Air Pollution Control – Part A Cyclone – Basic Principles

1. Air Pollution Control – Part ACyclone – Basic Principles Yaacov Mamane Visiting Scientist, CNR Rome, Italy

2. What is a Cyclone?

3. Cyclone Performance for various Application

4. Standard Cyclone Dimensions General guidelines: H < S W < (D-De)/2 Lb+Lc > 3D Cone angle = 7o ~ 8o De/D = 0.4~0.5, (Lb+Lc)/De = 8, S/De = 1

5. Conventional Cyclone

6. Stokes Law Gravitational forces are balanced by drag and buoyancy forces. This will lead to stokes law - settling velocity of particles.

7. When Fd = Fg the settling velocity is given: as follows. Where g – gravity acceleration d – particle diameter rp – particle density m - air viscosity For example Vt (1 mm) = 0.006 cm/s

8. For a particle moving at high speed Vc in a circle, the centrifugal acceleration is given by Vc*Vc/r. The centrifugal force is similar to the gravity forces Vc But Fc >> Fg The equivalent “settling velocity” of the centrifugal forces is taken from Stokes Law and is given by the following Equation:

9. Example For a particle of 1 mm moving in a 0.3 diameter circle at 18.3 m/s: vt = 0.68 cm/s While Vt stokes is only 0.006 cm/s!!!

10. Settling Chamber W v h Vt H Particle is entering a chamber at height h an horizontal speed V and settling velocity Vt, may fall inside the chamber. Time (Tl) to cross the chamber is L / V . Time (Th) to fall inside the chamber is h / Vt, thus we could define a collection efficiency h = Tl / Th = L Vt / H V L

11. The Collection Efficiency of a settling chamber used to collect large particles is given by the simple expression: • = L Vt / H V or • = L g d2 rp / H V 18m But V = Q / WH whereQ is the flow through the settling chamber, and thus h = L g d2rp / H V 18m= L W g d2rp / Q 18m = L W g d2rp / Q 18m This collection efficiency may be applied to a Cyclone where H and L of the chamber are equivalent to W and NpD0 of a cyclone

12. h = L Vt / H V for a cyclone may be written as: Since a cyclone is an elongated settling chamber 4W = Do = 2r For centrifugal forces

13. After rearranging the parameters the equation is now given by: And for a turbulent flow it is then expressed by the general term: Example: Calculate efficiency for a cyclone to collect 1 mm particles of density 1. Cyclone width – 15 cm, Vc – 18.3 m/s and N – 5. The efficiency is h = 0.023 And for a particle of 10 mm diameter h is larger than 1.

14. Let define a parameter of importance in particulate control, d cut , used to describe the properties of the cyclone, Where Dcut - cut diameter in mm m - is viscosity N - number of rotations Vc - tangential velocity rp - particle density W - entrance width of the cyclone

15. Pressure Drop Number of gas inlet velocity head Static pressure drop Power requirement K = 16 for normal tangential inlet = 7.5 for one with an inlet vane

16. Lapple Theory (laminar flow) Number of effective turns Gas residence time Terminal velocity Smallest collected diameter

17. 50% cut size The collection efficiency of any size dpj  (%) Overall efficiency Penetration Particle size ratio dp/dpc

18. Licht Theory (turbulent flow) (D in m)

19. Arrangement Parallel/ Battery Series Air Pollution Control Equipment, Theodore & Buonicore, CRC Press, 1988

21. Discharge Handbook of Air Pollution Control Engineering & Technology, Mycock, McKenna and Theodore, Lewis Publishers, 1995.

22. Advantages & Disadvantages Advantages: • Low capital cost • Ability to operate at high temperatures • Low maintenance requirements because there are no moving parts Disadvantages: • Low efficiencies for fine particles • High operating costs (due to pressure drop) Cyclones used for removing wood dust