Re: Vacuum cleaner blowers

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

having worked with a variety of fans from little tiny 1 cm diameter up to
40 foot monstrosities modified from a helicopter... I can provide a sort of
taxonomy here...

First, some basic rules of pushing air...

Centrifugal blowers generally are for high pressures and low volumes (where
high pressure is a few PSI/tens of kPa).
Fan type blowers (muffin fans, airplane propellors, etc.) are generally for
large flows and low pressure (typically a few inches/cm of water column..
i.e. 1/10 psi, 1 kPa)

The flow is VERY dependent on pressure drop for all blowers, and there is a
definite "sweet spot" where the efficiency (in terms of air moved at a
given pressure for electrical power in) is best.  Try to push a propellor
type fan against 1 psi, and it just won't work..

Noise bears no connection to efficiency/size/etc.... the acoustic power
from a fan is a tiny, tiny, tiny fraction of the electrical or mechanical
work involved.  Noise is mostly influenced by small changes in design, and
low noise fans are developed empirically (read, expensive cut and try)

For propellor type fans, operating the fan in a shroud or even just a hole
in a wall, greatly improves performance, as long as the gap is less than
say, 2% of the fan diameter (i.e. it's gotta be pretty close)

Some basic equations.... For most fans, flow speed (and hence volume) is
proportional to rotational speed.. double the fan speed, double the flow,
assuming same pressure rise across the fan.  Centrifugal fans have much
narrower ranges where this is true)

Doubling the pressure drop for the same flow, doubles the HP/watts.

Doubling the flow (or linear speed), requires 4 times the HP/watts (it goes
as the square... you're moving twice as much material (mass rate) at twice
the speed)



You've got high speed centrifugal blowers (vacuum cleaners are the prime
example).  They have a series wound motor and spin very, very fast.
Typically, they will run on AC or DC (brushed series wound motors are also
called "universal motors")  
>From Herwig's list, the following fit in this category:
> blower
> vacuum cleaner
> shop vac
> Hoover canister vacuum cleaner

They vary somewhat in performance, and are often advertised as being rated
in "Amps" (aieeee!).. but, to a first order, the fan power scales as the
electrical power.  The Sanyo data sheet provides a representative sample,
and you can just scale up and down according to rated electrical watts.
The rotor typically spins at >10,000 RPM
Figure on 1-2 psi pressure, 50 CFM... 

A leaf blower is sort of in this class, with much higher flows, and a bit
lower pressure....  flows would be in the 250-500 cfm area, pressure much
less than 1 psi
Auto turbochargers are special case... 250-500 cfm, but at 10-30 psi
pressure... (lots o' horsepower to run that pump...)  3:1 pressure ratio is
a practical limit (although you can do better with careful design)


You've also got propellor types... muffin fans, most cooling fans, etc...
Often run at lower speeds (some line synchronous rate: 1750 RPM, 1120RPM,
600 RPM, etc. (all in the 60 Hz world!)), to keep mechanical stresses low.
Fast speeds cause problems from: Tip speed approaching mach 1 (6 foot
diameter fan turning 3000 rpm has a tip speed of just under Mach 1),
vibration, centrifugal stresses, etc.  As mentioned above, think high
volume, low pressure for this sort of thing...(typically, these are rated,
in the US anyway, at 1/8"-1/2" watercolumn backpressure... 1/2" WC is about
0.02 psi...)

>From Herwig's list:
> fan
> muffin fan
>bathroom ventilator fan (some)

Then, you've got low speed centrifugal blowers, of which the squirrel cage
blower is a good example.. These spin at reasonable speeds (1000-3000 RPM),
and have performance midway between the centrifugal compressors
(turbochargers, leaf blowers) and a regular old "fan"...  They'll work
against fairly high back pressures (compared to a bladed fan)... say 0.5 psi..
>squirrel cage blowers
>some small cooling fans
>some bathroom fans

There are also axial compressors... basically fans in a tube, some with
stator blades.(trade name: vaneaxial)... high flows, a bit higher pressure
than a regular fan. In a axial flow turbojet, for instance, they use a
whole bunch of rows, each of which increases the pressure just a bit.. but
overall a high compression ratio (all those 1:1.05's multiply up... after
10-20 stages...)  You see these in their low pressure form in air
conditioning systems which have to drive a lot of ductwork (where the
static back pressure is high...)



Tesla list wrote:
> 
> Original poster: "Herwig Roscher by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <herwig.roscher-at-gmx.de>
> 
> Jim Lux wrote:
> > Since folks are sometimes using vacuum cleaner motors
> 
> Jim, all,
> 
> Living in a different part of the world, I've some problems when
> looking for cooling air sources. While it is easy, to convert CFM to
> m^3/h, it's difficult, to find the difference between
> 
> centrifugal blower
> leaf blower
> squirrel cage fan
> bathroom ventilator fan
> .....
> 
> Could you or somebody else please specify the related motor
> power or roughly sort them by increasing CFM?
> 
> TIA and cheers,
> 
> Herwig
> 
> ---------------------------------------
> Greed is the root of all evil!
> ---------------------------------------