# What is this metal pipe between the motor and tank on my upright air compressor, and why is it HOT?

So, I have an upright compressor (pictured below) that has a metal pipe leading from the motor to the tank. I had my compressor running and an air line was resting up against this tube and it has heated up the plastic enough to stink up my garage (It didn't melt it thankfully). I touched it and it burned my fingers like touching cookware. I assume it's normal operation, and I assume (But could be wrong) that that tube is how compressed air gets into the tank, but I don't understand why it's so hot. An explanation would be greatly appreciated.

Unfortunately, I couldn't find a picture of the compressor from the backside showing the tube, but this is the compressor. The tube leads from the rear of the plastic housing to the top of the tank.

• Seems strange that the manufacturer did not put a protective guard or something to keep you from getting burned, considering it's placement on the unit. On my compressor it is tucked in a location much less likely to be touched. I bet there are warnings in the user's manual though. Commented Aug 31, 2016 at 5:08
• @JimmyFix-it Given the owners manual was thicker than my biology textbook, I'm willing to bet you're right. Commented Aug 31, 2016 at 14:13

Compressing gas makes it hotter. This is the principle exploited by fire pistons to set fire to things. It is also used in a Diesel engine to ignite the fuel.

For an ideal gas P V ∝ T. As you increase pressure (P) the volume (V) reduces, however air is not perfectly compressible, so when you double the pressure, the volume is reduced by less than half, therefore temperature (T) must increase.

Another way to look at this is that your compressor is doing work on the air. Work is energy, some of that energy ends up in the form of heat.

Also, any kind of motor (electric or other) produces heat.

Pipes made of metal are good at conducting heat into adjacent hoses and fingers.

The opposite is also true, the reason that CO2 fire extinguishers have a plastic guard or handle on the nozzle is to prevent your hands freezing onto the outlet pipe as you discharge the gas - allowing it's pressure to decrease as it leaves the container.

• A running air tool gets noticeably cold. That's from the high pressure air reducing pressure and now having too little heat. It is the opposite and inevitable payback from the compressor getting hot. Commented Aug 31, 2016 at 19:15

You might be able to find a finned replacement line that will dissipate the heat better. These lines getting hot is normal, however some compressors will add fins to the line so that it dissipates the heat into the surrounding air quicker, and thus doesn't get as hot and become a burn hazard.