Learning About Cyclones:: Section 1Cyclones are simple mechanical devices for removing relatively large particles ( > 5 to 10 m) from gas streams. Actually, they are more commonly applied than you may realize.They are used by Environmental Engineers to remove particulate pollutants. Chemical Engineers and Mechanical Engineers to collect valuable particle products. Industrial Hygienists to sample particles in working environments. Pharmaceutists to classify their powdered drugs. You can easily spot them in the wood product industry, mills, cement plants, and other industrial facilities. As their operation is one of the least expensive for removing/collecting relatively large particles, they are also often used as a pretreatment device to reduce the load of fine particle collection devices.:: Section 2 Above are photos of cyclones taken at the Constance Theatre at the University of Florida. From the left image you can see where the dirty air comes in, the dust is discharged and the clean air is expelled.

Above is a photo of a cyclone for stacking sampling. Compared to the cyclone at the Constance Theatre, this cyclone is pretty small. The cyclone used by industrial hygienists is similar in shape, but even more compact so that it can be clipped on a worker's shirt for sampling aerosols in the work place.:: Section 3Principle Mechanism: Impaction A cyclone uses centrifugal force to separate particles from a gas stream. The operation is similar to that of an impactor. Figure 1 shows the schematic of an impaction plate. The gas stream passes through a narrow nozzle at a high speed. When an obstacle (i.e. impaction plate in this case) is in front of the stream, the stream can easily make a turn to avoid the obstacle. Particles in the stream however, may have a different fate. Larger particles, due to their higher inertia, won't be able to follow the gas stream well, and therefore they will deviate from the gas stream, and they will impact on the plate.

Figure 1: The schematics of an impaction nozzle and an impaction plate.Impaction, as can be seen, is the mechanism for the collection in the system, and it is more effective for larger particles. The most important parameter that affects impaction is called Stokes number (Stk). Stk includes both flow velocity and particle diameter and is defined as:wherep = particle densitydp = particle diameterU = flow velocity = air viscosityDj = nozzle diameter

In a T-intersection, the traffic accident rate is higher if a turning vehicle cannot make a successful one. Using the analogy of impaction, evaluate how the following parameters affect the traffic accident rate: (a) vehicle size; (b) vehicle speed; (c) lane width.

:: Section 4Particle Movement in a CycloneThe same principle is used in a cyclone except that the flow follows a circular path as shown below. This is a top view of particle movement in a cyclone