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


CCASA 3.0 Chocolateoak

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.