When considering types of fans for applications here are some of the different advantages that ball bearings and sleeve bearings each have.
Ball bearings have one of the best life expectancies of the different types.
They are better suited for higher temperatures with an average of -70 through 80 degrees C range (this can vary dependent on lubricant inside).
They can also be placed in different orientations (vertical, shaft center line, perpendicular, or parallel) without drastic reduction of life expectancy or increase in noise production.
However the disadvantages of ball bearings is they can be relatively expensive.
SLEEVE BEARINGS (pictured in the rectangle below)
Advantages of Sleeve bearings is that they may produce less noise at lower speeds and are generally lower cost.
However the disadvantage is that they should only be placed in vertical orientation as other methods of mounting can cause loss of the lubricant inside and shorten the life of the fan.
SEALED SLEEVE BEARING
This is a sleeve bearing that is designed to have a lubricant collection cup so that it can catch the lubricant and recirculate it - extending the life of the fan.
Disadvantage is that it is more expensive than a normal sleeve type but still generally not as much as a ball bearing.
Click here to view a list of DC fans with sealed sleeve bearings