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Boiling Water

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I've started using an instant-read digital thermometer when I boiler water for Loretta's coffee. It's gotta' be 190° for a good cup of drip coffee. Recently, I wanted to watch the temperature rise to 212° and then sit there, waiting for that additional 970 BTU's per pound in order to start boiling, but as soon as the temperature hit 212°, it started to boil. The temperature rise to boiling as a function of time is linear.

I understand the concept of latent heat, but how does that additional 970 BTU's get transferred to change the phase of water from liquid to gas?
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab

Comments

  • Leonard
    Leonard Member Posts: 903
    edited May 2019
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    Water can exist at 212 in both a liquid and gaseous state.

    Also guessing there are strong convection water currents swirling around and your meter is reading the average temp of them. Likely it's only 212 right at bottom of the pot, and cooler above.

    I remember watching water start to boil, a thin stream of bubbles leave bottom of pot, then condense before they make it to the surface. This is evidence top water is cooler.
  • Zipper13
    Zipper13 Member Posts: 229
    edited May 2019
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    Not sure if it's a noticeable effect here, but I assume the probe is resting at the bottom of the pot. The water pressure at the bottom would be maybe 0.25psi (6 or 7 inches deep of water) greater than atmospheric. And a liquid under pressure can takes more heat to evaporate I believe. so i bet it gets hotter at the bottom than at the surface at a boil?

    EDIT: after rereading the OP, I'm not sure this comment is relevant.
    New owner of a 1920s home with steam heat north of Boston.
    Just trying to learn what I can do myself and what I just shouldn't touch
  • mikeg2015
    mikeg2015 Member Posts: 1,194
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    I understand the concept of latent heat, but how does that additional 970 BTU's get transferred to change the phase of water from liquid to gas?

    The electric resistance element heats the glass pot on one side. Water on the other side of the glass absorbs the heat. At the surface, the water evaporates and cools off the surrounding water.

    If the pot is 6” tall, the boiling point of the water at the bottom is probably 213F. IF you had a bolted down sealed lid on a heavy cast iron pot, at 30psig it would boil at about 275F.

    So the 970 BTU’s gets absorbed into the surrounding water, and is simultaneously cooled as it boils.


  • JUGHNE
    JUGHNE Member Posts: 11,111
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    The phase change is what absorbs the extra 970 btu's.

    The same thing happens in refrigeration.....vapor to liquid (condenser/radiator releasing heat) and back again (at boiler/evaporator adding heat) giving or absorbing up a large amount of btu's.

    The boiling and condensing temps of the fluid vary greatly but still work the same.
  • Zman
    Zman Member Posts: 7,607
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    The water does not all boil off at once. The 970 btu's is consumed when the entire pound of water makes the phase change.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,109
    edited May 2019
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    Ah ha! So I just didn't wait long enough. Boiling, the pot of water is still absorbing the 970 BTU's and once it does, it turns to steam.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • mikeg2015
    mikeg2015 Member Posts: 1,194
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    Just remember this. Evaporation and boiling absorb heat, and condensation releases heat. It takes energy to change a phase with no change in temperature.

    Fun camping trick is to take a paper cup full of water and set it in a campfire but not under direct flame. The paper will only burn once the water is boiled off even though the surrounding air is over 500F, above the burning point of paper.

    This is how a double boiler is used in cooking. It prevents what your heating up from getting above 212F.
  • Alan (California Radiant) Forbes
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    I used to work in a pear orchard in Oregon. There were overhead sprinklers that were turned on when the temperature dipped below 32°F. As the water froze, it released heat which protected the trees.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    CLamb
  • JUGHNE
    JUGHNE Member Posts: 11,111
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    That was an old method of protecting root cellars that got close to freezing. Place buckets of water in the room.
  • Alan (California Radiant) Forbes
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    When it gets hot, my honeybees will place droplets of water around the hive and then line up at the entrance and start fanning their wings. Evaporative cooling.


    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    JUGHNE
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
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    This is how a double boiler is used in cooking. It prevents what your heating up from getting above 212F.


    Conversely, a pressure cooker enables your cooker to heat up and deliberately goes over 212F, typically to about 250F @ about 15psi.
  • nibs
    nibs Member Posts: 516
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    @Alan (California Radiant) Forbes How many bees do you have? (hives)
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,109
    edited May 2019
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    I have a very small yard, so just one. You?

    Email: berkeleyradiant@gmail.com
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • JUGHNE
    JUGHNE Member Posts: 11,111
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    Alan, does the presence of the hive in an urban area cause any concern with the neighbors?

    We have a 2 acre lot (block) and about 8 hives which we have not got started up yet.

    They might be a good deterrent from trespassers.
    Most people will not go near hives.
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,109
    edited May 2019
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    We have lunch in our backyard which is about 20 feet away from the hive and don't see any. My neighbors don't see any either. They fly straight up when they leave the hive.

    The greatest activity is within 15 feet in front of the hive where they land and take off.

    Years ago when I lived somewhere else and kept bees, my neighbor called me and said bees were all over his backyard. Turns out they were yellow jackets going for his BBQ steaks.

    We only have problems on warm summer nights. The bees that didn't make it home before nightfall will fly towards any light. We can't keep our windows open, at least those facing the backyard.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    JUGHNE
  • Zipper13
    Zipper13 Member Posts: 229
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    JUGHNE said:

    Alan, does the presence of the hive in an urban area cause any concern with the neighbors?

    We have a 2 acre lot (block) and about 8 hives which we have not got started up yet.

    They might be a good deterrent from trespassers.
    Most people will not go near hives.

    I live in a fairly dense neighborhood of 6,000sf lots. There are 5 hives in the yard across from me. Never noticed an issue with their bees.
    New owner of a 1920s home with steam heat north of Boston.
    Just trying to learn what I can do myself and what I just shouldn't touch
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
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    A fried of mine, and his wife, and a very young friend of mine, and I were having lunch at a table on their front porch. Some bumble-bee size bugs showed up, interested in our cokes. My very young friend of mine was afraid of insects in those days. My older friend assured her: "Don't worry. They do not eat much." I took the drinks inside and brought out water instead. The bugs lost interest.