DUST EXTRACTION – HOW TO PROFIT FROM REGULATORY COMPLIANCE – By Gus Bishop for Nederman Limited

INTRODUCTION

Process industries are at the vanguard of British manufacturing industry and particularly for the ongoing recovery of the British economy. Efficient production relies on bulk materials being stored, conveyed and presented for further processing in consistently good dust-free condition. The cost of such dust control can be converted into a value-adding benefit. Furthermore, whilst legislation demands a safe working environment, compliance can bring positive benefits that far outweigh the costs. This paper discusses appropriate dust control options and how to turn such “costs” into added value.

Survival in business depends upon profitability – on obvious statement indeed. But let us consider some drains upon resources that can become chronic and thereby a constant millstone around the neck of positive development. Consistently good product quality, reliability of production with no unplanned stoppages, consistent performance for a dedicated workforce (with minimal “churn”), predictable energy use, always optimized  and indeed a good long term reputation as an employer locally and with customers are all pointers to a well found, stable company. So, how can dust extraction work towards these aims when it is often initially regarded as a necessary and costly appendage to production equipment that simply costs money and adds yet another level to maintenance requirements?

Minimising energy requirements

Always relevant to any manufacturing activity, it is particularly important to a dust extraction system. In some process industries, the single largest motor on site may well be that which drives the main dust extraction fan. Even if it is not the largest, the total motor power for extraction will nonetheless be significant. A little thought at the design stage will pay back several times over as time goes by. Obviously the most efficient motors and fans will be chosen, but the power demand will depend on otherwise mundane elements of the installation such as air velocity in ducting, the route of the ducting itself and the likely resistance of the dust filter/extractor unit. Narrower ducting may be easier to install, perhaps be less intrusive and certainly will save a little on capital cost. However, the effects on operating costs could be quite dramatic.

Ducting uses energy

Consider this. A typical duct may have a resistance of about 2000 Pascals/100m at an air velocity of 30 m/s. Reducing the air velocity to 20 m/s would reduce the resistance to about 700 Pascals/100m – that is, a saving of almost 2/3 of the force required to move the air. If the dust would remain buoyant at 15 m/s, then the resistance would drop to around 330 Pascals/100m. The savings in noise alone would make this worthwhile, but energy cost savings could be dramatic.

Furthermore, the extra cost of the ducting installation (probably smaller than you think!) could be offset by the cost saved by installing a smaller fan. The pressure generated and the resulting airflow volume do not follow each other in direct proportion if parameters such as speed of rotation are varied. For example for a radial (centrifugal) fan, if the speed is increased, then the airflow volume increases in direct proportion to the ratio of the new speed to the old speed. However, the static pressure will increase in proportion to the square of the speed ratio and the power required will increase in proportion to the cube of the speed ratio.

Different but equivalent effects result from changes to fan impeller diameter, blade length and so on. Therefore, at the design stage, the dust control suppliers should make a careful assessment of the range of pressures and airflow volumes required. In many instances, inverter speed control may be employed to satisfy operating conditions, if the pressure/volume requirements can be satisfied. In this way the most efficient air mover may be selected.

A cheaper form of regulation may be the provision of fixed dampers in key parts of the duct work. Whilst this option may have some effect on energy consumption it is inherently wasteful because of the energy required to accelerate the air through the damper – energy that is not put to any useful purpose. Noise generated by dampers may also be an issue, particularly if the dust extraction point is a manned workstation, such as a manually operated machine, or an activity taking place within large hoods or booths. In general, the more precise the fan selection together with ducting designed for minimal resistance, the more efficient will be the system.

A happy dust filter/collector

Clearly the correct choice of dust filter/collector is vital and well found suppliers who design, manufacture and install their own equipment are likely to have gained beneficial knowledge and experience. Companies such as Nederman Ltd., with a wide range of offerings encompassing bag and cartridge technology, various grades of pre-separators and  waste processing equipment can offer an holistic approach, working in partnership with the end user to ensure proper compatibility.

A centralised dust filter plant may be the most appropriate approach or perhaps individual units may be installed close to the points of use. There are many and various arguments for either approach, but within a process plant downtime requirements for maintenance may favour individual units if their diversity would reduce the risk of an unwanted total stoppage in the event of a fault.

However, if the filter units are selected with conservative filter speeds and the filter media is carefully selected  to work with the material and its local atmospheric conditions, then failure should be rare indeed, provided that manufacturers’ maintenance schedules are followed. Of course, conservative filter speeds may add a little to the anticipated size and capital cost, but the payback in terms of energy use, lack of faults and  consistency of product quality will bring great satisfaction and peace of mind to the plant operator.

In many process applications, the ability of the dust extraction equipment to de-dust granular or pelletised products consistently is very much a part of the whole process. For example, in agricultural seed cleaning and packing minimal dust in the product is a useful selling point – cleanest seed achieving the highest price. Farmers will never be keen to pay for a lot of dust with their seed. Similarly, in powder or additive coating processes it is important to remove just the right amount of dust, consistently. The dust extraction equipment supplier must gain the best possible data about the material to be handled and agree to sample testing if there is any doubt, before finalising the design.

Compliance pays

Workplace environmental legislation (DSEAR, COSHH, ATEX et al) protects the workforce, but effective control of workplace air quality can bring other benefits to the employer. It’s compulsory, so why not make it work in your favour? There is no doubt that operatives who work in a good atmosphere will be more effective. Their knowledge that they work for a progressive and caring employer will engender loyalty resulting in lower turnover of staff. An added benefit will then be a reduced requirement for training new people.

Effective dust control is an on-going commitment to be included in the general maintenance regime of the plant operation. However, if it is truly effective, then this fact alone will reduce the overall maintenance costs. So, an installation that firstly may have been perceived to be just an imposed cost can in fact produce real benefits and actually result in a useful reduction in the cost of the total operation. To achieve this, however, care with the initial design process and the choice of a competent supplier are absolutely vital.