R&D Overview

At the same time the Department of Minerals & Energy (DME) was putting increasing pressure on the industry to bring dust levels down to acceptable limits. The old dry fan systems, originally developed by CDC and used for many years, were no longer up to the job. New ideas and methods were required to meet the DME’s demands.

CDC set about tackling these problems, using well-established principles but applying modern technology and different systems to meet the challenge. Our solution was the CDC integrated dust suppression system. The application of this three-part system, when used in conjunction with the principles confirmed by tests undertaken by the CSIR’s methane and coal dust explosion test centre at Kloppersbos outside Pretoria, brought dust levels down to below the required upper limit 5 mg/cu m and close to 2 mg/cu m.

The integrated dust suppression system has made a huge impact on coal mining, with an immensely improved working environment, greatly improved safety, reduced downtime, increased productivity and reduced maintenance costs. All this has been achieved with no increase in real costs as CDC’s prices for its dust suppression equipment, apart from inflationary effects, remained unchanged.

So the customer is getting a far superior product at no extra cost – a vindication of CDC’s commitment to R&D.

The CDC wet fan

The CDC wet fan was developed using the CSIR’s computer simulation programs to design the blade shape, size and configuration. The early units produced just over 6,5 cu m/sec at a pressure of 2 bar. With constant development and attention to manufacturing detail, this same fan now produces 10 cu m/sec at 2 bar and with improvements in operating efficiency it absorbs very little increase in power.

The current range of impellers have pressed steel blades, jig assembled and dynamically balanced and hard-chromed to give a totally reliant and long-life unit.

The range of sizes is expanded from one to six, covering a range of volumes from 4 cu m/sec to 17 cu m/sec.

Alongside the development of the fan was the development of the scrubber box, with over 85 per cent of the dust eliminated in the fan. The design of the scrubber box could be developed to reduce system resistance and improve water spray elimination.

The CDC mist eliminators operate at air speeds of up to 12 m/sec and still deliver dry air from the back of the scrubber box.

Different types of boxes have been developed, with a side-outlet type becoming popular in recent years. We have also developed a lightweight "disposable" box for mines commonly experiencing roof-falls.

The increased performance of the units together with their smaller size have allowed mines to select smaller units than before and to mount them in better-protected positions. This has resulted in better dust control, less mechanical damage due to roof-falls and overall better performance at much lower cost.

The electric motor

The electric motor used in CDC wet fan scrubber units was developed jointly by CDC, Sasol Coal and Transvaal Electric Company. They developed this small-frame low-inertia water-cooled motor to minimise the risk of overheating and failure of the motor under the arduous working conditions encountered in underground coal mines.

The initial brief was for a small low-inertia motor which, coupled with the light low-inertia fan impeller, meant that starting currents and times could be reduced and so dramatically reduce the thermal cycling to a minimum. This all but eliminated the problem of dust and water ingress into the motor through the flameproof path.

Over the years much attention has been given to improving mechanical and electrical reliability. The sealing arrangements have been improved to eliminate the ingress of water and dust to the motor and improve bearing life, while the rotor design has been enhanced to cope with the high repeated loads of constant starting and stopping.

On the electrical side improvements were made to the windings and insulation, the VPI system was introduced and a strict policy of using high grade materials throughout was adopted to ensure good performance, long life and reliability.

The overall effect of this development work is to produce a unit with long life, reduced downtime and increased productivity. It is extremely rare to have a motor burn out and most of the motors that are returned to us for repair are damaged by external forces rather than motor failure.

Filters and spray blocks

CDC has kept in step with the industry in its progress towards increased automation and reduced exposure to "human error" that heavy reliance on operators implies. Every article in the dust suppression system is looked at with a view to reducing the demands on the operator.

In the past the filtering system on the water supply to the booster pump and then to the spray nozzles was dependent on frequent backflushing to maintain water pressure and quality.

CDC has introduced a semi-automatic backflush filter incorporating the on/off valve to ensure that the filters are flushed frequently, virtually eliminating the possibility of clogging and guaranteeing good water quality. Once the valve has been closed, in order to re-open it the unit has to go through a backflushing cycle before the water can be turned on again.

We have also made the job of keeping the spray blocks clean easier and more efficient.

The efficiency of the spray fan system is dependent on good quality well-filtered water to ensure that the spray nozzles remain clean, but even with good filtration impurities can get into the water going into the spray blocks. Previously the cleaning procedure required unscrewing of the sprays from the block, cleaning them with a pricker and then re-installing them – not a task that an operator will tackle readily and eagerly!