Boiler classification
The boilers can be classified on the basis of many factors. For instance, based on the service rendered by the boilers they can
be classified as follows:
Industrial boilers : This type of boilers are usually low, medium and high pressure boilers with low to high capacity steaming rates that mainly renders service for the process requirements such as heat exchanging, steam-naphtha reforming, cracking, air-conditioning, in petrochemical plants, oil refineries, fertiliser complexes, textile manufacturing, metallurgical process etc. Utility Boilers: Thermal power stations across the globe use high-capacity oil-fired or coal-fired boilers that generate steam at supercritical pressures and temperatures which in turn are expanded in turbines to produce power. Industrial cum utility boilers : These boilers are intended to produce power for captive consumption and steam alone for process needs.Boiler is essentially a
heat exchanger. According to the flow of hot flue gases and water or steam water mixture in tubes or shell side this
heat exchanger can be classified as:
Water-tube boilersFire-tube boilers.Large scale steam production facilities are most probably water-tube boilers in which water or steam+water mixture is circulated through the tubes and waterwall tubes while the flue gases heat those tube contents from outer surface inside a furnace chamber.
In fire-tube boilers it is extremely opposite. Hot flue gases that are generated in the process is passed through the tubes of a fire-tube boiler in order to make use of the heat of gases before they are discharged into the atmosphere. Fire-tube boilers are, in that sense,
waste-heat recovery units. Feed water flows through the shell side to evaporate, picking up the heat of waste gases. High pressures and temperatures and rating could not be achieved in this boiler.
Based on the method of firing, the boilers can be classified as follows:
Internally fired boiler.Externally fired boiler.
Based on the method of water or steam or steam+water circulation boilers can be classified into three types:
Natural circulation boilerForced circulation boilerOnce through boiler
Based on the nature of draught, boilers can be classified as:
Natural draught boilersForced draught boilersBalanced draught boilersBased on the number of drums a boiler has, it can be fit into one of the following types:
Single drum boilerBi-drum boilerMulti-drum boiler
The seventh edition of 'Perry's chemical engineer's handbook' (edited by Robert H. Perry and Don W. Green) speaks of the
subcritical-pressure boilers and the
supercritical-pressure boilers.
The former range in design pressures upto 18.6 M Pa (2700 psia) and in steaming capacities upto about 2955 Mg/h (6,500,000 lb/h). The later designed to operate at pressures upto 34.5 M Pa (5000 psia). Based on working pressures we can simplify normal industrial boilers as low, medium and high pressure boilers. Pressure in Indian industries is expressed as Kg/ Cm
2 and steaming capacity in Tonnes/ Hour.
According to the arrangement of drums and tubes the boilers are known as:
Horizontal boilerVertical boilerInclined boiler
Corrosion in boilers Boiler water treatment
