Resources - Cyclones

A bit about how this neat piece of kit works.

Chapter 1

A cyclone is really a sort of ‘spin–drier’ for fluids that have small pieces of solids or liquids in them. The fluid input stream can be a gas, a vapour or even a liquid with solids or liquids dispersed within it — the only criterion being that the suspended particles have a higher density than the fluid they are suspended in.

You will most probably be familiar with them in the home in vacuum cleaners but they were invented many decades before that and have been used in saw mills and the chemical industry.

The biggest cyclones I have seen have been on the vapour outlet from a number of boilers and were around 2m in diameter as I remember them. The boilers were for a plant that had to distil over a hundred tonnes of ethanol a day so that should give you some idea of the scale that these things can work at. The particulate stream in that case was droplets of the liquid that was being distilled and that was fed back into the boilers.

The cyclone works by having a dirty stream that goes into a conical cavity at a tangent to the surface so that the fluid flows around the circumference. By doing that, any suspended matter that is big enough and heavy enough will tend to travel in a straight line rather than around in the circle that the fluid is taking, thus coming into contact with the side of the cyclone.

This carries on for a while as the fluid works its way down the cylindrical portion and then it starts to get interesting.

Whilst the speed of the fluid remains constant (simplistically speaking, anyway), the circumference of the cyclone’s body, as the fluid works its way downwards, gets smaller and therefore the rate of change of direction increases (conservation of momentum) and the tendency for particulates to find themselves thrown out of the stream increases.

Now that it is as low as it is going to get, the fluid stream now starts a journey upwards, at this new, high rate of rotation, with any particulates being thrown out of the stream but now into the descending stream.

The long distance travelled by the ascending stream give a lot of opportunity for this to happen on the way back up again and as a result, particulates are well separated out from the exiting stream.

Designs can vary and can have the conical section missing completely — being replaced by just a flat piece of metal. However, you get less pressure–loss if you accelerate gasses slowly so some sort of short cone will work better than no cone at all — reducing the pressure–loss means that it runs more efficiently.

Another design modification is to point the fluid inlet downwards and have a helix shaped into the top so that by the time the fluid has made one complete circuit of the cylindrical part, it is one rotation–height down, thus avoiding coming into contact with the inlet stream again.

Cyclones are used to remove: solid particles from gas streams in sawmills, vacuum cleaners and vents; spray droplets from the vapour output of boilers in the chemical industry; solids from liquid feeds in water treatment and so on.

	A company’s most valuable employees are the ones that don’t work for it any more.

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