1. Field of the Invention
A method for controlling an automatic stall-prevention of regulated-rotation-speed low-pressure centrifugal fan and a control automatics therefor.
2. Description of Background Art
Numerous compressors and centrifugal fans of different types, depending on their various applications, are known in the art. Particularly in industry, centrifugal fans, compressors and radial fans are widely used to achieve a pressure difference in piping. A problem for all centrifugal fan is generally known to be stalling. In other words, stalling is a characteristic state for all centrifugal fans, which occurs when the volume flow rate is too small in relation to the speed of rotation of the impeller. In this case the angle of incidence between the flow and the blade changes to be so disadvantageous that the flow disengages from the surface of the blade. Backflow is then able to occur in the blade passage and the impeller loses its pressure-increasing ability.
In this way, cyclical pressure fluctuations are generated that excite the natural frequencies of the structure surrounding, among other things, the piping. Pressure fluctuations create fatigue loading in the piping and in the structures. In addition, the temperature of the flow can significantly rise when the impeller, via the losses, continuously gives thermal energy to the gas, but the effective flow can be very small.
Problems occur particularly in processes in which resistance to the flow greatly changes. When using a centrifugal fan for producing low pressure in these types of processes, a stalling state must be prevented by giving leakage air to the centrifugal fan in a controlled manner.
Conventionally, centrifugal fans that have a constant speed of rotation have been used. In this case stalling is prevented with an automatic leakage air valve, which receives control from the current of the drive motor of the centrifugal fan. In the stalling state the current of the drive motor is smaller than in the normal operating range. The electric current also fluctuates strongly. The control logic of the centrifugal fan can easily be programmed to detect a stalling state and to eliminate it by means of leakage air.
Another problem is that in a stalling state the low pressure also fluctuates with a rapid cycle, and this situation is detrimental from the viewpoint of the process. A device that has a regulated speed of rotation has been launched in the low-pressure centrifugal fan market as a new technology, the stall control of which cannot be implemented with conventional technology.