Reducing combustion noise by flow control

The problem

Typical A-weighted noise levels generated by a dryer oven, used in malting grain, reached up to 95 dB. A low-frequency 'boom' at a tone of 82 Hz dominated. As the boom occurred only at low firing, it was diagnosed as flame instability that developed into an oscillation at a resonant frequency of the combustion tube. The precise cause of the instability was determined to be an excessively weak ratio of fuel to air at low firing.

The solution

To overcome this problem, a motor-controlled valve, previously used for controlling weld fume extraction, was adapted and fitted to the combustion fan intake. Triggered by a micro-switch, the valve was used to reduce the air flow at low loads and so improve the combustion characteristics.

The cost

£100 per unit. (1995)

The result

An overall noise reduction of about 8 dB with the low-frequency tone being reduced by 21 dB. The bought-in units had no effect on operations, could be retrofitted and satisfied hygiene requirements.

Source

Consultants were Industrial Noise and Vibration Centre.