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A chimney is a system for venting hot gases and Smoke from a Boiler , Stove , Furnace or Fireplace to the outside Atmosphere . They are typically almost vertical to ensure the hot gases flow smoothly, drawing air into the combustion through Convection . The space inside a chimney is called a '' Flue ''. Chimneys may be found in buildings, steam locomotives and ships (for the latter, the US term is '''smokestack''' (colloquially, '''stack''') and the UK term is '''funnel'''). Chimneys are tall to disperse pollutants in the exhaust over a greater area reducing the concentration of toxins to a safe level and to increase the draw. The term chimney may also be applied to natural features, particularly in Rock Formation s. HISTORY Romans used tubes inside the walls to draw smoke out of bakeries but real chimneys appeared only in northern Europe in the 13th Century . Industrial chimneys became common in the late 18th Century . Chimneys have traditionally been built of brick, both in small and large buildings. Early chimneys were of a simple brick construction. Later chimneys were constructed by placing the bricks around tile liners. To control downdrafts venting caps (often called ''chimney pots'') with a variety of designs are sometimes placed on the top of chimneys. In the eighteenth and nineteenth centuries, the methods use to extract Lead from its ore produced large amounts of toxic fumes. In the north of England , long near-horizontal chimneys were built, often more than 3 km (2 miles) long, which typically terminated in a short vertical chimney in a remote location where the fumes would cause less harm. Lead and silver deposits formed on the inside of these long chimneys, and periodically workers would be sent along the chimneys to scrape off these valuable deposits. CONSTRUCTION Due to brick's limited ability to handle transverse loads, chimneys in houses were often built in a "stack", with a fireplace on each floor of the house sharing a single chimney, often with such a stack at the front and back of the house. Today's Central Heating systems have made chimney placement less critical, and the use of non-structural double-wall metal piping allows it to be bent around obstructions and through walls. In fact, modern high-efficiency furnaces do not require a chimney and can vent sideways through a wall. , 1514]] Industrial chimneys were typically external structures, as opposed to being built into the wall of a building. Most often they were located near a central boiler, and the gases carried to it with external ductwork. Today the use of reinforced Concrete has almost entirely replaced brick as a Structural component in the construction of chimneys. Refractory bricks are often used as a lining, particularly if the type of coal generates smoke containing acids. Modern chimneys sometimes consist of a concrete Windshield with a number of Flue s on the inside. The 300 metre chimney at Sasol Three consists of a 26 metre diameter windsheild with four 4.6 metre diameter concrete flues which are lined with refractory bricks built on rings of Corbel s spaced at 10 metre intervals. The reinforced concrete can be cast by conventional formwork or sliding formwork. The height is to ensure the pollutants are dispersed over a wider area to meet legislative or safety requirements. CHIMNEY DRAUGHT OR DRAFT When coal, oil, natural gas, wood or any other fuel is Combusted in a stove, oven, fireplace, hot water boiler or industrial furnace, the hot combustion product gases that are formed are called Flue Gas es. Those flue gases are generally exhausted to the ambient outside air through a chimneys or so-called stacks (sometimes referred to as smokestacks). The combustion flue gases inside the chimneys or stacks are much hotter than the ambient outside air and therefore less Dense than the ambient air. That causes the bottom of the vertical column of hot flue gas to have a lower Pressure than the pressure at the bottom of a corresponding column of outside air. That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney. That movement or flow of combustion air and flue gas is called "natural draught/draft", "natural Ventilation" , "chimney effect", or " Stack Effect ". The taller the stack, the more draught or draft is created. It should be noted that not all ovens, boilers or industrial furnaces rely upon natural draught or draft. Many ovens, boilers and furnaces use fans or blowers to accomplish the same objectives, namely: the flow of combustion air into the combustion chamber and the flow of the hot flue gas out of the chimney or stack. Designing chimneys and stacks to provide the correct amount of natural draught or draft involves a great many factors such as:
The calculation of many of the above design factors requires trial-and-error reiterative methods. Governmental agencies in most countries have specific codes which govern how such design calculations must be performed. Many non-governmental organizations also have codes governing the design of chimneys and stacks (notably, the ASME codes). As a "first guess" approximation, the following equation can be used to estimate the natural draught/draft flow rate by assuming that the Molecular Mass (i.e., molecular weight) of the flue gas and the external air are equal and that the frictional pressure and heat losses are negligible: : DRAWBACKS A characteristic problem of chimneys is they develop deposits of Creosote on the walls of the structure when used with wood as a Fuel . Some types of wood, such as pine, generate more creosote than others. Deposits of this substance can interfere with the airflow and more importantly, they are Flammable and can cause dangerous chimney fires if the deposits ignite in the chimney. Thus, it is recommended — and in some countries even mandatory — that chimneys be inspected annually and cleaned on a regular basis to prevent these problems. The workers who perform this task professionally are called Chimney Sweep s. , 360 M high, serving Trbovlje Coal Power Station ]] Masonry (brick) chimneys have also proved particularly susceptible to crumbling during Earthquake s. Government housing authorities in quake-prone cities like San Francisco and Los Angeles now recommend building new homes with stud-framed chimneys around a metal Flue . Bracing or strapping old masonry chimneys has not proved to be very effective in preventing damage or injury from earthquakes. Perhaps predictably, a new industry provides "faux-brick" facades to cover these modern chimney structures. Other problems include "spalling" brick, in which moisture seeps into the brick and then freezes, cracking and flaking the brick and loosening mortar seals. chimneys on the Casa Milà ( Barcelona , Spain ), by Antonio Gaudí .]] DUAL-USE CHIMNEYS Some very high chimneys are used for carrying antennas of Mobile Phone services and low power FM / TV -transmitters. Special attention must be paid to possible Corrosion problems if these antennas are near the exhaust of the chimney. In some cases the chimneys of power stations are used also as Pylon s. However this type of construction is not very common, because of corrosion problems of conductor cables. Cooling tower used as chimney At some power stations, which are equipped with plants for the removal of sulfur and nitrogen oxides, it is possible to use the Cooling Tower as a chimney. (At plants without flue gas purification, strong corrosion would arise in the cooling tower). This can be seen in Germany at the Power Station Staudinger Grosskrotzenburg and with the Power Station Rostock . TRIVIA Tall cylindrical chimneys often survived explosion disasters without damage, which can be seen in pictures of destroyed factories after World War II. This inspired engineers after World War II to build cylindrical TV towers. REMARKABLE CHIMNEYS SEE ALSO EXTERNAL LINKS
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