Boilers

 

BOILERS

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Introduction:

A Boiler is a completely packed furnace in which fluid (mostly water) is heated at a certain temperature. Heating a fluid does not necessarily mean boiling it. And finally, the heated or vaporized fluid exits the boiler to be used in various heating processes and applications such as water heating, central heating, powering the turbine for power generation, sanitation, and much more.

 

 

Figure(1): A Boiler.

 

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Materials used for boilers:

 

The material used for the main part (the pressure vessel) is usually “steel or alloy steel”.

Stainless steel (of crystalline structure) is used for the parts that are always wet during the heating process to avoid corrosion and to insure proper safety due to stress corrosion cracking. Also “Ferritic stainless steel (of body-centered crystal structure) is sometimes used at superheated sections that are not exposed to boiling water. Other metals such as copper and brass are used to fabricate smaller-sized boilers. However, cast iron is also an option for fabricating the heating vessel/heating part of the boiler serving the purpose of a household/domestic water heater.

 

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Energy source:

The most commonly used heat source for combustion in a boiler is several fuels such as wood, coal, oil, or natural gas. Some large-scale boilers also use nuclear fission for generating steam. Some boilers are heat recovery generators, meaning they use waste heat from industries to boil or heat the fluid.

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The efficiency of a boiler:

There are two ways in which the efficiency of a boiler is calculated:

 

·       Input-output method (direct method): This way is more common to find the efficiency of a boiler.

 

*formula: Boiler efficiency= power generated/ power supplied

                                              =Q * (Hg-Hf) /(q*GCV) *100%

                

Where,

Q = Rate of steam flow in kg/h

Hg = Enthalpy of saturated steam in Kcal/kg

q = Rate of fuel used in kg/h

Hf = Enthalpy of feed water in Kcal/kg

GCV = Gross Calorific Value in Kcal/kg

 

·       Heat-loss method (indirect method): To get the efficiency of the boiler indirectly, these parameters are needed:

 

Ultimate analysis of fuel (H₂, S₂, S, C, moisture content, ash constraint).

Percent of O₂ or CO₂ at flue gas.

Flue gas temperature at the outlet.

Ambient temperature in degrees Celsius and humidity of air in kg/kg.

GCV of fuel in Kcal/kg.

Ash percentage in combustion fuel.

GCV of ash in Kcal/kg.

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Configuration of boilers:

·       Flued boiler.

·       Fire-tube boiler.

·       Water-tube boiler.

·       Flash boiler.

·       Fire-tube boiler with a water-tube firebox.

·       Section boiler.

 

·       Fire-tube boiler: In this water is gradually filled in a boiler barrel with a small part left to be filled to accommodate the steam also known as “steam space”. This boiler is almost used in all steam locomotives. The heat source is placed inside the furnace also known as the “Firebox”. This firebox is always surrounded with water to maintain the temperature inside the firebox below the boiling point. These types of boilers usually have low production of steam in comparison to other types of boilers.

 

·       Water-tube boiler: Here in this type of boiler the tubes are filled with water and are arranged inside a furnace in the most possible configuration. This boiler offers more production of steam as compared to the Fire-tube boiler. These are also good at maintaining good pressure, hence are used at the place requiring a high-pressure application.

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Figure (2): A water-tube Boiler

 

 

Figure (3): A fire-tubed boiler

                                                          

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Mountings on a boiler:

 

There are a few fittings and devices which are necessary for the smooth and safe operation of a boiler. The different accessories on the boiler are:

 

·       Water level indicator: As the name suggests this part of the boiler indicates the water level constantly and is also known as the water gauge.

 

·       Fusible gauge: This is like a limit switch that protects the boiler from damage due to overheating for low water levels.

 

·       Feed check valve: The function of the feed check valve is to control the supply of water to the boiler and to prevent the exception of water from the boiler when the pump pressure is less as the pump is stopped.

 

·       Blow-off cock: A blow-off cock performs two functions. It may be discharging a portion of water when the boiler is in operation to blow out mud scale or sediment periodically and it may empty the boiler when necessary for cleaning, inspection, and repair.

 

·       Safety valve: The function of the safety valve is to release the excess steam when the pressure of steam inside the boiler exceeds the rated pressure.

 

·       Pressure Gauge: The function of the pressure gauge is to measure the pressure exerted inside the vessel. It is usually constructed to indicate up to double the maximum working pressure.

 

·       Man-hole: The man-hole is provided in the boiler at the bottom where all the mud and other impurities are collected. The boiler is provided with many mud-hole/ man-hole. These man-holes are used for cleaning and maintaining the

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Accessories of the boiler:

There are a few accessories that fit the boiler and increase its efficiency. The accessories are as follows:

·       Feed pumps: Feed pumps are used to feed high-pressure water into the boilers.

 

·       Injector: The function of the injector is the same as that of the feed pumps, however, it is placed at places where there is no space to fit the feed pumps.

 

·       Evaporator: An evaporator is used in the high-pressure boiler which is placed after the air in the way of flue gasses water and tube. An evaporator is hence a unit that consumes the energy of the flue gasses in the boiler.

 

·       Superheater: Superheaters raise the temperature of saturated steam without increasing its pressure. And with this, the saving of fuel can be achieved.

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Conclusion:

 

Boilers are the most important part of an industry. In various industries, water-tubed boilers are used because of their high efficiency and safety. The boiler is a way to get cheap working substances to produce energy that is then used in various other applications.

 

 

 

 

 

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·       Institute Name: BRACT's Vishwakarma Institute of Technology, Pune.

 

 

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