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This question already has an answer here:

Why is a compressed air tank cold to the touch when it’s in a compressed state before use?

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marked as duplicate by sammy gerbil, Chris, JMac, stafusa, Kyle Kanos Feb 16 '18 at 13:22

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  • $\begingroup$ Are you implying that you believe that a metal air tank full of compressed air tends to be colder than if the same metal air tank were empty? $\endgroup$ – Samuel Weir Feb 14 '18 at 1:09
  • $\begingroup$ The tank gets cold when you let air out of it. It is not cold just sitting around doing nothing. $\endgroup$ – Ben51 Feb 14 '18 at 1:10
  • $\begingroup$ Gas cylinders in general tend to feel cold to the human touch because of the high thermal diffusivity of steel. The internal pressure (before use and well after filling) isn't a factor. $\endgroup$ – Chemomechanics Feb 14 '18 at 1:39
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    $\begingroup$ Possible duplicate of Why does cold metal seem colder than cold air? $\endgroup$ – sammy gerbil Feb 14 '18 at 14:47
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Observations over centuries have given us the ideal gas law .

enter image description here

where :

P is pressure, V is volume, and T is temperature

n = number of moles
R = universal gas constant = 8.3145 J/mol K
N = number of molecules
k = Boltzmann constant = 1.38066 x 10^-23 J/K = 8.617385 x 10-5 eV/K
k = R/NA
NA = Avogadro's number = 6.0221 x 10^23 /mol 

This says that, for a given volume, when the pressure falls in a fixed volume, the temperature falls. That is exactly what is happening when compressed gas is let out from a closed bottle. It is extremely apparent if you make a mistake in changing a camping gas little bottle and the gas starts leaking.

Now at the molecular level, temperature is proportional to the average kinetic energy of the compressed molecules. When a bunch of them leaves as the valve is opened, due to the pressure the tail of the high kinetic energy distribution will be leaving, lowering the average kinetic energy of the rest in the bottle. This is temporary, because the thermodynamic equilibrium with the surroundings will bring again the bottle temperature to room temperature ( with less pressure in the bottle according to the ideal gas law).

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  • $\begingroup$ as a side comment, think of a pressure cooker, high pressure in fixed volume, higher temperature . $\endgroup$ – anna v Feb 14 '18 at 8:15
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    $\begingroup$ The question was about "before use",so it's not really clear how this applies here. $\endgroup$ – JMac Feb 14 '18 at 13:09
  • $\begingroup$ This is not correct. The cited ideal gas law by itself cannot explain a temperature change. In an ideal gas, you need an adiabatic expansion(see en.wikipedia.org/wiki/Adiabatic_process#Derivation_of_P–T_relation_for_adiabatic_heating_and_cooling) where work is done for this to happen. Also, when you expand compressed air through a valve (throttle), you get cooling by the Joule-Thomson effect (en.wikipedia.org/wiki/Joule–Thomson_effect) which relies on the fact that air is not en ideal gas. This Joule-Thompson effect is used for cooling in common air conditioning systems. $\endgroup$ – freecharly Feb 14 '18 at 13:17
  • $\begingroup$ @freecharly I am comparing the gas in the bottle before and after some of it, the higher perentage of molecular energy part mostly, has left. The effect en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect is an effect on the leaving gas. . It may be that the outside of the bottle is cooled also thewarehouse.co.nz/p/gascraft-camping-backpacker-stove/… the cooling gas, [I do not want to experiment with my camping gas heater :( ] $\endgroup$ – anna v Feb 14 '18 at 16:42
  • $\begingroup$ @annav - Anna, how could you achieve that only the higher energy fraction of a gas leaves the container? This will probably not happen when you let the high pressure gas out through a common valve into atmospheric pressure. You would have to employ a Maxwell's demon. You are right to better stay safe and not do experiments with the camper gas heater. You could also use a typical hair o deodorant spray container. $\endgroup$ – freecharly Feb 14 '18 at 17:10
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Why is a compressed air tank cold to the touch when it’s in a compressed state before use?

If the cylinder has reached the ambient room temperature then the important parameter is the difference between the cylinder’s temperature and the temperature of your fingers.
Heat will flow from a hot body to the cold body and if the temperature of your fingers have a higher temperature than the temperature of the cylinder then heat will flow from your fingers to the cylinder.
This will raise the temperature of the cylinder in the locality of your fingers.
Because the thermal conductivity of the material (steel?) of which the cylinder is large the heat transferred to the cylinder from your fingers will quickly flow into the rest of the cylinder, this effect reducing the temperature of the cylinder in contact with the fingers and not raising the temperature of the rest of the cylinder by very much.
As the cylinder acts as a heat sink whose temperature does not rise very much because of its bulk and your fingers are not getting enough “body heat” to maintain a constant temperature the temperature of your fingers drops which gives you the sensation that the cylinder is cold.

If you touched the ground on which the cylinder was resting you would not feel that the ground was as cold as the cylinder even though the ground and the cylinder were initially at the same temperature.
This is because the thermal connectivity of the ground is so much less than that of the cylinder so the temperature of the ground around your fingers rises and this means that the rate of heat flow from your fingers would be reduced, thus your fingers would be cooled less - the ground does not feel as cold.

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There is no reason that the compressed air tank should have a lower temperature in the compressed state when the pressure was not changed for a couple hours. Metal, however, feels "cold" to touch even when at environment temperature due to the high thermal conductivity. Further, if air was let out shortly before, there could be a cooling effect due to the air expansion.

Added note: For a possible cooling effect due to recent gas expansion, which seems similar to the cooling of aerosol spray containers, I found this question with answers on SE: Why does the gas get cold when I spray it?

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Why is a compressed air tank cold to the touch when it’s in a compressed state before use?

Here's a conjecture which you may verify by doing some simple experiments if you have available an empty cylinder and a cylinder filled with compressed gas:

Compressing air sufficiently leads to liquefaction of some of its components, in particular water vapor (see this). If no precautions are taken to the contrary, a cylinder of compressed air contains liquid in equilibrium with its vapour at ambient temperature (assuming sufficient time has been allowed for equilibration). Your hand being warmer than the ambient, touching the highly conducting cylinder wall leads to transfer of heat into the liquid inside and its absorption by phase change of liquid-water to water-vapor. Compressed air cylinders are required to be kept out of direct sunlight to avoid gas expansion by direct heating and perhaps also due to phase change (although no one seems to mention this explicitly). This is perhaps why compressed air cylinders feel cold even before use.

P.S. Of course an experiment must be done to verify that the cooling effect is not exclusively due to cylinder wall being metal as some of the answers have suggested.

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