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Formation of carbonic acid

fusionman Member Posts: 4
I was reading an older article from Dan that said "insulated return lines keep the condensate warmer and that stops Henry's law from kicking in. Cooling condensate absorbs carbon dioxide from the boiling process, making the condensate acidic. Insulate the returns and they'll last longer because the condensate won't be so nasty."
So my 3 part question is... At what temperature does carbonic acid begin to form? is it a sliding scale? If i keep the condensate neat saturation will that prevent it?



  • Jamie Hall
    Jamie Hall Member Posts: 20,487
    It is a sliding scale (it's surprisingly close to linear, for most gasses). You can't prevent it -- if there is air present, and there always is in dry returns, the carbon dioxide in the air will dissolve in the water -- as will the oxygen. Here's a useful article on the subject: https://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html

    What the table doesn't show -- but is relevant -- is that it will reach saturation fairly quickly. Here is a half way decent article on carbonic acid...

    Condensate in a steam system has almost no buffering capacity at all -- being distilled water, after all -- but, fortunately, there is remarkably little of it and if the piping is correct, all of it disappears at the end of the steaming cycle. Corrosion of the pipes in wet returns is a problem, but much more related to the presence of oxygen in the water than to the relatively weak acid pH around 5.7) from dissolved carbon dioxide.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • PMJ
    PMJ Member Posts: 1,197
    I can't resist pointing out that the totally closed natural vacuum system uses much less water and processes much less air in and out. Obviously then much less corrosion of all kinds. Just a thought if the project was to do something really significant with regard to corrosion. I imagine we can agree that there isn't any way to argue that scrubbing all the piping with fresh air on every cycle is a plus. The original systems didn't do this.

    I've never seen any evidence that anything about these systems turns the water acidic anyway. The water in my system is always higher PH than my tap water and the vast majority of it has been in there a very long time with no additives of any kind.
    I've added so little over the years this can't be about the supply water.
    1926 1000EDR Mouat 2 pipe vapor system,1957 Bryant Boiler 463,000 BTU input, Natural vacuum operation with single solenoid vent, Custom PLC control