Steam Boiler Working principle and Types of Boiler

Transcription

1 Steam Boiler Working principle and Types of Boiler Definition of Boiler Steam boiler or simply a boiler is basically a closed vessel into which water is heated until the water is converted into steam at required pressure. This is most basic definition of boiler. 1

2 Working Principle of Boiler The basic working principle of boiler is very simple and easy to understand. The boiler is essentially a closed vessel inside which water is stored. Fuel (generally hydrocarbon and coal) is bunt in a furnace and hot gasses are produced. These hot gasses come in contact with water vessel where the heat of these hot gases transfer to the water and consequently steam is produced in the boiler. Then this steam is piped to the turbine of thermal power plant. There are many different types of boiler utilized for different purposes like running a production unit, sanitizing some area, sterilizing equipment, to warm up the surroundings etc. 2

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4 Boiler properties: (i) Safety. The boiler should be safe under operating conditions. (ii) Accessibility. The various parts of the boiler should be accessible for repair and maintenance. (iii) Capacity. Should be capable of supplying steam according to the requirements. 4

5 (iv) Efficiency. Should be able to absorb a maximum amount of heat produced due to burning of fuel in the furnace. (v) It should be simple in construction. (vi) Its initial cost and maintenance cost should be low. (vii) The boiler should have no joints exposed to flames. (viii) Should be capable of quick starting and loading. 5

6 CLASSIFICATION OF BOILERS According to what flows in the TUBE 1. Water tube. 2. Fire tube. There are mainly two types of boiler water tube boiler and fire tube boiler. In fire tube boiler, there are numbers of tubes through which hot gases are passed and water surrounds these tubes. Water tube boiler is reverse of the fire tube boiler. In water tube boiler the water is heated inside tubes and hot gasses surround these tubes. These are the main two types of boiler but each of the types can be sub divided into many which we will discuss later. 6

7 Water Tube Boiler A water tube boiler is such kind of boiler where the water is heated inside tubes and the hot gasses surround them. This is the basic definition of water tube boiler. Actually this boiler is just opposite of fire tube boiler where hot gasses are passed through tubes which are surrounded by water. 7

8 Water tube boilers, vertical striate tubes 8

9 Types of Water Tube Boiler There are many types of water tube boilers, such as Horizontal Straight Tube Boiler. Bent Tube Boiler. Cyclone Fired Boiler. Horizontal Straight Tube Boiler again can be sub - divided into two different types, Longitudinal Drum Water Tube Boiler. Cross Drum Water Tube Boiler. Bent Tube Boiler also can be sub divided into four different types, Two Drum Bent Tube Boiler. Three Drum Bent Tube Boiler. Low Head Three Drum Bent Tube Boiler. Four Drum Bent Tube Boiler. 9

10 Two Drum Bent Tube Boiler 10

11 Three Drum Bent Tube Boiler. 11

12 Four Drum Bent Tube Boiler. 12

13 Advantages of Water Tube Boiler There are many advantages of water tube boiler due to which these types of boiler are essentially used in large thermal power plant. Larger heating surface can be achieved by using more numbers of water tubes. Due to convectional flow, movement of water is much faster than that of fire tube boiler, hence rate of heat transfer is high which results into higher efficiency. Very high pressure in order of 140 kg/cm 2 can be obtained smoothly. Disadvantages of Water Tube Boiler The main disadvantage of water tube boiler is that it is not compact in construction. Its cost is not cheap. Size is a difficulty for transportation and construction. 13

14 Working Principle of Longitudinal Drum Water Tube Boiler, horizontal or Inclined or inclined Tubes 14

15 Working Principle of Cross Drum Boiler The working principle of cross drum boiler is same as longitudinal drum boiler. The feed water is fed to the cross drum through feed water inlet. Then this water comes down through the downcomer pipe and enters into inclined water tube placed in hot chamber. Here, the water becomes hot and steam is produced in the water which comes into steam chamber. Here in the steam drum steam is separated from water in natural way. 15

16 Cross Drum Water Tube Boiler Construction of Cross Drum Water Tube Boiler Cross Drum Boiler is essentially a variant of the Longitudinal Drum Boiler. In Cross Drum Boiler the steam drum is placed at cross ways to the heat source as shown in the figure. Here, the down-comer is fitted on the bottom of the drum and riser is fitted on the top of the drum via a horizontal tube as shown in the figure. 5 o to 15 o inclined water tubes are connected with downcomer and riser tubes in same manner of Babcock-Wilcox boiler. 16

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18 Water tube boilers are classified as 1. Horizontal straight tube boilers (a) Longitudinal drum (b) Crossdrum. 2. Bent tube boilers (a) Two pass (b) Three pass 3. Cyclone fired boilers 18

19 Fire Tube Boiler As it indicated from the name, the fire tube boiler consists of numbers of tubes through which hot gasses are passed. These hot gas tubes are immersed into water, in a closed vessel. Actually in fire tube boiler one closed vessel or shell contains water, through which hot tubes are passed. These fire tubes or hot gas tubes heated up the water and convert the water into steam and the steam remains in same vessel. As the water and steam both are in same vessel a fire tube boiler cannot produce steam at very high pressure. Generally it can produce maximum 17.5 kg/cm 2 and with a capacity of 9 Metric Ton of steam per hour. 19

20 Normally horizontal return fire tube boiler is used in thermal power plant of low capacity. It consists of a horizontal drum into which there are numbers of horizontal tubes. These tubes are submerged in water. The fuel (normally coal) burnt below these horizontal drum and the combustible gasses move to the rear from where they enter into fire tubes and travel towards the front into the smoke box. During this travel of gasses in tubes, they transfer their heat into the water and steam bubbles come up. As steam is produced, the pressure of the boiler developed, 20 in that closed vessel.

21 Fire tube Boilers 21

22 Types of Fire Tube Boiler There are different types of fire tube boiler likewise, external furnace and internal furnace fire tube boiler. External furnace boiler can be again categorized into three different types- Horizontal Return Tubular Boiler. Short Fire Box Boiler. Compact Boiler. Again, internal furnace fire tube boiler has also two main categories such as horizontal tubular and vertical tubular fire tube boiler. 22

23 Advantages of Fire Tube Boiler It is quite compact in construction. Fluctuation of steam demand can be met easily. It is also quite cheap. Disadvantages of Fire Tube Boiler As the water required for operation of the boiler is quite large, it requires long time for rising steam at desired pressure. As the water and steam are in same vessel the very high pressure of steam is not possible. The steam received from fire tube boiler is not very dry. 23

24 Comparison of fire tube and water tube boilers Particulars Fire-tube boilers Water-tube boilers Position of water and hot gases Hot gases inside the tubes and water outside the tubes Water inside the tubes and hot gases outside the tubes Mode of firing Generally internally fired Externally fired Operation pressure Limited to 16 bar Can go up to 100 bar Rate of steam production Lower Higher Suitability Risk on bursting Floor area Not suitable for large power plants Involves lesser risk of explosion due to lower pressure For a given power it occupies more floor area Construction Difficult Simple Suitable for large power plants More risk on bursting due to high pressure For a given power it occupies less floor area 24

25 Cont Particulars Fire-tube boilers Water-tube boilers Transportation Difficult Simple Shell diameter Large for same power Small for same power Chances of explosion Less More Treatment of water Not so necessary More necessary Accessibility of various parts Various parts not so easily accessible for cleaning, repair and inspection More accessible Requirement of skill Require less skill for efficient and economic working Require more skill and careful attention 25

26 Steam Boiler Efficiency The percentage of total heat exported by outlet steam in the total heat supplied by the fuel (coal) is called steam boiler efficiency. 26

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28 Boiler terms Shell: Consists of one or more steel plates bent into a cylindrical form and riveted or welded together. The shell ends are closed with end plates Setting: The primary function of setting is to confine heat to the boiler and form a passage for gases. It is made of brick work and may form the wall of the furnace and combustion chamber 28

29 Grate: it is a platform in the furnace upon which fuel is burnt Furnace: it is the chamber formed by the space above the grate and below the boiler shell, in which combustion takes place. Water space and steam space: the volume of the shell that is occupied by the water is termed as water space while the entire shell volume less the water and tubes is called steam space 29

30 Mountings: The items which are used for safety of boiler are called mountings Accessories: The items which are used for increasing the boiler efficiency are called accessories Water level: The level at which water stands in the boiler is called water level 30

31 Refractory: insulation material used for lining combustion chamber Foaming: Formation of steam bubbles on the surface of boiler water due to high surface tension of water 31

32 Scale: A deposit of medium due to extreme hardness occurring on the water heating surfaces of boiler because of an undesirable condition in the boiler water Blowing off: The removal of mud and other impurities of water from the lowest part of the boiler. Accomplished with the help of blow off cock or valve Lagging: Insulation wrapped on the outside of the boiler shell or steam piping 32

33 Boiler accessories Feed pumps: Used to deliver feed water to the boiler. It is desirable that the quantity of water supplied should be at least equal to that evaporated and supplied to the engine Two types of which are commonly used as feed pumps are (1) reciprocating pump (2) rotary pump 33

34 Injector Function of injector is to feed water into the boiler It is commonly employed for vertical and locomotive boilers and does not find its application in large capacity high pressure boilers Also used where the space is not available for the installation of feed pump 34

35 Economizer Is a device in which the waste heat of the flue gases is utilized for heating the feed water Economizers are of two types Independent type Integral type 35

36 Air Pre-heater The function of the air pre-heater is to increase the temperature of air before it enters the furnace. It is placed after the economizer. Flue gases pass through the economizer and then to the air preheater Degree of preheating depends on Type of fuel Type of fuel burning equipment, and Rating at which the boiler and furnace are operated 36

37 Types of air preheaters I. Tubular type II. Plate type III. Storage type 37

38 Super heater The function of a super heater is to increase the temperature of the steam above its saturation point The super heater is very important accessory of a boiler and can be used both on fire tube and water tube boilers. 38

39 Advantages of super heated steam Steam consumption of the engine or turbine is reduced Erosion of turbine blade is eliminated Efficiency of the steam plant is increased Losses due to condensation in the cylinders and the steam pipes are reduced. 39

40 Steam separator The function of a steam separator is to remove the entrained water particles from the steam conveyed to the steam engine or turbine. It is installed as close to the steam engine as possible on the main steam pipe from the boiler 40

41 According to principle of operation the steam separators are classified as follows Impact or baffle type Reverse current type Centrifugal type 41

42 High pressure and low pressure The boilers which produce steam at pressures of 80 bar and above are called high pressure boilers Examples: Babcock and Wilcox, Velox, Lamont, Benson boilers The boilers which produce steam at pressure below 80 bar are called low pressure boilers Examples: Cochran, Cornish, Lancashire and locomotive boilers 42

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