This permissible unit from Fletcher scrubs dry up to 9,000 cfm of air.

The need for this product is evident, as dust concentrations downwind of the continuous miner can exceed regulatory standards and overexpose roof bolter operators. In underground mining, onboard flooded bed scrubbers are used on continuous mining machines for respirable dust collection. Though these systems are proven, dust bypassing the continuous miner scrubber may expose roof bolter operators to high levels of respirable dust when working downwind of the continuous miner.

J.H. Fletcher & Co. was a pioneer in dust collection, receiving patents as early as the 1950s. When maintained, Fletcher’s Mine Safety and Health Administration (MSHA)-approved onboard dry dust collectors are proven to remove the respirable dust generated from roof bolting. With this background, Fletcher’s research and development department developed the MSHA-approved Fletcher dry air scrubber. The machine is a stand-alone, self-propelled, electrically powered ventilation fan. The machine has two operating modes: tram and ventilation. While in tram mode, the fan cannot be operated. The dry air scrubber is engineered to be positioned between the rib and vent curtain. The unit then works to clean contaminated air before it enters the roof bolter work space, working to provide roof bolter operators with a cleaner air supply and lowering their exposure to respirable dust.

The design reflects Fletcher’s diversity in the underground mining sector, with the ability to fit into a variety of mines. The dry scrubber is 16.5 ft long and 4.5 ft wide, equipped with 12-in. crawler pads.

The dry scrubber pulls a portion (up to 9,000 cfm) of the air flowing between the rib and the vent curtain into the filters on the inlet side of the fan. Filtered air is directed out of the machine and back into the main air stream. A pressure sensor arrangement is used to monitor filter conditions to alert the operator when the filters need to be replaced. A hydraulically operated hood and pivoting electrical controller allows access to the air filters.

The fan speed and resulting airflow is controlled by a variable frequency drive (VFD). This allows the mine to select an airflow controlled with a feedback loop to automatically adjust the fan speed to maintain a constant airflow output as the filters are loaded with dust.

NIOSH laboratory tests found that the dry scrubber averaged greater than 95% respirable coal dust removal while being operated at two different fan airflow speed quantities of 3,000-9,000 cfm. Underground studies found a collection efficiency of 99.2%.

Other potential applications for the dry scrubber include belt conveyor transfers, underground truck dumps, single entry air cleaning, and diesel particulate matter removal.