Frostproofing evacuated tube SHW

DamianB

Hi all,



I'm new here.  Seems like a good place.  I'm trying to help a friend of mine design a vacuum tube SHW assisted space heating system.  Our biggest concern is freeze proofing.  It can go down to -20f here in Utah.  I've read differing things about the heat pipe type evacuated tube collectors, with some saying that they have had freeze problems that have damaged the heat pipes.  To avoid this I am thinking that using evacuated tubes with u-pipes inside, set up to drain back, would be a good alternative.   My questions are A)  Is this a better way to go than heat pipes?  And B)  If this is in fact a better way to go then is the best configuration for drain back accomplished with the u-pipes mounted upside down or would it be better to mount them "on their sides" i.e. with the u-pipe tubes running essentially horizontally and the header pipes running essentially vertically.  Thanks in advance for any input or questions for clarity.



Best,



Damian

Kevin_in_Denver_2

Antifreeze

If you use u-tubes, you're probably stuck with antifreeze. If you use antifreeze, you should plan on "steamback":

http://www.solarwebinars.com/fundamentals-in-steamback-design-11082010/



The reason I wouldn't use drainback with u-tubes is because they won't drain right side up, and they won't purge the air reliably if they are mounted upside down.



But back to your original issue, heat pipes will not have freeze damage unless you get a bad batch, heaven forbid.

DamianB

And On Their Sides?

Thank you Kevin,



So, if it is within protocol here to discuss brands, what is a good brand of heat pipe collector that will likely not suffer a "bad batch"?



Antifreeze is not preferred.



If I may be so bold (or foolish), what is the problem with u-pipes mounted on their sides?  Would not that allow for both draining and purging?



Thanks,



Damian

Kevin_in_Denver_2

U tubes

SunMaxx is a Chinese collector, like all the e tubes. But they have invested a ton of money in their US distribution and support. Apricus has been around longer, but don't seem to be as well organized.



A U tube collector on its side might work, but I'd want to test it with filling it during dry stagnation. Also, getting the tilt just right would be very tricky. Tilt it one way it might airlock, tilt the other way, it won't drain.

DamianB

Thank You and One More Question

Hey, thanks Kevin.  We've been looking at Sunmaxx.  I have one of their demo kits and the tubes seem well constructed.  I accidentally dropped one of the tubes on a concrete floor one day and it just bounced.  They have u-tubes available as well.



I suppose I will talk to them about the heat pipes and the freeze issue.  Maybe that is an option but I think we'll go with u-pipes on their sides. 



With the air purging issue, what is the problem in practice if an air bubble is trapped in a u-pipe?  I'm thinking that if it was truly huge it could interfere with the heat transfer and possibly cause issues with uniformity of flow through the tubes.  Is there some other big sleeper issue that I am missing?



Thanks Again,



Damian

Kevin_in_Denver_2

Airlock equals no flow

Bubbles usually get flushed out of hydronic systems by fast -moving water.



The fluid velocity in a drainback solar panel is relatively low during the filling phase. If the there's a bubble, there's no heat collection. Steam bubbles could be another problem with u tubes.



Any pipe smaller than 1/2" is risky to drain if it has a low slope. 1/4" can hold several inches of water due to surface tension/capillary action.



If you are developing a relationship with sunmaxx, they have a few engineers on staff you could ask.

Monday's webinar is the perfect place to ask if they recommend u-tubes in drainback:

http://www.solarwebinars.com/solar-hot-water-webinar-schedule/



Remember, evacuated tubes are only preferable to flat plates when you need water over 160F all the time.

Bob Gagnon plumbing and heating

Panel Choice

I agree with Kevin, I have a collector array with 120 evac tubes and a flat panel collector alongside it, and for lower temperature heating, I would definitely use the flat panel.



Thanks, Bob Gagnon 

DamianB

Okay, so flat plate, even at 30f outside?

Thank you both.  Maybe I am making this way too complicated.  The reason we were thinking of using evacuated tubes was, well it gets pretty cold here in the winter (Southeastern Utah, elev. about 5,000 feet) and people I know with flat plate domestic hot water systems say they don't put out that much in the winter.  I guess what you are saying is that the difference between heating water to 130-140 degrees for hot water and ??? degrees for in floor is a whole different thing because running the panels at that lower temperature will mean much lower radiant losses, which is what is eating the flat plates' lunch when used for hot water.  Am I getting that right?  Average winter daytime temps are usually mid 30s.  Would you still go flat plate at even, say 30f average outside temperature for winter in floor heating assist?



Thanks again, really appreciate the help,



Damian  

hot_rod

where there at, and

what you're asking them top do. So you need your local climate date and define the operating temperature of the collector.



You can download the data from the various types of collectors at the SRCC website and compare output at different conditions. Or run a simulation on one of the many programs available. Download RETScreen for free and run some simulations. Or a program like F-chart, T-sol or others.



These programs will use the local weather station data to generate the simulation.



Download this PDF and read the part about inlet fluid parameter. You can build a graph of the collectors you are considering and see their efficiency at different operation conditions.



www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_3_us.pdf





The example shows an evac tube and flat plate performing the same with 20F ambient and 95F return fluid temperature to the collector.



As the outdoor temperature drops below 20 the evac tubes have an edge.



Remember cold, short winter days offer less solar radiation available. Look up solar radiation available tables at the NASDA site. A collector with a higher efficiency cannot generate energy. 10 or 15% of nothing is still nothing.



Weigh all the options including life expectancy, snow loading, maintenance, etc.,



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