h a l f b a k e r yWhy not imagine it in a way that works?
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SImply use air itself within the room or cabinate instead of a special gas, as a refrigerant. No fan required over evaporator.
Air may not liquify after compression, like Freon does, but my guess is that it is unnecessary. Recycle the air through compressor and evaporator. Another benefit is that
it will pasturize air due to succesive heating and cooling.
http://en.wikipedia.org/wiki/Latent_heat
[hippo, Jun 16 2011]
Personal Open Loop Air Conditioner
Personal_20Open_20L...20Air_20Conditioner
Unashamedly blatant self-promotion. [Wrongfellow, Jun 16 2011]
ROVAC
http://books.google...%22%20rovac&f=false
Air HAS been used as a refrigerant. It works, and the device does indeed pump a lot of air, but the biggest side-effect is that humidity in the air turns into ice crystals that join the fast-moving air exiting the device. Ouch! [Vernon, Jun 16 2011]
Vortex tube cooling
http://en.wikipedia.org/wiki/Vortex_tube
[ldischler, Jun 17 2011]
Einstein-Szilard refrigerator
http://en.wikipedia...nstein_refrigerator
Large, and perfectly formed. [8th of 7, Jun 17 2011]
Standard for Airliners
http://www.boeing.c...ommercial/cabinair/
-not impossible or even impractical- [DIYMatt, Jun 18 2011]
[link]
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The entire refridgeration industry are kicking themselves that they didn't think of thiS. |
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Seriously, why is world hung up o liquification ? If you see Boyle's law, just compression should be enough. |
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Because the gas/liquid phase change involves large amounts of
energy. |
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Air isn't used because enormous volumes are required for even
modest heat pumping, it would be pretty noisy, and the pipework
would have to be massively strong. |
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Big commercial systems use ammonia, but there's the toxicity
issue. |
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Widely Known To Be Impractical |
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As per Boyle's law, If we compress a certain medium the energy used to compress, is released as heat. This is independent of extent of volume change. If there is no loss of energy, the energy used to compress, HAS to get converted into heat, whether liquification occurs or not. |
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I think, Bolye's law and principle of conservation of energy should explain the viewpoint. But it might be incorrect. |
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latent heat of vaporisation |
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It'd work: all you need is an air-compressor and a radiator. |
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If you used a car turbocharger that gave 3.5psi boost, and say the air indoors is 20ºC and outdoors is 40ºC, you'd have roughly 95ºC air inside the system immediately after the turbo, and -25ºC air coming out the valve. |
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VJW: while you are correct about the energy transfer in compressing a gas, the practical issue comes in the scale. |
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If you are not using the latent heat of the fluid to transfer energy, then you need either a very large volume of gas or a very large pressure change to capture the same energy. |
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In my workshop is a 4HP compressor. I estimate the heat transfer (heating at the cylinder and cooling when pressure is released) is comparable to a domestic freezer, which consumes less than 1HP. |
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Twizz : Cooling efficiency will also depend upon how long was the compressed gas was kept for cooling, in a heat sink( radiator/cooling pipes); More the amount of time, more efficiency. |
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//More the amount of time, more efficiency.// Not bothering to cool to ambient is a waste. |
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You probably want a "spit valve" in between the rad and the cold-air-out valve: release the pressure enough to dehumidfy the air before returning it to the room. |
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In essense we are saying that energy is getting "leaked" somewhere. Otherwise this process ( of using air) can not be less efficient. |
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Whenever a gas (or a absolutely anything, actually) is compressed, it heats up and when expanded, it cools down. I felt that liquification of a gas is just a milestone (or a visible indication, nothing more) in compression process. |
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// energy is getting "leaked" somewhere // |
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'snot getting leaked at all. The basic thermodynamics is still the relationship between inflow and outflow temperature indoors, and the outdoors (heatsink) temperature. It's just easier to move a small amount of compressed-gas or liquid around than it is to compress and move around a larger amount of gas. |
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The calculations should be easy(ish) enough.
a) Find the specs for a normal air conditioner which performance you want to emulate: CFM and input/output temperature of the air (you might have to make some assumptions about outdoors air temperature)
b) Using Boyle's law a few times, pick a pressure-differential that matches the input/output temperature (based on an outside-ambient temperature of whatever.
c) Calculate how much power is required to move the CFM amount against your pressure differential.
d) now compare with the power requirements of the "normal" A/C. |
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// I felt that liquification of a gas is just a
milestone (or a visible indication, nothing more)
in compression process.// |
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As has been mentioned: latent heat of
vapo[u]ris[/z]ation. The energy needed to boil a
given volume of liquid, starting from just below
its boiling point, is usually far greater than the
energy needed to change its temperature by a few
degrees (whilst keeping it as a liquid, or keeping it
as a gas). This is why your kettle can boil a pint of
water quickly, but will take much longer to boil
dry. |
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The fact that the circuit is pressurized doesn't
really make much difference. In the cooling side
of the circuit, the liquid refrigerant is being boiled
by the heat inside the fridge. This process
extracts energy from the cabinet, which is why it
works. |
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The compression circuit only serves to adjust the
boiling temperature of the refrigerant, so that it
boils at fridge-temperatures but can then be
condensed at (or above) outside temperatures. In
each case, the majority of the energy absorbed
(inside the fridge) or given up (outside the fridge)
is a result of the liquid/gas or gas/liquid
transitions. |
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You could use air or any gas you like, but the
efficiency will be completely pants unless you use
high enough pressures to bring the boiling point
up into the range of the temperatures you
want to maintain. |
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As to //my guess is that it is unnecessary//: this is
why textbooks and the Webternet exist. My guess
is that a vacuum dirigible can be made from Coke
cans, but there are convincing arguments out
there as to why my guess is wrong. |
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If you're going to compress air, run it through Ranque-Hilsch vortex tube. You can easily get thousands of BTUs of cooling by splitting the air into a cold stream and a hot stream. They're convenient devices if you have plant air and an electrical cabinet that needs cooling, but they can also cool a room. |
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// energy is getting "leaked" somewhere // |
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I think in case of air (or some other gas which is tough to compress) relatively larger part of energy is wasted in compressing piston and metalic walls of the pump.
[edit]I mean, the work is done to compress the metalic piston*itself* and even the walls of the cylinder are getting pressed againt the air. This needs energy. In case of Freon, a larger part of input energy is consumed to compress the gas ( which is our intension) and lesser part is consumed (unintentionally) to compress the metalic body of pump itself. |
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Well, if you had the outflow press against a piston, before being distributed to the room, you could connect it to the compressing piston. In the case of going from 30C in the room (the inflow) to 0C from the A/C (the outflow), theoretically you could get a little over 90% of the pumping energy back. |
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If you put the pump outside then you don't have to worry about the heat of compression leaking into the room. |
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I (still) maintain that the only difference between the methods (if you connect an outflow piston to the compressor) is that a phasing cycle has less friction mostly due to the smaller radiator. (However I could be wrong for some cooling values: the phasing cycle requires is closed, so it does require an extra radiator.) |
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As [FlyingToaster] may be saying, you need to make the expanding air do some work, or it doesn't really cool. (No, I don't understand why.) So you'd need to rig an expander turbine, a shaft going elsewhere, and a way to use that energy. Which shoots your simplicity oot the windae. |
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(I once made a counter-flow air-powered cooler, and it didn't work for squat. Then I learned about the work physics and gave up on it.) |
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And, as people keep pointing out, you'd need massive amounts of air. |
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Take your average, noisy, blowy air-conditioner, with all the air pumping out of it into your room. Now, take an equal volume of air per minute, and stuff it INSIDE the air-conditioner body, with TWO heat exchangers. |
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//As [FlyingToaster] may be saying, you need to make the expanding air do some work, or it doesn't really cool. (No, I don't understand why.) //
me too. Don't you think this cotradicts with boyle's law ? |
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Anything (mostly)in this world, liquid/solid/gas cools when expands and viceversa. |
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//you'd need massive amounts of air. // |
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Amount of air available is actually is only roomful, since room is part of the closed cycle. We are not taking in outside air for the time being. |
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[bb] Well, you've got the air coming out under pressure anyways, so why not use it ? The compressor already has a flywheel, just stick the other piston onto it to help push. |
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The figures I used, 30ºC in and 0ºC out, translate to approximately a 10% difference in volume for the same molar mass at a constant pressure. Work = Pressure x Volume. If you use x energy to compress the warm-air, the cool-air that comes out will have approximately 0.9x pressure-related energy you can tap as it expands to ambient pressure. |
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His idea is still simpler than a closed cycle in that it doesn't involve an indoors radiator. |
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[VJW] I think where phase-change is superior is that you can pack a *massive* amount of heat into a phase change *without* changing the temperature. |
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Other than that... it should work and doesn't involve chemicals. Adding a rarefactor to help run the compressor should make it work just as economically, except for mechanical losses and friction. <drums fingers on table a bit>... actually I don't see much of a difference, except mostly the volume of air moved and the amount of substance moved through the radiator to dissipate the same amount of heat. |
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Weird, the more I think about it, the closer the efficiencies look between (regenerative)open-cycle-air and closed-cycle-phase-change systems. |
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// I don't see much of a difference // |
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Pressure, pressure, low mass, high flow rate .... |
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using NH3 as an example, its heat-capacity is, I think, 17% better than air, so less molar volume, okay fine. Ammonia's pretty light so there's your "low mass", though a quick check of WP it seems some refrigerants are quite heavy. |
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I don't get it. The liquid isn't what's being pumped, the gas is. And you're never going to get negative pressure on the hot side because if outdoor ambient was cooler than room temperature you just open a window, stick a fan in maybe. |
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// The liquid isn't what's being pumped, the gas is // |
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Boiling point −33.34 °C; it's an excellent refrigerant, apart from its toxicity to humans (and why should that matter ?). And its phase transition is just in the right place for commercial deep-freezers, or blast chillers. It's simple and cheap to make, too. |
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It can be used in absorbtion refirgerators too, although that cycle isn't much used in air conditioning. But it could be done. |
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Heat pumps aren't rocket science (no second-order differential equations) |
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what's that got to do with it ? look at where the compressor is (and has to be) on an A/C unit. The hot gas is what's being pumped. Who cares if it turns to liquid on the other side ? That just means it has to stay in the radiator longer to let all that phase change energy out. |
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//absorbtion// Einstein Lizard ? |
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Okay, this is my first but; |
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MFD - widely known to exist. Jet airliners cool their air by radiating heat away from the compressed engine blead air, then expanding it into the cabin (link). |
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But, but outside temperaures at those heights are around -30c, aren't they ? Why does one need to futher cool the air ? |
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Because the air that has been compressed has been heated by the very act of compression. |
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The AC needs to work on the ground, too. |
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