Application Conditions:
Very high levels of dust, high levels of noise, some flying rock possibilities, limited measuring possibilities, build-up on instruments, short range.
Level Technology Choice:
We have seen radar, laser, ultrasonic, microwave switch, photoelectric switch and tilt switch used in this application. With build-up of dust and some condensation present, technologies that do not have a self-cleaning capability, struggle to stay a reliable system. Ultrasonic frequencies that are too high such as 30 kHz, 40 kHz, 50 kHz and 70 kHz do not self-clean, as the pressure wave with each pulse of these frequencies are very low. The mistake comes with specifying the instrument by range instead of how bad the operating environment is going to be. Radar will need air purging with dry instrument air. Laser will need to be protected; as if the lens scratches the instrument will fail. Laser will also need to be purged with instrument air to keep the lens dust free. Microwave will work, but only as a switch. Photoelectric will work if the dust levels are very low, which is not the case. Tilt switch will work as a high-level switch only but not fail safe.
A non-contact technology, like a 20 kHz acoustic transmitter, is the best choice because of its narrow beam, self-cleaning capability, not affected by background noise in the crusher and will see through dust. Acoustic transducers have the ability to see through dust. Particles of dust in suspension are micron in size. The ability of a particular frequency to see through dust is governed by the wavelength of the frequency. 20 kHz has a wavelength of 16mm and can see through particles in suspension <1.6mm.
By controlling the “Choke Feed” to the crusher, we can minimize wear and tear on the crusher as well as increasing the operating efficiency. A Polyurethane rubber cone to reduce breakage caused by flying rock is also suggested.
