Www.thecementgrindingoffice.com

7/21/2019 Www.thecementgrindingoffice.com

1/17

2 Static separators:

2.1 Cyclones

2.1.1 Introduction:- In cement manufacturing industries, large-sized cyclone separators are used as main process equipments in

significant numbers for handling high volumetric flow rates of dust-laden gases.

- The cyclone is a simple mechanical device commonly used in the grinding circuits to remove relatively large particles

from gas streams.

- Cyclones are often used as precleaners to remove more than 80% of the particles greater than 20m in diameter.

Smaller particles that escape the cyclones can then be collected by more efficient control equipment like bag filters

and electroprecipitators.

- Cyclones are relatively inexpensive since they have no moving parts and they are easy to operate.

- The most common type of cyclone is known as reverse flow cyclone separator

2.1.2 Advantages of cyclones:

- Low capital cost

- Ability to operate at high temperatures and pressures

- Low maintenance requirements because no moving parts

- Constant pressure drop

- Can separate both solid and liquid particles, sometimes both simultaneously

2.1.3 Disadvantages of cyclones:

- Low efficiency especially for very small particles

- High operating costs in case of high pressure drop

- Subject to erosion or clogging if abrasive solids are processed.

2.1.4 Principle of operation:

- The spiral pattern of gas flow is developed by the manner in

which the gas is introduced. It enters along the side of the


2/17

cyclone body wall and turns a number of times to spiral down

(external vortex) to the bottom. Particles in the gas are

subjected to centrifugal forces which move them radially

outwards, against the inward flow of gas and towards the

inside surface of the cyclone. When the gas reaches the

bottom of the cyclone, it reverses direction and flows up the

center of the tube, also in a spiral fashion. This spiral fashion is

also called inner vortex and fine particles are carried with the

air and leave the cyclone through the immersion tube.

Solids at the wall are pushed downwards by the outer vortex

and out of the solids exit.

- Gravity has been shown to have little effect on the

cyclone's operation.

See the figures on the right side and below.


3/17

2.1.5 Forces affecting the particles

- We consider a reverse flow cyclone with a cylindrical section of radius R. Particles entering the cyclone with the gas stream

are forced into a circular motion.

- The forces acting on a particle following a circular path are drag, buoyancy and centrifugal force (Fd, Fband Fc).

- The balance between these forces determines the equilibrium orbit adopted by the particle.

- The drag force is caused by the inward flow of gas part the particle and acts radially inwards.

Considering a particle of diameter x and density pfollowing an orbit of radius r in a gas of density fand viscosity ,We have the tangential velocity of the particle be Uand the radial inward velocity of the gas be Ur. If we assume that

the Stokes law applies under these conditions then the drag force is given by:

- The centrifugal and buoyancy forces acting on the particle moving with a tangential velocity component Uat radius r are,

We can neglect the buoyancy force.

- And at a steady state, we have:


4/17

2.1.6 Flow Characteristics

- The rotational flow in the forced vortex within the cyclone body gives rise to a radial pressure gradient.

- This pressure gradient, combined with the frictional pressure losses at the gas inlet and outlet and losses due to changes in flow

direction, make up the total pressure drop.

- The pressure drop, measured between the inlet and gas outlet, is usually proportional to the square of gas flow rate through the

cyclone.

- A resistance coefficient, the Euler number Eu, relates the cyclone pressure drop pto a characteristic velocity:

Where fis the gas density

- The velocity vis based on the cross-section of the cylindrical body of the cyclone:

Where Qis the gas flow rate and Dis the cyclone inside diameter


5/17

- The Euler number represents the ratio of pressure forces to the inertial forces acting on a fluid element.

- Value is practically constant for a given cyclone geometry, independent of the cyclone body diameter.

2.1.7 Mechanical parts:

- Tangential inlet volute

- Cylindrical section

- Immersion tube

- Conical section

- Discharge (rotary valve, pendulum flap)

2.1.8 Cyclones families:

- Conventional

- High efficiency

- High capacity

See the figure on the right:

2.1.9 Design of the cyclones:

- Dimensions:

a = Height of tangential inlet

b = Width of tangential inlet

De = Diameter of air outlet tube

S = Immersion length of outlet tube

D = Cyclone diameter

h = Length of cylindrical section

z = Length of conical sectionH = Cyclone length

B = Diameter of material outlet

- On the sheet below, we can have a good idea of the standard cyclone dimensions for each family:


6/17

- Regardless of the configuration selected, we must follow the following recommendations:

a S to avoid the by-pass of the particules from the input section directly to the tube exit

b (D-De)/2 to avoid an excessive pressure drop

H 3D to keep the tip of the vortex formed by the gases inside the conical section of the cyclone

The inclination angle of the cone of the cyclone should be 7-8 to ensure a quick slide of the powder

De/D 0,4-0,5, H/De 8-10 and s/De 1 to ensure the operation with the maximum efficiency

2.1.10 Cyclones scale-up:

- The scale-up of cyclones is based on a dimensionless parameter, the Stokes number,which characterizes the separation

performance of a family of geometricallysimilar cyclones.

The Stokes number (Stk50) is defined as:


7/17

- It is interesting to find that, for well-designed and well-known cyclones, there is a direct correlation between Euand Stk50:

- For Stairmand high-efficiency cyclones: Stk50= 1,4/10000 and Eu= 320

- For Stairmand high-capacity cyclones: Stk50= 6/1000 and Eu= 46

2.1.11 Cyclone's efficiency:

- A model widely accepted is use for determining the efficiency of a cyclone.

- In this model, Ne is the number of revolutions the gas falling in the outer vortex.

The equation is:

See the "Design of cyclones" section to know the parameters

- With the model of Lapple (1951) which is an empirical relationship in order to calculate the cut size (50% of efficiency), we have:


8/17

Where:

Viis gas inlet velocity in m/h(range in m/sec: 15-30 m/sec) and

is the air viscosity in kg/m.h

b is the width of the tangential inlet in m

p is the solid density in kg/m3

f is the air density in kg/m3

- The efficiency (i) of any size of particle is given by the following formula:

Where Diis the particle of reference of a range

- The overall efficiency of the cyclone is a weighted average of the collection efficiencies for the various size ranges and is

given by:

Where mi is the mass of particles in a certain range and


9/17

Mis the total mass of particles

This efficiency can be undervalued with the concentration of solid particles in the air flow rate.

Then, when the concentration is higher than 2 gr/m3, a correction is applied:

where 1is the efficiency found,

C1is 2 (gr/m3),

2is the new efficiency and

C2is the concentration in dust

2.1.12 Cyclone's pressure drop:- In the evaluation of a cyclone design, pressure drop is a primary consideration. Because it is directly proportional to the

energy requirement, under any circumstance, knowledge of pressure drop through a cyclone is essential in designing

a fan system.

- Many models have been developed to determine the cyclone pressure drop but one of the well accepted is the model

of Shepherd and Lapple (1939). The formula of pis:

K is a constant:

K = 16 for tangential inlet without neutral inlet vane


10/17

K = 7,5 if tangential inlet with neutral inlet vane and large cyclones

It is better to keep a pressure drop lower than 2,5 kPa.

2.1.13 Design modifications and consequences:

2.1.14 General methodology for the design of cyclones

- 1. Select a configuration (conventional, high efficiency or high capacity)

- 2. Select a speed at inlet (15-30 m/sec)

- 3. In function of the flow rate importance, it is useful to have a 1st estimation of the cyclones number

- 4. Calculate the diameter of the cylindrical section of the cyclone D

- 5. Calculate the other dimensions of the cyclone on the basis of the table for the selected configuration

- 6. Calculate the pressure drop

- 7. To analyze if D and pare excessively large. Analyze the possibility of using various cyclones in parallel.

For nccyclones in parallel repeat items 2 and 3 using the value of Q/ncin place- 8. Calculate efficiencies for fractions and the total

- 9. Compare the calculated efficiency with desired. If you do not achieve the desired value, use a larger value of speed inlet

- 10. Estimate the cost of the cyclone


11/17

2.1.15 Example of calculation:

- We want to install cyclone(s) with following data:

- ventilation flow rate: 160 000 m3/h

- temperature: 90 C

- altitude: 1 000 m

- dust concentration: 80 gr/m3

- dust density: 1 600 kg/m3

- desired pressure loss: 150 mmWG

- Space restriction: maximum height of cyclone: 9 m

- Typical feed of the cyclone:

Dimension % residue cumulated size intervals mass in gr

% residue partial

1 99 01 2,2 1

2 96 12 6,6 3

5 91 25 11 5

10 85 510 13,2 6

16 71 1016 30,8 14

32 59 1632 26,4 12

45 35 3245 52,8 24

63 20 4563 33 15

90 8 6390 26,4 12

140 1 90140 17,6 8

> 140 0 0

220 100

- Calculate:

- number of cyclones and cyclones dimensions


12/17

- pressure drop

- efficiency

- check if all is OK

- Calculation of air density:

- pressure (ph) at 1000m over the sea level with the formula:

- air density (f) with the formula:

where t is the gas temperature in C

Following the methodology:

1 Configuration:

We choose: High Capacity Stairmand


13/17

2 Speed inlet:

We choose: 20 m/sec

3 Cyclones number first estimation:

At the moment, we choose: 1 cyclone

Then, the flow rate for calculation is: 160 000 m3/h or 44,444 m3/sec

4 Cyclone diameter calculation:

With the formula:

5 Other dimensions of the cyclone:

6 Pressure drop calculation:

With the formula:


14/17

Note: we selected K = 16in this case

7 Check if diameter and pressure drop are OK:

Pressure drop is Ok because 1380,1 Pa = 140,7 mmWG < 150 mmWG

There is a problem with the height of the cyclone: 11,280 m > 9 m

Then we calculate again for 2 cyclones, Q becomes 160 000 m3/h divided by 2 = 80 000 m3/h =22,222 m3/sec

Using the same formula, we will have:

Note, the pressure drop is the same because dimension parameters are of the same configuration!

Now, it is OK. The cyclone have a height of 8 m < max. 9m

8 Efficiencies calculation:


15/17

and:

where = 0,0765 kg/m.h

Then, we use the equation:

where Diis the average size of each interval

size intervals Average size

01 0,5

12 1,5

25 3,5

510 7,5

1016 13

1632 24

3245 38,5


16/17

4563 54

6390 76,5

90140 115

For example, we have for 0,5 :

Results for all sizes:

size intervals Average size

01 0,5 0,00128302177287949

12 1,5 0,0114298777003086

25 3,5 0,0592209424589986

510 7,5 0,22423535743116

1016 13 0,464793283874561

1632 24 0,747467079330783

3245 38,5 0,883947753305672

4563 54 0,937438968817134

6390 76,5 0,967817554669971

90140 115 0,985498639024517

The next step is to multiply i by the partial residue of the interval (or mass),

and to make the sum for all sizes in order to have the total efficiency:


17/17

size intervals Average size % residue partial

i Xmi

01 0,5 0,00128302177287949 1 0,00128302177287949

12 1,5 0,0114298777003086 3 0,0342896331009258

25 3,5 0,0592209424589986 5 0,296104712294993

510 7,5 0,22423535743116 6 1,34541214458696

1016 13 0,464793283874561 14 6,50710597424385

1632 24 0,747467079330783 12 8,9696049519694

3245 38,5 0,883947753305672 24 21,2147460793361

4563 54 0,937438968817134 15 14,061584532257

6390 76,5 0,967817554669971 12 11,6138106560397

90140 115 0,985498639024517 8 7,88398911219613

71,93 %

As we have a concentration of 80 gr/m3 in the inlet of the cyclone, we modify the efficiency with:

Conclusion, the efficiency is 85,66%

9 Checking of the efficiency:

If it is not enough, we have to recalculate with a higher inlet velocity

Documents

Www.thecementgrindingoffice.com