Drainback Questions

Keep on Walkin

I am looking for some feedback.

I have been reading up on indirect drainback systems and I want to install one in my  house. I have access to evac tube heat pipes and plan to use them in the system. Freezing is an issue so I will fill the system with 50%

water glycol mix as I plan to put the drainback tank in the

attic.  BP of PrGly water soln is 225F @ 15psig.



I learned that you keep the drainback system under pressure (about 15psig) to prevent boiling cavitation during the  initial pump up and that you need a minimum velocity of 2 ft/sec to bring about the siphon and clear the air out of the upper leg. Make sense so far?



The following  scenario causes me some concern.  If during the day, my solar storage tank is fully charged with heat, the pump turns off and the collector manifolds clear of fluid.  If there is a call for heat in the solar buffer, the pump will turn on and send the glycolsolution into a possibly 300deg manifold, causing flashing and perhaps a hammer.  Is this detrimental to the system?  Will this cause glycol degradation of any consequence?





One possible solution

is to have the controller lock the pump out at high manifold temperatures, but this effectively

shuts off the system until the manifold temp drops, which might be several hours on

a sunny day. Is this situation significant in reducing the efficiency of my system?  I live in SE PA.



Another possible option is to allow the solar storage tank to reach  higher

temperatures and thus have less frequent  shutdown of the solar heat transfer

system.  What max temperature should I allow the solar tank to reach?  It will feed the DHW tank which would have a tempering valve on it.





Thanks for your help.  I am new to this site.

Fortunat

controller settings

If the system is shut down on high tank temp but then the tank cools (because someone uses some water), but in the meantime the collectors are very hot, you should NOT turn the pump back on. Yes you'll damage the glycol, but you may also damage the collector from the thermal shock and more likely than not you'll make a hell of a racket as the fluid splashes into the hot collector and flashes to steam.



Most modern controller have a setting you can adjust which will prevent the pump from coming back on once the collector is above xxx degrees.



Yes, you might end up losing the rest of the day of production but it is worth it.



Set the tank high limit as high as you can tolerate (we use 180 in most of our systems) to minimize how often this happens (and make sure to use a good mixing valve to keep the water at the tap at a reasonable temperature)



hope that helps,



~Fortunat

www.revisionenergy.com

Keep on Walkin

Is drainback the way to go?

So is drainback the way to go? How many of these lock-out days does one see in the mid-Atlantic? Anyone have any experience with these system lock-outs being a problem?



 If they only occur in the summer, it might not be a big deal.  But if they happen frequently and someone takes a long mid-day shower, the system can't put heat back into the energy store until maybe the next day.



Should I be looking at a pressurized glycol system instead?  It has it's own issues.

Fortunat

drainback is fine

If the tank and collectors are properly sized, you shouldn't end up in this position very often. Usually you'll only overheat the tank if nobody is around to use water and you have several nice days in a row.



And remember that if the system is shut down on high temp, it means you have a whole tank of very hot water. So even if it doesn't come back on after an early afternoon shower, you probably still have more than enough hot water stored to get you through to the next sunny day (if the tank is appropriately sized).



~Fortunat

ABSolar

Flash Back in the Drainback

Hey Fortunat,

Would not the htf (at say 2.5 feet per second flow velocity) rapidly cool the collector header manifold with the presumably (worst case scenario) low temperature htf from storage (say delta t of 200 degrees).  Do you think the sudden thermal contraction on the manifold will stress it's shape or connections?  I'm thinking that the htf first flashes to steam as the fluid first enters the manifold, and this is a change in phase which absorbs 1000 times more energy per pound of htf than if the htf remained in liquid state, still transferring heat.  But the htf that is following is absorbing and cooling this steam cloud that just formed and shot out the other end.  Just trying to "see" in my mind what really happens.  Perhaps you can more accurately describe the moment by moment event.  Short question - can't the manifold "take it"?

Fortunat

ask Kevin

Can the collectors  'take it'?



I don't know. Depends on the collectors, I guess. I think Kevin from Denver did some testing like this recently and was successful at making a collector fail from thermal shock. Surely even if it survives, it must be hard on the collector.



As for whether the pump can overcome the steam bubble, I suppose that depends on the details. I think eventually it probably will. But in the meantime, I think you end up with some undesireable effects including a steam hammer sound, thermal shock in the collectors, and the possibility of bringing steam down from the collectors thus damaging things that are in the return line (flow meters or expansion tanks or whatever).



In general, I wouldn't think the small extra production would be worth it. Remembe that we want theses systems to last atleast 15-20 years.



~Fortunat

Kevin_in_Denver_2

Negligible Losses, very rare damage

1. Steam hammer is a troubling sound for the average homeowner to hear, so I recommend avoiding it.

2. I've never had damage nor heard of thermal shock damaging a flat plate collector or copper header piping.

3. I cracked only one glass tube in an older Apricus panel with thermal shock... SunMaxx/SunRain seem immune. These were extreme conditions with the collectors at 400F.

4. Assuming a 3 panel system, normal daily hot water usage always prevents tank overheating, so the "family vacation" is the only opportunity for this event.



As Fortunat says, this fairly rare event would have a negligible effect on system performance. Here's the scenario: After a minimum 3 day vacation, the family returns and showers around noon that day. You'd miss a half day worth of sunshine, but since the tank was so hot to begin with, there's plenty of hot water left for the next morning's showers too. The solar fraction during this time is still 100% even though some available energy wasn't collected.

ABSolar

On Board with this thinkiing

Thanks for your inputs, Kevin & Fortunat.  I'm on board with the logic.  I've got a large commercial domestic only drainback using the Sunmaxx tubes and was pondering the consequences of the event.  Unfortunately, I didn'e have time on the installation to generate the conditions for a first hand observation of flash steaming, so I could only theorize w/o 1st hand data. 

Kevin_in_Denver_2

Response to ABSolar

One thing to worry about is if the tank is designed to be at atmospheric pressure and the vent /overflow is small in diameter. Then the thermal shock event could pressurize the tank slightly.



If it's a closed drainback system, a pressure relief valve could pop.

ABSolar

More info on steam flashing

check out the link to this Apricus install manual, page 26.  It does some volume math concerning steam events in their manifold.  I think the gist is that some steam gets pushed back into the expansion tank (in drainback, that would read "gets pushed back into the drainback tank").  I think the additional body of water present in the tank + the metal containment body would have a rapid cooling effect of the steam, thus condensing it before a pressure relief event.  Whaddya think?

http://www.carbonsmart.us/pdf/A7-05.4.1.4-PB--Apricus_Solar_Collector_Installation&Operation_Manual_Nth-America10-19-09.pdf

Kevin_in_Denver_2

How many collectors?

ABsolar, your last comment is probably right in typical residential systems. In a large commercial system, the answer depends on the size of the drainback tank, how many collectors, and the PSI rating of the PRV. A 150 psi pressure only relief valve would probably prevent problems in most cases.



The Apricus manual didn't really explain anything to me because it doesn't specify the thermal mass of a collector.