What is Boiler ? and their classification:-
As we all are know about the boiler “boiler is close vessels in which steam is generated by the burning of fuel and that heat transfer to feed water”
As per Indian boiler act 1923 boiler means “any close vessel exceeding 22.75 liters in capacity which is used for generating steam under pressure and including any mounting or other fitting attached to such vessels ,which is wholly under pressure ,when steam is shut off.”
And also boiler is the heat exchanging equipment’s that produce steam. Chemical energy is released from it through fuel of coal, oil or natural gas; then the energy is transferred to water through heat transfer process, so as to change water into steam, which supply the necessary heat energy in industrial production; besides, the steam could be changed into mechanical energy by steam motivated machinery, or electric energy by steam turbine generator. So the main task of boilers is to effectively convert chemical energy in fuels into heat energy of steam. Therefore, recent boilers are also called steam generators. Function of waste heat boiler is utilizing waste flue gas released from cement kiln to produce super heated steam in compliance with stated temperature and pressure Water heater (coal economizer) is in the bottom hearth, heating the water from water supply main tubes by flue gas, and then delivering to the steam manifold.
Steam manifold is on the top of the boiler, playing the function of water storage and steam-water separation in the boiler steam-water circulation. Evaporator is above the water heater (coal economizer), heating water in the evaporator into steam-water mixture by heat released from flue gas. The steam-water mixture comes into the steam manifold through cooling wall upper header and ascending tube. Super heater is on the topside of the hearth, heating steam into super heated steam in compliance with temperature requirements; the super heated steam arrives at steam turbine through main steam tube and main steam valve.
Boilers and their classification:-
Boilers are widely used with many patterns; they are normally classified according to following methods:
Classified by purpose:-
Power station boiler:
mostly are boilers of large capacity and high parameters; fire room burning is with high heat efficiency; outlet working medium is super heated steam.
being used for industrial production and heating, they are mainly boilers of low pressure, low temperature and small capacity; mostly there is fire bed burning with rather low heat efficiency; when the outlet working medium is steam, boilers will be called steam industrial boiler; when outlet working medium is hot water, boilers will be called hot water boiler.
used as power for ships; normally low or middle parameters are applies; mostly are fuel boilers. Boilers are of small volume and light weight.
Classified by structure:-
Fire tube boiler: flue gas flows in the fire tube; boilers could be made with small capacity and low parameter; heat efficiency is rather low but structure is simple; there are low requirements on water quality; operation and repair is convenient.
Water tube boiler: gas and water flows in the tube; boilers could be made with small capacity& low parameter or large capacity& high parameter. Power station boilers are all water tube boilers; the heat efficiency is rather high but the requirements on water quality and operation level are rather high.
- Classified by circulation method
Natural circulation drum boiler:
are the boilers equipped with drums; working medium in the evaporating heating surface is circulated through pressure difference generated from density difference between steam-water mixture in ascending tube and water in down comer. These kinds of boilers could only be used below critical pressure. (Critical point: 374.15℃, 22.13MPa. Pressure maximum limit of the natural circulation system is 18MPa. When there is pressure higher or equal to reaches critical pressure, density difference between water and steam is not enough to create natural circulation).
Multiple forced circulation drum boiler:
it is also called assisted circulation drum boiler. Drum and circulating pump are provided; working medium circulation is established through working medium density difference and circulating pump pressure in the circulation circuit. Working medium in the evaporating heating surface is circulated through density difference between water and steam-water mixture, mainly relying on pressure head of boiler water circulating pump. This kind of boiler could only be used under critical pressure.
Boiler of low circulation ratio:
be equipped with steam-waterseparator and circulating pump; working medium circulation is mainly established by circulating pump; could be used for subcritical pressure and supercritical pressure; there is low circulation ratio at 1.25 to 2.0
there is no drum; water supply relies on water pump pressure; the boiler generates steam by single passing heating surface. These boilers are applicable for high pressure and critical pressure.
Combined circulation boiler:
there are recirculating pumps. When boiler load is low, operation is in recirculating manner; when boiler load is high, operation is in concurrent manner; these boilers are applicable for sub critical pressure and supercritical pressure. These boilers are developed comprehensively from concurrent boilers and forced circulation boilers as improvement to concurrent
- Classified by boiler outlet working medium pressure:-
Low pressure boiler: normally pressure is lower than 1.275MPa (13kgf/cm2)
Middle pressure boiler: normally pressure is 3.825MPa (39 kgf/cm2)
High pressure boiler: normally pressure is 9.8MPa (100 kgf/cm2)
Superhigh pressure boiler: normally pressure is 13.73MPa (140 kgf/cm2)
Subcritical pressure boiler: normally pressure is 16.67MPa (170kgf/cm2)
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