Chimneys release industrial by-product gases into the air, with pollution levels controlled by environmental policies. The natural draft process is used to direct fumes out of the chimney, and factors such as height, diameter, and gas composition are considered in design. Mathematical calculations and air dispersion modeling are used to ensure compliance with local standards. Acid rain can be prevented with equipment installed inside the chimney, and scale models are used to observe vapor droplet reactions.
A chimney is an industrial structure similar to a chimney. Gases that are by-products of industrial processes are released into the air at heights from which they can be dispersed above the ground. Pollution levels of carbon monoxide, nitrogen oxides and sulfur dioxide can be controlled according to environmental policies and government regulations. Some piles can be as high as 1,300 feet (396 meters).
The fumes are directed out of the chimney by the air pressure. The air rising through the chimney is hotter and less dense than the air outside the structure, so its pressure is lower. The air pressure, being higher outside, pushes the air and gases up and out the top of the chimney, in a process called natural draft or the chimney effect. This process can be combined with other industrial processes such as flue gas desulfurization, in which sulfur dioxide is altered by absorption, neutralization or oxidation and converted into a solid compound. Solid waste is removed from the equipment by the gas stream before going up the flue gas stack.
Factors that go into the design of a flue include its height and diameter, the amount of combustion air required, and the temperature of the gas after it is out of the combustion zone. The composition and density of the gas and the frictional resistance of the gas with the flue materials are also important design considerations. It is also important to consider how much heat is lost from the gas inside the pipe and the elevation above sea level.
Before you can build a chimney, all of these factors need to be factored into mathematical calculations. Organizations such as the American Society of Mechanical Engineers (ASME) and national government affiliated agencies have established guidelines on how to calculate flue design. Computers are also being used to model what happens to the air once it leaves the chimney, using a technique known as air dispersion modeling. Only when the engineers know they have created a design that complies with local standards can they proceed with the construction phase.
Acid rain sometimes falls around an exhaust stack and in the local area surrounding it. The equipment can be installed inside the chimney to remove particulates and acids. Another important step in designing the finished product is building a scale model and observing how the vapor droplets react in the structure and as they enter the surrounding air.
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