- A steam generator generates steam at the desired rate at the desired pressure and temperature by burning fuel in its furnace.
- Steam generators are used in both fossil-fuel and nuclear-fuel electric generating power stations.
- A steam generator is a complex integration of furnace, superheater, reheater, boiler or evaporator, economiser, and air preheater along with various auxiliaries such as pulverisers, burners, fans, stokers, dust collectors and precipitators, ash-handling equipment, and chimney or stack.
- The boiler is part of the steam generator where phase change occurs from liquid (water) to vapour (steam), at constant pressure and temperature. However, the term “boiler” is traditionally used to mean the whole steam generator.
- BASIC TYPES OF STEAM GENERATORS
Based on application:
Classification of boilers can be made in different ways. From the point of view of applications, they can be
(a). utility steam generators
(b). industrial steam generators
(c). marine steam generators
- Fire-tube boilers have been used in various forms to produce steam for industrial purposes and for hauling railway locomotives and river launches.
- They are no longer used in utility power plants and steam locomotives have also mostly disappeared. However, they are still often used in industrial plants to produce saturated steam at the upper limits of about 18 bar pressure and 6.2 kg/s steaming capacity.
- For small steam requirements, fire-tube boilers are suitable. They have certain inherent advantages like:
(1). low first cost
(2). reliability in operation
(3). need of only unskilled labour
(4). less draught required
(5). quick response to load changes.
- A fire-tube boiler is so named because the products of combustion pass through its tubes or flues, which are surrounded by water. They may be either:
Examples: Cornish boilers, Cochran boiler, Lancashire boiler, Locomotive boiler etc.
(a). Externally fired: locomotive type boilers, Lancashire boilers, horizontal return tubular (HRT) boiler etc.
(b). Internally fired: Scotch-marine boilers, Cochran boiler, Babcock Wilcox boiler package boilers etc.
Externally fired boilers:
- Fig.1 shows a typical externally fired fire-tube boiler in which the furnace is outside the boiler shell.
- Coal is entered manually by shovels on to the grate by opening the fire-door. The products of combustion flow through the tubes which are immersed in the shell containing water.
- A fusible plug made up of a low melting point alloy (lead-based) is installed on the roof of the crown m the furnace. If the water-level in the shell falls below a certain level the fusible plug melts due to overheating and water pours down through the hole formed and puts out the fire.
- There is a spring-loaded safety-valve provided to the boiler pressure within the safety limit.
- Water-tube boilers were developed to resolve the shortcomings of fire tube boiler and permit increases in boiler pressure and capacity with reasonable metal stresses.
Examples: Babcock-Wilcox boiler, Stirling boiler, La-Mont boiler, Benson boiler etc.
- The water-tube boiler, where water flows through the tubes and flue gases flow outside them, puts the pressure in the tubes and the relatively small- diameter drums, which are capable of withstanding extreme pressures of the modern steam generator.
REQUIREMENTS OF A GOOD BOILER
Different requirements of a good boiler are given below. In general boiler is supposed to generate large quantity of steam at desired pressure and temperature quickly and efficiently.
(a). It should be capable of generating steam at desired rate at desired pressure and temperature with minimum fuel consumption and cost.
(b). It should have sufficient steam and water storage capacity to meet fluctuation in demand and to prevent fluctuation in steam pressure or water level.
(c). Boiler should have a constant and thorough circulation of water.
(d). It should be equipped with all necessary mountings.
(e). Boiler should have capability to get started quickly from cold.
(f). Its construction should be simple and have good workmanship for the ease of inspection and repairs i.e. easily accessible parts.
(g). Boiler should have its heating surface nearly at right angle to the current of hot gases for good heat transfer.
(h). There should be minimum frictional power loss during low of hot gases and water/steam i.e. pressure drop throughout the system should be minimum.
(i). Tubes should be so designed so as to have minimum soot deposition and good strength against wear. Boiler should have a mud drum to receive all impurities.
(j). Boiler should have strength to withstand excessive thermal stresses.
(k). Boiler should occupy less floor area and space.
For deciding the boiler for any application, generally following criterion are made:
(i). Steam pressure requirement
(ii). Steam temperature requirement
(iii). Steam generation rate
(iv). Initial cost and constraints
(v). Running and maintenance costs
(vi). Availability of fuel and water
(vii). inspection and maintenance requirements
Fire tube boiler
Water tube boiler
It is limited to 20-30 bar. In case of waste heat boilers, it can be more.
It is virtually unlimited within metallurgical and design limits.
Limited to about 20 MW. Within design limits.
It is virtually unlimited.
All commercial fuels and treated waste can be used.
Any fuel can be used. Also, the furnace size is large.
It is package ready for work site.
It is to be shop assembled or erected at site.
Normally 80-85% gross calorific value but can be further increased using accessories.
Normally 8-90%. Gross calorific but can be further increased using accessories.
Generally, for heat supply.
Generally, for power and heat supply together.
Frequent inspection requirement. It is more than in water tube.
Inspection requirement is less than in fire tube boiler, boilers.
- In 1905, Dr. F.G. Cottrell, Professor of Physical Chemistry at the University of California, conducted a series of laboratory experiments that resulted in the development of the first commercial electrostatic precipitator.
- It was an immediate success and the precipitator soon came to be widely used in power plants, smelters, steel plants, paper mills and many other industries.
· The principal components of an electrostatic precipitator (ESP) are two sets of electrodes insulated from each other. The first set is composed of rows of electrically grounded vertical parallel plates, called the collection electrodes, between which the dust-laden gas flows. The second set of electrodes consists of wire, called the discharge or emitting electrodes that are centrally located between each pair of parallel plates.
Ash handling system:
- Boilers burning pulverized coal (PC) have dry bottom furnaces. The large ash particles are collected under the furnace in a water-filled ash hopper.
- Fly ash is collected in dust collectors with either an electrostatic precipitator or a baghouse.
- Draught refers to the pressure difference created for the flow of gases inside the boiler.
- Boiler unit has a requirement of the expulsion of combustion products and supply of fresh air inside furnace for continuous combustion. The obnoxious gases formed during combustion should be discharged at such a height as will render the gases unobjectionable.
- A chimney of stack is generally used for carrying these combustion products from inside of boiler to outside, i.e. draught is created by use of chimney.
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