In this article we will discuss about the design and constructional details of various boilers used in power plants for steam generation.
Design of Lancashire Boiler:
It is fire tube, stationary, horizontal straight tube, internally fired, natural circulation boiler. Normal working pressure of this boiler is about 15 bar and steam generation capacity is upto 8T/hr.
The main parts of the boiler are as follows:
(1) Boiler Drum:
This boiler consists of a very large boiler drum. The size of the boiler drum is approximately 7-9 metres in length and 2-3 metres in diameter. This is a very old type of boiler, in this boiler, the meeting edges of the plate are connected by means of rivetted joints.
Steel plates are rolled to form shell and the meetings edges of the shell are connected by means of rivetted joints. The alternate shells are made smaller in a diameter, so that the longer shells of required length can be obtained by inserting one shell into the other and rivetting over the circumference.
The end plates of the boiler are flat. These flat end plates, when they are subjected to steam pressure from inside, they will have a tendency to bulge out. In order to prevent them from bulging, they are strengthened by means of stays.
There are 2 types of stays:
(i) Longitudinal Stay:
It consists of round bar which extends from one end plate to the other and are connected to the end plates by means of lock nuts.
(ii) Diagonal Gusset Stay:
Consists of a triangular plate, which is rivetted to the shell plate and end plate.
(2) Fire Tubes or Flue Tubes:
Through the boiler drum passes 2 fire tubes.
Part of each of fire tubes is corrugated, so as to take up expansion and contraction when the boiler gets hot and cold. Also by providing corrugations surface area increases. These tubes are also made of number of shells. The ends of each of the shell are flanged outwards and the flanges of the two adjacent shells are connected by means of rivets, so as to get shells of required length.
It is to be noted that, the centre line of fire tubes is slightly below that of the centre line of boiler shell. Because fire tubes are provided approximately in the centre of water, so that heat can be transferred uniformly in all directions.
(3) Grate, Brickwork Bridge etc:
Combustion of fuel takes place over the Grate, Top surface of the grate is made up of fire-bars. Below the grate Ash-pit is provided.
When the combustion of fuel takes place, hot gases are generated and the Brickwork Bridge deflects the gases to move upwards.
It gives access to the inside of the boiler drum. It will be sufficiently large enough for a man to pass through it, for cleaning and inspection purpose.
The boiler is fitted with various mountings. Mountings are necessary for the safe and efficient operation of the boiler.
The term mountings refer to the items such as:
(i) Safety Valves –
(a) Dead weight safety valve,
(b) Spring loaded safety valve,
(c) High steam low water alarm.
When the pressure of steam in the boiler drum increases beyond safe value, then these valves will open and allow the surplus steam to escape to the atmosphere, till the pressure of steam in the boiler drum comes to normal working pressure.
(ii) Pressure Gauge – It indicates pressure inside the boiler drum.
(iii) Feed Check Valve – Water enters the boiler drum through feed check valve. On the inner side of the feed check valve, perforated feed pipe is fitted for feeding the water uniformly in the boiler dram. Below the feed pipe scum tray is fitted which separates scum and is removed through the scum valve.
(iv) Water-Level Indicator – It indicates the water level in the dram.
(v) Blow-Off Valve – It blows out impurities collected due to gravity.
(vi) Steam Stop Valve – Through this steam will be taken out for use.
(vii) Antipriming Device – It is always desirable that the steam should leave the boiler as dry as possible, for that purpose an anti-priming device or steam collecting pipe is provided in the steam space. It consists of a pipe and rectangular openings are provided in its bottom surface.
It is fitted by means of baffles from inside. When the wet steam (steam and water particles) enters the pipe, the water particles being heavier than steam, after striking the baffles fall back to water space and steam almost dry will be taken out through the steam stop valve.
Accessories are used for improving the efficiency of boiler plant. Generally Super heater and Economiser are used as accessories.
(7) Path of Flue Gases:
When the combustion of fuel takes place, hot gases are generated and the brick work bridge deflects these gases to move upwards.
During their first pass, these gases flow from the front end of the boiler to the rear end through central tubes. During their I-pass, they pass through the super heater, then they enter the common bottom flue at the rear end. Now their II-pass starts, during this gases flow from rear end to front end through the common bottom flue.
At the end of II-pass, the gases are bifurcated into two side flues, when the flue gases enter side flues, their III-pass starts end during their Ill-pass, they flow from the front end to rear end through side flues and during this pass they heat the water from sides.
After their third pass they meet in main common flue, from where they pass through the economiser and then they are exhausted through the chimney.
Design of Locomotive Boiler:
It is a Fire tube, Horizontal, Multitubular, Internally fired, Natural circulation boiler.
As shown in Fig. 11.8 it consist of a large cylindrical shell. This shell is fitted to a rectangular fire box at one end and smoke box at other end. In between the smoke box and fire box a number of tubes are fitted.
When the combustion of fuel (coal) takes place over the grate, hot gases are generated. These hot gases rising from the grate are deflected by the Fire brick arch, so that they come in contact with the complete heating area of the fire box. The dampers as shown, control the flow of air for combustion. These hot gases then flow through the tubes, heat will be transferred to water and water will be converted into steam and accumulates in the steam dome and gases afterward are exhausted to the atmosphere through the small chimney.
The regulator can be operated from the cabin by turning the lever. In order to have superheated steam, the steam is passed through the super heater tubes. These super heater tubes are of smaller diameter and are laid in the large diameter fire tubes as shown. Then this superheated steam is supplied to the engine. This boiler is fitted with various mountings as shown in Fig. 11.8.
Design of Cochran Boiler:
It is a Fire tube, Stationary, Multi-tubular, Internally fired, vertical boiler.
Cochran boiler is made in many sizes capable of producing steam upto 3 T/hour and working pressures upto 17 bar. This boiler gives Thermal efficiency (ƞth) of 70% with coal firing and about 75% with oil firing.
The boiler consists of a cylindrical shell, with its crown having hemispherical shape. Such a shape of the crown plate, gives sufficient strength to withstand the bulging effect.
The furnace or fire box is also crown shaped or dome shaped, which deflects the unburnt fuel back to the grate. Combustion of fuel takes place over the grate. Hot gases generated in the fire box, enter into the chamber C, though a short flue pipe F and strikes the boiler shell plate or Back plate which is lined by fire bricks. The back plate directs the gases into the smoke or fire tubes. The gases after passing through the horizontal tubes enter the smoke box SB and from where they are exhausted through the chimney.
The hot gases passing through the horizontal tubes, give their heat to the water and in doing so, convert water into steam, which gets accumulated in the steam space.
Design of Babcock Wilcox Boiler:
It is a water tube boiler.
It mainly consists of 3 parts:
(i) A Horizontal Steam and Water Drum:
This is the main part of the Babcock Wilcox boiler. The size of the boiler drum is very much smaller, when compared with boiler drums of fire tube boiler of same capacity. Hence these are simpler from transportation point of view.
(ii) A Bundle of Steel Tubes:
It is provided in the combustion chamber.
(iii) Combustion Chamber:
It is the space above the grate and below the drum where the combustion of fuel takes place. This complete chamber is divided into three separate chambers. These chambers are so arranged, that, the gases can flow from one chamber to the other and give out heat in each chamber gradually. Hence the first chamber is hottest and the last chamber is at lowest temperature.
The front end of the boiler is connected to the uptake header by a short tube and the rear end is connected to the down-take header by a long tube. In between the headers a number of tubes are fitted. The ends of the tubes in the headers are either expanded or welded in order to have gas tight joint. These tubes are arranged in zig-zag manner, so that more surface area of the tubes is exposed to hot gases. These tubes are sloping, downwards towards the rear end. The inclination of the tubes with the horizontal, will be in between 5-15°.
A mud collector is provided at the bottom most position of down take header for collecting and disposing the mud or any other suspended impurities. The entire boiler except the furnace in suspended on steel structure, so that the boiler can expand or contract freely without producing the cracks into the brick work, which encloses the boiler and furnace. This brickwork is lined from inside by means of fire bricks. Doors provided give access for cleaning inspection and repair purpose. Dampers provided are used for regulating draught.
This boiler is fitted with various mountings viz., safety valve, steam stop valve, pressure gauge, water level indicator, fusible plug, feed check valve, blow-off valve etc. This boiler is also fitted with super-heater as shown in Fig. 11.10.
Design of Sterling Bent Tube Boiler:
It is a –
(a) Water tube,
(b) Multi drum and
(c) Externally fired boiler.
The boiler mainly consists of –
(i) Steam drum A.
(ii) Water drum/Mud drum B.
(iii) Bottom headers C and D.
(iv) A bundle of tubes and
The steam drum A and water drum B are connected of by means of a number of tubes. The ends of the tubes are bent so that they can be connected to the drums radially as shown in Fig. 11.11. The ends of the tubes in the drums are expanded in order to have gas tight joint. The water drum is connected to the water headers C and D as shown in Fig. 11.11 by means of 2 pipes on either end (front and rear end). The upper steam drum is connected to the bottom headers by means of a number of tubes on either side.
Liquid fuel such as L.D.O. (Light Diesel Oil) can be used as a fuel and burners are provided at the front end. The boiler is suspended over the steel structure. The steel structure is enclosed in red brickwork and which is lined from inside by means of fire bricks.
E—are the doors which give access to the inside of the combustion chamber for inspection and repair purpose. F—are the baffles which divide the combustion chamber into 3 separate chambers (I, II and III). These 3- chambers are so connected that the hot gases can flow from the I-chamber to III-chamber and they are made to pass through economiser and finally they are exhausted through chimney. The water in the tubes will absorb heat in each chamber, so the temperature of flue gases will go on decreasing towards the rear end.
The boiler is fitted with various mountings viz.:
(a) Spring loaded safety valves
(b) Pressure gauges
(d) Water level indicators
(e) Feed check valve.
(f) Steam stop valve,
(g) Blow off valve.
Also superheater and economiser are connected in the boiler circuit as accessories.
Water will be pumped by means of a centrifugal multi-stage feed pump. Water first enters the bottom header of the economiser and when it rises to through the tubes, it recovers some of the heat of the flue gases and becomes hot. The hot water will then be collected into the upper header of the economiser and from where it goes to the upper drum.
Water is filled in the drum upto a pre-specified level, so that tubes, water drum, headers and vertical tubes are flooded by means of water.
We know that the I-chamber is at the hottest temperature, II-chamber is at the medium hot temperature and III-chamber is at the lowest temperature. So, due to this difference, in temperature, difference in densities of water is created and water will rise from water drum to steam drum. As the water rises, some of the water gets converted into steam. Water and steam then enters the steam drum A. In this drum steam gets accumulated in the steam space and the water which has not converted into steam; flows down to bottom headers through vertical tubes, and from bottom headers water goes to the water drum and again the cycle repeats.
The steam so generated will be wet, in order to have superheated steam, it is made to pass through the superheater and then it is utilised for power generation purpose.