Be sure to use the best fan for the job
CDC Technical Paper

Some mines have opted for a hooded system for their dust suppression, wherein the hood is integrated into the cutter boom and is connected to the scrubber by means of a flexible duct.

This arrangement increases system resistance and therefore care must be taken in selecting the correct fan and scrubber system that can accommodate this increased load.

It is perhaps worth examining what happens when the system resistance is increased on a typical axial flow fan curve.

The axial flow fan has an unusual curve, which starting from zero air flow generates maximum pressure, which falls off quite rapidly to a parallel section where air flow changes dramatically with little or no fall in pressure, to the last section of the curve, where it falls off fairly steeply to maximum air flow at zero or very low pressures.

It is on this last section of the curve that all axial fans are designed to operate, since on this section the air volume varies more predictably with changes in air pressure.

It is also in that section of the curve where the fan efficiency is at its greatest and also at its most stable. For systems where pressure will vary by relatively small amounts, the fan operating points are chosen around about two-thirds to three-quarters of the way up this part of the curve. This is certainly true of most wet fan scrubber box systems.

Increasing system resistance (usually from blocked screens) will push the operating point of the fan over to the parallel section where air volumes will vary enormously with small changes in system resistance pressure, as system resistance continues to increase.

Eventually air flow will cease altogether, with the fan going into stall. At this point the load on the motor will drop significantly, since little or no work is being done on the air.

This stall should not be confused with the stalling of the motor, which never happens on a fan. In fact, with big fans it is common to start the fan in stall to reduce motor load until such time as the motor has run up to speed, when the dampers are opened and the system resistance is reduced and the fan moves to the operating point on the curve.

CDC fans are selected with the operating point between 50 and 60 per cent of the way up the curve. This allows a generous margin for increased system resistance and certainly is sufficient to accommodate the increased resistance from a ducting system using the hood design. By adjusting the configuration of the screen and de-mister, CDC can fine-tune their boxes to give almost the same performance with the ducted system as they can with the open scrubber system.

Suggestions that the motor could burn out or be overloaded are just not true as they are patently contrary to the fan laws and characteristics!