Heat/Temperature is the issue, but the Temperature is related to the Pressure. First I would like to introduce you to two equations. The ideal Gas Law, and the Adiabatic Compression/Expansion law.
PV=nRT. Ideal Gas Law.
PV^1.4 = Constant. Adiabatic Compression/expansion Law. 1.4 is the ratio of specific heats for air (gamma).
We can do a little re-organization of these laws to formulate the following equations:
(P1V1)/(P2V2) = T1/T2
V2=(P1/P2 * (V1^1.4))^1/1.4
Remember all the temperatures need to be on an absolute scale, either Rankine (F+460) or Kelvin (C+273)
So lets say you didn't remove any of the heat from the compression process, how hot would the gas come out at 3000, and 4500 psi?
In the above equations, we have the ratio of the input to the ouput pressures so P1/P2 = 200 (for 3000 psi) or 300 (for 4500 psi). We are also concerned about the ratio of the volumes of the gas (V1/V2), so we'll assume that V1 =1, and solve for V2
From the adiabatic compression, V2=(1/200 * 1^1.4)^1/1.4
V2 = .0224 , or 2.3% of its previous volume for the 3000 psi compression
Similarly, for the compression to 4500 psi, V2=(1/300 *1^1.4)^1/1.4
V2= .017 or 1.7% of its previous volume.
Lets assume you start at room temperature, 72 degrees Farenheight, or 532 degrees Rankine.
Plug this back into the ideal gas law equations, and...
For 3000 psi compression
P1/P2*V1/V2 = T1/T2 ==> 1/200 * 1/.0224 = 532/T2
T2=2340 R or 1880 F. That's more than hot enough to melt aluminum.
For 4500 psi compression
1/300*1/.017=532/T2
T2 = 2713 R or 2253 F. That's about the temperature that steel melts at.
Now the above equations assume the compression is adiabatic, that no heat is removed. I was trying to point out how important the heat removal is. That unless the heat is properly removed, the temperatures will skyrocket in your compressor.
Add the presence of additional oxygen in the air (Nitrox) because of continuous mixing, and the degredation/oxidation (of oil) occurs even faster, and the issue of heat removal become more important. Most corrosion rates are a function of temperature and oxygen content, as the temperature and oxygen content increase, the rate of degredation/oxidation/corrosion increases.
Lessons learned: make sure you have good flow of air around your compressor. Keep it clean, remove any dust, or oil that may accumulate on the interstage cooling coils. Your compressor will last longer and be happier if you keep it in an air conditioned room.