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Two fans in series better than one larger one?

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Posted by crispin.proctor on May 25, 2008, 9:20 pm
Hi Folks,

I hope this is the right place to ask a question like this.

I am builing a solar heating solution for my workshop. It consists of
a heated cavity, a series of pipes and fans to push the air down to
the heat store (large volume of stone) and another fan to return the
air to collector as it will still be above ambient temperature.

All things euqal, I was wondering about the fans: There will be two
working together; one pushing the into the store, one pushing the air
back into the collector. As this is a (mostly) sealed system, these
fans will compliment each other. The plumbing is 68mm PVC gutter
downpipes. I chose that because it's wide ID, cost and east of use. As
it happens, I have a couple of 60mm PC style fans (http://
www.maplin.co.uk/module.aspx?ITAG=SPEC&ModuleNo0258&doy%m5#spec) .
As the pipes ID is 61mm, filing down the corners on the fans makes a
nice tight fitting round fan.

The fans have the following job:
1) Push cool (heavy?) air up from the store into the collector. This
is a head of about 1.7m (About 5"6).
2) Push hot (light?) air down into the bottom of the store. This would
be around 1.9m. Also, a small amount of pressure is needed here as it
has to force the air through a series of holes.

My question is this: If I find the fans do not have enough muscle,
would simply adding another fan at each end, in series, help? The
reason I would like to do this is because it's simple. Simply shoehorn
another fan into the pipe.
Drawback is double the current. As this setup runs on batteries and a
solar panel, I need to be conservitive.

My second option is to build a box which will house a bigger fan and
connect that into the pipe. This is not ideal / neat. Also, larger fan
still means more current.

Finally, does proximity matter if in series? Butting the two pans
together, they become very noisy. Turbulence I imagine. Spaced around
100mm apart, they become "normal" noisy.

Thanks for any feedback.


Posted by Morris Dovey on May 25, 2008, 9:49 pm
crispin.proctor@gmail.com wrote:

Note that the spec there doesn't provide any info whatever about that
fan's air-moving capability (specified here in CFM, cubic feet/minute).

Without that info the best you can do is make a WAG - and mine is that
you'll want a more capacity than a muffin fan is likely to provide.

Maybe. Probably not much. Consider: if the first fan moves air into the
pipe at 30 CFM, and the second moves air out the other end at 30 CFM,
then where's the gain (other than perhaps in fan longevity)?

Third option: calculate how much air you need to move, then go with the
plenum size and fan that satisfies that requirement - then consider
whether your panel can supply adequate power...

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by Robert Scott on May 26, 2008, 2:16 am
 The only reason for having two fans is if the backpressure (air resistance) is
so high that it significantly depresses the airflow rate of a single fan.  Two
identical  fans running near their maximum flowrate are no better then one fan
of the same type - never mind a larger one.  But if volume is your problem and
not backpressure, then get one large fan - or two fans in parallel, not in

Robert Scott
Ypsilanti, Michigan

Posted by Solar Flare on May 26, 2008, 10:32 pm
 If there was no backpressure in front of any blower then the air flow could
be infinite with no energy required.

All the factors obviously have not been considered in your loose statements.

Posted by J. Clarke on May 26, 2008, 11:43 pm
 Solar Flare wrote:

So air flow into a vacuum is infinitely high?  The tiniest pinhole
leak in the Space Shuttle instantaneously vents the entire cabin?

Further, "no back pressure" doesn't mean "no pressure", it means that
the pressure differential across the fan is zero.

Looks to me like you're more interested in showing people how smart
you are than in actually providing helpful information, and failing
miserably at both tasks.

To the OP--go with what Scott said.  The fan maunufacturer should have
tables of flow rate vs back pressure.

to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

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