Boiler What is Boiler? A closed metallic vessel in which the water is heated beyond the boiling temperature by the application of heat by the combustion of fuels to convert it into steam. The function of the boiler is to supply the steam at constant pressure with desired quality by maintaining the steam generation rate and the steam flow rate equal. Classification of Boiler: Based upon relative position of water and flue gas (hot gas) carrying tubes, 1) Fire tube boiler 2)Water tube boiler Based upon location of furnace, 1) Externally fired boiler 2) Internally fired boiler Based upon method of water circulation, 1) Natural circulation 2)Forced circulation Based upon direction of principle axis of the shell, 1) Horizontal axis boiler 2)Vertical axis boiler 3) Inclined axis boiler Based upon pressure of the steam produced, 1) High pressure boiler 2)Low pressure boiler Based upon usage, 1) Stationary boiler 2) Portable boiler 3) Locomotive boiler 4) Marine boiler Fire tube Boiler: In the fire tube boiler, the hot gases produced by the combustion of fuel are passed through tubes around which the water circulates as shown in the figure 1.1. Example: Lancashire boiler, Cornish boiler. Water tube Boiler: Fig: 1.1 ` In water tube boiler, the water circulates inside the tubes while hot gases produced by the combustion of fuel flows around the water tubes externally as shown in the figure 1.2. Example: Babcock and Wilcox boiler, Stirling boiler. Fig: 1.2
Comparison of Fire tube and Water tube boiler: Description Water tube Fire tube 1 Passage of hot gases and water circulation Here water circulates through the tubes and the hot flue gases surround them externally Here the hot flue gasespasses through tubes which are surrounded by water circulation 2 Position of furnace Furnace is situated outside the boiler Furnace is situated within the boiler Lancashire Boiler: Lancashire boiler is a horizontal, internally fired, natural circulation fire tube boiler. In this boiler steam is produced at 15 bar, at the rate of 8500 kg per hour. The steam produced in this boiler is used for processing purposes in sugar mills and in chemical industries as the produced at low pressure with low rate of production. Construction: Fig: Lancashire boiler Lancashire boiler consists of a horizontal cylindrical shell placed on a brick work as shown in the below figure. Two large flue tubes of diameter about 0.4 times the diameter of shell are fitted inside the shell which extends over the entire length. In each of these flue tubes two furnace grates are provided at the entrance of flue tube and the space underneath the grate is the ash pit(ash collector). The arrangement of brick work and shell creates a rear enclosed chamber for each of flue tubes at the rear end of the boiler which in turn is connected to one bottom central channel at the underneath the shell and which in turn is connected to two side channels at the front end. Two side channels are connected at their rear end to a common rear passage which is connected to chimney as shown in the figure. The boiler shell is mounted with necessary mountings like feed valve, steam stop valve, pressure gauge, water level indicator safety valve etc as shown in the figure. Working: Fill the boiler shell with water to ¾ of its volume by the help of feed valve which will submerge both the flue tubes and the remaining space inside the shell is for collection of steam. When the fuel is supplied to the furnace and burnt, the hot gases produced will passes along the flue tubes towards rear enclosed chamber and mean while heat transfer take place from the hot gases to water through the walls of the flue tubes. The hot gas from the rear enclosed chamber is next entered into the bottom central channel and passes towards entrance of the boiler shell. In passing heat transfer takes place from the hot gas to water through bottom portion of the boiler shell which is exposed to bottom central channel. After passing along the bottom central channel, the hot gas divided at the entrance of the boiler shell and enters into the both side channels towards rear exit passage which in turn connected to chimney. In passing through side channels heat transfer take place through side portions of the boiler shell which are exposed to side channels. The steam accumulated in the steam space is collected with the help of steam stop valve.
Fig: Different views of Lancashire boiler Babcock and Wilcox Boiler: Babcock and Wilcox boiler is a horizontal, externally fired, natural circulation water tube boiler. A high capacity boiler produces steam at 40 bar at the rate of 40000 kg per hour. This boiler is used in thermal power stations as it produces steam at higher pressure with higher rate of production and also this boiler is capable of producing steam very quickly to the higher amount as and when required. Fig: Babcockand Wilcox Boiler Construction: Babcock and Wilcox boiler consists of 4 parts as shown in the figure. 1) Steam and water drum 2) Water tubes 3) Furnace 4) Super heater 1) Steam and water drum: It is horizontal cylindrical shell which is suspended with the help of iron girders which are resting on iron columns. The boiler shell is mounted with necessary mountings like feed valve, steam stop valve, pressure gauge, water level indicator safety valve etc as shown in the figure
1 Feed valve, 2 Water level indicator, 3 Pressure gauge, 4 Safety valve, 5 Steam stop valve, 6 Down take header, 7 - Up take header, 8 Descending water tube, 9 Ascending water tube, 10 Chain grate stroke, 11 Mud box with blow off pipe, 12 Baffle plates, 13 Way to chimney, 14 T tube. 2) Water Tubes: These are number of inclined water tubes at an angle of 15 degree through which water circulates. The water tubes are arranged one below the other in a serpentine fashion to form vertical row with 4 to 5 water tubes as shown in the figure. Both the ends of the tubes are connected to end boxes which are called as headers. The header which is at right side is called down take header and on the left side is called up take header. Both the headers are in turn connected to boiler with the help of pair of tubes. Descending water tube connects down take header to boiler and ascending water tube connects uptake header to boiler. Each set of vertical row with pair of headers arranged one behind the other in required number. A mud box is connected at the bottom of the down take header with blow off pipe connected to it is to collect contaminants and to remove from the water tubes. 3) Furnace (Chain grate stroke): The furnace is provided at the bottom of up take header which consists of continuous moving chain in between two rollers. The coal is fed at the front end of the chain grate stroke is burnt on the moving grate and residual ash falls at the other end of the grate in to the ash pit. 4) Super heater: It is number of u tubes secured at each end to the horizontal connecting boxes and placed in the combustion chamber underneath the boiler drum. The upper box is connected to T tube which is situated in the steam space inside the boiler drum and allows steam to enter and passes it to super heater and then to steam stop valve by passing through lower box. Working: Fill the boiler shell with water to 1/2 of its volume by the help of feed and the remaining space inside the shell is for collection of steam mean while water tubes, headers also filled with water.
When the fuel is supplied to the furnace and burnt, the hot gases produced will passes through the particular path guided by the baffle plates. The produced hot gas moves upwards around the portion of the water tubes lying in between the combustion chamber and below the boiler drum, then down wards around the portion of the water tubes in between baffle plates, and then once again upwards between the baffle plate and the down take header and finally passes out to chimney. During the path of hot gas, the water extracts the heat from the hot gas and converted to steam, then the mixture of steam and hot water raises to up take header and then collects in boiler drum. The steam collected in steam space passes to the super heater by entering into the T tube in turn connected to steam stop valve by which super heated steam can be extracted. Advantages and Disadvantages of water tube boiler over fire tube boiler. Advantages: 1) Steam can be produced more quickly 2) Steam at higher pressure can be produced 3) Higher rate of evaporation 4) Sediment deposition is less 5) Suitable for any type of fuel 6) More effective heat transfer 7) Failure of water tubes will not affect boiler operation 8) Occupies less space 9) Easy maintenance 10) Easy transportation. Disadvantages: 1) Not suitable for ordinary water 2) Not suitable for mobile application 3) High initial cost hence not economical Boiler Mountings and Accessories The devices which helps boiler to work satisfactorily, efficiently, safely and for easy maintenance are called boiler mountings and accessories. The devices are divided into two categories based upon work as, 1) Boiler Mountings: The devices which helps to control the steam generation, measurement of steam properties and safe working of the boiler. The boiler mountings are 1) Water level indicators: It indicates water level in the boiler drum and guides to maintain constant level of water in the boiler and thereby avoids overheating damages. These are fitted at the front end of the boiler for clear vision. 2) Pressure gauge: It indicates the pressure of the steam inside the boiler to maintain the pressure to a desired safer limit. These are also fitted at the front end or top of the boiler for clear vision. 3) Safety valves: It maintains the safe pressure inside the boiler. If pressure crosses the safe limit, the excess steam will blow off from the safety valves automatically and thereby maintains safer pressure limit, preventing the explosion of the boiler. Commonly used safety valves are Dead weight safety valve and spring loaded safety valve. 4) Steam stop valve: It regulates the supply of steam for the required rate whenever necessary and shut off the steam when not required. It is situated at the top of the boiler drum. 5) Blow off valve: Its function is to empty the water in the boiler drum when required for periodical cleaning (removal of sediments collected at the bottom of the drum) and inspection. It is fitted at the lowest part of the drum.
6) Feed check valve: Its function is to feed water in to the boiler drum continuously and also regulates the rate of the flow of feed water and prevents escape of water from boiler drum due to higher pressure inside the boiler drum. 7) Fusible plug: It is safety device and its function is to protect the boiler from the explosion due to the over heating due to lower water level. Whenever temperature exceeds safety limits, the plug itself melts and allows the water and steam to escape and to extinguish the fire in the furnace. 2) Boiler Accessories: The devices which helps to improve the efficiency of the boiler and to enable proper working of the boiler. The boiler accessories are 1) Economizer: It is heat exchanger which is placed in the hot gas path so that it can exchange the heat between hot gas and cold water passing through the heat exchanger. It improves the overall efficiency of boiler by reducing the fuel consumption because the feed water supplied to the boiler will be at a higher temperature. 2) Air pre heater: It is heat exchanger which is kept in between economizer and chimney which exchanges heat between hot gas and cold air passing through heat exchanger. Supplying preheated air to the furnace produces higher furnace temperature and accelerates combustion of fuel and there by increases thermal efficiency of the plant. 3) Super heater: It is heat exchanger which is fixed in line of hot gas path which recovers heat from hot gas to increase the temperature of saturated or wet steam to super heated temperature by passing steam into this heat exchanger. 4) Feed pump: It feeds the water at a high pressure in to the boiler as pressure inside the boiler is higher than surrounding pressure. 5) Steam separator: It separates condensed water particles from the steam which is entering into the turbine. It is fitted at the entrance of the turbine 6) Steam trap: It is used to drain off the collected condensed water in the steam pipe lines. It is connected to small by-pass pipe from the main steam pipe to pass the collected water.