Thermodynamics and Heat Engine

LESSON - 27 STEAM BOILERS/GENERATORS, CLASSIFICATION OF BOILERS, LANCASHIRE BOILER

27.2. CLASSIFICATION OF BOILERS

Because of differences in features and characteristics of different boilers, the boilers can be classified on the basis of the following:

Content of tubes Firing method Mode of heating Pressure of steam Mode of circulation of working fluid (water) Purpose of utilization Position and number of drums Gas passage

Nature of draft Nature of heat source Circulation of working fluid Nature of Fluid used Boiler shell material Type of Fuel Shape of tubes and their spatial position Boiler size and rating

27.2.1.   Content of tubes: The boiler is either Fire-tube boiler or Water-tube boiler, depending upon the content flowing inside its tubes.

Fire-tube boiler. In this boiler, the products of combustion/hot flue gases flows inside and the water flow outside of the boiler tubes. Figure 27.1shows the fire-tube boiler.

The examples of the fire-tube boilers are

(a)    Cochran, (b) Lancashire, (c) Cornish and (d) Locomotive boilers.

 

Fig. 27.1. Fire-tube Boiler

Water-tube boiler. In water-tube boiler, water flows inside the tubes and the hot flue gases flow outside the tubes. Figure 27.2 shows the water-tube boiler.

The examples of the water-tube boilers are

(a)    Babcock and Wilcox boiler and (b) Stirling boiler.

Fig. 27.2. Water-tube Boiler

Comparative advantages and disadvantages of fire-tube and water-tube boilers:

Sr. No.	Parameters	Fire-tube boilers	Water-tube boilers
1	Rate of steam generation	slow	quick
2.	Suitability for power plants	unsuitable	suitable
3.	Operating steam pressure	Limited to 24.5 x 105 N/m2	Limited to 200 x 105 N/m2
4.	Chances of explosion	Less	More
5.	Risk of damage due to explosion	Much more	Much less
6.	Water treatment	Not very necessary as minor scaling would not go far enough to cause overheating and tube-bursting	Required as scaling will lead to tube-bursting
7.	Floor space required	Much	Less
8.	Cost and construction problem	More	Much less
9.	Transportation	Inconvenient due to large size of the shell	Comparatively easier
10.	Skill required for efficient operation	Less	More
11.	Operating cost	Less	High
12.	Overall efficiency	Upto 75%	Upto 90%

27.2.2.  Firing method: The boiler is either internally fired or externally fired boiler, depending upon the position of the furnace with respect to boiler.

Internally fired boiler: The furnace is provided inside the boiler shell and is completely surrounded by water cooled surfaces as shown in Fig. 27.1. Lancashire, Locomotive, Cochran and Scotch boilers are common examples of the internally fired boilers.

Externally fired boiler: The furnace is provided outside the boiler (Fig. 27.2).The furnace of this boiler is simple and easy to construct and can be easily enlarged. Babcock and Wilcox boileris the example of externally fired boiler.

27.2.3.  Mode of heating: The boilers may be classified according to mode of heating as

Fired boiler: The water receives heat for steam generation by burning of fuel.

Non-fired boilers: The water receives heat other than that produced by burning of fuel.

 27.2.4.  Pressure of Steam: Boilers may be classified according to steam pressure in the boiler.

Low pressure boiler: the steam pressure in boiler is below 80 bar. Cochran, Cornish, Lancashire and locomotive boilers are low pressure boilers.

High pressure boiler: the steam pressure in the boiler is higher than 80 bar. Babcock and Wilcox, Lamont, Velox, Benson, power etc. are examples for high pressure boilers.

 27.2.5.  Mode of circulation of working fluid (water): Depending on water circulation, the boilers are classified as Natural Circulation boiler or Forced (i.e., positive) Circulation boiler.

Natural Circulation boiler: In this, the water is circulated by natural convection current or by gravity which are created due to temperature difference. The boilers of low capacity such as Lancashire, Locomotive, Babcock and Wilcox boilers, etc fall under this group.

Forced (i.e., positive) Circulation boiler: In this, the water is the circulated by mechanical means (pumps). The forced circulation helps in to increase the water circulation along with uniform heat distribution throughout the boiler water thus results in rapid steam generation at high pressure. The boilers of high capacity such as Lamont, Velox, power boilers, etc. fall under this group.

 27.2.6.   Purpose of utilization: Boilers are also classified according to nature of service as follows:

Stationary/land boilers: Boilers that are used for stationary plants are called stationary or land boilers. Most of the industrial boilers used for processing work and power plant boilers used for power generation are of stationary boilers.

Portage boilers: Boilers which can be easily dismantled and transported from one place to another place for temporary use at sites.

Mobile boilers: The boilers which are fitted on vehicles and continuously move from place to place are called mobile boilers. The boilers used to run Marine and locomotive are the examples of mobile boiler.

27.2.7.  Position of boiler: The boiler may be classified as (a) horizontal, (b) inclined and (c) vertical,according to position of the boiler.

      Number of Drums: The water-tube boilers may be classified as (a) Single    (b) Multi-drums,according to number of drums.

27.2.8.  Gas Passage: The fire-tube boilers may be classified as  (a) Single pass (b) Multi Pass, according to number of gas passages through a cylindrical shell.

27.2.9.  Nature of draught: According to this, the boiler may be classified as

Natural draft boilers: In this the draft is produced by natural circulation of air and gas. In effect the chimney, boiler and the outer air constitute a large U-tube as shown in Fig 27.3.  The column of gases in the chimney is one vertical limb; the other limb is a column of cold outside air and the lower horizontal portion of the U-tube represents the boiler. The difference in density/pressure in the column of hot gases in the chimney and column of cold air in the other limb of U-tube cause the natural circulation of air. Factors which increase the draft:  •  The temperature of the colder, more dense, outside air  increases the draft. •  The temperatures of the hotter, lighter gases in the chimney increases the draft. •   Higher height produces more draft. Forced draft boilers: Here the draft is produced by mechanical means with one or more electric motor or steam turbine driven fans. When using a forced draft system, the entire furnace casing must be strong enough to withstand the internal pressure and must be carefully sealed against outward leakage as furnace casing is under a positive pressure.

Fig. 27.3.  Natural Draft

27.2.10. Nature of heat Source: Depending on the nature of heat source, boilers can be classified as:

Fuel fired boiler – These derive their heat energy by combustion of fuel which may be solid, liquid, or gaseous.

Waste heat boilers– recovers heat from the hot waste gases of other chemical reactions.

Electrical powered boilers– generate steam by the application of electrical energy.

Nuclear powered boilers– Utilize the energy of controlled thermonuclear fission reactions to generate steam.

27.2.11. Circulation of working fluid: Depending on the circulation of working fluid, water-tube boilers can be classified as (a) Once through boiler and (b) recirculation boiler:

Once through Boiler: In once through boiler, there is no recirculation of water. The feed water enters from one side of the water tube and leaves the tube as steam. The Benson boiler is an example of once through boiler.

Recirculation boiler: In this boiler, only a part of water is evaporated and the remaining water is circulated.

27.2.12.  Nature of Fluid Used: On the basis of working fluid used in the boilers, the boilers are classified as (a) Steam boilers (b) Mercury boilers.

In steam boilers, water is used as working fluidand in Mercury boilers, mercury is used as working fluid.

27.2.13. Boiler Shell Material: The boilers are also classified on the basis of its shell materialas (a) Cast iron boiler, (b) Steel boilers, (c) Copper and stainless steel boiler.

The cast iron boilers are suitable for low pressure; steel boilers are suitable for high pressure and copper and stainless steel boilers are suitable for miniature type of boilers.

27.2.14. Type of Fuel: According to the use of fuel in boiler, the boilers are classified as

a)      Coal fired boiler: Pulverized coal fired, Stoker fired and Hand fired

b)      Gas fired boiler

c)      Oil fired boiler

d)     Wood fired boiler

e)      Bagasse fired boiler

27.2.15. Shape of tubes and their spatial position

Depending on the shape of tubes the boilers may be classified as a) Straight tube boiler and b) Bent tube boiler.

Based on the inclination of tubes the boilers may be classified as a) Horizontal boiler,  b) Vertical boiler  and c) Inclined boiler.

27.2.16.  Boiler size and rating: Based on their size and rating, the boilers are classified into (a) commercial boilers, (b) residential boilers and (c) oil fired boilers.

Commercial boilers:  Heating surface = 11.98 – 331.756 m²

Gross heat output = (10827-300) kW

Residential boilers: Heating surface = 1.486 – 27.313 m²

Gross heat output = upto 30 kW

Oil fired boilers: Heating surface = 1.486 – 27.313 m²

Gross heat output range is of upto 30 kW

27.3.  FIRE-TUBE BOILERS

As discussed under boiler classification, in a fire-tube boiler, the flue gases flow inside one or more number of horizontal or vertical tubes passing through the cylindrical shell and water in the cylindrical shell surrounds these tubes.

• Lancashire boiler,

• Cochran boiler 

• Locomotive boiler

come under this category and are discussed in detail in this lesson and next lesson.

 27.3.1  Lancashire Fire-Tube Boilers

 Features and characteristics

•  Lancashire boiler is

                  - a stationary or land type,

                  - fire-tube,

                  -  horizontal straight tubes,

                  -  internally fired,

                  - natural circulation boiler.

•  It can be worked upto a normal working pressure range of 15 bar and its steaming capacity varies from 1400 to 5500 kg/h.

•  Its shell size ranges from about 5.5 metres to 10 metres in length to 2 to 3.5 metres in diameter.

•  Because of the simplicity of its design and ease of operation, the boiler is reliable and popular.            

•  Its maintenance is easy and operating costs is less.

Uses

This is widely used in chemical industries and sugar mills where steam is used for power generation and process work.

 Construction details:

Figure 27.4 shows the front view, sectional side view and top view of a Lancashire boiler and its brickwork setting. The parts are as follows:

Boiler shell: The cylindrical shaped boiler shell is made by steel plates which are either riveted or welded. It contains two parallel flue tubes which run throughout the entire length of the shell. In this shell water is converted into steam. It is a pressure vessel which is designed to withstand high steam pressure.

Flue tubes: Two tapered flue tubes are passed through the shell for the flow of first pass of the flue gases from front to the back of the boiler.

Furnace with fire grate: Each of flue tube has its own furnace with grate of about 2 m length arranged at the front end of the shell.

Fire bridge  is provided at the back of the grate prevents the fuel from falling over the end of the furnace and is also used for deflecting the gases of combustion upwards.

Doors are provided in the front brick work through which the coal is fed to the grates where its combustion takes place.

Dampers are provided in the flue passage for controlling the flow of flue gases and amount of air entering the grate by restricting the passage of the flow. This controls the generation of steam. These dampers are iron doors which slide up and down in the grooves (in the side flues) by means of chains, ropes or pulley.

Fig. 27.4. Lancashire Boiler  

Water level gauge indicates the level of water in the boiler and enables the attendant to regulate the supply of feed water.

Dead weight safety valve is for safety against excessive pressure building up in a steam boiler. 

Steam stop valve is mounted on the top of the boiler and is used to open and close the supply of steam for use.

Man hole is provided for the entrance of a man inside the boiler shell and is required for cleaning, inspection and repairing of the boiler.

Low water high steam safety valve is for blowing out steam if the steam pressure is higher than the working pressure and blowing out steam when the water level in the boiler is low.

Bottom flue directs the flue gases from back to front along the bottom of the boiler shell.

Side flue The side flue on both sides of the shell directs the flue gases from front to back along the side of the boiler shell.

Main flue The main flue directs the flue gases from side flues to chimney.

Ash pit is for depositing ash.

Blow of cock is situated at the lowest water space of a boiler to serve three purposes: (i) to remove the precipitated sludge or loose scale at intervals (ii) to lower the water level whenever the water level in the boiler has become accidentally too high (iii) to empty the water in the boiler whenever required for periodical inspection.

Blow off pit is for disposing of the blow off water.

Gusset stays is for securing and stiffening the four corners of the shell.

Perforated feed pipe is used for uniform feeding of water in the boiler during steam generation.

Anti priming pipe is fitted inside the boiler drum or shell and is also known as steam drier. It is used for separating out suspended water particles contained in the steam and allowing as far as possible the dry steam through the stop valve.

Fusible plug is mounted at the required lowest safe water level in a boiler.This device is intended to operate if the water level in the boiler falls too low, by permitting a discharge of steam into the furnace, thus quenching the fire.

U-tube Superheater is for superheating the wet or dry saturated steam.

Working:

(i) Path of Flue gas: The flue gases complete their path in three passes in boiler before they finally are discharged to the atmosphere.

First pass: The hot flue gases from the fuel burnt in the furnace first travel up to back of the boiler through flue tubes and then at the end of first path they are deflected by dampener in the downward direction towards the bottom flue to start second pass.

Second pass: There after they flow in the bottom flue from back to the front of boiler as shown by the arrow in the front view of Fig. 27.4. As the bottom flue is situated below the water shell the hot gases heat the lower portion of the shell.

Third  pass: After completing second pass, the flue gases are then bifurcated into separate paths in the side flues and complete their third pass by travelling from front to back of the boiler in the side flues as shown by the arrows in the sectional top and side view of Fig. 27.4. Flue gases in side flue thus heat the side of the water shell.

After completing three passes, the two streams of the flue gases from side flue meet again in the main flue where they finally discharged to atmosphere through the chimney.

(ii) Path of steam flow: The feed water is supplied uniformly to the shell by a perforated feed pipe controlled by a feed valve. When the boiler is heated, the steam is generated and collected in steam space over the water surface of the shell. The steam in contact with the water surface always contains water particles. To remove these water particles, the steam in the steam space first passes through the anti-priming pipe before it comes to the steam main pipe through a steam stop valve or it may move to the superheater. The circulation of water in the shell is shown by solid arrows in the sectional side view.

 (iii) Superheater arrangement for Lancashire boiler: Figure 27.5 shows the arrangement for producing superheated steam in Sudgen’s hair pin type Superheater of Lancashire boiler. The arrows show the path of the hot gases, when superheater is in use.  When superheated steam is desired to be produced, damper to superheater U-tubes is opened and the other damper to bottom flue is closed as shown in Fig. 27.4. Then the flue gases from flue tube pass on to the superheater U-tubes at the back of the boiler before it travel towards the bottom flue. In this case, valve V₁ and V₂ are opened and valve V₃ is closed so that steam from shell passes to the superheater U-tubes and after getting superheated passes to the main steam pipe outlet. By adjusting the gas damper, the temperature of the steam coming out of the superheater is always maintained constant irrespective of the amount of steam through the superheater. If the superheated steam is not required, the damper to the superheater U-tubes is closed as shown in Fig 27.4 and the other damper to the bottom flue is opened thus the hot gasses by passing superheater U-tubes and flows directly into the bottom flue and also valve V₃ is opened  and valve V₁ and V₂ are closed and passing boiler steam directly out to main steam outlet through valve V₃.

 Fig. 27.5.  Sudgen’s hair pin type Superheater of Lancashire Boilers

 (iv) Draft system: The supply of air to the grate is usually done naturally with the help of a chimney.