Alternative Expansion Tank Idea?

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Old 04-03-15, 04:36 PM
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Alternative Expansion Tank Idea?

Conventional expansion tanks are large with an air-to-water interface, which requires continuous air separation from the circulating water and re-injection into the tank. Bladder-type tanks continuously allow air to diffuse (leak) across the bladder, depleting the air cushion. The bladder can eventually rupture.

50+ years ago I was involved in naval nuclear propulsion. The reactor plants had a closed circulating water system. They had an expansion tank, called a "pressurizer." It was a vertical tank with electric heaters in the bottom. A steam bubble was maintained in the pressurizer to keep a controlled pressure on the main system.

Such a pressurizer could be designed for a residential hot-water heating system? Of course, the pressure would be limited to the saturation pressure of the water temp in the pressurizer, which would be a bit above the normal operating pressure of residential hot-water heating system.

I'm putting on my flack vest.
 
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Old 04-03-15, 05:11 PM
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I'm putting on my flack vest.
I can hear the masses chambering rounds as we speak!
 
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Old 04-03-15, 05:25 PM
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I can hear the masses chambering rounds as we speak!
I will pass any ideas or comments on to the U.S. Navy
 
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Old 04-04-15, 04:58 AM
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I'm not an engineer and I don't know a whole lot about the physics involved, but wouldn't there be a risk of over-pressurization and resultant explosion? For example, what if the electric heaters under the pressure tank failed to shut off? Yea you could incorporate several "safeties" into the tank control circuit, but then you're talking about more cost and more things that can fail over time (how many "runaway" aquastat stories - and/or failed pressure relief valve stories - have been posted to this board over the years?).

As we all know, the more complex home heating systems have become, the harder they are to maintain, and the more they are prone to shutting down in the middle of a 0-degree night, for one reason or another.

Then there is the risk of a poorly-informed DIYer tinkering with the controls, trying to get the system back up and running.

Not to mention the regulatory headaches that would be involved (yearly inspections, special licensing requirements for installers, etc.)

Off hand, it doesn't sound like anything I'd want to deal with.
 
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Old 04-04-15, 10:57 AM
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Gil,
Just a thought. As you said, this was 50+ yrs. ago and was probably cutting edge technology.
I think the better question is How do they do it today.
 
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Old 04-04-15, 12:19 PM
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Spott, they still do it the same way: Google "pressurized water reactor pressurizer"

I understand Rockledge's concerns, but they are routinely dealt with for electric domestic hot-water heaters and for electric boilers.

A concern might be the gradual loss of system pressure if utility electric power is interrupted. Reactor plants, whether naval or land-based, have back-up power sources. But, if a residential heating system were depressurized, it could be restored by re-energizing the electric resistance heaters immersed in the pressurizer.

I'm confident that this system could be made to work satisfactorily - but probably not a huge market for the idea
 
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Old 04-04-15, 07:22 PM
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I understand Rockledge's concerns, but they are routinely dealt with for electric domestic hot-water heaters and for electric boilers.
Gil, wouldn't the steam element of your proposed expansion tank put it into a different category than a "simple" electric water heater or boiler? And wouldn't the expansion tank be subjected to much higher temps on a continual basis, and hence leave much less margin for error? I think that's the part of it that I'm hung up on.
 
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Old 04-05-15, 12:52 PM
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For a system operating at 15psi, the temperature of the water inside the expansion tank would need to be 250deg (the saturation temp at 15psi). The expansion tank would have to be designed for that in accordance with the ASME pressure vessel code. A typical residential hot-water heating boiler is designed for 30psi, so the expansion tank should be designed for the same conditions.

There is another design issue. The expansion tank would have a steel nozzle on the bottom to connect to the main hydronic system. Room-temp water and 250deg water would be able to flow back and forth through the nozzle, causing serious thermal expansion/shock issues. The navy's solution was to install a thermal sleeve inside the nozzle and extending into the pressurizer. The thermal sleeve is fillet welded to the nozzle toward the end of the nozzle discharging into the main system and rolled with a loose friction fit at the end of the nozzle connected to the pressurizer, along with a mixing diffuser. (Perhaps the navy has since come up with a more elegant solution than the thermal sleeve, even though it worked fine.)

I agree - this idea has no commercial prospects, even though it would work fine.
 
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