Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A portable air conditioner

  1. Mar 13, 2005 #1

    Pengwuino

    User Avatar
    Gold Member

    One problem that came up for my father and my aunt was that they did not want to work (cleaning my grandfathers garage, hes in a nursing home now :-/) during the summer in the garage. Now i was wondering how feasible is a portable air conditioner? If say, at the 'hot coils', instead of air, you put a lot of water but not so much as to make it non-portable. Then after a while, you'd have to dump the water and refill it of course. But how long could you run the air conditioner before the water becomes too hot to work as a heat transfer unit. Theres a big danger of course of pouring out like, 600 degree water ... a lot of which would vaporize as u begin pouring it. The water would be enclosed by a variation of a dewar flask i suppose to keep the water from leaking off too horribly. What does everything think about the feasibility of this lol.
     
  2. jcsd
  3. Mar 13, 2005 #2

    russ_watters

    User Avatar

    Staff: Mentor

    The water will heat up relatively quickly, so this sort of thing wouldn't work (about as fast as if you put it on the stove at "medium"). There are, however, two kinds of portable air conditioners: water and air source. Unfortunatly, both need a heat sink. An air source portable has a duct that you send out a window to discharge heat. A water source unit is connected to a spigot and a drain (or just discharged outside)
     
  4. Mar 13, 2005 #3

    Pengwuino

    User Avatar
    Gold Member

    Ah so it would heat up pretty fast... dang. Wonder if you could setup somethign where you connect the unit to a faucet and water comes in and u drain out through another hose going back into the sink (but make it flush with the drain so the water doesnt make contact with the air in the sink)... or i dunno :D Maybe ill find anothe rway to eleviate working in a hot as hell garage in the summer with no a/c.
     
  5. Mar 13, 2005 #4

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    You could look into an evaporative cooler. You fill them up with water, and they cool the air, but aren't suitable for applications where moisture is a problem unless you use them in conjunction with a dehumidifier.
     
  6. Mar 13, 2005 #5

    Pengwuino

    User Avatar
    Gold Member

    where does the hot air go though? actually wait, how does it work?
     
    Last edited: Mar 13, 2005
  7. Mar 13, 2005 #6

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    The air is drawn through water-soaked pads within the machine, where it evaporates water. Since evaporation requires heat, the temperature of the air drops. This cooled air is then circulated around the room.

    The problems arise when the input air is already damp, because it finds it difficult to hold more water so the air temperature cannot drop.

    Air conditioners work on a closed cycle (which is why you close the windows and doors for them to work properly), evaporative coolers work on an open cycle (so you need somewhere for the heated up air to escape).

    Wait for russ to get in here, he'll know much more about it.

    Edit: oh, he's here already.
     
  8. Mar 13, 2005 #7

    Pengwuino

    User Avatar
    Gold Member

    Wait so these evaporative coolers need a place for the hot air to escape? I thought the air comes out cooler. And i dont understand this idea. I thought the air flowing over i guess these pads woudl transfer energy to the colder pads and come out at a cooler temperature. Why would it "evaporate", i thought that means it would get to like 100C.
     
  9. Mar 13, 2005 #8

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    When the cold air comes out of the cooler, it will gradually get heated up by the air in the room. When it's been warmed up (by the heat inside your house) you ideally want to get rid of it, replacing it with more nice cool air from the cooler.

    If you drag fresh, dry air (from outside) over some wet pads, the water evaporates into the air, cooling it down. You're right that the air flowing over the pads tranfers energy to the pads, this is how the air becomes colder. It's like when you sweat; the sweat evaporates off your body. This requires energy (in the form of heat), so you become colder.

    As for water needing to be 100 degrees Celcius before it will evaporate, well you know this isn't the case because a puddle will evaporate on a dry day. Water molecules on the surface will 'borrow' enough energy from the surrounding molecules to allow them to evaporate.
     
    Last edited: Mar 13, 2005
  10. Mar 13, 2005 #9

    Pengwuino

    User Avatar
    Gold Member

    I dont get it though, what does it mean to evaporate compared to vaporize?
     
  11. Mar 13, 2005 #10

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    Same thing really. Evaporation is when a liquid changes into a gaseous state below its boiling point, which happens when the surface is exposed and unconfined. Vaporisation is just when a liquid changes from its liquid to its gaseous state.
     
  12. Mar 13, 2005 #11

    russ_watters

    User Avatar

    Staff: Mentor

    That's the water source air conditioner I was describing. They aren't very common though.

    Regarding evaporative coolers - they don't work unless the humidity is really low. But Pengwuino, evaporation happens at any temperature, yet the energy required to change phase is the same as with boiling. That energy absorbed (in the breaking of chemical bonds keeping water liquid) means the remaining water and the water vapor must lower their temperature to keep the system energy constant. This is how a cooling tower can cool water below ambient temperature.
     
  13. Mar 13, 2005 #12

    Pengwuino

    User Avatar
    Gold Member

    Whats a cooling tower and how does it work.... i might have a project for that kind of concept :D
     
  14. Mar 14, 2005 #13

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    Cooling towers are those massive stacks you see at power stations, which cool whatever substance has been forced through the turbines to generate power.

    They work, in exactly the way, that Russ told you they work.
     
  15. Mar 14, 2005 #14

    Cliff_J

    User Avatar
    Science Advisor

    Pengwuino - to your original question, think of the typical window mounted air conditioner, if it was a larger one (just big enough to keep a 2 car garage reasonably comforable) it would have something like a 12,000 BTU capacity.

    In the industry, this would be called a one ton unit, and a home might have a 3 ton or 4 ton unit depending on its size. This ton rating means in a 24 hour period of operation it pumps enough heat to melt x tons of ice into water. So you could build a historic air conditioner with a large payload of ice, about 83 lbs per hour or so. (close enough, you could look up latent heat of fusion and do the exact math from 32F to 70F and so on...)

    Ice has much more heat capacity than water - think of a ice filled drink on a hot day and how quickly it starts to warm up once the ice in it melts. So you'll need a lot of capacity to keep an volume as large as a garage pleasant in the summer.

    Cliff
     
  16. Mar 14, 2005 #15

    russ_watters

    User Avatar

    Staff: Mentor

    They make them for building HVAC systems as well. I even saw one a guy made to water-cool a computer. It was a 3' high pvc pipe with a shower head at the top. It would be an interesting project to make that work for a water source air conditioner (heat pump).

    My boss's water source air conditioner is made by Koldwave and they still make portable water source A/C units.

    Ebay has one - 12,500 BTU (typo in the title says 100,000), for $225, opening bid.

    With an open system, you'd use a lot of water (about 3 gal/min). For my boss, that's not a big deal - he has well water, so he doesn't pay for it - but if you wanted to do something like this and feel like a little project, bulding a cooling tower could be fun. I can help with the performance calculations, if you want.

    HERE is the cooling tower a guy made for his computer.

    edit: I just discussed this with my boss - we may do this just for fun (yes, we're nerds).
     
    Last edited: Mar 14, 2005
  17. Mar 16, 2005 #16
    I may be a novice in this field, but has anyone considered using a cooler based on the stirling cycle? It's most often used to cool superconductors in infrared sensors (I would think), but I suppose they could operate with most temperature deltas.

    Something that has been bothering me about stirling cycle cooling is what the further cooling requirements are, and what part of the machine gets cold and what gets hot. Is it the side where heat is normally applied that gets cold, or is it the other one?
     
  18. Mar 16, 2005 #17

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    The side to which heat is supplied is the side which gets cooled, but is still warmer than the side to which heat is rejected. I think...
     
  19. Mar 16, 2005 #18
    Where are you located (just generally, such as SW USA, or NE USA, UK, etc.)?

    How long of a project are we talking about here? Several days, a week, a month?

    How big is the space to be cooled? (Again, generally speaking.) Also, how full is the space? This might effect air movement.

    I have some ideas, but it depends on the specifics.
     
  20. Mar 16, 2005 #19
    to superconduct, to cool at those temperatures, you would use a conventional ac unit but more heavy duty. You have a compressor in the condensing unit and a cooling coil. This is how it was done in the early 1900's when super cooling first came out. Basically, as new substances were being developed that could evaporate at lower and lower temperatures, these were used as coolant and the substance being supercooled would be the substance loosing heat. Once this new super cooled substance evaporates and cools, then it is used to supercool a substance that evaporates at an even lower pressure. It sounds confucing but its one big cycle.
    Im not sure how they o it now, but this is the simplest method I've heard of.
    Regards,

    Nenad
     
  21. Mar 17, 2005 #20

    minger

    User Avatar
    Science Advisor

    Nenad, seems your referring to a Cascading Refrigeration Cycle.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: A portable air conditioner
  1. Air conditioners (Replies: 11)

Loading...