Feed tube resonance

The geometry of many decanters is such that there is a risk of resonant vibration of the feed tube at frequencies around the bowl speed. To counter this, feed tubes have been made slightly tapered, and made of lighter materials such as glass fibre and even carbon fibre, and sometimes composite material. 

As discussed in Section 2.2.8.2, care has to be taken in avoiding resonance of the feed tube. The concentric floe tube helps with the rigidity. Tapering the feed tube and making it of lighter materials also help. The resonance frequency of the feed tube is reduced when feed is admitted to it. It is not unusual for the feed tube s resonance frequency to be higher than bowl speed when empty, and lower than bowl speed after feed starts to flow. 

Each component of the decanter has its own natural frequency, even the stationary components, which could resonate sympathetically if this frequency is close to the bowl speed. The feed tube is a good example, being a long, thin tube. Apart from the inverse square relationship with length, resonance frequency is also proportional to the fourth power of diameter in its simplest form. Unless care is taken the feed tube can be caused to resonate like a tuning fork. The design engineer thus endeavours to maximise diameter and... 

The dispersion of pasty pigments with shock waves was studied experimentally using the shock-wave generator with a resonance tube of 0.033 m. This allows achieving air velocity equal to 163 m/s at 19.8 Hz of the reciprocating movement of the piston. The pasty feed was located 0.05 m from the resonance tube outlet at its axis. Such a distance was found to be optimal over the range from 1.5 to 5 times the tube diameter. 

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