The reasons are the follows:

1.The nose is lack of the oil:

The lubricating oil has an role in the lubrication and cooling. If the nose is lack of the oil, the cooling effect will be reduced and then the temperature of the nose will rise.

2.The effect of the lubricating oil is not good:

The oil cooler has a block.

3.The thermostatic valve has a failure:

If so ,  this will result in the lubricating oil directly enters into the nose not passing the oil cooler . And then the temperature of the nose will rise.

4.The oil stop valve has a failure:

If the oil stop valve can not open timely, this will result in the lubricating oil can not form the recycle the circulation in the air compressor.  And then the lubricating oil can not have an cooling effect on the nose.

5.The nose has the friction:

When the air compressor operates normally, the lubricating oil forms the oil film between the female and male helical rotors and this will prevent the female and male helical rotors from touching directly. If touching directly, plenty of the heat will be produced. At that time, the lubricating oil can not completely play the role in the cooling effect. So the temperature of the nose will rise.