Triple pipe heat exchanger

[Lumberjack]

I was looking over the heat exchanger in this posting:

http://www.fieldlines.com/story/2007/9/21/2192/03162

And I realised I could do one like it only with three pipes instead of two.

I was thinking:

1. /2 inch pipe for solar collectors with glycol
   
3. /4 inch pipe for the radiant heating system and storage
   
1. inch pipe for the domestic water preheat
   
   

Would this satisfy the double wall requirement?

Would this be a good configuration or should the domestic water be at the center?

Any idea on how many feet of this I would need? The one in the posting is about 20 foot and seems to work ok...

[GaryGary]

Hi,

I think you would need some type of thermally conductive material between the two pipes  where no water is flowing in order to get any kind of descent performance.  

The double wall heat exchangers are required to leak fluid to the outside when they fail so that you can detect that you have a problem.  I think that some of them use a thermally conductive paste that is water soluble between the two pipes so that when there is a leak in either pipe, it washes out the paste and leaks to a place that is visible.

Gary

[Lumberjack]

I am planning water in all three pipes....

[electrak]

I am thinking about doing something simular, I am going to use ethelene glycol in a diesel at atospheric pressure though a heat exchanger with propolene glycol at about 20 psi going into the house( 90ft. ) to another exchanger to the hot water tank at about 40 - 60 psi. so if I do have a leak it will go tords the bad stuff. Might not be to code but should keep you safe

[Lumberjack]

I am not sure you would get the flow rates you need for your application with 1/4 inch step ups. Copper pipe does go up to 3 inches though and doubles in price with every step up. Also you cannot have any internal joints so you are limited to about 20 feet or so.

I am looking at four 7 foot long pipes in a two down and two back arrangement. I am figuring about 200 for pipe and 200 for fittings. This is an improvement over the 700 or more for copper coils but it may not be legal per say. I considered a longer arrangement but a 20 foot pipe would be very easy to damage with a bend in the middle and would need more support etc.

I suppose I should not gripe too much considering it is one of the cheaper parts of the system....

[jonas302]

take a look at flat plate heat exchangers on ebay I ten plate heats my domestic water I think around $120  I heat the house with a 60 plate around 300

I have seen 30 plates  on there for around 100 after I bought mine of course

[fcfcfc]

Hi:

There are allot of variables here. Since you mention space heating I am assuming you are putting together a large thermal array..??.. If the array is only going to be about 60sqft or less, I wouldn't bother with space heating but design the system for DHW only. Regular smooth pipe in pipe HX's develop laminar flows quickly which is why you need so many feet of exchange surface. Always try to put the hottest fluid (solar array loop) on the inner pipe and work the pickup fluid on the other pipe and run the fluids in opposite directions. Insulate the HX well and only use Propylene Glycol/distilled water (50/50) in the collector loop.

.....Bill

[Lumberjack]

I am looking at a system I intend to expand large enough to supply most of my space heating needs and preheat domestic hot water. (yes your collectors are being considered.) The radiant floor heating and solar install will likely end up as separate projects But I want to make sure they will be able to tie properly together.

I kind of suspected that the hottest water should go on the inside. Thanks for confirming that.

[fcfcfc]

Hi: If you are doing it in two stages, start with an off the shelf 80 gallon electric tank (electrically dis connected) and use a small flat plate exchanger, maybe 20-25Kbtu's and have one side of that on the potable water side with the 80 gallon tank and a small bronze circ, like a TACO-006. You then could set up a small drain back tank (20 gal open air) with the cast iron pump for the collector loop and the other side of the HX. Drain back is the most efficient because there are no heat exchangers on the collection side, for the most part percentage wise, once you expand the system. Both pumps run when the collector loop activates. When you expand the system to include SP, your remove the 20 gallon drain back and switch it to a 300-600 gallon non pressurized tank which then becomes the main heat storage tank and the "drain back tank" so to speak. Make sure you plumb the small heat exchanger for the DHW in the collector return line first, so it has "first grabs" at the higher array temperature. The only problem I have found with big non pressurized tanks is bacteria control. In my current 350 gallon, I have 10% Ethylene Glycol to prevent growth but at the price of antifreeze these days that's expensive. You can always go to garages and filter the old stuff, but for a 600 gallon tank that's 120 gallons because anti from cars is already 50/50. What I need to find is a toxic liquid that I can use about 1-5 gallons of for 600 gallons of water, but not so "BAD" that I need an EPA suit to use it.... Anyway, that's a tough problem and a WIP.

[Lumberjack]

300 gallons of anti freeze is very costly which is precisely why I dont want a lot of storage on the collector side of things. I intend to plumb the storage onto the radiant side instead and only store the excess. The collector circuit I plan on either a closed loop with glycol or a drainback system but with as few gallons in it as possible. Drainback is the safer choice I think, if the collectors can deal with it but I may be forced to install a failsafe overheat system which would require a closed loop instead.

I did some rework of the three pipe idea and realised that I can eliminate the middle pipe and cut my cost in half. The center 1/2 inch pipe would carry the collector and the 1 inch outer pipe would be split into two loops with one loop for domestic hot water and the other loop for the radiant heat. Each loop would be two 5 foot lengths tied at one end in a down and back configuration giving ten feet of exchange area. The effective loop actually grew by a foot on each pipe. I could also adjust the length of the pipes to customise each loop to the amount of exchange I actually want.

I figure if I put the hotter radiant loop first and the cold DHW loop second, I also wont be stealing heat from the radiant and heating water which is the cheaper of the two expenses. I also think I will get a much better exchange with the DHW in this arrangement.

I was also looking at the cross section of the triple and decided there was not enough space between each section which would also lead to a high resistance to water flow. Eliminating the 3/4 inch pipe doubles the space availible and should more then cut flow resistance in half. I did consider the 1/2inch inside 3/4 inch arrangement but realised it would take four pipes to get the same flow rates and I would need more then four to do the DHW as well as the radiant.

Yes I know I could get a plate exchanger but what fun would that be? Besides I suspect they wont transfer nearly as many BTUs as I want and I would still have to insulate it heavily and It would take either two exchangers or a more expensive 3 circuit one to do the job. I do intend to check the cost of them out more closely but I suspect a commercial made unit is actually going to run more for the system I intend.

[Lumberjack]

BTW FCFCFC, I asked how your collectors handle stagnation and freeze conditions by e-mail and on your old thread , I would appreciate a reply when you get a chance....

[fcfcfc]

Hi: Sorry, are your initials BL..??.. I am behind on my emails because I am putting up my new building and I am pretty much dedicating 90% of my time to that, which is causing a backlog on everything else.

They do handle stagnation but I don't recommend it. Now with that said, I had a conversation about 2 years ago where this question was asked, and one of the two other parties in the thread on this, took what I said and completely twisted it around into meaning that the units had a big problem with stagnation, blah, blah, blah. And the more I tried, patiently, to make it as clear as humanly possible, the more they twisted it. Needless to say I was so angry I started to have homicidal impulses, so I flagged there email addresses as spam and ended the communication. To put it simply, they take it but it is still abusive and it will take its toll over time no matter what ANY evac mfg tells you, and I mean ANY of them. There is no reason to stag collectors if the system is done right.

Changing subject back to the design of the system, what type of radiant system do you have..??.. Low mass, high mass, etc.....

BTW, you don't need 300 gallons of antifreeze, you need 60 gallons of PURE antifreeze for a 600 water tank, 60 gallons Glycol, 540 gallons water. This will give a ~10% concentration.

.....Bill

[Lumberjack]

In our climate I believe a 50/50 mix is called for to prevent freezing. I suspect freeze is very likely to ruin the manifold. 10 percent I would guess is enough to stop bacteria I suppose But I would guess there are safer additives like those used in cooling towers that might be a bit less. - A new question for the list!

Oddly enough yes my initials are BL but I always go by lumberjack everywhere online. I picked it up because of my snoring while on hiking trips..... I understand being busy, life sure does heat up some years.

Guessing from your question I would say a low mass install or more precisely under floor joist. I am still in planning for the whole thing right now and there is still a lot of stuff I need to find out before I begin. This years projects will mostly involve conservation measures like extra insulation and air leak detection. My current place is a rental so not only must everything be cost effective It has to get approval from the owner as well. ( I am considering buying it but I seriously doubt the owner will sell his grandparents house....)After the kids go in a few more years I will be getting a house for sure and a full bore install will be done on it. Sooner or later, it is only a matter of time.

So stagnation is a problem for everyone. I guess it means I need to come up with a failsafe way to bleed off excess heat in the collector that does not rely on pumps. Some form of thermosyphon controlled by a thermostat to radiate over a preset temp might do since All that really needs to be done is keep the temp below 200F. That also puts drainback off the list as sooner or later stagnation will occur....Add search for a valve that will open at about 180 or so to the list....

[fcfcfc]

Hi: Let me try and clear up these points. If you are already clear that's fine. First in a drain back the 10%EG is for bacteria only and has nothing to do with freezing. Freezing is not relevant in drain back. That is the whole purpose of drain back is to negate freezing. Now there are BTU benefits also, but primary is no heat exchanger on a non pressurized drain back system (large open tank).

Second, your system is fine going up to the boiling point of water as long as the tank components are rated for it. With a low mass radiant you need a good amount of water storage, I mean at least 600 gallons for a non pressurized tank. I get away with 320 on my DPDB systems but they are pressurized and the boiling point of water even at 30PSI is 272 DegF, so boiling is not an issue. And I don't have any heat exchangers at all.

Stag can be avoided using the combination of proper steep mounting angle, proper storage size and a simple 12VDC MSI inverter and a couple of batteries and relay. For a few hundred dollars it is easy to put together a backup power "board" that will run a system for a couple of days, giving you a nice window of action if the power failure becomes extended. In light of a pump failure, the best thing is have a spare sitting there to cut down on down time. AGAIN, if they do stag for a few days its not the end of the world. Your not going to bust tubes or they won't turn orange or pink..LOL.. Think of it as going to a rock concert... when you come out your ears are ringing but by the next day you are fine... BUT do that for 20 years and you most likely will have a problem. I have had tubes here deliberately staging 24/7 for 18 months!!! They still get very hot and look fine... the point is simply don't beat up on them if you don't have to. In allot things, heat is a lifespan killer....

[Lumberjack]

Thanks for the info, your are trying very hard to make my life a lot more difficult I will give it some more thought before selecting a final design.

Have you considered perhaps chlorine instead? It would be a lot less dangerous in the event of an accident but you would have to re-dose it every three months...

[fcfcfc]

Hi:

Chlorine is a real bad choice along with Bromine. They are highly corrosive especially to copper and are dissolved gasses so they tend to leave the water. I am afraid it is not a simple problem to solve. I will find a solution but I have to do some more chemical research....

Thanks for your thoughts though....

[Catch66]

Hello,

Lumberjack,

"I am looking at four 7 foot long pipes in a two down and two back arrangement"

From my DHW system experience 4'x 12'Panel. My first exchanger started with

5..1/2" x 3/4" x 30" counter-flow. glycol enters at top 3.0 gal. max.flow.

When I first was able to test it was near September last fall. I had roughly

a 16 degree differential. I purchased a 20 plate exchanger,

installed my differential was much better but still had separation, So purchased final

30 plate exchanger and installed,set it to have a single pass through both HX with water side single pass. I plan on adding a Evacuated tube system later as a high temp return tied into flat plate hot outlet, so just planning for later.

My system is closed 16# on glycol side and city water on the other.

Good Luck on your final decision.

Chuck

[Madscientist267]

I know little to nothing about solar water heating (although obviously intrigued), but one thing that came to mind in reading this; if the concern is only to keep bacteria at bay, what about an alcohol mix? It's inert in regards to reaction with the metals, and while lowering the boiling point slightly, I wouldn't think it would be all that much in a ratio that is just sufficient for antiseptic properties...?

Steve