) says the friction factor
f = 0.11(12eps/(3.7dh)+68/re)^0.25 = 0.0327, if that applies to such fast
air in tiny pipes, so on a good day, the friction loss of a 100'straight
pipe with air density rho = 0.075 lb/ft^3 is f(12l/dh)rho(v/1097)^2
= 0.0327(12x100/0.5)0.075(2934/1097)^2 = 42" of water. (the answer is
always 42.) but your pipe will have some direction changes, raising
the loss, since the drum isn't 100' long...
>my "basic air conditioning" book gives a formula for calculating the hp.
>hp = v*p/(6356*e)
>where:
>v is in cfm
>p is pressure
>e is efficiency...
>>...my book says that fan efficiencies range from 70 to 90 %.
that might work better for larger fans. norman saunders, pe, estimates
that a fan's efficiency is about the same as the blade diameter in inches.
>this puts us in the range of an $11 8cm computer case cooling fan,
>rated at 12vdc and 2.28 w. (see the "fans" section at:
>>http://www.aboutcomputerparts.com/) it will "blow" 32.4 cfm @ zero
>>static pressure and 0 cfm @ 3.4"h20 min.
what a remarkable little fan. grainger's excellent 3" 12v 2.4w 4wt35 fan
moves 35 cfm in free air, 15 at 0.2" h20, 10 at 0.3", 6 at 0.4", and
0 cfm at 0.5" h20. rotron's 11w ac 3" fan (grainger 5c152) moves 34 cfm
at 0 sp and 0 cfm at 0.15" h20. neither moves any air at 42" h20.
>may jesus bless,
may he grant you common sense and humility, and a desire to appreciate
nature's behavior as it is, rather than as you wish it to be, as well as
a desire to check your calculations from more than a single direction
before posting this sort of confusing nonsense again.
nick

