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Check valves for passive solar hot water system - Page 4

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Posted by Loren Amelang on October 26, 2007, 4:57 am
 
On Wed, 24 Oct 2007 18:01:51 -0700, mark.fox@gmail.com wrote:


There at least used to be a special thermosyphon check valve, a brass
housing with a very lightweight plastic ball that floated up against
the seat. You of course had to install it in the proper location and
orientation...   Can't Google up any at the moment.


For lowest energy and noise, it is hard to beat the El SID drivers:
<http://cgi.ebay.com/Ivan-Labs-EL-SID-SID20PV-20-Watt-5-0-GPM-Solar-Pump_W0QQitemZ150121632359QQcmdZViewItem>
(Available many other places as well...)


Definitely you still need a check valve, and with low power pumps you
face almost the same issues as with thermosyphon - a normal check
valve will just about eat all of your pressure budget. I ended up
making my own floating ball checks, but they haven't been
problem-free.

Google finds mention of floating ball checks, even some with stainless
balls, but I see nothing you can buy.

Anybody else have an extremely low pressure check valve to recommend?

Loren

Posted by Sundug on October 26, 2007, 1:00 pm
 
On Oct 24, 8:01 pm, mark....@gmail.com wrote:

The SHW stystem on my home uses a PV powered, magnetic drive pump, and
a brass swing check vale. It`s been working well for over 16 years.
The pump and check valve are necessary because the collector is above
the HE tank.
 In a seperate system on my shop, the collector is below the HE tank,
so no pump or check valve are needed. In a thermosyphon system, the
bottom of the tank/HE needs to be above the top of the collector to
work well. Check it out here-
http://home-and-garden.webshots.com/album/69453274eglrgF
Doug


Posted by Sundug on October 30, 2007, 12:20 pm
 On Oct 24, 8:01 pm, mark....@gmail.com wrote:

There is NO need for any sort of check valve in a properly designed
thermosyphon system. That would be where the bottom of the Heat
exchanger or tank is above the top of the collector. Warmer fluid
rises, colder fluid settles and stays down in the collector naturally.
I know this by experience and the laws of physics. Doug

DIY Solar Article can be found here:
http://www.solarfacts.net/articles/experiences/experiences_living_on_solar_-_doug_kalmer.html

DIY Solar Hot Water Articles can be found here:

http://www.solarfacts.net/articles/diy/diy_solar_hot_water_heater_-_doug_kalmer.html

http://www.solarfacts.net/articles/diy/diy_solar_hot_water_heater_two_-_doug_kalmer.html

Gallery of Solar Homes- second one down-
http://www.jc-solarhomes.com/gallery.htm


SDHW and house-
http://community.webshots.com/user/sundug



Posted by mark.fox on October 31, 2007, 1:34 pm
 Doug, thanks for the links and your suggestion. Let me explain why I'm
querying about check-valves for a thermosyphon system. My concern
comes from experience designing passively heated and cooled
greenhouses which seems to contradict common experience with
thermosyphon systems.

I understand the reasoning that people use and even have a fair bit of
trust that it is correct. However, exactly the same "laws of physics"
that Doug stated are what concerns me. During the day in a
thermosyphon system, there are no problems. The heat gathered at the
collector drives the liquid from its intake (at the bottom of the
collector) out its outlet (at the top), up to the inlet on the storage
tank (at the top of the tank) down through the heat exchanger, out the
tank's outlet (at the bottom), and back down to the inlet on the
collector. The heat exchanger adds to the force driving this loop.
Where there is a change in temperature of the liquid (the collector
and the heat exchanger) the fluid is given a push. Natural convection
also helps. In the system described all of these work to drive the
loop in the direction we want during the day. But it's a different
story when the sun goes down.

At night, the collector is cooling the liquid. This will tend to drive
the fluid down through the collector (from outlet to inlet, ie.
backwards). The stored heat in the tank will tend to drive the fluid
up (outlet to inlet, also backwards). The fact that we've arranged the
tank above the collector gives the system a tendency to keep the cold
fluid in the lowest parts of the system and the warm fluid in the
highest, but is that tendency enough to overwhelm the other two
tendencies that want to drive the fluid backwards? I honestly don't
know. I can say without a doubt that, if there is a reverse flow at
night, it will be much less vigorous than the flow during the day. So
you may still win and collect a positive balance of energy.

A check-valve will stop the reverse flow at night (and possibly during
the day as well if the cracking pressure is high enough to overwhelm
the passive forces driving the system). It also allows one some
freedom in placing the collector and tank.

So that's my reasoning. From what Doug is telling me (thanks again
Doug) and some articles I've read, it's not a problem. However, I'll
carefully observe my system for the first few nights regardless.


Mark




here:http://www.solarfacts.net/articles/experiences/experiences_living_on_ ...

down-http://www.jc-solarhomes.com/gallery.htm


Posted by nicksanspam on November 1, 2007, 9:50 am
  

Like this, viewed in a fixed font?
 
 --------------<--   ---
|           |     |
|           |     |
|   tank    |     |
|   100 F   |     |   H
|           |     |
|           |     |
|           |     |
|           |     |
|-----------      |  ---
|                 |
|                 ^   S
|                 |
|                 |  ---
v                 c      
|                 o  110 F
|                 l
|                 l
|R                e   C
|                 c
|                 t
|                 o
|                 r
|                 |
 -------->-------    ---


Like this?
 
 -------------->--   ---
|           |     |
|           |     |
|   tank    |     |
|   100 F   |     |   H
|           |     |
|           |     |
|           |     |
|           |     |
|-----------      |  ---
|                 |
|                 v   S
|                 |
|                 |  ---
^                 c      
|                 o  40 F
|                 l
|                 l
|R                e   C
|                 c
|                 t
|                 o
|                 r
|                 |
 --------<-------    ---


I don't think so, with the piping arrangement above. If the backflow is
very slow, the H pipe will be 40 F, no matter how well it is insulated,
and the separation pipe S and the collector C and the return pipe R will
also be 40 F, so the water in the S, C, and R pipes will all be the same
density and balance out, with no backflow pressure, which leaves heavier
water in pipe H and lighter water in the tank and a backflow rate that
decreases as we add insulation to the H pipe. The vertical separation S
doesn't seem to do any good here, except to increase the flow resistance.

OTOH, a different piping arrangement with less tank stratification would
seem to prevent backflow, even with no separation S:

 -----------
|           |
|           |   Do Solaharts work this way, with a horizontal hot dog
|   tank    |   above a tilted flat plate?
|   100 F   |
|           |   Should the tank have an "upside-down dip tube" inside
|           |   to enhance stratification by delivering collector output
|           |   water to the upper part of the tank? Is that the same as
|           |   a well-insulated H pipe?
|-----------|
|           |
|           c      
|           o  40 F
|           l          Are there other reasons to have a separation S?
|           l          Something to do with conduction or induced
|R          e   C      currents within the tank?
|           c
|           t
|           o
|           r
|           |
 -----------

Nick


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