Why do air compressors spit and crackle when you're draining them? I always assumed it was flakes of rust getting pushed through but today I got hit with crazy shrapnel walking past our brand-new draining shop compressor. Thought I got hit with a shotgun.
It almost stops periodically then a big crackle and it flows again for a while.
This is also not a little petcock drain, this is 3/8 iron pipe with a full-flow 1/2" ball valve
Ice from adiabatic cooling? The tank temp doesn't seem to be cold, nor is the escaping air below freezing, nor does it seem like 150 psi should be enough pressure drop to freeze water.
The volume of air compressed in the tank is pretty huge - the air compacts, but the moisture within it can't. That water condenses at the bottom of the tank - the more the compressor runs, the more water collects. Over time, the inside of the tank rusts, and when venting the tank it sputters and spits chunks.
^^Yep, this is why in cold climates you have to drain your tanks every night.^^
BTW it is the air dryer that does it.
Down here I've seen small chunks of ice spit out of a compressor drain. But I'm talking one of those little bitty drains that looks like a radiator drain valve. The ice flew out maybe pinhead size and melted when it hit the floor.
nderwater wrote:
The volume of air compressed in the tank is pretty huge - the air compacts, but the moisture within it can't. That water condenses at the bottom of the tank - the more the compressor runs, the more water collects. Over time, the inside of the tank rusts, and when venting the tank it sputters and spits chunks.
I know all about that, but this is a brand new, two-day-old compressor with 3/8" and 1/2" pipe to drain it. I don't care how much water is in there, there isn't any flaking rust yet. 
Every compressor I've ever seen, old or new, large or small, always does this. Adiabatic cooling could describe icing in the valve, but the adiabatic effect is much greater AFTER the air hits atmoshpere than it is in the valve, so one would expect the freezing to happen first outside the outlet where the pressure drop is the greatest.
Me no understand. Physics, check. Logic, check. Intelligence, check. Why compressors spit? No friggin clue
I'm going to go with the ice. Adiabatic cooling is taking place as the compressed air is exiting through the valve, 'cause believe it or not that is where the air is doing most of its expansion. I used to run a jackhammer for a living and any temp south of about 75 or 80 and you could see ice chunks flying out of the exhaust port. 1/2" ball valve to drain the compressor and I'm not surprised that it felt like a shotgun when you got hit with it.
Edited to account for fat fingers.
I see what you're saying. Ever hold a lighter or match to light the stream from an aerosol can? The flame follows the mist back to a certain point near, but outside the nozzle - there's a threshold at which the outgoing vapor still has too much density to ignite. What you're seeing is the same phenomena, only with cooling instead of combustion. Once the expelled mist disperses enough, the vapor is able to crystallize.
curtis73 wrote:
I don't care how much water is in there, there isn't any flaking rust yet.
Ehh depends.. Most tanks aren't usually painted or coated on the inside.. I'll also bet you could have been hit by a small chuck of weld slag..
Is this s HF compressor? 
Adiabatic cooling is what used to lead to carburetor icing, which can happen even on very hot days. On many FI engines, you can feel that the throttle body is much cooler than the rest of the engine (and that's why many engines run hot coolant through them). So that gets my vote.
Is this s HF compressor?
nope. brand new Curtis brand 300 gallon 360v industrial
