A boiler is a closed vessel   in which water or other liquid is heated in order to generate steam or vapor which is then used for other external processes. Water is a useful and cheap medium for transferring heat to a process. When water is boiled into steam, its volume increases about 1600 times, generating a force that is very explosive. This can be achieved by combustion of wood, natural gas, coal or oil.  Electric steam boilers on the other hand use resistance to produce the required heat. The chemical energy from any of these external fuel sources is converted into heat which is then transferred to the water through radiation which is the transfer of heat from a hot body to a cold body without a conveying medium, conduction which involves the transfer of heat by actual physical contact and by convection, the transfer of heat by a conveying medium like air or water (EuropeanCommission, 2006).

Since the development of the first boilers in the 18th century, boilers have evolved so as to increase their efficiency and low-cost design as well as pay more attention to air pollutant emissions like carbon monoxide and hydrogen chloride. These types of emissions depend on the type of fuel used and the load factor of the boiler.  The power of a boiler is determined by the required steam mass flow rate, temperature and pressure. The amount of input fuel required depends on the fuel energy content and on the overall energy efficiency. A boiler’s performance is characterized by its steam pressure and temperature.  Saturated steam is steam at boiling temperature for a given pressure, which is what most boilers produce and make use of.  If more heat is supplied and the steam pressure rises above the saturation temperature at a given pressure, then the steam becomes superheated steam. This kind of steam though at a higher temperature, can decrease the efficiency of the steam generating plant.  If more heat is supplied to the superheated steam, it becomes supercritical steam which can be used in power generation (USEPA, 2004). 

A closed boiler is one in which  all the steam generated is returned to the vessel  in form of water and is reused while an open boiler is one that does not return water to the original vessel.

The boiler system comprises of three major parts which are: the feed water system, the steam system and the fuel system. The feed system supplies water to the boiler and regulates it to meet the systems demands. The steam system is responsible for collection and control of the steam produced in the boiler. This system is regulated and checked using pressure gauges and is directed to the point of use through an efficient piping system. The fuel system includes all the equipment used to generate the required heat which is dependent on the type of fuel used in the system (Hartford, 1911).

There are three basic types of boilers which are used for industrial uses. These are the fire tube, the water tube and the fire box boilers.  In the fire tube boilers, heat passes through the tubes which are surrounded by the water being heated. These tubes are arranged in banks so that the heat produced can pass through the vessel many times before escaping. Fire tube boilers are relatively small in size compared to the other type of boilers.

In water tube boilers, heat is made to pass through the tubes which contain the water.  These tubes are then interconnected to a steam outlet for distribution to the plant system. These types of boilers are the most commonly used because they are larger in size and can therefore withstand greater pressures and temperatures, though their initial and maintenance costs are higher.

 In a fire box boiler, the hot gases from the fire box which is the space where the fuel is burned are channeled into the tubes where they heat the water.

Water is supplied to the boiler from the boiler feed water plant also known as the demineralizer plant.  The demineralizer removes all salts present in the water by removal of hydrogen ions which are replaced with sulphuric acid. This water should be free of any foreign materials that could cause harm to the boiler and also decrease its performance. Some of these harmful substances include oxygen, positively charged ions of calcium, aluminum, sodium and zinc. There are also other negatively charged ions like carbonates, bicarbonates, silica and fluorides which could harm the boiler efficiency.  The removal of oxygen is usually done in the de-aerator located after the ion exchanger.

The de-aeration of the condensate returning from the process ensures that the water is free of oxygen bubbles that may inhibit heat transfer. In   de-aeration, the dissolved gases are removed by preheating the feed water before it is allowed to enter the boiler.  The removal of these gases is very important to the boiler equipment longevity as well as safety of operation. De-aeration can be done by chemical de-aeration, mechanical de-aeration or both. The chemical treatment is used to remove harmful substances that could cause build up in the heat transfer equipment. The economizer is used to preheat the water entering the boiler. This helps reduce fuel cost making the boiler more efficient (Shields, 1961).

The water vessel in a boiler is connected to the heat source by metal rods which heat the water and convert it to steam. The steam is allowed to collect in the dome before exiting the boiler. The function of the dome is to force the steam to become highly condensed in order for it to exit the boiler with a large amount of pressure. A boiler also contains a drain which removes impurities from the water vessel and a chimney to allow heat to escape once it has passed the water vessel.  It is vital for all boilers to have safety valves in order to allow excess steam to be released in order to prevent explosions.

The heart of a boiler is a pressure vessel which is a closed container designed to hold gases or liquids at a pressure. This pressure vessel is usually made of steel or wrought iron. This pressure is obtained from an indirect source or from the application of heat from a direct or indirect source.  If not properly maintained, boilers can be a source of serious injuries and can lead to huge losses in form of property destruction. Thin and brittle metals that make up some parts of the boiler could rapture or poorly welded seams could open up leading to violent eruptions of the pressurized steam. Collapsed boiler tubes could also spray the hot steam they contain into the air injuring the around (Reeves, 2001).

Even with the best pretreatment programs, boiler feed water often contain some degree of impurities which normally accumulate in the boiler. The increasing accumulation of dissolved solids may lead to carry over of boiler water into the steam which may cause damage to the piping system as well as the process equipment. These suspended solids could also lead to the formation of sludge which will lead to a reduction of the boiler efficiency as well as it s heat transfer capability.

In order to avoid these problems, water should often be discharged from the boiler in order to control the concentrations of the suspended and dissolved solids in the boiler.  Discharging of the surface water is usually done in order to get rid of the dissolved solids while the discharging of bottom water is done in order to remove the sludge from the bottom of the boiler.

Boiler blow down i.e. discharge of water from the boiler is a very important aspect of the boiler maintenance. Lack of proper blow down can lead to increased fuel consumption, extra chemical treatments for the boiler as well as increased heat loss. Also, since the blow down water has the same temperature as the boiler water, it can be reused in the boiler operations once removed. However, excessive blow down can lead to wastage of water, energy and treatment.

The two major types of boiler blow downs are intermittent and continuous blow down.  Intermittent blow down is done by manually fitting a valve at the bottom of the boiler which is removes the unwanted parameters.  It requires large short-term increases in the amount of feed water put into the boiler which leads to a substantial amount of heat energy being lost. Alternatively, continuous blow down involves the steady and constant dispatch of small stream of concentrated boiling water being replaced with steady and constant inflow of feed water.

The various energy efficiency opportunities in a boiler system can be related to combustion, heat transfer, water quality, avoidable losses and blow down. To maximize a boiler’s efficiency, the stack temperature should be designed to be as low as possible. Nevertheless, it should not be very low such that water vapor in the exhaust condenses on the stack walls. Automatic blow down controls that sense and respond to the boiler water conductivity and pH should be installed in order to reduce uncontrolled continuous blow down. In oil and coal fired boilers, soot should be removed as it acts as an insulator against heat transfer.

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