Building a Flow bench, motor speed:flow

[Greg Hermann]

To do effective work at 25 or 28 inches of water pressure (the two head
flow testing "standards"), you are going to need a regenerative pressure
blower rather than a fan to do it. These are available, used in industry
for stuff like aerating the "liquid" in sewage treatment plants. Siemens
and Gast are two outfits who make this type of blower, but there are plenty
or others out there besides them. A web search on "pressure blower" would
turn 'em up.

To put it into plain language--you are NOT going to make this kind of
static pressure at such a low (!!!)  flow rate with a simple fan on any
kind of a practical basis. With a fan wheel of any practical size, and at
any practical fan rpm, you would put the fan into aerodynamic "stall"
_LONG_ before getting up to that level of static pressure !!!! If you want
to do your testing at a non-standard pressure--say 5 or 6 inches of H2O,
then that's another story, but you would also be talking much smaller flow
numbers and a big drop in accuracy.

To do 600 cfm at 33" water column or so (allowed for some loss in the
ducting and flow measurement orifice here), it will take about a 5 HP,
maybe even a 7.5 HP motor to drive the thing. (Already corrected for blower
efficiency here, regen blowers and pumps are NOT known for being terribly
efficient critters.)

You probably want to have an assortment of flow measuring orifices (say 6
or so, different sizes) so that you can do accurate testing down at lower
flow rates (thus staying in the useful part of the "range" of the
differential pressure sensor used for flow measurement).

Variable speed drive is a wonderful way to go, much better than a
restrictor orifice, but also more $$$. Emerson makes some variable AC
drives that would do the job right and less $$$ than most.

Again, more $$$, but static pressure sensors are available. My approach
would be to control the speed based on the TEST delta P, and use a separate
delta P measurement (across a known, fixed orifice upstream of the test) to
measure the flow rate. You are also going to need decent temperature,
humidity, and absolute pressure instrumentation if you want to get anything
resembling repeatable test results. Mamec Sensors (Minn., USA) makes all
the sensor type stuff you would need to do it right, and for reasonable
$$$.

Greg

At 4:44 PM 1/28/02, The Punisher wrote:
>Restrictor orifaces SUCK (pun intentional) The electronic controlled
>superflow 600's have all the traditional manometer and oriface's ect.. But
>the little computer just trys to maintain the test pressure by regulating
>the motor speed. it then uses some sort of map sensors inside the
>controller(might be like megasquirt) to calculate the flow. IIRC you just
>run it on the biggest oriface and wait for the reading to stabelize (takes
>several seconds). Very simple.
>Also instead of one big,expensive,complex blower, just use several small
>inexpensive ones in parallel. Thats how alot of flow benches are made.
>
>
>>From: "The Dupuis" <dupuis10 at telusplanet.net>
>>Reply-To: diy_efi at diy-efi.org
>>To: <diy_efi at diy-efi.org>
>>Subject: RE: Building a Flow bench, was: Measuring air flow
>>Date: Sun, 27 Jan 2002 14:51:18 -0700
>>
>>Gus Mahon uses a leaf blower that makes 13" fully stalled.  I think this
>>would work but you'd have to use more than one, or measure flow AND
>>pressure
>>drop and calculate go get to a standard known pressure drop.  Obviously,
>>with the correct air pumps you'd regulate airflow to arrive at a known
>>pressure drop and then measure the airflow, but with a pump that small,
>>you'd have to run it wide open and measure both.
>>
>>I don't know how fast you hope to spin the turbo with an AC motor, as there
>>are none that spin faster than 3550 rpm.   How powerful and fast a DC motor
>>do you have?  interesting prospect though, being able to regulate the speed
>>of a DC motor to vary airflow rather than use a restrictor orifice.
>>
>> > -----Original Message-----
>> > From: owner-diy_efi at diy-efi.org [mailto:owner-diy_efi at diy-efi.org]On
>> > Behalf Of 944Technologist
>> > Sent: January 27, 2002 4:17 AM
>> > To: diy_efi at diy-efi.org
>> > Subject: Re: Building a Flow bench, was: Measuring air flow
>> >
>> >
>> > I was looking at using a leaf blower. It has a dead gas engine
>> > but it looks
>> > easy enough to convert it to electric. Or add an electric motor to a
>> > 924Turbo Turbocharger that I have laying around.
>> >
>> > FR Wilk
>> >
>> > From: "Eric Aos" <eoa at spartek.com>
>> > > The furnace blower fan doesn't have enough vacuum (I tried one
>> > once). Get
>> > > the fans from http://www.grainger.com/ , you can order the same
>> > ones that
>> > > are used in a shop vac.
>> >
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>>
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>
>
>
>
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