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Compressor drain...

It was probably about 65°, but it will do it pretty much any time of the year.

Is it because it's dry (no humidity)?

I've never seen ice buildup like that out of a compressor drain. Mine just has a barely visible vapor (I have a cooling system on the head before it goes into the tank).
 
Is it because it's dry (no humidity)?

I've never seen ice buildup like that out of a compressor drain. Mine just has a barely visible vapor (I have a cooling system on the head before it goes into the tank).

The humidity yesterday was over 40%, so it was definitely not dry.
 
I was still curious as to why the ice would form, so I did a little research and this is what I came up with. It happens to be the last response that explains it. Makes sense to me.

From the reddit engineering forum:

I'm not sure why theres a plethora of wrong answers here - it's an engineering forum!

The tank of air cools because of adiabatic expansion - it's happening so fast that there's not enough time for significant amount of thermal energy to enter the system and keep the system temperature propped up. This results in an expanded gas with lower energy (PV in the gas equation), and more importantly lower energy density, which directly leads to lower temperature. Then the ambient water freezes. Tada!

This is the exact same phenomenon used in air conditioners and refrigerators (albeit with different gasses).

From a math standpoint, the compressed air is losing energy because it is performing work on the outside air as it expands. Q is basically zero because it's adiabatic, so all of the energy change comes from the mechanical work done by the air.

 
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