heat exchanger size?

[hamlet jones]

I have an old sphericel propane tank made from 1/4 inch steel, 150 gallon size.  After washing it out, I want to built a thermal storage tank with itI'm planning on building a super insulated firebox underneith my spherical tank, with perhaps four 3inch flues welded up the center of my tank (like a vertical fire-tube steam boiler). Don't worry, it'll have all the safety reliefs, sight glass, low water alarm, automatic fill, thermal shutdown, etc.  I'm going to place it outside my county home, so the fire insurance isn't corrupted.  I plan on circulating the heat away with a series of pipes connected directly to the storage medium (water), yet for my hot water needs, I'll be wanting a heat exchanger, because

I don't want the rusty storage water coursing thru my home plumbing, so maybe a copper heat exchanger is in order (copper/steel = low galvanic corrosion).  

Question:

I'm guessing a coil of 1/2 inch to 3/4 inch copper tubing, but I have no idea how long I should make it.  Anybody have any idea how much surface area I'll need for an effective heat exchanger?

Thanks in advance,

Hamlet

[ghurd]

I'm no expert, but its been here a while, so I'll try.

The more the better. Too much is never enough.

It would be most efficient to leave the house at room temperature.

You are supposed to use the extra heavy CU.

A factory made wall/floor heater has good surface area, so it may be cheaper sometimes.

The more BTUs, the more surface area needed.

The 'propane guy' gave me heck for asking about something like this. Seems propane is absorbed or soaks into the steel of the tank. Something about explosions. He thought it was the dumbest question he was ever asked! Didn't mind telling me that either.  A friendly sort. NOT!

Antifreeze 50/50 has rust preventitives and holds and carries more heat.

Some extra CU soldered to the heat exchanger would increase the effective surface area. Like CU washers every 1/2". Or a scrap plate soldered on like this O. Thats the plate on the pipe, best I can do. New CU plate costs more than more pipe I bet, but it doesn't have to be thick.

My folks house has something just like car radiators inside the ducts. Stacked and tipped to fit like this. // The original furnace blower moves the air.

The air moves from left to right. The hot water moves right to left through the radiators in series.

Car radiators are great heat exchangers. Could get a lot of heat out of a brand new $100 radiator and big, high volume 120v muffin fan or 4. Just bought a new one myself, and 2 core was only $20 more than single core, good deal for something like this.

This could be way off base... Logic suggests its a good idea is to have a LOT more water in the house than at the fire. Maybe 10X more. Hot water is trying to cool from 180' to 75'F. Cool water is trying to go from 75' to 700'F.  The cool water is going to get hot faster than the hot water gets cool. So the house water needs a lot more surface area and time compared to the boiler water.

I think Nothing To Lose is working this area too. Check his posts.

Just 2 cents. Did anyone see this?

G-

[hamlet]

Thanks for your thoughts.  Perhaps I wasn't making myself clear:  i need to have some idea of how much surface area a WATER-to-WATER heat-exchanger needs to be.  In the house, I'll use an old radiator for the water-to-air thing (heating the garage/house), but I'll be using tubing in my boiler/furnace for collecting the heat from the boiler fluid (again, H2O), a closed system that won't/doesn't mix with my potable hot water.

The propane or the stink that they put in it WILL sink into the steel, and give you a good flash-over.  After washing the thing out, I'll fill it with CO2 (dry ice) before welding.  I forget how many pounds I'll need. Anyway, be safe.

[ghurd]

The do-hicky at my folks heats the house AND the hot water. Natural gas.

Never been inside the exchanger. The whole thing is about 20" tall, 16" dia.

I think there are 3 sets of pipes. Cold water in / hot out for water heating.  

Recycling hot out, cold back for the home heating, in a 1" CU pipe.

And a recycling cold out, hot back from the fire (I don't believe it has water in this loop. Don't know for sure.).

The exchanger is the only 'holding tank'. It is on demand. When the 'tank' temp gets low the circulator and fire kick on. I imagine domestic hot water just flows through tubing in there, but it never runs out cold. Same with the closed loop for home heat.

It is a real big house to heat. There is a small commercial operation that uses hot water constantly (2 or 2.5 gallons/minute) for about 5 hours a day, 3 days a week.

IMHO That is a lot of heat fast from an exchanger that size!  I don't know the surface area, But that tank is not very big for all the pipe running through it.

Maybe the guts are really fancy. Or maybe it takes a lot less surface area than I would have expected.

G-