Engineering Design Approach To Chimmeys

Chimneys are used for throwing burnt or unwanted gases from the inside of the buildings or factories into the atmosphere. Various kinds of chimneys are used today which can be very high and narrow, and a detailed engineering approach must be applied. Otherwise, it will be very disastrous if that chimney collapse due to its weight or the wind.

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Parameters In Chimney Design

First of all, the height and the length of the chimney must be defined. These dimensional parameters change according to constructional requirements and out of our topic. Once these parameters are delivered to us, we can make calculations of that chimney according to our parameters.

As mechanical engineers, our parameters are the self-weight of the chimney and the maximum wind pressure that would occur in the area where the chimney is built.

Calculation Of The Wind Effect On Chimney

Strength of Materials, Third Edition

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Because of the wind pressure, there will be a bending moment and because of this bending moment, bending stress will occur at the bottom of the chimney structure.

First, we need to calculate the wind pressure before finding out the bending moment occurrence. Take a look at the formulation that is used to calculate wind pressure:

In this formulation, ‘K’ is a constant, ‘y’ is the height component of the chimney, ‘n’ is the positive exponent, ‘A’ is the projected area that wind force in effect on the chimney, ‘C’ is the shape factor that changes according to the shape of the chimney which is 0.65 for circular cross-section, 1.0 for square or rectangular cross-section.

Once the wind pressure on the chimney is calculated, bending moment and bending stress can be easily calculated;

If all the parameters are substituted, the bending stress is;

In here, ‘D’ is the outside diameter of the chimney and the ‘d’ is the inside diameter of the chimney. ‘h’ is the height of the chimney.

Calculation Of The Self-Weight Effect On Chimney

The calculation of the weight of the chimney is very simple. You need to calculate the total volume of the geometrical shape of the chimney and multiply it with the material density of the chimney.

In this formulation, ‘ρ’ is the density of the material, ‘g’ is the acceleration of gravity which is generally considered as 9.81 m/s^2.

Effect Of The Cross-Section Of The Chimney

You may deal with the other types of cross-sections of chimneys. If the cross-section shape changes, it will affect the area moment of inertia which is used to find the bending stress. And it will affect the shape factor ‘C’.

So, you can change the calculations according to different shapes of cross-sections.

Conclusion

Calculations of combined stresses on chimney design are very simple like above.