Boiler Draught (Types, construction and working)

Boiler Draught (Types, construction and working)

1. Prof . Avesahemad S. N. Husainy Assistant Professor, Department of Mechanical Engineering Member of ISHRAE and ASHRAE Applied Thermodynamics I Unit No 2 Boiler Draught (Lecture 6) https://www.youtube.com/channel/UCPVNu2txpcsFU2eZf0DPRsg Sharad Institute of Technology College of Engineering , Yadrav ( Ichalkaranji ) NAAC ‘A’ grade accredited institute, NBA accredited programs, An ISO 9001:2015 accredited institute. 21/06/2020 ASNH 1

2. 21/06/2020 ASNH 2 Introduction of Boiler Draught Boiler draught may be defined as the small difference between the pressure of outside air and that of gases within a furnace or chimney at the grate level, which causes the flow of air/hot flue gases to take place through the boiler. The draught is necessary to force air through the fuel bed/ grate to aid in proper combustion of fuel and to remove the products of combustion i.e. flue gases to the atmosphere after they have given their heat to water being evaporated in the boiler. Classification of Boiler Draught

3. 21/06/2020 ASNH 3 1. Natural or Chimney Draught: In this case the amount of draught directly depends upon the height of chimney. It is produced due to the difference in densities between the column of hot gases in the chimney and a similar column of cold air outside the chimney.

4. 21/06/2020 ASNH 4 2. Artificial Draught: Far producing much draught, the chimney height has to be increased considerably, which is neither convenient nor economical. Also, since the draught depends upon the climatic conditions, some mechanical equipment's are used for producing the required draught and the draught so produced is called as the artificial draught . i) Forced Draught: In a Forced draught system, a Fan or Blower is provided is shown in figure which forces the air in the combustion chamber. In the combustion chamber combustion of air and fuel takes place and hot gases generated. These gases are forced to pass through the flues, economizer, air pre-heater and then they are exhausted after recovering heat of flue gases.

5. 21/06/2020 ASNH 5 ii) Induced Draught: In this system, the Blower or Induced Draught fan is located near the base of chimney. The air is sucked in the system, by reducing the pressure through the system below atmosphere. The flue gases, generated after combustion are drawn through the system and after recovering heat in the economizer, air-preheater, they are exhausted through the chimney to the atmosphere.

6. 21/06/2020 ASNH 6 3. Balanced Draught: It is always preferable to use combinations of I.D. and F.D. instead of Forced or Induced draught alone . If Forced Draught alone is used then the furnace cannot be opened for firing or for inspection. Because the high pressure air/gases inside the furnace will try to blow out, and there is every chance of blowing out of the fire completely and the furnace may stop . If Induced Draught fan alone is used, then also furnace cannot be opened either for firing or for inspection. Because the cold air will try to rush into the furnace, which reduces the effective draught . To overcome both these difficulties Balanced Draught is used. In this case I.D. fan and F.D. fan are provided

7. 21/06/2020 ASNH 7 Steam Jet Draught Construction of the system is simple and easy to maintain. Thus, maintenance cost is also low. In steam jet draught system a small portion of generated steam exhausts through a nozzle and the kinetic energy of the jet of high-velocity steam drags the air or flue gases in the boiler system. We can categorize the stream jet draught in two types. One is natural stream jet draught, and another is force steam jet draught.

8. 21/06/2020 ASNH 8 In forced stream jet draught, we feed a portion of the steam produced in the boiler to the entry point of the furnace through diffusion pipe. Due to the kinetic energy of steam, there will be a draught at the entry point because of which fresh air gets dragged to the grate and then to the economizer then to preheater then ultimately to the chimney. Forced Stream Jet Draught

9. 21/06/2020 ASNH 9 Natural Steam Jet Draught In natural steam Jet draught, we fit the steam nozzle in a smoke box attached to the bottom portion of the stack. The steam forcefully enters in the smoke box due to which flue gases created in the furnace get dragged in the smoke box because of the resultant draught created the kinetic energy of the steam jet. This process of creating draught in this way is called natural jet steam draught.

10. 21/06/2020 ASNH 10 Perform-Reform-Transform