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In power plants boiler feed water controls are responsible for regulating the level of water in the boiler steam drum, by ensuring that the supply of feed water to the boiler keeps in step with the changes in boiler steam production .
Boiler is a closed vessel that is subjected to be heated by some other source in which water or other fluid is heated. Boiler that generates steam by oxidizing a liquid was built late 1960’s which is one of the largest low pressure steam boiler . Similar to the combustion the liquid is oxidized which a large volume of fuel which is oil through which the air was force to oxidize. Twenty four feet in diameter and ninety feet tall with thousands of square feet of heating surface it made about 25,000 pounds per hour.
In steelmaking operation the boilers used were operated at 100 million BTU/h(British Thermal Unit per hour) that uses 500°F air. It is wise that operating this type of equipment for getting out most steam saves fuel that would have to be used in generating steam.
Normally the equipment which is an open container or small one heating water is not a boiler. The hot water heating equipment used at homes is not considered as boiler unless it holds more than 120 gallons. An internal diameter of 6 inches or less is another limit on size of a boiler. The boiler design have certain codes and the exceptions found in the code are occasionally stretched to create boilers that, by definition, are not. Fired air heaters are also not considered as boilers unless the air is 5 under pressure. Any application that heats air or any other gas is generally called as furnace which doesn’t contain the heated fluid in an enclosed vessel. There are many boilers unique to their respective industry. Asphalt heaters, flux heaters (a raw material that becomes asphalt), many forms of waste heat boilers and equipment like recovery boilers (used in the paper industry) which convert product can be encountered by burning it. The principles discussed shows the understanding of unique boilers which by virtue of their uniqueness and are understood clearly by reading their operating and maintenance instruction manuals. This section contains general descriptions of the basic elements of a boiler plant to provide a basic understanding of the systems and equipment.
In boiler the fluid which is heated or vaporized exits the boiler for the use in various processes heating applications and processess. It is generally a device that is used to create steam from water by applying heat energy. Although the definitions are flexible, usually older steam generators were normally termed as boilers that works at low to medium pressure of 1-300psi but when the pressure above that is said as steam generators.
Wherever a source steam is required a steam generator or boiler is used there. Its size and form depends on the application. Typical small boilers are used in mobile engines such as portable engines, steam powered road vehicles, steam locomotives that forms an integral part of the vehicle. Boiler type of large separate steam generators are used in industrial installations, stationary steam engines and power stations connected to the point-of-use by piping..
Types of Boiler
There are various types of boilers used in various industries few of those types are listed below
Pot boiler is also called as “Haycock boiler”: a primitive “kettle” where a fire heats a partially filled water container from below. This type was in use during 18th Century. This type of boiler generally produces and stores large volumes of very low-pressure steam, often hardly above that of the atmosphere. These could use wood or most often, coal as fuel. Efficiency was very low.
Normally in all steam locomotives this type of boiler is used where the boiler barrel is filled partially with a small volume(steam space) left above to accommodate steam. In order to maintain the heating surface temperature slightly below the boiling point, the water has kept permanently surrounding the furnace or firebox. The length of hot gases is kept extended long way as the furnace is located a the end of fire-tube, thus the surface is augmented by heating the surface which can be increased by reversing the direction of the gases through another parallel tube or bundle of multiple tubes. Alternatively gases were taken along the sides and then under the boiler through flues(3-pass boiler). Mostly solid fuels are used as fuel in this type of boiler and is comparatively has a low rate of steam production.
In water tube boilers, possibly in number of configurations water tubes are arranged inside the furnace. Normally there is lower and upper water tubes connecting large drums with lower ones containing water and upper ones with steam and water. In other types like monotube boiler through succession of coils water is circulated by a pump. Ths team production rate is high in this type of boilers but the storage capacity is less than the above. Wter tube boilers are generally preferred in high pressure applications since the pressure in water/steam is contained within small diameter pipes that can withstand the pressure with a thinner wall.
Flash tube boilers have strong tubes that are arranged closely through which the water is pumped. As the tubes are kept very hot and so feed water is quickly flashed into steam and superheated. The advantages of this type are less weight and take less time to raise steam from cold water. It is easier to overheat than an ordinary boiler as there is no ;arge reservoir presents to maintain the tubes from high temperature even if water flow is interrupted or inadequate.
Figure Flash boiler
Multi tube boiler
In 1828 Marc Seguin in france designed a two pass boiler formed by multiple tubes. Scotch” marine boilers are of similar to that design used for marine purposes with natural induction. In 1829 a muti-tube one pass horizontal boiler made up of two units was suggested by henry booth, the treasurer of Liverpool and Manchester railway. It has multi barrel consisting of two telescopic rings mounted with 25 copper tubes inside that has a firebox surrounded by water spaces. Much of water space occupies the tube bundle and with vastly improved heat transfer. This type of fire tube boilers were later used by many constructors and has been built ever since.
Superheated steam boilers
In the superheated steam boilers the water is boiled to get steam and the steam is heated further in superheater, where as in other boilers the water is boiled and then the steam is used at saturation temperature(i.e.,saturated steam). This provides high temperature steam. As the higher steam temperature needs a higher flue gas exhaust temperature, thermal efficiency of steam generating plant is decreased due to this fact. To overcome this problem there are several ways like heating economizer or as combustion air heater in hot flue gas exhaust path. There are some practical limitations of using wet steam causes the turbine blades to be damaged because of the condensation droplets which can be overcome by using superheated steam that provides unique safety concern as the escaping steam is completely a superheated vapour.
The super heater works as a coil on an air conditioning unit and the steam piping (with steam flowing through it) is directed through the flue gas path in the boiler furnace. This area is typically maintained between 1,300 °C (2,372 °F)-1,600 °C (2,912 °F). Some super heaters are convention type (absorb heat by radiation) and some were radiation type (absorb heat via a fluid i.e. gas) and some are a combination of the two. In both the types the extreme heat in boiler furnace/flue gas path will also heats the steam in the super heater steam. It is important to that pressure in the superheater is not raised while the temperature of the steam in the superheater is raised.The pressure remains constant as that of boiler as the turbine or moving pistons offer a “continuously expanding space”. The water droplets are entrained in the steam and that is the significance of the process of superheating steam is tto prevent damage to the turbine blading and/or associated piping.
2.3.8 Hydronic boilers
This type of boilers are used for residential and industrial purpose of generating heat. These boilers are typically used in central heating systems (combined with domestic water heating) fitted to houses in European countries. This type boilers operate in the way of heating water and through the radiators throught out the houses or baseboard heaters circulates the fluid which can be of any means such as gas, oil, wood etc..Nastural gas is the most usual and economical choice in built-up areas. Most of these type of systems are fitted with the condensing boilers for greater efficiency.
Hydronic boilers are said to be condensing boilers as they condenses water vapour in flue gas and captures the latent heat of vapourization of water produced during combustion.
The main reason for the use of hydronic systems in recent days are,
Although the installation is expensive due to the cost of aluminium and copper, it is has advantages of more efficient and economical than air-forced systems.
Occupies less space and includes less use of metal.
Copper gets heated through conduction and radiation, whereas the thermal conductivity of air is low and also provides less fluctuating temperatures as the copper baseboard pipes hold heat longer time before releasing than air.
Any kind of combustion by-products or dust, mold etc,. were not introduced into living space and also interior air is not dried out sooner.
Fuels such as wood, coal ,oil or natural gas were used for boiler as a source od combustion. Resistance immersion type of heating elements were used for electric steam boilers and for heat recovery steam generators source is mainly heat rejected from other processes.
Historically, Serious injuries and property destruction occurs in boilers were due to the poor understanding of engineering principles. Poor constructions such as rupturing of metal sheets, poorly welded or riverted seams easily makes caution that leads to violent eruption of pressurized steam.
A boiler which is allowed to boil dry due to loss of feed water is more dangerous. Even if water feeded in empty boiler damages the superheated metal sheets which leads to violent explosion which can’t be controlled. Leaking of steam supply lines causes the boiler to be drained. These type of causes and damages can be prevented if the level of the boiler water is controlled properly at a constant rate along with the feedwater and steam flow.
In boiler pressure or temperature is controlled by maintaining the flow of fuel and air. Level is maintained only if feed water is controlled. It is important for an operator to be trained about the flow control, which increase or decreases the values depending on the direction of flow. Flow control by means of changing the flow rate is called “modulating” control, whereas other one is ON/OFF control which involves with the feed tank valve open when the level reached bottom of the tank and shuts when it reaches at the top.
In a boiler plant when the steam that leaves out of boiler contains more energy than the feed water flow rate then the steam pressure falls. Then some of the energy in the fuel goes up the stack as flue gas. So the sum of flow inside boiler should match up the flow out energy. Most of the fuel furnace boiler temperature ranges from 1200 to 3200°F
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