External versus internal heat exchanger

ecky

Looking for opinions. I am connecting 6-4x8 flat plate solar panels to a 1500 gallon wood-fired boiler. I am using a corn-based glycol and my research indicates a required flow rate of 8-9 GPM. My question is whether to go with an external heat exchanger (probably plate versus tube-in-tube) or to use 1/2" black steel pipe (30-50') running internally in the boiler (near the bottom of course). The second configuration would avoid buying a heat exchanger and a second pump. The first configuration would require one circulating pump. Any advice / suggestions / concerns?

Karl_Northwind

what?

hold on.......

What are you doing and why?



is this a GARN?

The Garn Forum seems to be gone, at least at Garn.com

let's hear a little more about what you're trying to do and we'll all be able to give you a better idea of what the best solution to your application is.



when how and why would you be dumping solar heat to the boiler? what panels are you proposing using?  what will the heat be used for?



More information and some details please.



Karl

ecky

Solar with Garn for DHW and radiant floor

Sorry Karl. Did not want to step on anyone's toes by mentioning brand names. So here is what I am trying to do - integrate solar hot water with a Garn WHS1500 (holds 1500 US gallons - 5000 L). The Garn is going to provide space heating (2 floors of radiant heat in the house) and DHW. By integrating solar, I would presumably shorten the wood heating season (spring / fall). In the summer the Garn would become a heat dump and provide the DHW (hopefully) without having to fire the Garn. The 6 panels are used flat plate 4' x 8' collectors. I was hoping to use a single solar circulator (Laing Solar DC-5) which is supposed to produce a max of 7 gpm. By running 1/2" black pipe back and forth inside the bottom third of the Garn I would avoid a heat exchanger and a 2nd solar pump (Shurflo 2088). I am wondering if this is a good idea (the downside being if a leak developed inside the Garn I would have to drain it). Not sure how much black pipe I would need to simulate a heat exchanger. I do know that I have to use black pipe in order to avoid the problem of dissimilar metals. I choose a 1/2" thinking it gives me the best surface area to volume ratio.



Hopefully that gives you a better idea of what I am hoping to accomplish. As a bit of an aside, I am also looking for a DC pump that will push about 5 gpm through a 1" ID pex from the Garn (non-pressurized) to the house.



As to the Garn forum - you are right. It has moved to Facebook.

Karl_Northwind

as I suspected.

It's certainly worth a shot, but I think what you're going to find is that those 6 panels will not heat that tank up alone to the point where it's going to provide satisfactory heat to your house.



Just looking at the energy available from a normal 4x8 collector, you can expect (slightly optomistically) to get 32,000 btu per sunny day per panel in the shoulder seasons say 50 deg outside and 100 degree fluid in the solar system. so on a day where you're going to get 192,000 Btu, and probably soak that up into the house and DHW, you're going to raise the temp of that tank maybe 15 deg F, if you're not pulling any heat out of it, and if you don't figure losses to the outside (unless it's in a space where the lost heat will benefit you)

as long as you're only expecting to use it during the sumer and shoulder season, and not expecting great things from it in the winter (more on that later) I think you'd be OK.



I think you'd be much better off using a double pumped system, and if the pipe runs are short, you could use 3/4 pipe and the laing D5 would be fine. if longer, you would bump up to 1" pipe.   With most DC systems, it usually ends up being a bit lower flow rate than desired, because the equipment is limited.

if you're going to move approximately 200,000 btu over 4 hours, find a flatplate HX that you can move 50,000 btu thru with the temp ranges you want, (say 10 deg drop on the solar side at 4-5 GPM) and use a second Laing on the tank side.

2 laings will work well off a single 45 w pv panel well.



the amount of iron pipe you'd have to put in the garn would be a serious pain, and not save you much money, and have serious pressure drop. 



in the winter you might be better off and get more performance sending the heat from the solar into the water returning to the garn (using a zone valve and some simple controls) possibly using that same flatplate HX. Is the heating system in the house low temp?  would it reasonably be delivering water back to the garn below 100F? That's where solar works really well in the winter. 



this sort of solar design stuff is what I do.  great fun.

karl

ecky

Good suspicions!

Hi Karl!



My intent was never to heat the house entirely with the solar. It won't even be close. It should shorten the heating season on either end though (I hope). I don't expect anything from it in the winter (I'm at the 45th parallel - Ontario). Actually that is not entirely true. I am expecting it to keep the Garn from freezing should we go on a holiday. That is not an unreasonable assumption I should think because the rate of heat loss would decrease as the temperature differential between the Garn and the outside approach. Add the relatively large thermal mass and the odd bit of heat from the panels - I am hoping the Garn will stay "liquefied"!



As for the heat exchanger I REALLY appreciate your thoughts on that. The runs are a max of 50 feet from the Garn to the supply end of the panels, and 30 feet on the return end. I am embarrassed to mention this but after talking to a friend who has 4 panels, we felt that we could run pex underground. I was going to use 3/4" copper for about10-12 feet from the return (hot) side and 4 feet before connecting to the supply end of the panels. Do I need to dig up some pex? Please say no. ;^)



So a 2nd Laing pump will work between the Garn and the HX even though the Garn is not pressurized? As for the pipe it was not so much a cost savings, but moreso to "simplify" the system with one solar pump instead of 2 on either side of a HX.



Piping to the return could work nicely but if I use an external HX, I was just going to plump it into the bottom of the Garn which is where the return pipe comes in anyway. By doing that I would avoid a zone valve and some controls would I not?



The return design temp is supposed to be 110 I believe. The house is 2 x 1150 sq ft straw bale house. The two radiant floors are concrete and earth.



Any further thoughts would be greatly appreciated.

Karl_Northwind

garn

I'd attach the circulator pickup in the cold return, and it'll pick up the cold return water, and then circulate thru the HX and have another dip tube (forgive me for not knowing offhand the location of the various ports of the Garn) that returns to somewhere else in the tank, opposite end if possible, and 2/3 of the way up from the bottom.  I know there are lots of ports on the front end of the garn. 

the solar loop will have such a small flow rate (3-4 gpm) that I wouldn't worry about it effecting the hydraulics of the rest of the system.  assuming you'll be running larger circulators to move the water thru the heating system.



do your best to keep the circulator as low as possible, and pull from the tank (put a ball valve between the circulator and the tank for service purposes) then feed it thru the HX and thru another BV (dont' forget a pressure relief valve) and then back into the garn.



I would (as a safety) put a thermostatic mixing valve between the solar and the pex (the copper run spacer is also a good idea) with the circulator pulling from the mixed port and feeding to the pex underground.  the cold from the HX will come from the underground, tee into the cold and then back into the collectors.  set the valve for 150, and it's a good safety.  caleffi makes some high temp high flow TMV's that I've used in this sort of application, or for heat dump controls.

any decent hot water collector will under worse case scenario, produce antifreeze pressures and temps that will damage pex. you will still have to run an extra wire to the other circulator.  that or run it all in copper.



the solar will surely keep the garn from freezing, but I'd be concerned with the piping and connections more than the mass of the boiler itself.  keep it all insulated so all your water is inside the insulation envalope and the mass of the boiler can keep it all warm.



What all are you heating with this monster? a garn is a big boiler for a single dwelling (strawbale superinsulated at that) is there a shop or something else being heated?



Karl

TonyS

In my home

I had a 2 panel solar system tied into a 80 gallon tank with a internal coil. The coil was made of soft copper and was made to be irreplaceable. I let my acid neutralizer go empty and it chewed that coil up in a month, at that time I was easily getting 130 to 160 degree water every day the sun came out. I didnt want to remove the stainless tank so I opted for putting on an external flat plate exchanger. This system is bad at best. Not only do I have to run a second pump, but I lose the natural stratification of the tank because of the pump and also lose more heat because of the lower insulation value of the armorflex on the external loop. Now even on a good day I barely make 120 degrees.

I am going to replace the tank with a Lockinvar stainless with a stainless coil.

ecky

re: In my home

TonyS

Thanks for the info. Sorry to hear about your troubles. You have raised a good point. I would suggest that the stratification (as mentioned by Karl) would not be an issue given the size of the Garn (1500 gallons). If I use black pipe I take away the issue of dissimilar metals but then lose efficiency as compared to copper. The heat exchanger itself will be very close to the Garn and in heated space so there would be loss as your suggested but relatively limited since the loop (HX to Garn) would be less than 10'. Since I can not use copper, the question becomes how much black pipe would I need to give me decent HX efficiency? Karl (see above) pointed out that I would experience serious pressure drop. It would seem that an external HX might - in my case - be the best solution. Comments?

ecky

Darn Garn

Hi Karl - thanks again for taking the time to help me through this. I really appreciate your efforts. First the easy stuff. I am only heating our house. The Garn is a big beast for the house. I liked the concept of efficient "batch" boils and it also gave me a dump for the solar panels. I have superinsulated the Garn barn (straw bale walls with a "ton" of Roxul) in order to limit standby losses and hopefully extend times between firings.



Your first paragraph is a great idea and makes sense - sort of. I obviously am new to this. Would there not be an issue if the solar pump was running but the pump running the primary loop to the house was not? Are you not going to be "sucking" water from the primary loop? If both the solar and the primary house pumps were both running then I see no prob. This is probably just a case of me not understanding pumps but thought I should verify. (Oh - and no need to apologize for not knowing all the ports on a Garn - I was surprised that you guessed it in the first place!)



Your second paragraph is confusing me. (Not hard to do!) In the first paragraph you suggest pulling from the return line. In the second, are you not saying pull from the bottom of the Garn? While on the topic of the HX loop, is there a reason why you suggest using a 2nd Laing pump instead of a DC Shurflo 2088 (which appears to be about $100 cheaper)?



Paragraph 3. Gotta draw out a schematic to get a grip on what you are suggesting. Will look at that tomorrow. Questions to follow I suspect.



Paragraph 4. Excellent point. The Garn barn is insulated - not sure how well. This winter will be the first test to see if that will be an issue. Thanks for the heads up - hadn't actually thought about the piping between the buried sections and the Garn and HX. They are in the building but how warm that will be is the question. As you have said, since it is all inside the insulated envelope, it will probably be OK.



Anyway, thanks so much Karl. Your experience and advice means a great deal to me. Cheers.

Karl_Northwind

the

I would tap off the back of the garn with full size pipe (2" iron pipe I think) and assemble a couple of tees and such, with shut offs should you want to tap off to heat a shop, chicken coop or something.  I would use the first tee to tie into the solar.  at a 2" pipe you should be able to send a couple of GPM into and out of the boiler with no interaction with the other loops.  flow check in the other loop wouldn't hurt, although any minimal thermosyphoning would help prevent freezing on both of those loops.

I am just more familiar with the laing pump.  as I recall ( short on time, class starts in 5, and it's bad form for the teacher to be on the computer late)  and as I recall the shurflow is for pressurizing water systems.  probably not duty rated for long term use with hot water. I'll have to check the specs on thepump later.

as for tony's problem, I think he's turning the tank over too often.  if he has a ball valve somewhere in the system, he could throttle back the flow on the DHW side a bit and see what temps he gets.  I have to assume he has a reasonably sized flatplate HX in there, and it's not too small.

gotta go . don't think I missed anything. more later.



Karl

ecky

loop

Ok but I will not be teeing off the back at this point. It is a very tight fit at the back of the Garn. The return comes in front so I could tee off of that. 2 Laings it is then. The one is pushing the envelope for performance. The performance graph shows about 7 gpm with virtually no head. I am unsure of what the head would actually be in a closed loop system. According to my readings with 192 sq. ft. of panels I should have 8-9 gpm. Can I actually get by with one of these Laing DC-5's?



Your suggestion for Tony seems reasonable.



Karl, you mentioned you were a teacher. What level?

TonyS

I will give that a try

I throttled back, That may help. Thanks

Karl_Northwind

teaching

I teach solar thermal as adjunct faculty at the tech college level as well as teaching classes from intro solar 101 to design and installation classes in the midwest and massachusetts (Alt E Store)



I've seen the laings run 200 square foot of collectors adequately (although probably not optimally) but the first time cost doubles if you step up to 2 of the.  the next bigger option is the march 809HS.  it's the largest (AFAIK) small DC pump out there that's readily available.  I don't think the 830 is available currently.  but they're all brushed pumps.  there's maintenance involved, so we try to steer clear of them.  the Conergy pumps are monsters.  25 gpm and up.  and a grand or more each.



if your panels and HX have relatively low pressure drop, I think you'll be fine with the single laing. you may be running .8 gallon per collector rather than 1.2 that you think you should, but most of the time, that won't substantially hurt your performance.  you'll see a 15 deg delta t on the collector loop rather than the 10 deg delta t that you would otherwise.  if you want to double the price of the solar loop pumping equipment, it certainly won't hurt.



there are also lots of ports on the front of the Garn for the electric elements.  you could easily fabricate a fitting for one of those, with a dip tube (to keep fluid from short looping) and have the solar pull from the return from the heating system, and return to the bottom far end of the tank. 



best,

karl

Karl_Northwind

let us know when you get some sun.

Keep in mind that not stirring the tank will not increase the heat gain much, but will keep the tank stratification better, which will get you a higher temp at the top of the tank (where it's most useful, as that's where the backup heater pulls from when you turn on the tap)  and may reduce your backup usage, even if the solar collection is sightly lower.



karl

ecky

Teaching

I am certainly lucky and appreciative to be picking your brain! Thanks Karl for taking the time.



I had a look at the March 809 and it appears to have a max head of 4.3 ft which is less than the Laing by my figuring. So I think I will stick with it. What I notice is that there are several versions of the Laing DC5 with different pump curves. You have a particular recommendation?



When you talk about a 15 degree deltaT rather than a 10 degree, I would have thought that was a good thing. I thought the greater the temp differential between the in and out of a HX the better (i.e. the more heat is being transferred from the solar to the garn.) Clearly I am misunderstanding this whole process.



In the last paragraph you are talking about short looping. Not sure what you are referring to. If I pull from the return pipe (as per your suggestion) to the HX and then feed back through one of the electrical ports (if using an external HX) to the GARN, how would I short loop? (i.e. what exactly is short looping?)

ecky

PEX as a heat exchanger

It has been suggested that one could use PEX as an internal heat exchanger inside the GARN. Does this remove the concern about pressure / pump issues raised by Karl earlier? Obviously one would need a longer run of PEX to compensate for the lower efficiency of heat exchange provided by plastic versus steel versus copper. Comments? Suggestions?

Bob Gagnon plumbing and heating

My Heat Exchanger

A guy from Indiana told me he had good luck with a pex-al-pex heat exchanger, I'd like to try it. I ran this homemade coil, when copper was a lot cheaper, out of 5 coils of 1/2" X 60' flex copper. Tony S, why is your water so acidic? I never ran a neutralizer and my coils are about 5 years old?



Thanks, Bob Gagnon

Karl_Northwind

March 809 HS

you must be looking at the 809, not the HS.  the HS deadheads at 15' head, and will move 6+ GPM a low heads.



I'd still recommend the Laing, and the D5 strong is the one I use.  with the Brass housing and the threaded connections.



the lower the delta T in the collectors, the better use you are making of every square inch of the heat exchanger you bought.



Short looping would be putting heated water into the garn close to where you're pulling it out to go to the heat exchanger, which would be the easiest point to go to in an otherwise stagnant tank.  you want to return the fluid ffrom the HX near the middle of the tank generally, and pull from the bottom.



a few feet apart in a tank that size would be just fine.



I don't have numbers to back this up(just the experience and observation of others) but you'd want to triple the amount of copper in square footage if you want to use pex as an immersed HX.   a larger amount of volume will help in intermittant use (such as DHW heat extraction from the tank) but I think you'd be better with copper for submerged coils.  you don't need storage capacity in those coils, you need lots of surface area with high thermal conductivity.



there are also access issues with the garn internals.  not somewhere I'd want to spend a lot of time.



Karl

ecky

March (of the penguins?)

Yes Karl - I suspect you are right (809 versus 809 hs). Laing it is for pumps - brass housing for sure, and threaded.



As for a lower delta T - I clearly don't understand why but will attempt to keep it low. It just doesn't make sense to me. I thought the greater the temperature differential between fluid going into the HX and that coming out, the better. Unless (possible light bulb here) you are saying that you want the temperature of the fluids (solar loop glycol and Garn hot water) to be within 10 degrees of each other. That would make sense. Is that what is meant by a delta T of 10?



OK - got the short looping concept. Thanks Karl for explaining. So why not put the return from the HX in near the top of the Garn where the water should be the hottest? (There is a fitting on the manhole which Garn states is for solar).



The longer coil of pex was to compensate for the lower heat exchange efficiency of pex versus copper versus steel. (Copper is not an option according to Garn - dissimilar metal issues. Black steel or pex would not be a prob.) You stated earlier however that pressure head (if I understood your correctly) would be an issue - so an external HX looks to be the way to go. Just for the record, the Garn is currently empty so access to the inside is not a problem at all - if that changes anything.

hot_rod

corrugated stainless

is another product commonly used for heat exchangers inside tanks. It should get along with the Garn. It has more surface area and the corrugations tend to blend the flow inside.



I wonder that CTTS gas line could be used, even with the jacket left on it would probably transfer better than pex. Your sure can coil it into a tight coil or unusual shapes.



hr

ecky

Stainless steel

"It has more surface area and the corrugations tend to blend the flow inside."



"I wonder that CTTS gas line could be used, even with the jacket left on it would probably transfer better than pex. Your sure can coil it into a tight coil or unusual shapes."



Great ideas HR, but does that still not bring up the issue of head. I was told (offline) that a head in a closed loop system is really a non-issue because once you get the fluid going it is kind of a push-pull effect and there is really no head to overcome therefor the circulator is not a problem. Karl suggests otherwise and I am inclined to go with his advice. This would mean that while the heat transfer issue would be better, I would now need a larger pump. This leads to another question. (Does it ever end?) Is a 90 degree corner in pex (or this SS stuff) the same head as a 90 degree fitting? I would assume not but assume there is some head adjustment necessary. And how does one calculate the head in a solar panel, or HX?

Karl_Northwind

head loss

The Caleffi paperwork lists a head loss for their 3/4" solaflex (of course it's really a curve) at 2 gpm (say 2 parallel loops of 2 gpm split from a total flow of 4 gpm) at .0375 PSI drop per foot of pipe.

if you need 100' of pipe (I don't know how much you'd need)  you'd have a pressure drop of 3.75 PSI for the loops.  that's 8.66 PSI of pressure drop. you'd be back up to either a pair of Laings or the HS 809. 



I have definitely looked at using the CSST piping for pool systems pulling heat from large flooded tanks. I've never needed to, but it'd be good for some applications.



If you go to flatplateselect.com, you can sign up for an account (free) and do some detailed HX specing.  or I can do it for you.

why not, I'll do it now.

Side A: 50% propolyne glycol, 130 to 110F, 4 gpm 0.2 PSI pressure drop, and Side B 100-120F 3.6 gpm 0.1 PSI



that's for a GEA flatplate 5x12x40 plate. you can find them easily on ebay.



that's moving about 36000 btus.  which is plenty for 5 collectors (I think that's what you had right?)

you could go a bit smaller easily, but you'd lose some efficiency.  There's no such thing as too big a heat exchanger.  untill you have to pay for it.  there's no such thing as too big an expansion tank, or too many isolation valves.



Cheers

Karl

tim smith

Re: csst tube for heat exchanger

Hotrod, looks like Easyflex has uncoated csst approved for water and sprinkler, that could be the ideal thing for that?  Good luck

Karl_Northwind

PSI vs head

sorry, typo: 3.75 PSI is 8.66 ft of dynamic head, obviously not 8.66 PSI.

still enough that it would hamper performance from the available sizes of PV driven pump.



karl

Karl_Northwind

suberged CSST

You could certainly decrease the head loss by using multiple loops of the CSST, to have decent flow rates. 

making sure the connections are good is key in these cases, and you'd have to assemble them in the tank.



I would certainly consider that as a potential.  If anyone has heat transfer curves for CSST tubing, I'd love to see them.  replacing/upgrading them in the tank would be a serious PITA.  at least a normal flooded tank, they'd be easy to pull up and replace.

 

I don't know when I'll get my big system up and running, but I have a bunch of cut off pieces of 1/2" CSST, and may try to develop some numbers.  as More CSST comes on the market in the US, it may be a better cost choice than copper coils. 



Just so you know, that bare 3/4" CSST is really pricy.  I hope it's really efficient. 



Karl

hot_rod

csst hx

I may be able to come up with some hx data. Our current solar tanks have a smooth steel coil, 1-1/2 inch tube size. 36 feet on the lower coil. It is a solid performer able to handle 200,000 BTU input.



I think a 1-1/4 or 1-1/2 CSST of the same length would be a good place to start a comparison.



The company that builds solar stainless tube also build these CSST type Hx coils, I will start there. It sure is easy to work with.



hr

Karl_Northwind

let us know

Please let us know when you have some performance numbers.  I build systems occasionally that have large water volumes and coil sizes. 

I'd like to see what that surface areas and the heat transfer rates are.



Do we know what flow rate will give us turbulent flow in 1 or 1.25 corrugated tube?



thanks

karl

TonyS

Carbonic acid

Many of the wells I work on are acidic Bob. Some below a #5 ph. They seem to be getting worse all the time, I assume from the larger amounts of CO2 in the atmosphere.

Many wells that use to use calcite now need calcite/magox mix to neutralize.

I have found when acidic water eats hard copper it tends to eat it evenly in a linear fashion. I remove copper pipe that is as thin as eggshell evenly throughout but soft copper is another story, it tends to get holes in it where the copper is bent, soft copper gets holes eaten in one part when the rest of the copper is as thick as new. I have found that it is much more likely to attack the cold water side much more rapidly than the hot water. This is because the co2 comes out of solution when heated. I cannot sell anything with soft copper in it in this area on a well because my warranty is based on the customer maintaining the acid neutralizer and most people call me to fill the neutralizer when they start seeing green stains in the tub...by then it is to late!

Karl_Northwind

csst

having dug into the pricing, the impressive thing is that 1 1/4" bare CSST is only about 40% more expensive than 3/4" rather than the doubling every time you go up a size like copper. 



what are everyone's thoughts on using larger tubing, say  1 1/4  vs  3/4"? 

bummer that the bare stuff for solar use is twice the price as the coated stuff for gas use.

hot_rod

with CSST

there are dozens of different profiles. The depth of the groove, the spacing between the ridges, the type of stainless used, these all lead to the cost and flex-ability difference. Also the type and quality of weld when you look closely inside varies from brand to brand.



I did get some spec from our manufacturer on flow rate required to get turbulent flow in our 1".



Calculating the heat transfer looks a lot more complicated than bare tube and depends on all the above.



Calculating the surface area and the temperature profile on the ridges and vallys, etc.



But a university in Germany has taken this on as a project and I will get more info. They can model different options but they will actually flow test and data log to get actual hard data not just computer simulation models.



hr

NRT_Rob

turbulent

can you really "go laminar" in a coil?



straight pipe I could see, but I would think the continuous curve nature of a HX coil would cause a "swirly" flow pattern just from inertia if nothing else.



no?