Gas generation in hot water storage

[Jens]

I have run across something rather unexpected and thought I would share it ...

My thermal storage system consists out of three 60 gallon hot water tanks. They are arranged as a sequence where hot water flows into the top of the first tank, out of the bottom of the first and into the top of the second, then out of the bottom of the second into the top of the third and from the bottom of the third back to the engine heat exchanger. The idea was to stratify the water temperature during a run and have the coldest water return to the engine. This works very well all around.
Tank #2 has two 3600 (?) watt dump loads in it to serve as a load to the engine.
Tank #3 has a 5 gallon external tank that serves as an expansion reservoir. There is one circulation pump that sucks water out of tank 3, sends it to the engine heat exchanger from and then it flows into the top of tank #1.
In theory, this is all nice and dandy. There should never be air in tanks one and two. Only tank 3 is vented to the atmosphere and will either be over-full when hot or slightly low when cold.

I have had issues with the expansion tank overflowing on occasion and I have finally tracked it down.

It turns out that the dump elements create air bubbles which get trapped in the top of tank #2 which is not vented. The air displaces water which then pushes out and overflows the expansion tank.

The reason this is so surprising is because I was always under the impression that any water gasified by a heating element would recombine to form water (condense) as it separates from the heating element and cools down in the surrounding water. This might happen to some degree but certainly not completely. BTW, this water has been essentially the same for over a year so any air should have separated out other than maybe air entrained due to water falling into tank #3 when it isn't quite full.

I will be venting the tank separately.

Anyhow, I hope this tidbit of info might be useful for someone.

[Geno]

That is interesting. I hope it doesn't take a year to eliminate the gas bubbles in my system. I also have 2, 950 watt, hot water heater elements in one of the barrels. Time will tell, I'm to far in to turn back now.

Thanks, Geno

[Crofter]

Jens, commercial systems that are susceptible to air lock problems have automatic air venters on high spots. Any electrolytic action anywhere will produce gas. I wonder if that could apply to Genos bubbles on the outside of the heatex? If that liquid is not isolated from your engine any slight combustion leak could also contribute.

The auto vents occasionally start to leak liquid and have to be changed out; for that reason a ball valve to isolate should be installed below them.  I have been on that mission a few times.

[LowGear]

So as the water comes out of the exchanger it is at it's hottest.  It goes into the top of tank 1 and starts to slowly cool assisting the flow downward.  This feeds into the top of tank 2 which experiences the same downward thrust.  And tank 3 much the same and back into the exchanger loop.  Here comes some of those questions:

a.)  Where are your heating elements? 

b.)  Unless you have rewired the tank only one element comes on at a time and the bottom one only when the upper thermostat reaches it set temperature.  Why do you need two rather than just the top one in tank 1 without the thermostat problematic opportunities?  Water heaters are thermosiphon driven unless a hot water demand is present.  (Or is that one element in two of the tanks?)

c.)  Isn't the engine water pulled out of the exchanger independent of this "load" loop? 

d.)  Got picture?

Oh yeah.  I agree with Crofter.  The automatic air vent at the top of our boiler just before the expansion tank simply stopped the annual "bleed the radiators on both floors" ceremony we used to endure each fall.  I'd probably put one at the top of each tank.

The really good news is that you start an at least half warm engine every time.

Casey

[cognos]

Observations.

1. It is unlikely that there is any entrained or dissolved air in the water after a year, unless there has been a need to top up the system regularly with raw water.

2. It is also unlikely that the elements should be getting hot enough generate anything but negligible "bubbling" at the element surface of water vapour. Any water vapour thus generated would be instantly absorbed (bubbles collapse) back into solution in the mass of non-boiling water. Water vapour would not be responsible for forming an air void (or "Lock) in a sealed tank that wasn't at 212° +...

3. Given 1 and 2, something else must be going on... dissimilar metals corrosion creating gas? If so, it could be hydrogen, so take care...
- as mentioned above, combustion gas leak?

I'd be tempted to put all 3 tanks on a common vent to eliminate this problem.

Just sounds a bit odd to me, and deserving of a bit of exploration!

Just my thoughts, I may be misunderstanding the problem.

[sailawayrb]

Hi Jens,

I agree with Cognos/Crofter.

Water will absorb air (if it can "see" any air) and cold water will hold significantly more air than warm water.  Small bubbles will appear in the water when it is then heated or depressurized, because this reduces how much dissolved gas the water can hold.  The other thing that comes to mind is some type of bacteria in the water that is generating a gas...such as an iron/sulfur bacteria (which don't need sunlight or O2 to live and can tolerate water temps as high as 140 deg F) that you can sometimes find in wells that produce a rotten egg smelling hydrogen sulfur gas.  Chlorination would take care of this bacteria borne source.  Venting should do the trick no matter what the source.

Bob B.