Hesch said:
Generally centrifugal air pumps/air blowers are used when low noise is required. [emphasis added]
While this statement is true, it isn't the main issue -- though it does imply a key point:
there is no fundamental difference between a pump and a blower or fan. Terminology-wise, "blower" and "fan" refer to air, so they can't be used to described water flow, but "pump" can refer to air or water. Regardless, the thing separating them when you focus on air is
pressure.
Fundamentally, any pump/blower/fan is a device that moves a fluid by generating a pressure difference. The difference between an air pump, blower and fan is colloquial in nature only: "fan" is generally used to describe low pressure, "blower" medium pressure and "pump" high pressure. So let's examine the different types of air movement devices:
1. Fan. Low pressure. Application: The thing sitting on your desk, blowing air at you. If you can see the actual fan, that means there is no ductwork attached to it and therefore it operates at low pressure. These are axial, bladed, like propellers. They are capable of only very low pressures; Less than 1" W.G.
2. Blower. Medium pressure. Application: HVAC air movement. These need to be able to generate some pressure so that they can force air through ductwork, filters and cooling/heating coils without those things having to be too large. Note, however, that for HVAC, "fan" and "blower" tend to be used interchangeably, though "fan" is the more common. It is only for specialty higher pressure applications (dust collectors, for example) that the term "blower" becomes favored.
These are almost all centrifugal. Applicable pressure: 1-10" W.G.
3. Pump. Higher pressure (above 10" W.G. or measured in psi, or to generate a vacuum). Most centrifugal, but sometimes positive displacement.
And another level:
4. Compressor. Anywhere from a few psi to hundreds. Depending on the flow rate and size needed, these may be positive displacement or still centrifugal.
So as you can see, there are no hard and fast rules and the terminology overlaps, but the key difference is in how much pressure vs flow you need:
-Low pressure and high flow = Axial
-Medium pressure and medium flow = centrifugal
-High pressure and medium flow = centrifugal
-High pressure and low flow = positive displacement (piston/diaphragm)
Regarding efficiency: for HVAC air movement, centrifugal fans are indeed generally the highest efficiency, generally running in the 65-85% range. You do sometimes see high pressure axial fans/blowers, but I'm not clear on the limitations that make them uncommon.