[Dabidler]

I was thinking about doing a plans of an air conditioner, but I couldn't think of a coolant that is easily found in the air.
Id like to know the compressibility of nitrogen or other gases easily found in the atmosphere.
My air conditioner will be doing this cycle:
Compress coolant into cylinder > Open the cylinder valve to maximum and let the gas flow through cooling pipes > Let the coolant escape into air. And some filter that lets only that coolant to pass through it or a way of filtering it. Thanks for any answers.

[curiouscat]

If the coolant is a component of air & its going to escape into air anyways, why do you need cooling pipes?

[Dabidler]

If the coolant is a component of air & its going to escape into air anyways, why do you need cooling pipes?

Before its going to escape into air, its going to rapidly depressurize, cooling the cooling pipes. Then it escapes into air.

[Enthalpy]

The attempt is puzzling. Maybe we miss something from the description.

To paraphrase Curiouscat, in an open cycle of an atmospheric component, you could just let the cool gas mix with the room's air, without any exchanger, couldn't you?

Then, since it probably takes much gas, the best must be to process the air without separating any constituents. It avoids a difficult step, and preserves the air's composition.

Normally, a cooler needs to dump heat somewhere. This is typically done at the end of a compression, when a gas or vapour has become warmer. Its heat is dumped outside the room by an exchanger, and only then, the gas or vapour is expanded (or evaporated, de-solved...) to produce a temperature below the initial one. Compression and expansion alone only produce the initial temperature, a bit warmer because of losses - no cooling. After all, heat has to go somewhere.

Anyway, around one or few atmospheres, air behaves closely like a perfect gas. The main difference is when humidity condenses.

[curiouscat]

To restate what Enthalpy  has explained:

The purpose of compression is to raise the level (temperature) of heat. Otherwise it won't flow to the outside of the room.

[Dabidler]

The attempt is puzzling. Maybe we miss something from the description.

To paraphrase Curiouscat, in an open cycle of an atmospheric component, you could just let the cool gas mix with the room's air, without any exchanger, couldn't you?

Then, since it probably takes much gas, the best must be to process the air without separating any constituents. It avoids a difficult step, and preserves the air's composition.

Normally, a cooler needs to dump heat somewhere. This is typically done at the end of a compression, when a gas or vapour has become warmer. Its heat is dumped outside the room by an exchanger, and only then, the gas or vapour is expanded (or evaporated, de-solved...) to produce a temperature below the initial one. Compression and expansion alone only produce the initial temperature, a bit warmer because of losses - no cooling. After all, heat has to go somewhere.

Anyway, around one or few atmospheres, air behaves closely like a perfect gas. The main difference is when humidity condenses.

Thanks, but if I release coolant directly into the room, its going to be very noisy. Thats why its just going to depressurize in the cooling pipes and go back outside to be released.
And a fan will try to cool the heat generated to as much closest to outside air temperature as possible, then when its going to depressurize... enough to frostbite you.

[Enthalpy]

Under reasonable pressure and temperature, nitrogen and oxygen have the same compressibility as air.

Compression is adiabatic quite accurately, so it would follow PV^γ=constant, where γ is the ratio of specific heat at constant pressure versus constant volume, or 1.40=7/5 for diatomic molecules that vibrate little. Better figures at
http://www.engineeringtoolbox.com/specific-heat-ratio-d_608.html
http://en.wikipedia.org/wiki/Compressibility_factor
only the figure after 1.40 changes. In other words: picking only one constituent of air, a difficult operation, wouldn't improve the thermodynamic behaviour - unless you go for argon...

PV^γ=constant
PV/T=constant
γ=Cp/Cv (the specific heats)
combine into
P varies as T^Cp/γ
V varies inversely as T^Cv/γ
where
Cp/γ=3.5
Cv/γ=2.5
for air, nitrogen, oxygen under usual conditions.

Though, pumps have a limited efficiency. The "isentropic efficiency" tells how big the temperature variation is, as compared to what would be expected from the pressure ratio. Example:
Start from 300K 1atm
Compression to 1.55atm, perfectly efficient adiabatic, would have brought air to 340K, a 40K increase
But a good 80% efficient pump increases T by 50K, to 350K.
The pump consumes mechanical power for 50K and achieves the pressure ratio of 40K only.

-----

Cooling by a nozzle is complicated and doesn't relate with isentropic nor isothermal cases, alas. Keep away from simplistic textbooks. I know no simple way to evaluate it.

If all the speed gained in the nozzle were converted to heat, the air would have the same temperature as before the expansion (some imperfect gases would cool through this process, because they behave a bit like a liquid that evaporates by expansion; not the case with air at 1atm) (other imperfect gases would heat). That would bring no benefit.

A trick is that the expanded air is cold before it brakes, so you can and do obtain a cooling effect where the air has still a high speed. It is possible, but difficult, to have a heat exchanger that operates on high-speed air and doesn't brake it much. The air that passes the exchanger and then brakes is warm and should be dumped outside. Not easy.

An other trick is to separate air into warm and cold flux in a whirl. There you do get one flux of cold air that, even after braking, is still cooler than the compressed air.

-----

Compressing and expanding a gas only looks simple, it's not. Because obtaining cold from an expansion is difficult, fridges and air conditioners use cycles less simple but practical, like the dissolution of ammonia in water, which also means more heat and cold from the same pumped volume.

By the way, if you make a decent air conditioner without ammonia, just propose it for the international space station. I can't grasp why they put a toxic gas in a confined environment.

[billnotgatez]

As an aside
There are methods of using ram air (bleed air) even though it is hot to provide cooling in aircraft.
http://en.wikipedia.org/wiki/Environmental_control_system_%28aircraft%29#Cold_air_unit
http://en.wikipedia.org/wiki/Bleed_air

Of course there must be some efficiencies to using the standard refrigerants
http://en.wikipedia.org/wiki/Refrigerant

[curiouscat]

Cooling by a nozzle is complicated and doesn't relate with isentropic nor isothermal cases, alas. Keep away from simplistic textbooks. I know no simple way to evaluate it.

One utility of those two cases, isentropic & isothermal seems that they offer two limiting cases that bound the performance of any real nozzle.

Correct? i.e. no matter what one does one cannot escape the envelope bounded by isentropic & isothermal.

[Enthalpy]

[...] No matter what one does one cannot escape the envelope bounded by isentropic & isothermal.

I too believe so, but don't have more solid proof of it. It could look like: in one case, the maximum possible amount of work is extracted from the enthalpy, in the other case, no work at all.

[curiouscat]

[...] No matter what one does one cannot escape the envelope bounded by isentropic & isothermal.

I too believe so, but don't have more solid proof of it.

To go outside those bounds (isentropic vs isothermal) would violate thermodynamics or so I assumed.

[Enthalpy]

[...] There are methods of using ram air (bleed air) even though it is hot to provide cooling in aircraft. [...]

[Advantages in] using the standard refrigerants [...]

Thanks! I didn't know airliners now have alternatives to bleed air.

An airliner needs much cooling power so turbomachines fit well, and to spread the cool in a long cabin, blowing directly the processed air must save much exchanger mass. While blown air is noisy in an airliner cabin, we don't hear the turbine, so this worry is solved.

For a house air conditioner or a fridge, hundreds of watt lead to very small turbomachines that are globally less good than piston machines. For instance a car turbocharger fits in a hand and processes already several 10kW, so 200W are inconvenient. Refrigerants typically use an evaporation or dissolution cycle, where the same pumped volume means more heat thanks to the change of state.

I proposed a tiny compressor-bearing-motor-turbine arrangement there
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2051&start=10#p23468
more in the 10kW range and primarily for spacecraft, where closed cycles of clean gas simplify some aspects, and suggested very compact heat exchangers there
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2051#p23419
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2051&start=10#p23477
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2051&start=30#p26099
and now that the ammonia cycle has repeatedly shown drawbacks in the confined habitat of the International Space Station, it could be a good opportunity to develop this air conditioner.

The same hardware could also convert sunheat to electricity in space, radioisotope heat to electricity, keep oxygen, methane, hydrogen liquid indefinitely, replace indefinitely liquid helium for space probes' cameras and Earth-bound uses, make fridges, heat pumps and air conditioners on Earth and for our astronauts - to my biassed eyes a good investment.