tank in a tank heat exchanger

[BruceM]

Hi Guys,
I'm finishing construction of my big (128SF) drainback solar panel for hot water heating (space and domestic).  I have built the 700 gallon insulated storage tank, ready to put in the liner.  My water pressure is just 12 psi, from a hill mounted tank. 

I'm interested in the "batch" tank within my tank heat exchanger for my domestic hot water, but am having a hard timing finding an affordable tank. I can't use a water heat tank since the outside of those tanks would corrode when submerged.  New galvanized steel tanks are out of sight, cost wise, and stainless steel pressure tanks seem insane as well.

My last resort is a copper tube exchanger, but in winter my incoming water is near freezing, so according to the figures I've seen, I would need at least 180 feet of 1/2" copper ( as 6-8 coils in parallel) at 2 GPM.  (38F to 105F).  This is $360 just for the soft copper, perhaps $420 all together for copper cost.  I think that even 180 feet is marginal.

Any ideas would be appreciated.

PS-  I can't use a big coil of 1" pex as the exchanger/batch heater as I don't tolerate hot water from pex for showers. Too much plasticizers smell, especially the first year. 

[mike90045]

......PS-  I can't use a big coil of 1" pex as the exchanger/batch heater as I don't tolerate hot water from pex for showers. Too much plasticizers smell, especially the first year. 

Crank the water temp up, over 150F if you can, and cook it out for a couple weeks. Then use it ?
  But don't expose the PEX to sunlight / UV kills it quick.

[Ronmar]

Sadly, having to deal with domestic water pressure, I don't see an alternative to the spendy SS pressure tank or the copper coils.  You might be able to save a little building a heatex bundle out of straight copper and elbows...  1/2" X 10' copper is $9.95 and 1/2 elbows are about .38 each in bulk(HD prices).  Depending on how long a bundle your tank could accomidate, you  could probably do 200' for under $250.  Of course you would be a little more complex with all the unions.

Another option might be a smaller heat exchanger loop in the top of the storage tank feeding a standard domestic water tank alongside the storage tank.  This would require a small circ pump and some temp sensing/control to circulate water thru the loop to "charge" the water tank.  the good thing about this method would be if for some reason there is not enough heat in storage to charge the domestic tank, eventually the heat level drops to a point where the normal electric elements kick in as a backup to maintain temp.  I am working on something similar for my radiant floor to maintain boiler tank temp, with the electric as a backup to guarantee warm floors when solar or my woodchip/pellet/wasteoil boiler is not able to provide:) 

Are you doing anything to deal with possible growth in the tank/solar storage water?  Copper dosn't react well with any form of chlorine added to the water...

[BruceM]

Thanks for your suggestion, Mike.  Alas, I've already tried that experiment before abandoning Pex for shower hot water.  I love Pex for most other uses. 

Ronmar, thanks for your thoughts.  I've thought about going to rigid copper at half the price of soft, but with so many fittings to fold it into my 5x5x6 foot tank it doesn't end up being such a bargain. 

I'll have to think more about the possibility of adding another pump to circulate the hot water through my propane hot water tank. This would allow a more modest copper coil in the storage tank.  Alas, with my basic pilot type propane water heater, I'd have to turn the propane off, since there is no practical way to suspend it's turn on.  (I wonder if there is some way to trick it.) If I end up needed more propane assist than I hope to achieve, it would be a real pain, and very inefficient.

My initial plan was to keep it simple and have no additional pump, use the storage heat exchanger/tank as a pre-heater in the periods of cloudy winter weather (very rare), but to allow for manual bypass of the propane heater so it can be turned off most of the year.  So even when the propane water was turned on and "in the loop", the incoming water would be pre-heated and little propane would be consumed.  The propane heater would have to maintain tank temperature with this approach, but not do any serious heating.

Used stainless steel, closed top barrels (30 and 55 gallon) are available on ebay, but I think 12 psi is too high for their flat top/bottom design.  Stainless pressure tanks seem to command very high prices.

On the biological front, I will monitor the water and will resort to silver and/or copper ion generators or non-chlorine biocides if need be.  Gary Reysa has had no biological problems with his copper collector /EPDM tank drainback system, and I hope my experience will be the same.

[mike90045]

instead of PEX, there is also flexible poly pipe, in garden (40psi) and NSF (100psi) grades.  We us it with a large (200') loop laid out in the sun, and outdoors, we smell nothing, and when I prep the pipe by dipping in boiling water to force it over barb fittings (backed with a hose clamp) I smell nothing.  But it will kink easily, and I'm not sure of it's pressure/heat ratio.

[Ronmar]

You don't have to trick the propane unit.  You just set the propane thermostat lower than the op temp of the circ pump system.  The circ pump draws heat from the storage tank and maintains tank temp untill it no longer has the heat in storage to do so. When it can no longer maintain temp, the temp then falls to a point where the propane kicks in normally... 

[BruceM]

Ronmar- Let me see if I understand this arrangement with the 2nd circ pump cycling hot water from the propane tank through a copper coil in the storage tank... It would require the pump and heat exchanger coil in the tank(140F max) to transfer enough heat to keep the tank above a minimum useable shower temperature.   I'd add another thermistor to the propane tank, with differential control between it and the storage tank temperature to turn on the pump. I'd draw the water off of the hose bib (bottom of the tank), where the ccccold winter water comes in, put it back in a tee at the hot outlet. 

It seems like I'd might still need about the same amount of the copper needed for the pumpless preheat coil in order to keep the tank temperature from dropping below the propane turn on point, since the storage tank temp may not be too far about a useable shower temp.   I suppose if I set the stock propane thermostat low enough, it might not kick on.

Any ideas where I could find a calculator or equations for copper tube in tank heatex

[Ronmar]

The calcs are fairly straightforward.  Copper has a heat transfer co-efficient of 231.  That is 231 BTU per hour, per sq/ft, per degreeF of temperature difference.  1/2" tube has a circumference of 1.57".  Multiply that by 10'(120") = 188 sq/in or 1.31 SQ/FT of surface area per 10'.  Inlet water from the ground is probably going to be in the 50F range and lets assume a safe target HW tank temp of 120F. 50 gallons of water weighs 400 pounds.  SO 90 degrees(50 to 140) X 231 BTU = 20790 BTU x 1.31 sq/ft = 27235 BTU.  1 BTU will raise 1 pound of water 1DegreeF.  27235 BTU divided by 400# of water means it would raise 50 gallons of 50F water 68F in 1 hour.  This assumes a lot of course, that the water temps around the copper maintains 140F and that you can turbulate the water inside the tube enough to maintain the biggest temp difference. A piece of solid copper wire run inside the 1/2" pipe will help to turbulate the flow thru the pipe.  That 140F target temp is of course going to drop as you extract heat from the tank.  Also the transfer rate slows as the temp differences gets closer to one another.  110 to 140 is only 30F.  So 30X231X1.31 is 9078 BTU/HR.  9078 BTU/hr will raise 50 gallons of 110F water 22.7F in an hour. 

You can take these basic numbers and apply them to various sizes of heat exchangers and varying tank temps to come up with something that meets your needs...

[vdubnut62]

Uh, now that I am completely dazzled by science and math, and as pertaining to the closed pressure tank question, um Beer Keg? They do come in stainless and aluminum, 12lbs of pressure would be no problem.
And if this option is off the table, never mind.
I'll sit down and be quiet now.
Ron.

[Ronmar]

Hey Ron, I think his heatex needs to sustain domestic water pressure, and I am not sure a keg would do that.  Emptying one and then pressure testing in a tank full of water might be entertaining though:)  "Hey Bubba hold my beer while I open this air valve"...

[BruceM]

Thanks for the black pipe idea, Mike.  I'm not normal since a sick building/aircraft composites injury, so I'd rather avoid plastics mixed with hot water.  

Thanks very much Ronmar.  I'll play with some numbers tonight to see how much copper I'd need.  Copper is king for long life, I think.  Some vendors claim 1/2" coils at 20" radius have good turbulence.  I'll have to check their performance figures against the equations to see how close they come.

I did find a bargain galvanized tank at my local Tractor Supply, which makes that approach tempting.  $269 for a 43 gallon tank.  Very little plumbing, and no extra pump is appealing.  Limited lifespan is the only downside.  

I also thought about SS beer kegs, Ron.  If I can find a cheap source it might be a good way to go. So far, no luck, and mail order is about $140 each for tanks bad enough to be decommissioned.  A bit of a plumbing frig with only the top hole but doable via the small pipe in a bushed down big tee method.   I'm going to check on availability tomorrow .  My water pressure is only 12psi but the tanks are rated around 130 psi.

The good news of the day-  we got the 32 foot by 4 foot high panel mounted on my shop south wall this morning before the wind got horrible. Jacks and blocks and straps- it went pretty fast.  Tomorrow we'll add the polycarbonate glazing and steel braces for tilting the bottom out about 33 degrees after the glazing is on.  (It's hung via 9 leftover 4" door hinges from the top edge.)  We got the 1" Pex (sloped to drain back) running to the storage tank installed this afternoon.  

[Dualfuel]

Dear Bruce,
This is a very interesting project and I would really like to see some pictures of it.... Solar water heating is the next big to do around here too.
I am not clear on the details. Is 12psi the pressure at your tap? So the heatexchanger tank within the storage tank would be exposed to 12psi on the inside and a few psi on the outside? waddya mean the water will be close to freezing? Do you mean the incoming tap water going to the storage tank?
Two possibilities come to mind...a brand new radiator old school copper....OR....why not use a stripped down domestic water heater? You mentioned corrosion...so make the storage water non-corrosive. Everybody here has to do that with their boilers...its not that much fuss.
I'm sitting here thinking about how I would have to do it...I would need the storage water to have anti-freeze and the heat exchanger to be absolutely leak proof or maintain constant pressure on the inside.

OK, I think I would have a surflo pump wired to start pumping when the panel warmed up, then I'd have the heat exchanger at the drainback, then the fluid leaving the heat exchanger would go to the storage tank. I think I would look at either those thermostatic zone valves for hydronic systems or hydraulic solenoids (12v), to make a manifold to heat the exchanger when the panel is "off".
Please take all this as mere armchair speculation...my main interest would be to build something like this with the parts I have on hand, and one thing around here that runs really hot and uses twelve volts are the hydraulic units. I'd even consider using oil as the panel/storage fluid, as here, freezing is more an issue then efficiency.
DF

[BruceM]

DF-  If you research hot water systems starting at builditsolar.com you can get a good education.  My system is a copy of the diy copper tube/aluminum fin design developed by Gary at builditsolar.  Closed systems to the solar panels are to be avoided if at all possible, drainback systems have the longest, collector service life by far, require no antifreeze.  Antifreeze in the storage tank is to be avoided at all costs-  it's insanely expensive and an ongoing cost as the antifreeze must be replaced.  Closed systems have a fatal flaw- if there is a system failure, the antifreeze gets overheated, and if at any time of the year you have excess capacity, you MUST have a system to either cover the panels or another active system to dump that heat.  Drainback systems drain in case of failure, have no antifreeze and have the only downside of requiring a lot more pump to get the flow started (though only slightly more once the return flow is established).  A drainback system can be safely turned off when your storage tank gets to it's maximum temperature.  Because of the expense of copper and aluminum for the collector, I decided that I would do drainback, single tank and single pump system, even though for me that meant having to build a mostly below ground tank, cutting through the slab in my shop floor.  It eliminates the need for a heat exchanger for the solar loop.  

I'll post some pictures of my Hiser (horizonal riser) solar panel and tank soon.  Yes, my domestic water pressure is 12psi from gravity pressure.  No boost pumps, no pressure tanks, just one 2000 gallon storage tank up on the hill.  I love the simplicity of it and oversized my pipes for the lower pressure.  It works fine.  I use a gas fired pressure washer with 75 feet of pressure line for cleaning patios and cars- the garden hose is the only place you notice the low pressure, especially on more than 30 feet of hose.

What methods could be used to prevent corrosion of a 43 gallon galvanized tank within my storage tank? 

[veggie]

+1 for Dualfuels' suggestion of a school radiator.
Or .. consider connecting 3 or 4 sections of 3" pipe in series to form a submersible radiator.
The corrosion allowance on sched.40 pipe means the thing will take 473.347 years to corrode through   .
Not sure of the resources you have, but a bit of welding is necessary to join each end with 3/4" pipes.
The "pipe exchanger" can be made from any diameter you choose to get the desired retention time.
Then lay it flat on it's side in the tank. Perhaps on legs which to raise it off the floor and place it in the center of the reservoir.

veggie

[Tom Reed]

Just thinking out loud here, why not buy a good a good 1/2" tubing bender and get some stainless tube. You can then bend up a shape to best fit your new tank. Or once a time I used to make heat exchangers by winding copper tube around a drum. Eliminating the fittings will save a lot of cost and failure points.