Plain steel expansion tank options..

[deerefanatic]

Hi all, since I'll be isolating my stainless milk tank with a heat exchanger, I am wanting to go with a true sealed pressurized system for my boiler. BUT, my heat exchanger on my stove isn't designed for pressure..... The ends are 1/4" steel, the firetubes are schedule 40 pipe, and the front and sides are made from a single bent piece of 1/4"..... But the back is a piece of 3/16" steel that's welded to the top, bottom, and sides. Under 10-15 psi of pressure it bows out noticeably. I'm worried over what it would do if pressure were to hit 30psi (pop-off valve pressure)... So, instead of a bladder tank, I was thinking of using a plain steel tank with very little precharge pressure. But, the air will get ingested into the water over time, which has the same effect as an open system..

So, I was wondering if one could use welder shielding gas? I use Praxair's stargon which is 80% argon, 8% CO2, & 2% oxygen. Seems since it's safe for use in shielding melted metal, it ought to be ideal as an air charge for hydronic water since everything in it except for the 2% oxygen is considered metal inert gas. (Where the term MIG welding came from)

What do you all think?

My other option is to weld angle iron across the back of the stove to beef it up to handle higher pressure, which may be the better way to go...

Looking for feedback here.

 

[Nofossil]

I thought that the European approach was interesting - basically, a small open-topped tank (maybe 5 or 10 gallons) mounted above the highest point in the system. You could float a layer of oil on the water to prevent evaporation and oxygen entry, and perhaps use a standard toilet tank float mechanism to automatically maintain the level. Water in this tank would never get hot,and the level would fluctuate based on system temperature. System pressure would be quite low.

 

[DaveBP]

You could float a layer of oil on the water to prevent evaporation and oxygen entry

I've been hoping someone on the forum here with some college level chemistry research skills would pick up on this. What kind of oil that is liquid at room temperature would be oxygen impermeable? Affordable? Nontoxic? No vile smell at hot water temps ( probably shouldn't be sticking your nose near vent pipes, anyway). I think a true oxygen barrier oil would be hard to find.

Those of us planning on large pressurized storage are cringing at the cost of expansion tanks in the required sizes so these alternative methods are interesting. I also thought about a smaller propane tank with a non oxygen gas in it. CO2 has a risk in that when it dissolves in water it forms carbonic acid and would need more chemical to keep the system water alkaline. Nitrogen is cheap and I'm pretty sure stays nonreactive and it dissolves less in water than air. There may be others and better.

Hansson might know how hot that tank-in-the-attic gets when the 'tide' cycles in and out. I seem to remember a photo of his tank with foam insulation around it.

 

[deerefanatic]

I too like the oil on top method... But then again, how much good does it do?

I'm leaning heavily toward simply re-enforcing the back of the heat exchanger to be able to handle pressure, then getting a bladder tank and pre-charging it to about 6psi.... That way system pressure should rarely exceed 10 psi (which in it's current state my boiler will handle); will be sealed; and should something go hay-wire, the boiler will be able to stand the 30 psi needed to pop the pressure relief.......

 

[DenaliChuck]

Someone on the board also suggested a layer of wax, but it seems that this would only work if the water in the tank was hot enough to keep the wax melted.

 

[deerefanatic]

Actually Nofossil is the wax man........ But, it only stops evaporation. It won't do anything for oxygen ingestion..........

 

[machinistbcb]

I used two 80 gallon air cushion expasion tanks for my set-up. It really should only allow a small amount of oxygen in to the tank as long as you have no leaks. I am no expert but seems like a pretty simple solution.

 

[deerefanatic]

so I should only need like 16 gallons or so maximum for my 150 gallon system capacity then...... Don't the air cushion tanks get water logged quickly though? That'd be where the oxygen is ingested..........