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Two fans in series better than one larger one? - Page 2

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Posted by crispin.proctor on May 27, 2008, 9:47 am
Folks, thanks for all your input.

I understand that the fans are, basically, pretty useless. However,
they _should_ do the trick for me. I am not looking to evacuate a room
in under ten seconds. I am simply looking to move hot air downhill
(vertical pipe) to a store. The back pressure will be there but
slight. As I am not looking to move vast volumes at gale speeds, this
is still not an issue. I have built a controller which does all the
temperature measurement and only turns the fans on when certain
thresholds are met. Also, it turns them on proportionally. When the
temp in the collector is 30c, they are running at 20%. At 50c, they
are full tilt. Or would that be "full tilt".

The idea of two fans in series, just to help each other out in a push
me, pull me scenario. I have decided to go this route as the fans are
cheap and cannot hurt. The longest run is the vertical 1.9m up and 1.9
down. I will have one at the top, right outside the collector (which I
suspect / hope will have an unhappy life as it will be sucking hot (if
all is well, very hot) air) The second fan will be at the bottom of
the pipe pulling it down (The upper one was pushing) and pushing it
through the spreader.

This setup will work (crosses fingers). I kind of tested it over the
weekend. I have built the spreader which is a simple Y junction of the
pipes followed by another slight bend to bring them parallel again
with a gap of 500mm between them. Into the parallel section, I have
drilled many many holes (5mm). Connecting a fan to the  inlet of the Y
creates enough of a jet of air at most holes to put out a match. Not a
hurricane but like I said, I am not looking for that.

Adding the 1.9m length of pipe to the setup, as expected, reduces the
effect somewhat. However, adding the second fan to the top of this,
the setup regains some of it's former glory.

Coupled with the fact that this is a closed system, the revers will be
happening on the other side of the heat store. There will be a fan
sucking the spent air out and pushing it back towards the collector.
There will be another fan at the collector sucking the air up the pipe
and pushing into the collector. Just for fun, I got the controller to
delay these two sets of fans to allow for inertia of the air. (Hey - I
was bored and it was raining outside)

A worrying factor I found yesterday was the point at which PVC becomes
malleable. It's low. Very low. I held the pipe over the stove to
soften it for the bending and fitting. Far sooner than I expected, it
went limp. Funny - my wife said the same thing....

Posted by Robert Scott on May 27, 2008, 1:54 pm
On Tue, 27 May 2008 02:47:29 -0700 (PDT), crispin.proctor@gmail.com wrote:

Since you have a functional test bed, you no longer have to guess about things
that are easy for you to measure.  To measure how much pressure differential
each fans sees, just take a length of small clear plastic tubing and put about
10" of water in the middle of it.  Then stick one end of the tubing upstream of
a fan and the other end downstream of the fan.  Let the middle of the tubing
hang down so the water gathers in the middle.  Make sure it settles in one
contiguous slug.  Separate segments of water separated by air bubbles will mess
up the measurement.  With the fan turned off, the water level on both sides of
the tube will be the same, since water seeks its own level.  With the fan turned
on, the downstread side will have higher pressure than the upstream side.  This
pressure difference will cause a displacement in the two water levels in the
tubing.  Measure that displacement very carefully.  That displacement is the
pressure differential in inches of water.  Fortunately, most fans are speced in
the same units - inches of water.  Look up the specs on the fan and see what the
flow rate is at the pressure differential you measured.  Then see where you are
with respect to these two extremes:

1. Suppose the pressure differential is so low that the specified flow rate at
your pressure differential is almost the same as the specified flow rate with
zero pressure differential.  If that is the case, a second fan in series will
not help.  But putting a second fan in parallel will almost double the flowrate.

2. Suppose the pressure differential is so high that the specified flow rate at
your pressure differrential is much lower than the zero pressure differential
flow rate.  In that case, a second fan in series will indeed help increase the
flowrate because the backpressure is your main limiting factor.

Before you discount the need for "hurricane force" winds, consider whether the
low flow you are willing to accept is going to make any measureable difference
in the temperature of your shop.  Last winter I built a hood that sat over my
woodburning stove in our attached sunroom.  The hood collected the rising hot
air and blew in into our air duct distribution system.  The woodburning stove
was never meant to heat the whole house - just the sunroom, which, until I made
my modification, was cut off from the rest of the house.  The results were poor
- not because the hood was not collecting hot enough air from the stove, but
because I used a small 8" fan.  In retrospect, I should have calculated before I
wasted my time building.  Don't make the same mistake I made.

Robert Scott
Ypsilanti, Michigan

Posted by nicksanspam on May 27, 2008, 5:09 pm

The fan pressure is likely to be less than 1/8" H20. A $0 Magnehelic
gauge might work better, or measuring flow with a $0 Kestrel digital
wind velocity meter or a $0 Dwyer vane air velocity meter.


Posted by daestrom on May 27, 2008, 8:48 pm

I've had terrible luck with just such an 'instrument'.  My forced-air
furnace for my entire house develops less than 1/8 inch water 'head'.  To
get decent measurements of such a small fan, you'll need a magna-helic gauge
that can read down below 1/10 inch of water or better.

A lot of fans are rated in 'total  head'.  This is the combination of static
pressure rise and velocity gain.  So many times the measured static head
across the fan (such as from magnahelic gauge or your manometer) only shows
you part of the story.  If the air is also accelerated from near 0 fps to a
relatively high velocity, then the 'total head' can be higher than 'static
head' by a fair amount.

This can be the case with 'ductless' fans that are not intended to create a
pressure rise into a closed system of ductwork but rather just 'get the air


Posted by Solar Flare on May 28, 2008, 1:22 am
 I think we discovered where the vaccuum is.

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