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Two small fans in series (airflow) better than one bigger one?

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Two small fans in series (airflow) better than one bigger one? crispin.proctor 05-25-2008
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Posted by on May 25, 2008, 5:17 pm
Hi Folks,

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

I am building 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 equal, 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&ModuleNo=30258&doy=25m5#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 conservative.

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.


Cheers,
Crispin

Posted by Zyp on May 26, 2008, 2:48 am
crispin.proctor@gmail.com wrote:
> Hi Folks,
>
> I hope this is the right place to ask a question like this.
>
> I am building 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 equal, 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&ModuleNo=30258&doy=25m5#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 conservative.
>
> 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.
>
>
> Cheers,
> Crispin

The axial fan you've choosen is efficient for free air flow, but not
designed for ducted air flow. Generally speaking, axial fans are not used
to develop pressure in a duct system. A blower is not as efficient, but
'is' capable of pressurizing a duct system. So to answer your question, no,
an axial fan is not a good choice. You should be using a blower to move air
in a duct.

--
Zyp



Posted by new jersey on May 26, 2008, 8:42 am
in my opinion you are playing with you self 68 mm=apx 3" pipe
to push any air through must be squirrel cage blower and even that
with that size pvc pipe would be useless but that again is my opinion
Tony


> Hi Folks,
>
> I hope this is the right place to ask a question like this.
>
> I am building 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 equal, 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&ModuleNo=30258&doy=25m5#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 conservative.
>
> 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.
>
>
> Cheers,
> Crispin



Posted by Mark on May 27, 2008, 11:58 am
On May 25, 5:17=A0pm, crispin.proc...@gmail.com wrote:
> Hi Folks,
>
> I hope this is the right place to ask a question like this.
>
> I am building 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 equal, 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=3DSPEC&ModuleNo=3D30258&doy=3D25m5#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 conservative.
>
> 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.
>
> Cheers,
> Crispin

read up on the PV spec curves of fans

also if you can use real electricity in your system, consider a small
heat pump, you can store a LOT more heat using a "phase change" like
the "latent heat" of freezing/meltng water...but this of course is at
32F so you need a heat pump to get the heat out of the water..

use the sun to melt the water and then the heat pump to pull heat out
of the water and freeze it...

Mark



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