[Gasification] blower suggestions?

[Mark Ludlow]

Hi Daniel,

Well, a simple inquiry about blowers has certainly elicited a wealth of
responses!

In my way of looking at things, shop vac motors are good for...well, shop
vacs and most just barely adequate for that. Unless you're lucky enough to
buy an old one, you'll find that anything recent is pure plasticrap, with
cheap bearings and high noise levels.

In the centralized dust collection system in my cabinet shop of times past,
plenty of suction and pressure head was provided by a 3-hp, 3,550 rpm
direct-drive, straight-bladed Rayco centrifugal blower. The blower motor and
impeller were totally isolated from each other (the motor being TEFC).

Tip speed is a function of both rpm an impeller diameter. A blower for an
air-impingment dryer I have been working on for the past year produces 57"
of static head and its closed impeller is slightly overdriven by a
belt-drive 30-hp, 3,550-rpm motor. Regenerative blowers produce several psi
of head pressure at 3,550-rpm but high air velocities mean high noise
levels.

For all of my projects I usually build scaled-down prototypes from
industrial detritus. Fans are easy because there's a lot of them sitting
around. It's difficult to imagine why anyone would select a 6-71 Roots
blower for such a simple project, but that was one suggestion I read. If I
need a fan that lasts, I would never choose a universal-wound motor, except
in desperation. Induction motors can be easily speed controlled (to 180
Hz--check your bearings first!) with inverter variable-frequency drives, and
used and new surplus drives are plentiful on the online auction markets. For
example: I recently bought a new Reliance Sensorless Vector 3-hp drive for
$79. It's no problem to derate it and run it on 240V single-phase. Goodbye
brushes and 1,000-Watt (resistive) load limitation of dimmers; hello the
ability to initiate control remotely, or automatically through a feedback
sensor (Pressure? Temperature? Air Velocity?). Overkill? Perhaps, but
industrial performance and reliability at bargain-basement prices!

Cheers,
Mark

-----Original Message-----
From: Daniel Chisholm [mailto:dmc at danielchisholm.com] 
Sent: Wednesday, August 08, 2007 4:58 AM
To: Discussion of biomass pyrolysis and gasification; mark at ludlow.com
Subject: Re: [Gasification] blower suggestions?

On Tue, 2007-07-08 at 13:00 -0700, Mark Ludlow wrote:
> Lets assume that the fan is driven by an induction motor without
> brushes.

Probably not.  High pressure blowers (e.g. the shop vac or central vac
ones we're talking about here) need high tip speed in order to produce
high pressures (and tip speed goes as RPM times diameter).

The ones I've taken apart use universal motors (i.e. a brush motor like
you'd see inside an electric drill, circular saw, angle grinder, etc),
and turn (SWAG) 10,000-ish RPM.

The nice thing about universal motors is that you can control their
speed with a $7 light dimmer switch (within reasonable limits - it has
to be kept going fast enough to get adequate cooling air).

Whereas an induction motor is inherently a constant speed machine, and
its speed can't be cheaply and practically varied (which took me a while
to wrap my brain around - if you reduce the applied voltage, e.g. with a
lights dimmer, it won't slow down, but it will draw more current in
response, and overheat!)

A $20 Princess Auto special blower (almost certainly from a shop vac)
that I have had sitting on my bench forever, uses a centrifugal blower
wheel (approx 4" dia.) for moving its main charge, and uses a small
axial fan for its cooling air.  This would be a better one to use for
sucking on a gasifier than my house's central vac, which passes all of
its air through the motor.

Jim, have you thought about pushing your air through your system, rather
than pulling?  It's a tradeoff of course, it makes some things such as
sealing and fuel feeding more difficult, and others such as fan
selection easier.


-- 
- Daniel
Fredericton, NB  Canada