|
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 gases. Those gases are generally exhausted to the ambient outside air through chimneys or industrial flue gas 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.
Designing chimneys and stacks to provide the correct amount of natural draught or draft involves a number design factors, many of which require trial-and-error reiterative methods.
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:[4]
| where: |
|
| Q |
= chimney draught/draft flow rate, m³/s |
| A |
= cross-sectional area of chimney, m² (assuming it has a constant cross-section) |
| C |
= discharge coefficient (usually taken to be from 0.65 to 0.70) |
| g |
= gravitational acceleration, 9.807 m/s² |
| H |
= height of chimney, m |
| Ti |
= average temperature inside the chimney, K |
| Te |
= external air temperature, K |
[edit] 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 sweeps. In the middle ages in some parts of Europe, a crow-stepped gable design was developed, partially to provide access to chimneys without use of ladders.
Masonry (brick) chimneys have also proved particularly susceptible to crumbling during earthquakes. 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.
[edit] 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 pylons. However this type of construction is not very common, because of corrosion problems of conductor cables.
The Dum Detí a Mládeže v Modranech in Prague, Czech Republic is equipped with an observation deck.
[edit] Cooling tower used as an industrial chimney
At some power stations, which are equipped with plants for the removal of sulfur dioxide and nitrogen oxides, it is possible to use the cooling tower as a chimney. Such cooling towers can be seen in Germany at the Power Station Staudinger Grosskrotzenburg and at the Power Station Rostock. At power stations that are not equipped for removing sulfur dioxide, such usage of cooling towers could result in serious corrosion problems.
[edit] Notable chimneys
[edit] See also
[edit] References
- , C.F. Saunders (1923), The Southern Sierras of California
- , Jules Verne (1872), Around the World in Eighty Days
- , Diagram of 25 tallest flue gas stacks worldwide
- , Natural Ventilation Lecture
[edit] External links
Wikimedia Commons has media related to:
|
Supertall self-supporting chimneys (at least 300 meters in height) |
|
|
|
|
(Source: Wikipedia) |