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Evaporative Air Cooling Conundrum

Sparkymania
Hi


This is going off topic on this forum, I know, but as I am registered with this one and not an A/C forum I thought I might give this a try.

Something different.

I understand the workings of refrigeration and A/C units.

However, there is a "cooler unit" (not described as an A/C unit ) that uses water rather than refrigerant.

Now the principle of A/C units is to move the heat, via the refrigerant, from inside to outside.

The air inside gets recirculated and cooled and dehumidified.

The one that uses water is called an evaporative cooler. There are large commercial and industrial units in hot and dry environments like south west USA where they are used extensively. The reason is they draw in air from outside, which is already cooler that the inside air, cools it and pushes the inside air out through vents. It also humidifies the air with the evaporated water. This is good in environments where the humidity is too low.

However, they are also found as small portable room units in this country. 

They work by water soaking a filter. The warm air is drawn in and as it goes through the wet filter the water evaporates transferring the heat from the air to the water thereby cooling the air. In these units the air is recirculated. The expelled air feels cooler than if you run it without water as it will then just act as a fan.

So it works.

Now here's the thing I don't understand. I've watched loads of YouTube videos and none of them explain this, only what I have explained above.

This goes for both types, industrial and small portable.

If the heat transfers from the air to the evaporated water and the evaporate is expelled with the cooled air, the heat being expelled should end up being the same. Why can you feel the cooler air and not the warmed up evaporate? In the case of an industrial unit why does the cooled air cool the area and the warm air not heat the area at the same time canceling each other out?


Hope this has sparked someones interest as a departure from the usual electrical topics.
  • 0
    Not really my area, but as my head sees it...
    If the heat transfers from the air to the evaporated water and the evaporate is expelled with the cooled air, the heat being expelled should end up being the same.

    Yes the amount of heat energy must be the same - but heat isn't quite the same thing as temperature. As background, consider that different substances have different heat capacities - chuck 1kWh at a 1kg of steel and it won't see anything like the temperature increase as when throwing the same kWh at 1kg of copper. Similarly converting liquid water to vapour takes heat energy (for no temperature rise) - so effectively the process of vaporization increases the heat capacity of the water/vapour as well. So to balance the books as it were, turning liquid water to vapour at the same temperature takes more heat, or if no extra heat is supplied, the temperature must drop, The so the exhaust air really is cooler. In the same way, sweat really does cool skin. Try Googling something like 'latent heat of vaporization' for better explanations.


      - Andy.
  • 0
    Thanks Andy

    I understand something about latent and sensible heat and the process of vaporization so understand your explanation to a point. 

    I have seen videos on this when researching refrigeration but still find the concept hard to "visualize" so to speak.

    Still, this puts a bit more understanding on it.

    So I can see how the expelled air would be cool.

    However, wouldn't the heat be released back to the air when the vapour condenses on surrounding surfaces reheating the room?

  • 0
    However, wouldn't the heat be released back to the air when the vapour condenses on surrounding surfaces reheating the room?

    Yes I think it must relase the same heat when it condenses. In a well designed system that shouldn't happen inside the building though (condensation risks all sorts of problems from mould growth to impaired thermal insulation to rotting timbers) - traditional buildings are a long way from air tight and newer ones should have some ventilation system - so hopefully the damp air is expelled and eventually condenses outside somewhere (e.g. in clouds to make new rain).

      - Andy.
  • 0
    I hope I understand the question, and that what follows makes sense in that context.

    This has quite a lot in  common with sweating and standing in a breeze - when you are in equilibrium at temperature T you the molecules on your surface have a mixture of energies that represent that temperature on average.  That means that some of the more energetic molecules have boiled off, but will return - rather as the steam above your coffee will to a degree rain back into it if undisturbed.  Others molecules in the group are less energetic than the average, so move more slowly and are more tightly bound,  but these do not interest us yet, but as they bump into other hotter molecules they may acquire energy and become more mobile..

    However in both the coffee and the sweaty body if there is a breeze, then the friskier molecules that have borrowed some energy 'from the bank of averages' to briefly break slightly free, are taken away on the breeze before they can give it back.

    The population of water molecules that remain, is now missing its most energetic members, now has a lower energy - so energy flows in to compensate, from the body or the coffee, leaving it cooler. (so you can catch a chill while sweaty if you stand in a breeze, and you can cool your drink by blowing on it.  - except for dragons  who according to some children's book blow on their food if it is too cold.)


    The work  that is done is in breaking the weak Van der Waals and hydrogen bonds between water molecules that by rights should have stayed a bit more together at that temperature, and the energy to do this is removed from the environment.
  • 0
    These evaporative air conditioners are an advertisng dream as they do work, in that they reduce the temperature.

    However it seldom feels cooler as the humidity has gone up so the normal body cooling by perspiration works less well.

    Definitely one to avoid unless you know enough about your situation to judge whether it will achieve the effect intended.
  • 0
    changing water from a liquid to a gas, or vapour consumes energy, and therefore cools the room


    they don't work over here as the climate isn't dry enough


    its the principle upon which cooling towers work, obviously on a different scale
  • 0
    Johno12345:

    they don't work over here as the climate isn't dry enough


    I am inclined to agree. Leaving aside any cooling of the exhausted air, the motor will heat the room.


    Heat from the incoming air is lost as the latent heat of evaporation of the water, but when the water in the air condenses, the heat is given back. If the walls are sufficiently porous, the moisture may travel through and be evaporated by the surrounding dry air; but if it condenses within the room, nothing has been gained.


    ETA: it may be that in the short term, the air within the room is cooled and the walls are warmed, but this will soon even out.


  • 0
    From your answers it looks like the small portable ones we get here work basically like a fan. A fan will work well if it is blowing air directly on to you as it moves the heat directly surrounding your body, coming from your body, away and replaces it with room temperature air which is cooler. This worked well at night over the last week. It does nothing if placed on the other side of the room.

    The portable evaporative cooler doesn't displace the warm air out of the building like the industrial South West American ones.

    It circulates it. So it works well if it is blowing the cool air directly at you but does nothing if it is placed on the other side of the room where the latent heat in the eveporate is released back into the room as it condenses.


    Thanks for your replies everyone.
  • 0
    Evaporative coolers work EXTREMELY well in 2 situations:


    1, if you want something to blow a stream of cooler air at you to help you sleep. The effect is then concentrated on you, and not at 'reducing the sensible temperature in the room as a whole' And they work a LITTLE better than the average fan, the air does feel sensibly cooler than just with the fan (I have a small unit that can do both... when the water pump is OFF, the air does feel slightly warmer on the skin) but the effect's not huge.


    2, in desert areas when applied to an entire building.


    Thinking southern california style 'swamp coolers'. They basically have a mesh of straw contained in a grid surrounding a fan... and a pan of water circulated by a small aquarium style pump to keep the straw grids wet.

     
    82efed77b8ea2cc595aa0a27134a03bd-original-swamp.jpg




    The fan blows an ENORMOUS amount of air into the house, therefore the inlet vent is usually in the entrance hall...


    This expels the cold air to the outer environment before condensation can happen.


    Oddly the tiny thing in the pic entering front left is not a cable of any kind, it's the water supply. They use about 1/8" microbore tubing  to keep the reservoir topped up. A common failing is that the ballvalve fails and ... yeah, the fan doesn't only blow cold AIR into the hall <grin>

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