are my fan choices too big ?

[ivanhoew]

hello everybody ,

i have a batcave im equipping ,and apart from a chassis dyno and other trinkets , i have a couple of fans from which to make a bench eg...












the question is are they too big ,too small, or just right ?

it seems to me in my state of total ignorance ,they are quite big ,but the motor ,a 4hp 3 ph ,is a lot less bhp than the cumulative load of vacuum motors used ..

i have a flow quick audie tech gizmo i use at the moment on a couple of vacuum cleaners .these are 1700 and 1100 watts .so will i get much the same flow from my 4hp motor ? even though the fans are bigger .

thanks
robert .

[Tony]

Difficult to say for sure.

If you already have the Audie, I suggest you fire up each blower and plot flow versus pressure with a range of different flow restrictions. Then plot a flow versus pressure curve.

The diameters of both blowers look rather small, so very high blower rpm will be required in both cases to reach useful flow bench pressures. Only way to know is to try it !

[Malvin]

This might help you look for spec for Fans

http://www.cincinnatifan.com/catalogs/H ... ternet.pdf

[Tony]

Great resource Malvin !!!!

The smallest rotor shown is 15 inches diameter.
For rotors smaller than 15 inches diameter, what is important is tip speed to determine developed pressure differential.
If your rotor is only (say) 7.5 inches, just double the rpm on the graph to give similar pressures as a 15 inch rotor.

If you wish to use rpm other than 3,500
Pressure is very nearly proportional to rpm (or tip speed) squared.

With a bit of mathematical gymnastics, it is possible to predict the developed pressure from any given rotor diameter and rpm from these blower curves Malvin has provided .

[ivanhoew]

greart ill have a measure ,maybe take them apart to measure the wheel dia ,and work out the gearing .
i do remember ,the paxton came off a chassis with a 15 hp 3ph motor and a plate rating it at 800cfm at 1.5 psi . but i could not run that big a motor on domestic mains i think .

[ivanhoew]

bit more from c fan...

http://www.cincinnatifan.com/catalogs/P ... ternet.pdf

bit of measuring and rpm working out should narrow it down .

[ivanhoew]

i think mine is the at800 .....god knows what rpm it runs at !!!!

[Malvin]

This should help you figure out the RPM It also goes into the pitch of the pulleys

http://www.gizmology.net/pulleysbelts.htm

These Calculators are nice but I still write it all down to do the math old habit's die hard.

[ivanhoew]

righty ho ,
well the iron case fan is an approximately 10 inch wheel ,and had a 3.33:1 ratio step up ,so it on a 2820 rpm motor so will be doing 9390 rpm .wow ! that seems a lot .

so from the fan data sheets above ..

25'' h2o ...620 cfm... 20 '' wheel @3500 rpm .

so a 10'' wheel @9240 rpm hmmm goes to find calculator...


so , 20'' wheel is doing 305 fps tip speed

10'' wheel is doing 409 fps. tip speed .

WOOF!!


SO
305 SQUARED IS 95481
409 squared is 167281

so 75.198% bigger .. hhmmm think iv missundrstood this a bit ...

ok back again ,so ,if pressure is proportional to the square of tip speed ,ill work out a ratio on the 20 incher ..


25 inches water >>95481(sqr of tip speed in fps)
so.....1 inch water = 3819
167281/3891=42.99 inches water . does that make sense ?

[ivanhoew]

incidentally,i had some success getting my first 3ph motor to run on single ph ,this one for the ex extraction on the chassis dyno ..

http://www.youtube.com/watch?v=O9B1OQ-FDtQ