Evaporation and Boiling Into Thin Air…

No, refrigerant evaporation and boiling are not allowed into air. Its destiny is in air conditioners. Sorry. It is harmful to you and me, and no way will we have cooling through air conditioners – if that happens.

Evaporation occurs at the surface of an exposed liquid


Nostalgic moments:

If you have been to How Air Conditioners Work page, evaporation part comes last in air conditioning cycle.

It has two good friends and a best buddy.

The good friends are the compression, and condensation section.

These two has prepared a nice and high pressure refrigerant.

And wow, the best buddy is the expansion, i.e. the throttling part.

This one, has done the breaking-the-big-chunk-into-manageable-pieces of the refrigerant, for evaporation to take place.


Evaporation in general:

Synthesizing from Kakaç’s "Fundamentals of Heat and Mass Transfer" book, it is a phenomenon, where atoms or molecules of a specific liquid on the surface part of its volume, overcomes its exposed surface tension, and escapes into the surrounding atmosphere.

I am overwhelmed myself. Ok, let me put an example. Starting with,

  • a glass of water as a specific liquid

  • the water surface is the exposed surface

  • the volume is how much water you put in

  • the atmosphere, if you’re still on earth, is our natural air

Evaporation occurs as the molecules at the surface has more energy than the molecules at within the liquidIf you leave the glass uncovered, the water will eventually disappear into the air. Put your little brother or sister out of the equation. They might spill it over before anything else.

If you leave it covered, evaporation will start at a higher rate than condensation. Eventually, evaporation rate will equal the condensation rate, and equilibrium of rate of atoms or molecules, leaving and entering the surface of the liquid, will equalise.


Those two paragraphs need explaining:

In basic understanding, every atom or molecule has a specific internal energy and,

  • these atoms will continuously move in random and linear motion (liquid or gas), or vibrate and rotate within its position (solid). More details in here

  • liquid atoms or molecules will have highest energy at the surface of its volume, compared to atoms or molecules beneath it

Let me elaborate on the second point.

Molecules or atoms of liquid (be it water or liquid refrigerant) inside the volume will have pulling force from neighbouring molecules or atoms of the same matter, in all directions.

Unlike its co-molecules inside the volume, counterparts on the surface will only have pulling force in downwards and sideways directions. What’s directly ahead? Just the surrounding vapour pressure.

This means that the internal energy of the molecules or atoms at the surface, will overcome the pulling forces from the neighbouring molecules or atoms, and evaporate into the surrounding atmosphere.

What does that imply? Yes, evaporation happens as long as the matter remains in liquid form, and not necessarily at the boiling temperature.

However, the evaporation rate does depend on temperature and pressure as these factors effect the "activeness" of the atoms or molecules.

So,


What’s with the equilibrium explained earlier?

Equilibrium of an evaporating liquid will be reached, if the rate of evaporation (molecules or atoms leaving the surface) equals the rate of condensation (molecules or atoms entering the surface) within a given space.

Let me use an example of a clear glass, half filled with water, and its top covered with clear cover. Initially,

  • evaporation will happen, but

  • due to closed volume, evaporation will occur until,

  • water vapour fills the air space and saturates it, and

  • condensation will happen, returning water vapour into condensed water

Evaporation and condensation equilibrium in a closed space will be achieved when the evaporation rate and condensation rate is the sameEvaporation and condensation will happen continuously from that point on. Equilibrium reached. The larger the air space, the larger the required water quantity, for equilibrium to reach.

But, the container has to be able to withstand the vapour pressure within it, for equilibrium to be maintained.

Example of my curious-childhood experience.

One fine morning when I was 8, my mom gave me a bottle of warm TANG for school. Well, she was in a hurry and there was no time to wait for the water to cool down enough.

No, the problem did not occur there. My mom bottled it for me just fine.

The problem occurred with me. I felt the bottle was quite warm and shook it. BAM! The lid came off, and off goes my favourite TANG.

What happened was, I agitated the warm liquid and that exposed more surface area per second. Hence, vapour pressure built up just enough to blow it off.

Please don’t try that at home. I was lucky then.

Oh yes, I just got the explanation quite recently, not then.


And that leads to our exploration into boiling:

Boiling will form large bubbles within the liquidBoiling, as evaporation, involves escape of molecules or atoms from liquid phase, into vapour phase.

However, boiling, as the definition stands, happens very rapidly, at the boiling temperature of a given matter. In short, it happens due to heat surrounding the matter is enough to initiate the boiling temperature.

The boiling temperature and the temperature at which vapour starts to condense, are the same, for a given matter. As condensation page explains, condensation of vapour does not happen at singular temperature. It is also a function of pressure. Similar concept applies for boiling.

Lower the pressure, lower the boiling temperature. Likewise for the reverse.


Fine, all the explanation. Where’s the application?

Still remember our refrigerant in air conditioners?

Illustration of refrigerant boiling in air conditioner evaporatorAfter throttling, where the large "chunk" of liquid refrigerant is broken into smaller refrigerant droplets, evaporation will start to happen.

This is followed by the travel into the evaporator with post code, YOUR-ROOM. The evaporating liquid at low pressure, will get heat from the room and boiling will start.

Heat from the room is more than enough to boil, say Refrigerant 12 (with sub-zero boiling temperature at 1 bar).

So, heat removed from the room is COOL provided to you!

That should explain how boiling and evaporation can cool a space. Shouldn’t it?



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