A Preface On The Boilers:
Many centuries ago, human being found out the power of steam, but using industrial steam boilers started from 1712. They were made of lead or wood producing pressure a little more than atmosphere. In 1725, Histech boilers with steel riveted plates and proportional pressure came into use.
(Figure 1) Schematic Section of a Boiler
By passing the time, it was determined that the best way of using boilers scientifically is to make them circular. In 1795, by producing rolled plates, circular boiler was made too. Water Tube boilers were made in 1873 (Figure 1). Water flows in slant pipes where the heat absorbs. Regarding extending the surface the heat will be exchanged in the best way.
By extending other parts such as super heater, deaerator economizer and air heaters, and so on, gradually, the primary steam boilers changed to current boilers with more capacities. Boilers advancement processes are mentioned below:
1. Temperature increase
2. Pressure increase
3. Boiler steam outlet increase
4. Efficiency increase
5. Easy to control
6. Reducing manufacturing, operation and maintenance expenses
7. Boiler lifetime increase
Boiler kinds:
Boiler duty is to turn liquid (water) into super saturated steam, but in industry, all devices turning liquid to super heated steam are called boiler.
Boilers are being used in steam production units for general, electrical and industrial uses, and depend on their planning can make use of nuclear, charcoal, mazut, gas, oil and natural gas as their fuel.
Based on different parameters, boilers classification is as bellow:
Boilers with nuclear fuel (reactor):
Using nuclear fuel for steam production contrary to fossil fuels has no bad effects (just a little controllable radioactive rays which are not harmful) on human and environment. Nuclear boilers have high efficiency in producing thermal energy. Moreover, oil and gas can be used not only as fuel but also for producing petrochemical material and many other industrial products. Nuclear power plants have a high efficiency for producing thermal energy, useful in producing electricity. In nuclear boiler installations a second fluid flows between the reactor and boiler, and in boiler, the heat transfers from fluid to water. Also, produced steam will circuit in a normal steam circulation.
Nuclear power plant boilers are designed in various types that their most important ones are Soft Water Reactor, Boiling Water Reactor, and Hard Water Reactors.
Fossil Fuel Boilers:
All industrial boilers Iran use fossilized fuels. Using fossilized fuels for producing electricity forms much carbon dioxide because the heat generated from burning the fossil fuels is three times more than the produced electrical energy. Upon the negative effects, the fossil fuels have on our life and environment, it is recommended to reduce using fossil fuels.
The followings are different fossil boilers:
1. Vessel boilers:
This kind of boilers consists of closed vessel in which heat exchange occurs out of it and water vaporizes in the vessel. Their efficiency is low -about 30%-, which are used just in industries with low-pressure steam.
2. Fire Tube Boilers:
In these boilers, tubes are coated with water and produced gases pass through the tubes. Heat exchange occurs between water and products. Furnace can be in or out of the boiler. Fire tube boilers efficiency is about 70%, which are used for steam production in low-pressure units.
3. WATER TUBE BOILERS:
In different kinds of these boilers (figure 2), heat exchange occurs by contacting the produced gases with the outer surface of tubes containing Water and steam and in conducting, replacement and radioactive ways. Small Diameter of the steam and water tubes that reduces thermal stresses on tubes is the advantage of these boilers to fire tube ones, which consequently are used in high-pressure industries. Their efficiency is about 85 to 95%.
(figure 2)
A water tube boiler consists of combustion chamber with tubes, steam and water headers and drums, super heaters, air heaters, economizer and preservers.
In this boilers, integrated fin coated vertical tubes are used since water tubes are under high thermal charge, pressure and temperature stresses effects should be designed precisely. Water tubes are assembled in vertical mode in combustion chamber, which are welded in stubs on upper and lower sides of headers. Headers reduce the number of tubes connected directly to drum. Temperature distribution in water tubes depends on heat transmission coefficient, thermal charge amount, thermal conduction coefficient, dimensions, and tubes engineering and fin structures. Fins cause equal distribution of thermal charge on tubes, reduce boiler heat gradient in one hand, and increase the heat exchange level, which causes more heat exchange on the other hand.
3.1. ONCE THROUGH BOILER:
Are non-drum boilers with super critical pressure, which are called Benson. Combustion chamber and wall tubes assembly are planned in a way that wall tubes feeding water passes the combustion chamber and wall tubes, and then vaporizes and conducts directly to super heaters, these boilers are called Non-drum boilers consequently. As a fact that Benson boilers are high Pressure boilers require high technology in manufacturing process, but because of having no drum are lighter in weight than under critical presser boilers (with drum).
In Benson boilers, circulation number is one. Since these boilers work, upper the critical pressure for increasing the tubes length unlike drum boilers plan the wall tubes diagonal in order to decrease the boilers height. Wall tubes in Benson boilers are thicker than drum boilers. When starting the Benson boilers for separating water and steam make use of cyclone, which uses from centrifugal force for separation. Because of being drum-less, you will not have any water and steam reserve for emergency uses.
Industrial Boilers Kinds:
1. Indirect Gas Heaters:
These heaters are from fire tube ones, which are used for heating the natural gas after pressure reduction installations (civil gas).
2. Direct Gas Heaters:
These heaters are from water tube kinds and called refinery furnaces too.
Tubes are directly exposed to fire and heat and the coil is exposed to replacement heat.
3. Chemical reaction heaters (reactors):
Drum shaped recovery boilers, which are used in petrochemical plants in which their coils are installed helical on the plates.
4. Recovery type boilers:
Recovery type boilers, which are used for making use of outlet gases energy in gas power plants, (figure 3). Heat recovery power plant is a combination of steam power plant and gas turbines for increasing the whole system Production. In this kind of boilers, gas turbine can operate independently.
(figure 3)
5. Recovery Boilers:
These boilers are used in melting furnaces, in metalwork plants, which are installed directly in melting circuit. There is not the possibility of separating them of working system.
6. Incinerator Boilers:
These boilers are used for devastating the civic litter and generating electricity.
These boilers should be equipped whit pro and after reserves (for collecting the litter and evacuating the ash).
7. Coal Boilers:
These boilers also should be equipped with pro and after reserve for reserving coal.
Boilers Manufacturing Parts:
Each boiler consists of different parts that each one is installed for a special aim. Based on the duty they have boilers parts are divided into following parts:
Pressure Parts:
All parts in which water as steam (tubes, headers and so on) passes through, having more inside pressure than environment are called under pressure parts (figure 4). Generally, all parts from boiler feed water pump to supper-heater outlets are called pressure parts, which are listed bellow:
Main Feed Water Pipe:
transfers water from boiler feed pump outlet to economizer inlet header
- Economizer Inlet Header:
Generally it collects or distributes fluid (water or steam)
A – Distributor header: header outlets are more than inlets
B – Collector header: header inlets are more than outlets
(figure 4)
Economizer Tubes:
Economizers increase boilers efficiently. Economizers increase the inlet water temperature to reach saturation, which prevents the reduction of drum water temperature. Economizers are installed in combustion gases outlet.
More heat exchange, needs more heat surfaces to maintain this fact fins are Installed on economizers tubes (fins increase heat surfaces).
The boilers burning fuel determines the use of fins. If the used fuel is from light Fuels (natural gas), the fins are used and. the fins are installed in two ways:
1. In line
2. Stager
Economizer Outlet Header:
Collects water from economizers and conducts to steam drum
Economizer Outlet Pipe:
Exchanges water from economizer outlet to steam drum inlet