h a l f b a k e r yNot so much a thought experiment as a single neuron misfire.
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The Personal Open Loop Air Conditioner consists of a compressor which
takes air from the atmosphere, pressurises it, and feeds the hot
compressed air through a radiator which cools it to near ambient
temperature. The compressed air is then fed into a pressure hose which
can be passed through a window
into the building. At the far end of
the pressure hose is a nozzle which allows the air to expand as it
flows out into the surroundings; it cools adiabatically and provides a
steady stream of fresh, chilled outdoor air which can easily be
directed at whichever part of the owner needs it the most.
Conventional air conditioners are inflexible. The cooling breeze
emerges from a large, heavy box which is often bolted down or even
built into the ceiling. This device produces its cooling effect from a
small metal nozzle on the end of a hose, which can easily be pointed
in any direction where it might be needed.
Conventional air conditioners are environmentally unfriendly.
Classical refrigerants such as Freon and other CFCs, and the complex
manufacturing processes which produce them, have deleterious effects
on the atmosphere and biosphere. This device dispenses with such
exotic substances, resulting in a cheaper and greener construction.
Conventional air conditioners continuously recycle the building's
interior air, making no effort to deal with things like oxygen, carbon
dioxide, farts and BO. This invention continuously replenishes the interior
air with fresh, oxygenated, scent-free air from outdoors, for a
pleasanter and more thorough cooling effect.
Hilsch Vortex tube
http://www.dissigno...20White%20Paper.pdf
[Ling, Aug 06 2008]
[link]
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So why don't regular air conditioners use
air instead of freon? |
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Very low efficiency if you aren't changing phase in your coolant. |
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//making no effort to deal with things like oxygen, carbon dioxide, farts and BO//. Hmm, ever done any HVAC design? I have. Your home unit may not do much in the way of air exchange, basically because your house isn't nearly as airtight as you think it is. For commercial installations, air exchange is one of the top 3 or 4 factors that go into the design. |
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I suppose you could bake this by seting up an ordinary storage type air compressor in a sheltered outdoor spot and running the hose inside. Yes, it would be inefficient, but if used for personal cooling when required might use less energy than cooling the entire house with a conventional air conditioner. Don't get too close to the nozzle. |
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"Efficiency" is determined by the heat-pump ratio; how much heating/cooling for a given energy input. |
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Closed-cycle machines using Freons achieve about a 3:1 efficency; 700W of input for 2000W of cooling. |
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The adiabatic expansion idea is still valid, and since the General Gas Law applies the thermodynamics are fairly simple. |
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Assume a 1 kW compressor motor. The compressor inducts air at 1 Bar and 293K. SInce PV=RT, if the pressure is raised to 5 Bar, the temperature will rise proportionately. |
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Pass the air through a radiator as {Wrongfellow] suggests, and with the aid of a fan the air can probably be cooled back to 303K fairly easily (better with a water spray). |
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Now, you'll get your aidabatic cooling at the end of the pipe; but - |
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1) The air will be at about 240K, quite capable of causing frostbite; |
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2) The noise will be horrendous; |
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3) the blast will blow you off your chair. |
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The high pressure is needed to "force" the heat out of the working fluid (large differential between fluid and ambient) so dropping the pressure reduces the cooling power. Running at a lower pressure means a much bigger dissipator at the compressor end. |
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not recirculating the cooled air is where you will suffer the greatest loss of efficiency. Supplementary evap cooling would help.... |
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/"Efficiency" is determined by the heat-pump ratio; how much heating/cooling for a given energy input./ |
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So using your numbers, a 100% efficient 1kW compressor (yeah right) will be able to compress 2.5 L/s from 1 bar to 5 bar. |
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The temperature drop is then 293-240=53K @ 2.5L/s, which equates to about 160 Watts' worth of cooling power. But this is from a 1000W compressor. |
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//basically because your house isn't nearly as airtight as you think it is// |
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If I thought my house was anywhere near airtight I'd also propose a second piece of hosepipe to equalise the pressure! |
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Now in my opinion these things are really, really cool. |
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A while ago I had the idea of attaching one of these to the outlet of a small computer keyboard vacuum cleaner, to provide a portable jet of cool air that you could keep in your pocket no matter where you go. |
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Unfortunately it's completely impractical because a Hilsch tube will only run off a ridiculously high pressure input. |
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Originally no Hilsch tube was mentioned, so I suppose
you were simply talking about compressed air. Here's
the thing: When the compressed air leaves the nozzle,
it cools down taking in energy from its surroundings,
but AT the nozzle the endings heat up extremely. You'll
have to grab that air and take it out of the room,
otherwise the total energy will be INCREASING the
heat, by letting in pressurized air into the room.
This is can be accomplished, but needs to be thought
through. |
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I don't think that's correct, [pashute]. Yes, friction in
the nozzle is a source of heat and a cause of
inefficiency (it would be more efficient, for example,
to use a pressure exchanger to transfer that energy to
compression on the hot side), but there is a net cooling
effect nonetheless. For example, when you let air out
of a bicycle tyre, both the valve and the released air
become cooler than ambient. |
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The fundamental reason why a single phase heat pump
is generally less efficient that a phase change heat
pump is is that the former cannot operate as close to
equilibrium. This is in turn because the expansion of a
gas occurs over a temperature range, whereas the
evaporation of a liquid occurs at a single temperature
(and similarly for compression/condensation). |
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BTW, it might be more meaningful to compare the
thermodynamic efficiencies (always less than one),
rather than the ratio of heat moved to high grade
power consumed - by which measure, an open window
has an infinite 'efficiency'. |
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But even more important is the question 'How
comfortable will I be, for how much power
consumption?' |
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