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solardan1959

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Reply with quote  #11 
cwwilson721
  That was my point also.  Using metal frames, copper pipes, and tempered glazing we are using the same materials as the manufactured and those have been tested into stagnation for long periods with little ill effects.  Every CVPC I have built does very poorly near stagnation and while I could not get it to blow a gasket, the results were never very pretty.  One time overheating and the pipes can look pretty bad and while it will continue to work it does not look very professional.
  Your concerns are very valid for CPVC and while many things can be done to reduce heat you are also taking away from the performance.  The foam plug is probably a very good compromise as it will heat up as required but in an overheat condition it will blow but as you stated, at what point is that.  It would be a very easy test but then several types of foam will need to be tested to hopefully find one that goes at about 190ish.  Easier to do would be to have a high level bi-metal snap switch that turns on at about that and uses a fan to suck out the hotter air.  The fan will then shut back off when it cools to 170ish.
   Many people have used CPVC for years with no ill effects but for insurance  I would use copper if I wanted it to last.

With a properly designed drainback, no water should be left in the system to provide steam.  I do not think the sun alone shining on empty pipes will cause them to swell though I guess I did have sag when I mounted them horizontally and only supported them about every three feet.  And letting it get to 220 plus in the collector. This could be avoided by using the wrap around flashing that would produce better heat by transferring the heat to the pipe and by providing support to the pipe. (or a snap switch powered fan)

Dan

SolarInterested

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Reply with quote  #12 
How about a Synthetic Muscle Made of Fishing Line to pull open a spring loaded vent door? Maybe not but I thought the science was neat and it is DIY

http://www.popularmechanics.com/science/health/breakthroughs/synthetic-muscle-made-of-fishing-line-is-100-times-stronger-than-the-real-thing-16514805


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GaryBIS

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Reply with quote  #13 
Hi,
This is just a thought on another temperature protection mechanism.

If the collector is left full of water when the pump goes off, and the collector plumbing is vented to the atmosphere, then the water in the collector will not go above the boiling point of water. That is, the water in the collector will boil and vent off steam, and the water temperature will just stay at the boiling point.  

The protection ends when all the water in the collector has boiled off.  A back of the envelope calc makes me think that the water might last about 3 hours in full sun with a 50 F ambient temp.

I think that CPVC or pex will take temperatures up to the boiling point of water without any significant damage -- I've had my CPVC and pex collectors up to 230F without any apparent damage or distortion.

I'm not suggesting this as a particularly practical way to protect a collector -- just a thought.

The other approach that I don't think has been mentioned is the bimetallic spring vents.  This would be like the automatic crawl space vents that open above a certain temperature.  The vent could allow air to circulate through the collector box when the temperature gets above its set point.
http://www.amazon.com/Energy-Saver-B-E-BROWN-Replacement/dp/B000DZCYM4
Don't know if one could get a bimetallic spring that would operate at the right temperature or not.


Gary


solardan1959

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Reply with quote  #14 
Gary,
Quote:
the collector plumbing is vented to the atmosphere,

What do you mean by that?  I'm not sure how you do this without water flowing out the vent unless you mean like a hot water pressure release valve.  Or does one long tube extend real high and under normal operation the water pressure is not enough to flow out that tube?

Thanks for the auto vents, while the ones you listed open at far to low a temp for this discussion, that may be the perfect solution to get my heat from the sunspace into the garage.

Dan 
GaryBIS

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Reply with quote  #15 
Hi Dan,

Not sure how one would vent the collector to atmosphere -- I guess a primitive way would be to run a pipe up from the top of the collector high enough so that the pump would not be able to pump the water out of it (ie higher than the static heat of the pump).  Or, maybe one of those air eliminator devices that are used on radiant floor systems might work?  

Not really saying its a very practical system as I described it -- just thought it might trigger some further thinking.

I just added this system the other day, and due to the way its constructed, I think that the CPVC used for its collector is protected by the water boiling idea: http://www.builditsolar.com/Projects/WaterHeating/CostaRicaThermosyphon/CostaRicaThermosyphon.htm
That is, as soon as water in the collector reaches boiling, the steam and water would be forced up into the tank, which is just above the collector, and since the tank is open on the top, the pressure could never build up.

I read a while back that some NREL work on plastic water heating collectors was trying to use phase change  for protecting the collector from stagnation damage -- this sounds like water boiling to me.  They did not give any details on how the system might actually work.


The bimetallic spring idea vents seems more workable.  For example, what happens if you take one of those foundation vents and wind the spring up tighter?  Does it move the operating temperature up some?  Could you get it up high enough to work as a collector vent?  
OK -- I'll order one and report back on what happens [smile]

Gary





gbwillson

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Reply with quote  #16 
Are these bi-metallic spring vents adjustable?
I couldn't find any that listed anywhere. Automotive thermostats can be purchased to open at different temps. While they are designed to be immersed in antifreeze, they do open if the air temp gets to the proper setting.

Another possible source for a anti-stagnation vent is from an old air cooled VW. My VW Beetle had a bimetallic spring that had dampers that controlled the airflow over the fins until the engine heated up. I'm guessing it's not a high value item or hard to find(YET).

Greg in MN[wave]
GaryBIS

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Reply with quote  #17 
Hi Greg,
Not sure on the adjustability of the bimetallic spring vents.
I ordered one online yesterday just see how it works and see if it can be adjusted and how much.

It might be more practical to just find a place that sells the spring alone, and build the vent part.  I just thought that it would be worthwhile to just see how the commercial foundation vent works.

I'm a bit skeptical about the automotive thermostat in that the flow opening on the ones I've seen is small -- not sure enough air would get through to be effective?

Gary
Garage_Hermit

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Reply with quote  #18 
Maybe the solution is some hydraulic relief, in the shape of a dashpot...

http://en.wikipedia.org/wiki/Dashpot

In particular, regarding an air-relief flap on our overheating collector...

A spring applies force to close the door, and the dashpot forces fluid to flow through an orifice between reservoirs (the orifice is often adjustable), which slows the motion of the door.

As I see it, the flap could be held closed by the spring pushing it, and the flap would be pushed open after the overheating collector gets to a dangerous level...

The door would be pushed in the opposte deiection by a dashpot = equals an oil cylinder.  As the dashspot gets hotter in the stagnating collector, the viscosity of the onboard oil changes, and the dashpot extends its rod, opening the flap.

or something like that...

I guess the commonest source for a used one, would be an automobile shocker ??

G_H

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Ky-Jeeper

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Reply with quote  #19 
GH, I believe i've seen them used as a sun tracker. I have something similar for a cold frame for my gardening.

Quote:
Originally Posted by Garage_Hermit
Maybe the solution is some hydraulic relief, in the shape of a dashpot...

http://en.wikipedia.org/wiki/Dashpot

In particular, regarding an air-relief flap on our overheating collector...

A spring applies force to close the door, and the dashpot forces fluid to flow through an orifice between reservoirs (the orifice is often adjustable), which slows the motion of the door.

As I see it, the flap could be held closed by the spring pushing it, and the flap would be pushed open after the overheating collector gets to a dangerous level...

The door would be pushed in the opposte deiection by a dashpot = equals an oil cylinder.  As the dashspot gets hotter in the stagnating collector, the viscosity of the onboard oil changes, and the dashpot extends its rod, opening the flap.

or something like that...

I guess the commonest source for a used one, would be an automobile shocker ??

G_H
Ky-Jeeper

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Reply with quote  #20 
Wished i could work with the temp range on this for opening vents. Over heating this will destroy it maybe. I probably try.

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