Oxygen depletion in unpressurized storage?

an unpressurized tank is normally used in conjunction with copper coil heat exchangers that are part of the boiler system. Boiler water passes through the coils and gives up heat to the water in the unpressurized tank. In this way the boiler system can remain closed and pressurized (as it should) while the tank is open or unpressurized. The primary advantage of unpressurized storage is that it allows you to build the tank in the location where it is to be located. Individual tank components can be brought in one piece at a time so if you have difficult access to your tank location this is a good choice. These tanks also allow you to add an additional coil for domestic hot water production - either as a direct heating method from the tank or as a pre-heat for an indirect water heater or dedicated fossil fuel or electric water heater. The disadvantages are that these tanks are normally relatively short and squat so take a large footprint and they typically use some kind of liner that has a limited life and is susceptible to damage from too high water temps. The advantage of pressurized storage is that it is a bit easier to plumb, normally takes a smaller footprint because the tank can be tall and narrow, provides better stratification and is not susceptible to high temperature damage and so can be run to higher storage temps. Disadvantage is that you must use an indirect water heater because there is no opportunity to add a coil for domestic hot water inside these tanks. An indirect is a great way to heat water with wood or fossil fuel, it just adds some cost. If you already have an indirect, pressurized storage is something to consider.

you shouldn't plan on using ferrous (iron, steel) materials in your unpress. storage tank. It will always have a signif. qty. of O2 in the water, because it is essentially "open" (even though its covered) to atmospheric pressure. Copper coils are fine in the tank as heat exchangers. The water that is running through the copper coil IS oxygen depleted because its part of a closed system. "open" and "closed" systems are very different and their distinctions are important with regard to oxygen, and should be attended to.

Fred61 said:

Assuming an unpressurized storage tank has a tight fitting cover and does not need to continually be "topped off," will the ogygen level in the water eventually be reduced to a level comparable to a closed system?

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I don't think you could seal it up enough to prevent some O2 getting in. The reason would be that to seal it up that tight, you would then build pressure as the water temp went up and this would cause another set of problems. With a quality boiler treatment, I wouldn't worry to much about the O2 levels as most piping is thicker than the walls on allot of OWB and it may be cheaper to replace it as apposed to using copper. The liner will probably need to be replaced at some point and you can check it then. Allot of tanks have been built this way. Each type of system has advantages and disadvantages.

Fred61 said:

Thanks guys! I wasn't thinking about the expansion and contraction of the water. I guess you could say that the tank is breathing as the heating and cooling takes place by exhausting upon heating and inhaling upon cooling. If the cover was tight enough it would be like a canning jar and collapse the tank or cover upon cooling.

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The expansion and contraction will be substantial- so make sure you build in an overflow, or else don't fill it all the way up before heating it. My tank which is "nominally" going to be 1350 gallons cold will hold about 1200 "actual" after it heats from basement temp to near 200F, the water expands, and then (if I ever let it cool off all the way) returns to 55F old farmhouse cellar temperatures and the water again decreases in volume with the temperature.

PS, some might view this as a bad thing- but one of the benefits of unpressurized storage is the ability to skip rather large and expensive pressure expansion tanks.

BTW Hot water holds less oxygen than cold water.

slowzuki said:

BTW Hot water holds less oxygen than cold water.

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Very true- and also water will retain less dissolved gases at low pressures than at high pressures (just as the bubbles come out of soda when you open the lid)

but- in an unpressurized system that is open to the atmosphere, and assuming that the temperature is not staying high all the time, you have to assume (since nature tends to dislike and thus work against differentials- it's that entropy thing) that oxygen will migrate back in, too.

Seems like it'll end up "semi-depleted"

I've thought about an unpressurized tank an the O2 issue. Basically non-ferrous materials need to be used. I also wondered if using polypropylene floating balls on the water to reduce the surface area would help.

Mike

The balls really only slow evaporation losses.

A bladder tank would work too.

There are a number of threads on here about dumping parafin wax into the tank to form a skim on top to slow evaporation. I have no clue what the gas diffusion through it would be like.

An unpressurized tank will always have dissolved O2. If the tank is open enough to allow access for checking the level, there will be enough oxygen to be a corrosion issue.
If there is any ferrous metal in the tank, it must be treated with corrosion inhibitor. I have always assumed that if the water is heating and cooling, air is coming into and out of solution. This seems to be borne out by the corrosion of ferrous materials that might be in the system (if there is no inhibitor).

I would be a little skeptical of permeation of any material except metal. A condom or any polymer might allow enough O2 permeation to be an issue. A metal "condom" probably would be okay.
Boy, there's a mental picture!

A test will ultimately show the answer, for the tank, that is!

Tom in Maine said:

I would be a little skeptical of permeation of any material except metal. A condom or any polymer might allow enough O2 permeation to be an issue. A metal "condom" probably would be okay.
Boy, there's a mental picture!

A test will ultimately show the answer, for the tank, that is!

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I will _not_ sign up to be a beta tester for the above-mentioned line of products ;~)

Does anyone know the details of the engineering and assembly of the huge vertically-expanding metal natural gas tanks like you see just south of Newark NJ if you go down the Jersey Turnpike (or was it the Garden State Parkway?) They have telescoping metal shells with cages around the sides to keep them straight and reinforced. They change dramatically in overall height depending on supply vs demand. I always wondered whether they had a bladder inside or, if not, how they kept the gas inside. Whatever it is, if it were feasible, it seems like it might be adaptable to water storage- and maybe in a simpler way, as the degree of expansion is less.

Does anyone know the details of the engineering and assembly of the huge vertically-expanding metal natural gas tanks like you see just south of Newark NJ if you go down the Jersey Turnpike (or was it the Garden State Parkway?) They have telescoping metal shells with cages around the sides to keep them straight and reinforced. They change dramatically in overall height depending on supply vs demand. I always wondered whether they had a bladder inside or, if not, how they kept the gas inside. Whatever it is, if it were feasible, it seems like it might be adaptable to water storage- and maybe in a simpler way, as the degree of expansion is less.

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Basically, they are like a bucket of water buried flush with the ground and a slightly smaller bucket inverted inside of that. Bubble gas up into the smaller bucket and it floats up above grade. The framework you see is just to keep it from tipping over and jamming as it gets higher. Take the gas out and it goes back down.

Most OWB are open systems, and if the water is properly treated, they do last for years without corrosion failure. Mine (bought it used, in use 5+ years when I bought it) I used for 10 years and still in apparent good condition. I treated the water regularly with sodium sulfite. I sold it last summer and no reports of problems from the buyer.