The bearings used in a fan and the quality of lubrication are the largest determinants of a fan's lifespan and noise. These estimates are assuming that fans are running in strenuous environments (50 deg C)--actual lifespan can actually be much longer. There are three common types of bearings in fans (and many sub-types).
Sleeve Bearings (~40,000 hours)
Sleeve bearing fans will be quieter than an equivalent speed ball bearing counterpart at lower speeds, but degrade much quicker and eventually will be louder than counterparts. They also must be mounted vertically or risk failure. They are generally ~35% cheaper than a ball bearing alternative thus being the cheapest option. Assume that a fan is sleeve bearing if a manufacturer doesn't state the bearing type.
-Rifle Bearing Variation--This sleeve bearing modification pumps liquid upwards from a reservoir to protect the shaft of the bearing thus allowing the fan to be mounted horizontally. This also extends the life of the fan marginally.
Ball Bearing (~65,000 hours)
Ball bearings are known for their long life and reliability. They are the gold standard in the fan world. There will normally be two rows of ball bearings (referenced style as dual ball bearings) in fans to ensure an even and smooth glide as the bearing rotates, but limited space on smaller (or cheaper) fans occasionally will only allow for one row of ball bearings. They will perform quieter than sleeve bearing fans when running at higher speeds and can be mounted in any direction.
Fluid Dynamic Bearings (~60,000 hours)
Fluid dynamic bearing variations often go by many other proprietary names (hydrodynamic or hydrostatic bearings, vapo bearings, SSO bearings, etc). They pump or suck fluids around the sleeve to minimize bearing abrasion. This results in the quietest bearing type available and a longer lifespan than sleeve counterparts. Fluid bearing manufacturers will often also modify their fan motors with additional magnet stabilization to further extend the lifespan (ie. Sunon's MagLev system or Noctua's SSO2 sytem). These are almost always the most expensive bearing type.
A fan's blade design is a large determinant on the amount of airflow and static pressure efficiencies. More blade surface area generally will mean that the fan will push more air for a given amount of revolutions. The less quickly a fan spins, the less noise it will make and energy it will take.
It's up to a fan engineer to maximize the surface area of the blades without compromising on the motor size (which would sacrifice static pressure capacity). One of the easiest way to determine the quality of the fan blade design is to measure the distance between the fan blade and the housing frame around it. Blades that are close to the frame are maximizing the allocated surface area. However, centrifugal force will expand the plastic of a fan blade over an extended period of time by a few millimeters. It is likely that a higher quality plastic was used if blades are very close to the frame as they won't have to account for as much blade expansion.
We are hesitant to outline coil design considering that a hundred-page white-paper would only begin to scratch the surface on the evolution of electromagnetic engineering. However, it may be useful to know that the fan's motor lies inside the fan hub--the area behind the sticker of the fan. The electrical draw (current) of the fan, normally measures in amps, is based on the quality of windings (wires that make up the coils) and the materials being used. This also affects static pressure efficiencies.
Fan energy efficiencies has significantly improved over the past 20 years primarily due to higher quality coil designs. DC (direct current) coils are going to be more efficient than AC (alternating current) coils, but access to a DC power supply is often more limited and requires a transformer. All DC fans that we offer use brush-less motors. Brush-less motors are marginally more expensive, but they significantly increase the lifespan of the motor. EC (electronically commutated) motors are a relatively new invention that will transform AC power to a DC motor internally--these will start trending much more in the next few decades as more manufacturers start integrating the technology.
We're happy to discuss any other technical questions on bearings, blades, or motors. Feel free to give us a call at 425-821-6400 anytime Monday through Friday 9am-5pm PST.