Radial fans (pumps, compressors) historically appeared much later than axial ones. But they turned out to be, as befits innovations, much more promising, better in many ways:

• hydrodynamic,
  
• energy,
  
• economic,
  
• noise.
  

The only drawback is the specially shaped channel-cases, which cause additional dimensions, weight and cost. Axial fans create a sufficiently directed flow even in a completely open space. Only in order to increase efficiency, channels / impellers are used (which, by the way, are struggling with centrifugal effects, which are parasitic in axial systems). For radial impellers, the collection and redirection of the diffused air is absolutely essential. However, the collecting "snail" can have only half a turn, be very flat (short along the axis), which, in combination with a flat motor, allows you to create fans with dimensions smaller than those of axial fans of the same performance. In connection with the predominantly oblong-narrow shape of fan coil units, most often they use impellers of the "squirrel wheel" type - with a large axial length and small diameter. An electric motor (also long and thin) usually has a double-sided shaft and is located between the same impellers, which makes it possible to perfectly balance the entire rotating part and minimize the load on the bearings.