Heat Exchangers - Built or Bought

[WireNut]

I was wondering if anyone here would be willing to share their knowledge and perhaps some pictures on what they're running for a heat exchanger in an open heat storage tank. The two options I'm looking at are a home-built copper coil system similar to this:

Source: http://www.builditsolar.com/Projects/SpaceHeating/DHWSpaceBoiler/DHWSpaceBoiler.htm

Or perhaps a manufactured solution like this:

(broken link removed to http://www.outdoorfurnacesupply.com/shell-tube-heat-exchangers/sp-series-155-000-btu-pool-spa-heat-exchanger.html)

Has anyone tried the second method (the heat exchanger style pictured)? If not that type but another, could you provide suggestions?

 

[hobbyheater]

This is a section from the Jetstream manual on making your own heat exchanger. If it would be any help to you, there is a complete section on building your own tank which I could send to you.
I have not personally used this system as my storage was in place and working before gasification was available.

 

[peakbagger]

American Solartechnics builds coils, they are not just a coil of copper, there are two main headers and then multiple smaller diameter tubes coiled that tie back into the headers. I believe the intent is it increases the coils response time by upping the surface area.

 

[hobbyheater]

(broken link removed to http://www.americansolartechnics.com/photos.html)

This the site that "peakbagger" suggested.

 

[zawetzel13]

I'm planning to build an open storage tank by this fall. I just ripped out all the Hot water baseboard out of my home and plan to re-use that copper with fins in the tank. Not sure how much of it I need to put in the tank, but I was hoping to browse this site in hopes to find answers to that. The tank will be 4' X 8' welded steel. I got a quote from a local welder that said he could do this for $800 (materials & labor). This will give me almost 1,000 gal of hot water to heat my 2,300+ sq ft home here in Maine.

I would like to know if anyone has used old baseboard heaters as their heat exchangers in the tank, and what kind of btu/hr/ft they were able to heat water in the tank. Just don't know exactly how many ft of the stuff I need to put in.

 

[WireNut]

Thanks guys,

hobbyheater -> I looked over that PDF file, and what you had looks alot like what I'm looking to build. I have never been able to find a retail supplier of finned copper coils (or soft finned copper tube). Do you know where I could find that, or do most folks just stick with the plain copper?

peakbagger -> Thanks, I had looked at them before for the pre-build open heat storage solutions, and they were a little more then my budget would allow. I'll have to check with them on the heat exchangers as I don't see any pictures of them on the website (just a picture of a copper coil on the page for the space heat exchanger).

 

[jebatty]

To the best of my knowledge, for tube and shell or for plate heat exchangers, what's important is not the published btuh rating, but the specs upon which that rating is based: Side A temp in and Side A temp out, Side A gpm, Side A pressure drop; Side B temp in and Side B temp out, Side B gpm, Side B pressure drop. I have seen ratings for heat exchangers that are very deceptive. Do your homework to avoid potentially expensive disappointment.

 

[tom in maine]

As some of you know, we make heat exchangers.
I am not going to go into details, there are enough online.

I have to comment on the use of copper fin tube with aluminum fins in an open tank.
The fins are not going to do anything good. They will corrode, but that is not going to help with heat exchange.
Installing fintube in a steel tank is a recipe for a BIG corrosion issue.

Use smooth copper or plate heat exchangers. Extended surface, even copper is not going to do much for performance.

 

[Bob Rohr]

As some of you know, we make heat exchangers.
I am not going to go into details, there are enough online.

I have to comment on the use of copper fin tube with aluminum fins in an open tank.
The fins are not going to do anything good. They will corrode, but that is not going to help with heat exchange.
Installing fintube in a steel tank is a recipe for a BIG corrosion issue.

Use smooth copper or plate heat exchangers. Extended surface, even copper is not going to do much for performance.

Once you start pricing soft copper coil I think you will find that a PHE is really the best heat transfer device for the price. No need to worry about dis-similar metals in an electrolyte (water) causing problems when the HX is external to the tank.

There are plenty of free online sizing and selection programs so you get exactly what you need in a PHE, instead of guessing at coil length size and headering method, plus soldering and assembly labor.

 

[tom in maine]

It is possible to source a plate hx for less than a retail priced commercially made coil, perhaps. One has to be aware of the quality of the PHE, and the fact that once you start pumping water out of a tank, you are disrupting stratification. A coil, being passive within the tank, is going to maintain some serious stratification (if so desired).
There is also no need for a tank pump which will have to be non-ferrous for moving tank water.
There is no guessing with a commercially made coil from a vendor who knows what they are doing ;^)

 

[Bob Rohr]

It is possible to source a plate hx for less than a retail priced commercially made coil, perhaps. One has to be aware of the quality of the PHE, and the fact that once you start pumping water out of a tank, you are disrupting stratification. A coil, being passive within the tank, is going to maintain some serious stratification (if so desired).
There is also no need for a tank pump which will have to be non-ferrous for moving tank water.
There is no guessing with a commercially made coil from a vendor who knows what they are doing ;^)

Hi Tom
Yes and no, pros and cons It's possible to maintain the stratification with some difuser pipes when pumping in and out of tanks.

But also two pump flows will transfer much more energy compared to one with a tube bundle in a tank of still water. PHE are designed with corrugated plates to enhance turbulence and get near perfect heat transfer.

Both are acceptable, but I have seen a fair share of "swing and miss" DIY heat exchanger rats nests. I would agree, unless you have experience assembling and designing a quality tube bundle HX, leave it to the pros. A simple mouse click will have one arrive at your door ready to go to work.

 

[WireNut]

Good points I'll have to look at. What kind of temperature difference will I see between the top and bottom of a 4 foot deep tank? I know it varies, but has anyone observed the temperature difference and can tell me what they've experienced? If it's a 10 degree difference, that's a huge difference that could net me another 8 hours without having to restart a fire.

 

[hobbyheater]

Good points I'll have to look at. What kind of temperature difference will I see between the top and bottom of a 4 foot deep tank? I know it varies, but has anyone observed the temperature difference and can tell me what they've experienced? If it's a 10 degree difference, that's a huge difference that could net me another 8 hours without having to restart a fire.

I can get the top and bottom temperatures within 10 degrees when heating the tank but it stratifies to 15 degrees difference as it cools and this has been consistent for 30 years. The tank is operated with static pressure with a open-to-atmosphere expansion tank on the floor above. The tank's capacity is 1046 imperil gallons. The gauge in the picture is at the bottom of the tank with the tank being 54" in height. The top gauge is 8" from the top of the tank. Heat loss from the tank is about 1,200 BTUs per hour which I think is pretty low.


The boiler and storage share the same water . The heat exchangers for DHW and house zones are each made of 120' of 1/2" copper and are at the top of the tank and run from one end of the tank to the other. Both have operated 30+ years trouble free.



Storage is largely above the height of the boiler so a simple one way check valve prevents backflow and loss of heat when the boiler is not firing. The storage tank is a old steam boiler with its flame tubes removed; 5/8 inch riveted plate steel.
The system that I have here is not typical of anything you will see on this site.