Steam boiler is a closed vessel in which heat produced by the combustion of fuel is utilized to generate steam from water, at desired temperature and pressure. Boiler is a device used for generating steam which is used for driving prime movers like steam turbines or industrial purpose like heating. Another words, boiler can generate steam with capacity the exceeding 25kg, and hot water above 100℃ under the pressure grater than 0.7MPa. Its applications include heating, power and industrial processes. For heating, the boiler can be used to provide the hot water for buildings and residential in winter. For power plant, the high capacity boiler can be used to generated the power to ensure the normal life and industrial production. For industrial processed, the boiler can be used all kinds of factory, such as food processes, chemical industrial, construction industrial, pharmaceutical industrial, textile and so on.
1. According to the difference of body structure and heating medium
(1) Fire tube boilers: structure is simple, the capacity is small, the requirement to the water quality is not high, less than 25t/h.
(2) Water tube boilers: structure is complex, the capacity is grate, the requirement to the water quality is high, more than 25t/h.
2. According to the difference of the capacity and combustion chamber
(1) Vertical boiler
3. According to the difference of the fuel
(2) Biomass boilers/Biomass steam boilers(Fuel: Biomass pellets, wood, palm husk, rice husk, sugar cane)
(4) Gas boilers/Gas steam boilers(Fuel: Natural gas, LPG)
(6) Electric boilers(Fuel: Electricity)
4. According to the difference of the working pressure
(1) Low pressure boilers(lower the 2.5MPa)
(2) Middle pressure boilers(between 2.5MPa and 6MPa)
(3) High pressure boilers(above the 6MPa)
5. According to the difference of circulation of water
(2) Forced circulation boilers
6. According to the difference of installation
(1) Mobile boilers (or) Portable boilers
(2) Assembly boilers
What is the high pressure boilers
When a boiler operates with working pressure above 6MPa, it is called high pressure boiler. The high pressure boilers are widely used for power generation in power plants. If the feed water pressure keep increasing, the saturation temperature of water rises and the latent heat of vaporization decreases. The feed water can be heated to saturation of temperature in the economizer to make use of waste heat for improving the thermal efficiency. Recovery from the exhaust gases escaping to chimney. Then the boiler supplies only latent heat of vaporization and super-heat. Thus, the boiler operation at high pressure will require less heat addition for steam generation.
Characteristic of high pressure boilers
1. Large number of small diameter tubes
This structure increases the heating area between surface area and volume-ratio through with the use of small-diameter tubes. It helps in high rate of the heat transfer to water flowing inside the tubes. Therefore, a large number of small diameter tubes are used for water circulation in forced circulation.Higher steam pressure and temperatureThe boilers generate steam at a pressure between 6MPa and 30MPa and temperature of 450°C to 585°C with two super heaters in series. For power plant, the use of such stream is very suitable for power generation. Meanwhile, it increases thermal efficiency of the plant and reduces the moisture contents in low pressure stages of expansion in the turbine.
2. Improved mode of heat transfer
Nowadays, the high pressure boilers use the heat transfer by radiation along with conduction and convection. The total heat-receiving equipment is divided into several parts. So they can easily be located in various zones of the furnace for most efficient heat transfer to the water circuit.
3. Pressurized combustion
During the boiler operation, the pressurized air is used in the furnace for increasing the fuel efficiency and thermal efficiency. Also, it gives large amount of heat in a small space.
4. High thermal efficiency
According to the firing methods, monitoring, furnace conditions, control flue gases and water velocity, the high pressure boilers have more superiority compared to low pressure boilers. Therefore, its thermal efficiency is more high.
5. Compact structure
For high pressure boilers, the high rate of heat transfer decrease the overall size, thus the structure of boiler becomes more compactness.
6. Improved the transfer rate
The high pressure boilers make use of the following measures to ensure the heat transfer rate:
(1) Evaporative of the water above serious pressure of steam.
(2) Heating of water by mixing superheated steam for high heat transfer rate.
(3) Improving the combustion air velocity on the tube.
Typical high pressure boilers
1. La Mount boiler
This boiler was developed in 1925 by La Mont. Usually, it consists of economizer, boiler drum, super-heater, evaporator, air pre-heater etc. La Mount boiler works on the principle of forced circulation. The working process can be depicted as follows: First, the feed water is pumped through the economizer and preheated water is supplied to the boiler drum. Then, the water is heated up by the hot gases flowing in the upward direction, which are later exhausted through the chimney high in the atmosphere to reduce air pollution.
The schematic diagram of La Mount boiler is below:
2. Benson boiler
Benson developed the boiler in 1927 in order to overcome La Mount boiler drawback, such as formation and attachment of bubbles on the inner surfaces of the tubes. The Benson boiler can be erected in a commparatively smaller floor area. And the space problem does not control the size of boiler used. Meanwhile, it is a high pressure, water tube, forced circulation boiler. It has unique characteristic of absence of drum and entire process of heating, steam formation and superheating takes place in a single continuous tube.
The schematic diagram of La Mount boiler is below: