VARIABLE PITCH IN MOTION
History
There are many applications where it is required to adapt the volume flow rate (reducing or increasing) according to the changing values of the process. Some decades ago, when the electronical speed control was at its beginning, the volume flow rate was adjusted as follows:
- By a damper (cheap solution, very low overall-efficiency),
- With an adjustable DC motor (very expensive motors and control units) or
- By impellers with variable pitch angle in motion (mechanically very complicated,
acceptable efficiency).
Advantages of variable pitch in motion
Only in a very few applications axial fans with variable pitch in motion have the following advantages:
- Possibly a little better efficiency with non quadratic system resistance-curve (example: mine-ventilation with weekly lengthening tunnels).
- No additional electrical losses (1 − 2%) by the operation with a frequency converter.
- Extremly high demands for an electromagnetic compatibility (but: we do not know any
example − even in military submarines the use of frequency converters is allowed)
Disadvantages of variable pitch in motion
- High capital expenditure, as technically/structurally very complicated.
- Low safety in operation, as the failure of many small components easily can lead to a failure of the whole fan unit.
- Very high maintenance costs because of the numerous mechanic wear and tear parts (bearings of the blades etc.).
- Very high risk of a "stall", as with a wrong pitch adjustment the fan will inevitably go into stall (= burble).
- Parallel operation very complicated, as a pumping (hunting) between two fans can be avoided only by using a very complex system control.
- Heat resistance very problematic, as combustible hydraulic oil is used. The whole hydraulic unit (also at the impeller) has to be thermically insulated. Varying the pitch in motion (forward, reverse, etc.) in a fire situation mostly not possible.
- For a quadratic system resistance curve (which is the most common one) optimum efficiency is only reached at one single working point, in all other working points lower efficiencies than frequency converter driven fans with fixed blades.
Conclusion
- Except for a very small advantage in efficiency in some applications, fans with variable pitch in motion give no relevant advantages. Therefore nowadays these fans present an old and overcome technology, which is only used for special applications (e.g. mines). The production of this kind of fans has nearly discontinued in Germany.
- From an economical point of view these fans with variable pitch in motion are too expensive at purchase and maintenance compared with frequency converter driven fans with fixed blades.
- From safety aspects fans with variable pitch in motion are to be classified much lower than frequency converter driven fans with fixed blades.
- All modern frequency converters can be equipped with network-filters and -chokes to realise very high electromagnetic compatibility.
For the reasons mentioned above we have stopped the development of axial fans with variable pitch in motion.
