In this article we will discuss about the objects and design of stack.

**Objects of Stack****: **

For keeping the maximum concentration of pollutant within allowable limit, the stacks are constructed for venting the industrial gases into atmosphere. The industrial gases which are discharged through stacks are diluted and dispersed into atmosphere according to the prevailing atmospheric conditions. Therefore stack serve as pollution controller for odorous gases or low concentrated pollutants.

**Following are the main objects of stacks: **

(i) To provide natural draft for combustion,

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(ii) To avoid re-entry of smoke into building, and

(iii) To disperse the pollutant into atmosphere.

**Design of Stack****: **

**Following data should be collected before design of stack: **

1. Gas temperature and its pressure range,

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2. Rate of emission,

3. Type of emission such as dust, fume, mist, gas or spray,

4. Concentration of pollutant,

5. Discharge velocity of stack,

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6. Composition of gas,

7. Particle size distribution,

8. Vertical temperature profile,

9. Mean mixing depth (MMD),

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10. Stability class,

11. Direction and velocity of wind,

12. Relationship of stack to adjoining structures and terraces, and

13. Draft required at the flue opening to the stack.

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The diameter of the stack can be determined on the basis of exhaust gas flow rate and on the exit velocity required. The wind flow over stacks create the negative pressure zone, how velocities cause down-wash. This reduces the effective stack height and not only increase the concentration but also cause the gas to re-enter the building cavity.

As a thumb rule, the stack velocity should be more than 1.5 times the maximum wind velocity. This increase of wind velocity also increases the plume rise and the effective stack height. But it increase the pressure drop, therefore the required horse power of blowers is increased.

**Procedure for Stack Design:**

The height of the stack should be such that it keeps the ground level concentration within permissible limits.

**Following is the procedure of design of stack: **

(1) Collect the basis design information’s as given above.

(2) Based on the stability class, determine the ground level concentration at various distances for various stack heights. The other approach, in the diffusion equation, substitute the allowable concentration of pollutants, and calculate the effective stack height for a given stability class. Determine the plume rise due to momentum rise and buoyancy rise and deduct from the effective stack height. This will give the height of stack above the ground level.

**Brink and Crocker have suggested the following: **

(i) The height of the stack should be 2.5 times the height of the surrounding building,

(ii) The velocity of the exit gases should not be more than 60 fps., and

(iii) The concentration at ground level varies inversely with the square of the stack height, and is maximum at a distance of 5-10 stack height.