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Thread: Transfer of heat

  1. #1 Transfer of heat 
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    Hi guys,

    Let us say for an eg, a room as a temp 45 Celsius and the environmental temp is more than the room temperature, lets say 50 Celsius. My desired effect is to reduce the room temperature i.e transferring heat from the room to environment without any external aids such as electricity, heating. Is it possible to do? If it is possible in what way it can be done?


    Last edited by kraman.157; January 10th, 2014 at 10:05 AM.
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  3. #2  
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    Quote Originally Posted by kraman.157 View Post
    Hi guys,

    Let us say for an eg, a room as a temp 45 Celsius and the environmental temp is more than the room temperature, lets say 50 Celsius. My desired effect is to reduce the room temperature i.e transferring heat from the room to environment without any external aids such as electricity. Is it possible to do? If it is possible in what way it can be done?
    This sounds like summer in Riyadh or Doha, or somewhere!

    You can get some cooling effect by evaporating water: this is the principle behind the wet and dry bulb thermometer method for measuring humidity. I can't tell you how much of a difference you can get. It works best in a dry climate of course.


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    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by kraman.157 View Post
    Hi guys,

    Let us say for an eg, a room as a temp 45 Celsius and the environmental temp is more than the room temperature, lets say 50 Celsius. My desired effect is to reduce the room temperature i.e transferring heat from the room to environment without any external aids such as electricity. Is it possible to do? If it is possible in what way it can be done?
    This sounds like summer in Riyadh or Doha, or somewhere!

    You can get some cooling effect by evaporating water: this is the principle behind the wet and dry bulb thermometer method for measuring humidity. I can't tell you how much of a difference you can get. It works best in a dry climate of course.
    Yes you are right by evaporating water you can cool but the big problem u ll face is water availability. You cannot simply convert water into vapor and leave it in the atmosphere. Moreover as I said I dont want to use any external aid. In your case I have to provide some external heat source ( and not the environment temp) to heat the water to change it into vapor.
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  5. #4  
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    Quote Originally Posted by kraman.157 View Post
    In your case I have to provide some external heat source ( and not the environment temp) to heat the water to change it into vapor.
    No, it's the temperature of the room that is evaporating the water, and it is this evaporation that is cooling the temperature of the room.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  6. #5  
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by kraman.157 View Post
    In your case I have to provide some external heat source ( and not the environment temp) to heat the water to change it into vapor.
    No, it's the temperature of the room that is evaporating the water, and it is this evaporation that is cooling the temperature of the room.
    I dont think he meant the environmental temp because boiling point of water is 100 celcius. But in my case the environment temp is only 50 celcius.
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  7. #6  
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    Quote Originally Posted by kraman.157 View Post
    Quote Originally Posted by KJW View Post
    No, it's the temperature of the room that is evaporating the water, and it is this evaporation that is cooling the temperature of the room.
    I dont think he meant the environmental temp because boiling point of water is 100 celcius. But in my case the environment temp is only 50 celcius.
    You wouldn't be boiling the water, just allowing it to evaporate in the room air (increasing the rate of evaporation by increasing the surface area of the water by some means).
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  8. #7  
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    That's exactly what he meant, evaporation and boiling are not the same, evaporation occurs at all temperatures as long as the partial pressure of water in the air is below the vapour pressure at that temperature.
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  9. #8  
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    Quote Originally Posted by kraman.157 View Post
    Quote Originally Posted by KJW View Post
    Quote Originally Posted by kraman.157 View Post
    In your case I have to provide some external heat source ( and not the environment temp) to heat the water to change it into vapor.
    No, it's the temperature of the room that is evaporating the water, and it is this evaporation that is cooling the temperature of the room.
    I dont think he meant the environmental temp because boiling point of water is 100 celcius. But in my case the environment temp is only 50 celcius.
    No, KJW is right. Water evaporates at any temperature at which it has a significant vapour pressure. Boiling point is merely the temperature at which the vapour pressure equals atmospheric pressure, thus allowing bubbles to form throughout the liquid and thus causing it to boil. At 45C, water will evaporate rapidly in a dry atmosphere, cooling as it does so due to absorbing its Latent Heat of Vaporisation from the surroundings. This is how sweat cools you down. If you were to hang a lot of wet cloths in a room with dry air at 45C, you might get a signifiant cooling effect, I think.
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    Quote Originally Posted by PhDemon View Post
    That's exactly what he meant, evaporation and boiling are not the same, evaporation occurs at all temperatures as long as the partial pressure of water in the air is below the vapour pressure at that temperature.
    Oh! I C. ok. Now the question is it possible? If so how will u reduce the pressure and maintain it?
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  11. #10  
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    Yes it is possible, you've seen puddles evaporate at temperatures below 100 C haven't you? You don't need to reduce the pressure, I was referring to the partial pressure of water (or "concentration of water vapour") not total atmospheric pressure.
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    Let us say in a dessert condition where the temp of the surrounding is 60 Celsius and the room temp is 45 Celsius. Now I wanted to cool the room to atleast 40 Celsius. What shall I do? what setup can I go for?

    Note: I dont want any external aids such as electricity or external heat
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  13. #12  
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    Evaporation of water would help as outlined above (may not cool by as much as you want though). Big blocks of dry ice may be better(?)
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    The problem with evaporative cooling, apart from the consumption of water, is:

    (1): It only works effectively when the environment being cooled isn't too humid.

    (2): It humidifies the environment, thus reducing the effectiveness of the body's natural cooling via sweating, as well as the continued use of evaporative cooling.

    Point (2) is significant because I personally would rather be in 40°C low humidity than 30°C high humidity.
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    Just add water. Even a relatively tight home will have some ventilation which will allow drier air in.

    This method works best in dry area. Note, in your OP, you talk about "transferring heat;" the evaporation techniques are converting some of the ambient heat into latent heat contained in water vapor.
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  16. #15  
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    Quote Originally Posted by KJW View Post
    The problem with evaporative cooling, apart from the consumption of water, is:

    (1): It only works effectively when the environment being cooled isn't too humid.

    (2): It humidifies the environment, thus reducing the effectiveness of the body's natural cooling via sweating, as well as the continued use of evaporative cooling.

    Point (2) is significant because I personally would rather be in 40°C low humidity than 30°C high humidity.
    Good point. I was going to add that I can imagine the cooling effect might work well in Riyadh where the air is totally dry, but would not work at all in, say, Dubai, Jeddah or Doha, as these cities are close to a warm sea and have an ambient humidity around 90%.

    Re point 2, I suppose a lot would depend on how much air exchange there would be with the surroundings to get rid of the humidity. Maybe a balance might be found that was not too uncomfortable - one would have to try it, I think.
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  17. #16  
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    Quote Originally Posted by exchemist View Post
    Good point.
    As a person who lives in a sub-tropical climate that is currently in summer, such matters are not far from my mind.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  18. #17  
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    It would seem that no one yet has answered the question in the opening post. The opening post explicitly mentions transferring heat from the cooler room to the warmer environment, thus ruling out evaporative cooling and other ways that merely cool the room without transferring the heat to the environment. The answer to the opening post is that work must be expended to transfer heat from the cooler room to the warmer environment. Thermodynamics is quite explicit in the minimum amount of work that must be expended to transfer a given amount of heat from the cooler temperature to the warmer temperature.
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  19. #18  
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    You're right looks like we all misread the OP
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  20. #19  
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    There are some nifty ideas about non-electrical ways to aircondition a hot place. There's the traditional unglazed terra cotta pot full of water in front of the source of air for your room. The air entering the room is cooled by flowing over the much cooler pot - which is always cool because of the water evaporating through the pores in the clay. (If that's not cool enough you can do the double pot system to refrigerate some drinking water. Pot-in-pot refrigerator - Wikipedia, the free encyclopedia

    If we're talking a whole house, there are ways to ensure you have a cool, damp room, even an enclosed verandah/greenhouse, at one end of the house. At the other end you have a room with a fireplace, yes indeedy, a fireplace. When you get the doorways and airflow set up, light a small fire and the cooler, damp air will flow through the spaces and up the chimney. With an appropriately designed house, this can work as well as an air conditioner. It also is the only method I know of that transfers hot air outside rather than cooling whatever air is inside.

    I never managed to do this at my previous house - no fireplace here anyway so I can't try it - but I always thought putting a large candle or two on a highish base inside the fire/chimney space would set up the necessary flow if I could get any consistent airflow in a large room with 3 doors.

    The biggest issue with any idea like this is restricting the airflow. One, you should have a reasonable height in a room. A conventional low ceiling makes this really hard. The other is the "Shut The Door!" problem. You need to have some kind of protection for doors and windows that have any possible link to the area you want to cool. Some kind of double door or air barrier so that people entering from the exterior don't bring a blast of hot air with them.
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  21. #20  
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    Hi guys,

    Let us say for an eg, a room as a temp 45 Celsius and the environmental temp is more than the room temperature, lets say 50 Celsius. My desired effect is to reduce the room temperature i.e transferring heat from the room to environment without any external aids such as electricity, heating. Is it possible to do? If it is possible in what way it can be done?
    Can anyone suggest an idea for the above quoted scenario? Pot in pot refrigerator cant be used for the above scenario.

    If we're talking a whole house, there are ways to ensure you have a cool, damp room, even an enclosed verandah/greenhouse, at one end of the house. At the other end you have a room with a fireplace, yes indeedy, a fireplace. When you get the doorways and airflow set up, light a small fire and the cooler, damp air will flow through the spaces and up the chimney. With an appropriately designed house, this can work as well as an air conditioner. It also is the only method I know of that transfers hot air outside rather than cooling whatever air is inside.
    BTW, can anyone explain the above quote said by "adelady" bit more clearly.
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  22. #21  
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    Quote Originally Posted by kraman.157 View Post
    Can anyone suggest an idea for the above quoted scenario?
    Did you read my previous post? Is human power allowed? If so, you might be able to set up a human powered generator to power a small air-conditioning unit (though I don't know if human power is sufficient). If no power source is allowed, then there is no solution to your problem as stated as it would require violating the second law of thermodynamics.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  23. #22  
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    Quote Originally Posted by KJW View Post
    It would seem that no one yet has answered the question in the opening post. The opening post explicitly mentions transferring heat from the cooler room to the warmer environment, thus ruling out evaporative cooling and other ways that merely cool the room without transferring the heat to the environment. The answer to the opening post is that work must be expended to transfer heat from the cooler room to the warmer environment. Thermodynamics is quite explicit in the minimum amount of work that must be expended to transfer a given amount of heat from the cooler temperature to the warmer temperature.
    Do you mean that it is possible to cool the room without transferring heat to the environment? My ultimate aim is to reduce the room temperature 45 C to 35 C. I dont bother whether heat is transferred to environment or not.

    The principle of evaporative cooling is like the dry hot air which passes by the surface of water will evaporate the surface water thereby cools. Am I right? If this is right is it possible to use it for mobile applications like in car, etc

    Prob 1: To reduce the temperature of a stationary application like house room whose temp is 45 C which is to be reduced to 35 C without any external aid.

    Prob 2: To reduce the temperature of a mobile application like car whose temp is 45 C which is to be reduced to 35 C without any external aid.
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  24. #23  
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    Quote Originally Posted by kraman.157 View Post
    Quote Originally Posted by KJW View Post
    It would seem that no one yet has answered the question in the opening post. The opening post explicitly mentions transferring heat from the cooler room to the warmer environment, thus ruling out evaporative cooling and other ways that merely cool the room without transferring the heat to the environment. The answer to the opening post is that work must be expended to transfer heat from the cooler room to the warmer environment. Thermodynamics is quite explicit in the minimum amount of work that must be expended to transfer a given amount of heat from the cooler temperature to the warmer temperature.
    Do you mean that it is possible to cool the room without transferring heat to the environment? My ultimate aim is to reduce the room temperature 45 C to 35 C. I dont bother whether heat is transferred to environment or not.
    Yes. The sensible heat is changed to latent heat of vaporization. The room will become cooler but more humid. Because of the increased humidity it may not be any more comfortable for people in the room.
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  25. #24  
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    Quote Originally Posted by kraman.157 View Post
    Do you mean that it is possible to cool the room without transferring heat to the environment?
    Yes, by storing the heat as the latent heat of some endothermic exergonic phase transition or chemical transformation, as in the case of evaporative cooling, but could also be from melting ice, dissolving particular salts in water, etc. The problem is that the materials will absorb only a limited amount of heat, and once the materials have fully transformed, they are spent.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  26. #25  
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    In order to reduce the temperature of the vessel you should first construct a heat exchanger. This will prevent the cooling area from effecting the climate within the vessel (humidity). Water is a good choice as a refrigerant. I would attempt to construct it to capture and use to wind outside to influence the evaporation of the water with in the heat exchanger. Also I would mount the exchanger as high as possible in the controller area (vessel), to capture the warmest air.
    Condensing plates on the roof of the controlled area can be constructed to capture dew, and funnel it into the heat exchanger. Wind is a necessary factor in this method, therefore 1 variable that would make it far more reliable for use in and automobile.
    Another concept would be thermal storage for capturing the cooler temps that occur at night.
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    One simple solution would be to bury a tube in the ground from the house/room/building to an opening as far away as possible up to about 35 - 50 meters. Then put a solar chimney at the highest part of the building. Hot air rises, so the chimney will vent out the hottest air, drawing in air cooled by the ground from the tube.

    Air layers in heat bands, so your coolest air will still be close to your floors. This is why people in hot climes tend to sit or crouch on the ground while in colder climes, we use chairs to sit 18 inches above the floors.
    I have 2 solar chimneys on my building, and draw in ground level cool air with no electricity. So, we only use the air conditioning when we want to lower the humidity.
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    Ninja'd by sculptor!

    That's very much like the system we had designed for us when we were planning house extensions a long time ago. Though we had very high ceilings in those rooms. We were planning on a sort of two-step ventilation process. Having an outlet (into the above ceiling space) in various rooms at the optimum point for best airflow through that space. The vents we were planning on using from the roof itself would have been the whirlibird style. The advantage of those, of course, is that heated air would be continuously extracted from the roof as it warmed - thereby preventing the rooms below from heating up quite so much in the first place. https://www.youtube.com/watch?v=KGkpvJGRNvk&hd=1

    For less used areas like outbuildings or sheds, it's cheaper just to use a pipe of some sort and ensure that the upper third/half is painted black - or at least a much darker colour than the rest of the roof and the pipe. The differential heating of that column of air gives you a bit of air movement starting at the top so that airflow through the room/ building starts at the right time and continues as the day gets hotter.

    -----------------------------------

    Of course, none of this is actually extracting heat from any space. It's just optimising displacement of hot air by cooler air.
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    Quote Originally Posted by keeseguy View Post
    I would attempt to construct it to capture and use to wind outside to influence the evaporation of the water with in the heat exchanger. Also I would mount the exchanger as high as possible in the controller area (vessel), to capture the warmest air.
    @Keeseguy can you pls explain the above quote a bit more clear.

    If I use thermal storage I need some external aid to operate the equipment right?
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  30. #29  
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    If you built an evaporation chamber at the highest point in the space to be cooled it would be effected by the warmest air in the space, accelerating evaporation with in the exchanger. The area of evaporation should receive as much air flow velocity from the outside as possible to increase the evaporation rate. Of course the effectiveness would be dependent on the humidity and wind.
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  31. #30  
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    sculptor's concept is a perfect thermal storage idea
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