J.H. Fletcher takes us through its dust control systems for roof bolters.
Fletcher’s work on dust control began in the 1950s. The first system design was a cone-shaped vacuum located under the drillhead. The vacuum collected cuttings and dust as they fell, transferred them to the back of the machine and released them into the dust box.
In the late 1950s, Fletcher introduced its internal dust collection system. The system uses a vacuum pump to create negative pressure at the drill bit to draw the drill cuttings through the bit and drill steel. This process allows the cuttings to be simultaneously vacuumed as the operator is drilling.
The pre-cleaner functions by processing out large particles before they are processed through the dust filter. This material is removed through a cyclone process, pushing larger particles to the exterior and allowing the air to flow upward into the dust system. The larger particles are disposed of on the underside of the machine. The pre-cleaner process reduces the frequency that operators must service the dust collection box.
After air is pushed through the pre-cleaner, it enters into a series of filtration systems. Once inside the dust box, the air enters into the dust bag. The remaining air is then vacuumed into a second cyclone process, removing any leftover debris. After the remaining air rises, it is then piped through a filter and the filtered air is then released back into the environment.
Fletcher dust bags capture and contain the discarded dust and cuttings. The dust bags increase filtration quality and improve clean-up. A study by NIOSH found respirable dust levels in the collector exhaust to be over two times higher than when a dust bag is not in place. The dust bag was also tested to show nearly 100% of the dust fed into the collector was captured within the bag.
Figure 1: Dust box with bag and pre-cleaner.
A newer options presented by Fletcher is the air canopy curtain. The device, consisting of a perforated plate, is placed on the underside of the operator’s canopy. Air is pulled in through a centrifugal fan at the rear of the machine, pushed through tubing and filtration to the front of the machine, and then blown down through the perforated plate. The air flowing through the perforated plate is pushed down over the breathing zone of the person working beneath the canopy, supplying filtered air to the operator. This system is backed by a NIOSH study, revealing dust reductions of 40 – 60% at a low air velocity.
Figure 2: J.H. Fletcher’s CHDDR roof bolter with air canopy.
Fletcher currently has a filtration car in development, designed to filter the mine air. It is a stand-alone, narrow machine that can be positioned between the rib and vent curtain. This machine is equipped with a vane axial fan, which can be varied in speed by a VFD controller. A portion (up to 9000 cubic feet per minute) of the air flowing between the rib and vent curtain is pulled into filters on the inlet side of the fan. Clean air is directed out of the machine back into the main air stream. Feedback loop signals the VFD for increased speed during the life of the filter.
Figure 3: J.H. Fletcher’s filtration car.
Written by J.H. Fletcher. Edited by Jonathan Rowland.
Read the article online at: https://www.worldcoal.com/product-news/05092014/world-coal-fletcher-underground-dust-control-systems-1292/