Dust and fume extraction

The intention of any local extract system is to reduce what could be a danger due to the pollution of the atmosphere by some me­chanical or chemical process. It is imperative that the contami­nant be captured as close as possible to its source by moving a mass of air across possible escape routes.

An alternative is to reduce the concentration of the contaminant below the danger level by introducing a large quantity of clean air and removing a corresponding amount of dirty air from the affected area. Local exhaust will, however, give more positive control than this so-called dilution method.

Types of extract system

There are many different types of extract system, among which the following are examples:

• Wood refuse plants — to keep saws, planes and other ma­chines working

• Fume exhaust plants — to extract fumes, which would be harmful to health, if allowed to escape

• Kitchen extract system — to remove unpleasant odours and steam

• Recovery plant — where dust produced in some mechanical process such as machining, milling or polishing, has a value

• Atmospheric pollution prevention.

Components of an extract system

All systems will use ductwork, fans and hoods to capture and transport the contaminants. It may also be necessary to incor­porate some sort of air cleaner or dust collector. The cost of these items can be considerable and it is therefore essential to ensure efficient capture of the contaminants so that the amount of extract air may be minimized. This is especially important where dangerous fumes have to be extracted. Some fumes may act as cumulative poisons, whilst some mixtures of fine dust particles and air become explosive and must be avoided.

Categories of particles to be extracted

There are two main categories of particle to be considered:

• Fine dusts, fumes, vapours and smokes which can gener­ally be dealt with by air movement.

• Heavier particles which need special precautions and should be caught in their trajectory.

Exhaust hoods have to be located in the path of the particles.

In both cases, it may be advantageous to use both blowing as well as exhaust opening i. e. a so-called “push-pull” system. This can help to reduce factory heating loads by decreasing the number of air changes. The blower air can be cold. Air can be recirculated into occupied spaces but only if the dust or fume is non-toxic, the collection efficiency is high and there are no small particles which could elude collection.

General design considerations

Circular cross-section ducting is preferred, as rectangular ducts produce lower velocities in their corners, leading to dust deposition and build-up. The inside of the duct should be kept as smooth as possible and free from projections. A means of identifying blockages and removing them by inspection doors and clean-out hatches should be incorporated.

The solid particles are not assumed to affect the flow in any way as the usual mixtures are from 0.1 m3/s of air per kg of dust for the heaviest exhaust duties down to about 10 m3/s of air per kg of dust for grinders etc. On a volume basis this is about 5000:1 down to 500000 :1. It is normally possible to ignore the effects of the compressibility of the air, although system pressures are usually much higher than those experienced in air conditioning etc.

Motion of fine particles, fumes and vapours

In fine particle control, an open inlet may be modelled on the as­sumption that it approximates to a point source, see Figure

21.62. (See also Chapter 3, Figure 3.36.)

Design may necessitate that the exhaust opening may need to be some distance from the source of dust emission, see Figure

21.63. For example, this could help the operator of a grinder. The force to capture a particle of known physical characteristics

Hood

Figure 21.69 Double canopy

Figure 21.62 Point source approximation

Figure 21.66 Exhaust opening with flange

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Figure 21.63 Extract at distance from emission Can be calculated, and from this the extract velocity can also be calculated. It is often possible to use a cross-blast to advantage by posi­tioning the extracting opening somewhat downstream of the point of dust emission, as in Figure 21.64. Instead of a point, the source of emission may be an area. An example of this is a pickling tank. In such cases the exhaust hood should overlap the area as shown in Figure 21.65. The minimum angle of the hood should be 35°. Any condensa­tion will then run down the surface of the hood. Any lesser angle and the condensate will “rain” back on the work and operator beneath. The efficiency of exhaust opening can be increased by flanging, see Figure 21.66. If the flange is brought out to the 5% velocity contour, then the volumetric flowrate is reduced to 70% of the value without this addition. Thus the duct size may be reduced and hence with the reduction in flowrate, there is also a reduction in fan size, motor power and running costs. Where a tank is against a wall than the extract flowrate may be reduced to 75% of that normally required for the same effi-

Figure 21.67 Exhaust hood against a wall

Condensation m warni a* crfbafTle

Gottenng to calcn condensate Needed generaty onty at «tart-up from cow as when the hood * reefty warmed up re-evaporabon

150 mm to 225 mm

Figure 21.68 Baffle type hood

Figure 21.64 Position of extract when cross-blast present

Ciency of collection with the sarrie cross-draught. This latter, however, may also be reduced giving even greater savings. The ultimate of this approach is the spray booth, illustrated in Figure 21.67. Where small volumes of fumes are produced, a large hood with corresponding large volume flowrates might be considered un­economic. The design might then be modified to the baffle type shown in Figure 21.68. Where larger volumes of fumes are produced, a double canopy hood can effect savings in extract flowrate. See Figure 21.69. Large hoods may necessitate more than one opening to reduce height, see Figure 21.70. Distances from hood periphery to extract opening must all be equal to maintain an even airflow, see Figure 21.71. In long hoods a solution is often a long box or plenum chamber above the hood. This has a longitudinal slot in its underside. As the pressure loss across the slot is large, it will tend to be the same for all the hood. The loss in the box is by comparison

Design gap determined by work carried out

Figure 21.65 Exhaust hood overlapping tank emission

Figure 21.70 Large hood with double extract

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Figure 21.71 Equalisation of distance from hood periphery to extract opening

Figure 21.72. Slotted plenum extract

Small and negligible from any part of the slot. To maintain ex­actly equal losses, the slot might theoretically be shaped as in Figure 21.72.

All the refinements in hood design detailed above tend to in­crease both the first cost and weight. In an age when energy costs and greenhouse effects were not so important as today they were therefore largely ignored.

Weight was an important consideration as hoods have usually to be suspended from roof trusses. Cleaning is also more diffi­cult with a baffled or double hood. Nevertheless these problems can be overcome and it behoves us all to consider again these solutions when energy costs are so important.

Interior lighting to a large hood may be necessary and here bulkhead water-tight light fittings should be used. Smooth exte­riors to these should be chosen, so that they may be easily wiped clean of grease, dust etc. If sheet metal ducts are used then earthing points for the lights should also be fitted.

In some industries, such as the automotive, a monorail system is needed above the tank and it is difficult to fit an extract hood. Side extract may then be the only possible solution as illus­trated in Figure 21.73.

This may be arranged at either side if the tank is wide, Figure 21.74.

Alternatively a “push-pull” system may be employed, see Fig­ure 21.75.

Where one end of the tank can be blanked off, then a modified form can be used. (Figure 21.76.)

This method is effective up to about 1 m wide with velocities of 10 m/s. Slots should be about 35 mm to 50 mm wide. Again the slots should be tapered along their length to maintain an equal

Figure 21.74 Double extract at sides

Extract dud about «UHi!• tewД twnQ bkMn it —i — J- ■111?”

Figure 21.75 “Push-pull" system

Figure 21.76 Modified “push-pull" system

1

Figure 21.77 Spray booth extract

Pressure loss, the widest point being furthest from the extrac­tion duct. Spray booths incorporate many of these features, albeit in a different configuration, (Figure 21.77).

Dust features

When heavier particles such as e. g. wood chips are being ex­tracted, then duct wall thicknesses should be increased and bends offset slightly at their beginning, where possible, to de­crease damage, see Figure 21.78. Damage can lead to leak­age into extract ducts with consequent loss of suction and pos­sible increase in fan power.

“Lobster backed” bends of less than 250mm diameter should be made up in 15° segments. Larger diameters should be in less than 15° segments to maintain the approximation to a curve. The inside of ducts must be smooth and care should be taken at gaskets in flanged ductwork to ensure that there are no lips.

With condensable fumes, low points in duct systems should be fitted with drain points. A drain should also be incorporated in the fan casing, see Figure 21.79. All bends should have an easy radius with a bend ratio R/D of 2 or more. Ducts should generally be not less than 100 mm diameter to obviate clog­ging, although this size should be used with discretion. Airtight­ness is important, as due to the higher pressure losses, leak­age may be quite large, “robbing” the furthest sections of the system.

If dampers must be used then they should be of the slide type, coming from the top of the duct so that no dust build-up occurs, as shown in Figure 21.80.

Figure 21.78 Offset bends

Sweep-up points should be self-closing and airtight. It is not normal to allow for the air quantity passing through them as they are only open for short periods. Electrical continuity should be

Figure 21.79 Drain point positioning

Arranged across flanged sections. Sparking would otherwise occur across gaskets due to the build up of static electricity.

Balancing of duct systems

In any dust or fume extract plant it is essential to “balance" the system to ensure that the design extract flowrates are achieved and that all points have adequate suction without overloading the fan-driving motor.

The alternative strategies are as follows:

1. Size the fan and all branches on a given extract velocity without allowing for any balancing. In fact the design flowrate will be achieved and the motor may be over­loaded. No proper control over the extract from individual machines will be achieved.

2. Balance branches by one of these methods:

A) Using dampers or blast gates or internal cones of a suit­able design

B) Re-arrange machines or re-route ductwork to equalise the pressure loss to all extract points

C) Reduce the size of branch ducts so that they have an ap­propriately high and equal pressure loss at the design flowrate.

3. Size as in method 1, but then calculate or measure the ac­tual air quantities flowing and then determine the new and correctly fan pressure.

Of the above, 2 c) is preferred as flowrates are controlled with­out the need for dissipating energy across dampers or cones. The velocities in short legs may however exceed the notional design figure.

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