Steam Boilers

Steam Boilers

Ø  Introduction-

The Boiler is an integral part of a thermal power plant. Its structure is like a closed vessel containing water and other accessories. The main function of a boiler is to generate steam by exchanging the heat produced by fuel combustion (fuel may be solid, liquid or gaseous) with water. Without boiler the cycle of steam/thermal power plant cannot be completed hence boiler is called as a heart of steam power plant.

Ø  Important terms in Steam boiler-

i)        Boiler shells- Shells are cylindrical in shape. It is made up of steel plates by welding or riveting together. The size of a boiler shell will decide the water containing capacity of a boiler.

ii)      Combustion chamber- This is the place generally located at the bottom of boiler used for actual burning of fuel.

iii)    Grate- It is the part of combustion chamber upon which fuel is placed. The material used for the grate is generally cast iron.

iv)    Furnace- The space above grate in combustion chamber is furnace. The actual heat exchanging process starts from furnace.

v)      Heating surface- The part which is actually exposed to the fire is nothing but a heating surface.

vi)    Mountings- These are the various fittings mounted on a boiler for its proper functioning.

They include- Pressure gauge, Water level indicator, Safety valve, etc.

vii)    Accessories- Accessories are integral part of boiler. All accessories together form a boiler and placed within the boiler shell.

They include- Feed pump, Economizer, Super heater, etc.

Ø  Types of Boilers-

I.          Babcock & Wilcox Boiler- 

Fig 1: Babcock and Wilcox Boiler

The main parts of Babcock and Wilcox boiler are as follows-

i)          Drum- It is a large water tank placed horizontally at the top of the boiler.  The capacity of drum is one of the factors to identify the capacity of boiler.

ii)        Down take header- It is placed at the downside of the boiler and receives water from the drum.

iii)      Up take header- It is placed at the front side of the boiler and transfers steam to the drum. 

iv)      Water tubes-These tubes contain water and are placed at the middle part of the boiler. The water in tubes gets converted to steam by exchanging the heat with hot flue gases. The inclination given to the water tubes is near about 10 – 15⁰ with horizontal. Water tubes are not completely filled so that the steam can be separated out easily.

v)        Fire door- It is the access given to ignite the solid fuel in the furnace.

vi)      Mud collector- It is the bottom part of the down take header used to collect contaminants in water like impurities, mud etc.

vii)    Baffles- Baffles are placed between the water tubes and they are mainly used to divert the flow of hot flue gases. 

Accessories & Mountings-

i)        Safety valve- It is one of the mountings of the boiler placed on the drum and works as a safety device. To relieve excess pressure is the main function of the safety valve. This valve gets operated automatically when the inside pressure cross the safety limit.

ii)      Pressure gauge- This is the device to measure the pressure inside the boiler vessel. It always acts as an indicator for operator.

iii)    Water level indicator- Operator can easily understand the water level within the boiler from outside with the help of water level indicator.

iv)    Steam stop valve- It usually regulates the flow of steam from boiler to the steam pipe or from one steam pipe to other. When this valve connects boiler and steam pipe then it is called as ‘Junction valve’ otherwise it is called as ‘Steam stop valve’.

v)      Super heater- The super heater is having a structure like heat exchanger which converts wet steam to dry one. Super heater always improves the quality of steam and hence the efficiency of steam power plant.

Construction-

Stephen Wilcox and George Herman Babcock invented this boiler in the year 1967. It is a natural circulation, externally fired, medium pressure, stationary horizontal water tube boiler in which water is flow in the inclined tubes (inclination is about 100-150). Water tubes are straight and attached to the header at each end. Horizontal axis drum is located at the top of the boiler along with its mountings like, safety valve, pressure gauge, water level indicator, etc. The man hole is provided for the maintenance of the boiler as shown in figure.

Working-

The fuel is supplied in combustion chamber through fire door to the grate where it is burnt. Burning of fuel will result into hot flue gases in furnace to boil the water. These hot flue gases are deflected by baffles to move upward towards water tubes. Water boils in water tubes and steam is generated due to heat exchange with hot flue gases. Such a steam goes to the drum through uptake header as a wet steam. As the steam is getting stored at the upper part of the drum, the pressure in that area is increased. To relieve the excess pressure safety valve is used as a mounting of the boiler.  The steam pressure for this boiler may be as high as 100 bar and steaming capacity is up to 27000 kg/h.

Further the wet steam is passed through the super heater where almost all suspended water particles in steam get removed and steam becomes dry steam. To get the best quality steam, dry steam should not contain any water molecules in it. Such steam is finally taken out by steam stop valve and supplied to the steam turbine whenever necessary.

At the bottom of the boiler mud collector is situated from where all the mud particles are removed through blow off cock.

Advantages-

i)          Solid as well as liquid fuel can be used

ii)        Minimum drought losses

iii)      It is natural water circulation water and hence becomes easy to operate.

iv)      Maintenance is easy as more space is available.

v)        Steam generating capacity is high.

vi)      Efficiency is more as compare to fire tube boiler.  

Disadvantages-

i)        The impure water can generate scaling problems within tubes and other parts of boiler hence pure water is necessary for this boiler.

ii)       Its maintenance cost is very high.

iii)   Water can be overheat if feed water supply is failed even for short period hence operation of this boiler should be carry out very carefully. 

iv) Most of the parts of this boiler are constructed to fix at one place so this boiler is non-transportable.

Applications-

This type of boiler is used where high pressure steam is required, generally at power generation industries.

II.          Cochran Boiler-

 

 The main parts of Cochran boiler are as follows-

i)     Boiler shell- It is the cylindrical cover of the boiler having a hemispherical top.

ii)  Furnace- The space above grate in combustion chamber is furnace. The actual heat exchanging process starts from furnace.

iii) Fire tubes- These are the tubes carrying fire and connect furnace at one end while chimney at other end.

iv) Fire hole- It is the hole at the bottom of the combustion chamber which is used to place the fuel.

v)  Furnace- The mediator between combustion chamber and fire tubes is a furnace having a dome shaped top surface. Due to this shape, un-burned charge and flue gases deflects back to the combustion chamber.

vi)  Flue pipe- It is a part having a pipe like structure connecting furnace and combustion chamber.

vii)  Chimney- It is the passage from where exhaust gases goes to the atmosphere.

viii) Fire brick lining- This is provided to the combustion chamber externally to protect it from overheating.

The accessories and mountings of the Cochran boiler are as listed below-

1. Safety valve

2. Pressure gauge

3. Water level indicator

4. Stop valve 

5. Blow off cock

Construction- 

The credit of manufacturing Cochran boiler goes to Cochran & Co. of Anman. It is a vertical boiler having multiple horizontal tubes placed in it. The water is filled in the boiler in such a way that it passes over the tubes.

At the lower part of the boiler combustion chamber is placed where actual burning of fuel takes place. Furnace is connected to combustion chamber through flue pipe. This boiler consists of a hemispherical shaped top to which mountings like steam stop valve, pressure gauge etc. are attached.

Working-

Through fire hole fuel is sparked into the combustion chamber. Hot flue gases pass through fire tubes and exchange heat to the water by convection process. After exchanging the heat with water, hot flue gases goes to the atmosphere through chimney. Water surrounding the pipes starts getting heated and finally steam is generated. Generated steam will start to accumulate at the top of the boiler. Steam stop valve is operated to remove this steam whenever necessary.

Advantages-

i)     Floor area required is less.

ii)        Initial cost is less.

iii)      This boiler can be transport from one place to another.

iv)      Volume to area ratio is high.

v)        Easy to operate.

 Disadvantages-

i)          Steam generation rate is low.

ii)        Pressure handling capacity is low.

iii)      Access for maintenance is difficult.

 Applications-

This type of boiler is used in Paper Industries, Chemical Industries, Refining industries, etc.