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A steam boiler or steam generator is a closed vessel in which water is heated, vaporized, and converted into steam at a pressure higher than atmospheric pressure. They are exclusively used in thermal power plants.
- The most important function of a boiler is either to produce steam or hot water.
- Steam boilers produce steam that powers turbines for power generation and other industrial heating uses.
- Hot water boilers heat water for domestic or commercial use.
- At an industrial level, a boiler is characterized as a vessel having the capacity of holding more than 22.75 litres of water.
- They are used to produce steam or hot water.
- The steam produced from boiling on a large scale is then used in power generation for different machines inside the industry.
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Key Terms: Fuel, temperature, Boiler, latent heat of vaporization, pressure, boiling point, steam, energy, heat transfer, fossil fuels, latent heat of fusion
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Working Principle of a Boiler
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The working of a boiler is quite simple. Water is stored in closed boiler vessels. It is heated with the help of a furnace which creates hot gases. The hot gases are made to come in contact with the boiler vessel.
- With the help of this heat energy, the ultimate aim of a boiler of converting the water to steam is achieved.
- Having a higher pressure than the normal atmospheric pressure helps to achieve the required temperature quickly.
- As per the ideal gas equation, we know that P α T, thus increase in pressure leads to increase in temperature.
- Different types of boilers are used for different purposes.
- The following figure represents a water tube boiler:
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Efficiency of a Boiler
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The percentage of total heat exported by outlet steam in the total heat supplied by the fuel (coal) is called steam boiler efficiency. It is given by
Steam Boiler Efficiency (%) = Heat Exported by the outlet steam / Heat supplied by the fuel × 100
Generally, a steam boiler has an efficiency of 80% – 88%.
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Parts of a Boiler
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The components of the boiler are divided into sections as given below –
Boiler Mounting
These components decide the performance of the boiler, they include:
- Pressure Gauge: For measuring the pressure inside the boiler, it is placed in the front of the boiler
- Water Level Indicator: As the name suggests, to indicate the level of water inside the boiler, it has three cocks
- Stop Valve: This valve is used to control the flow of steam from the boiler to the steam pipe
- Safety Valve: This is an important valve as it helps in preventing the boiler from explosion due to pressure. It is fitted in the drum. This valve blows off when the pressure inside the boiler exceeds the defined limit. There are two such valves.
- Blow-Off Clock: It is used to empty the boiler and is fitted at the bottom of the drum
- Grate: It is the furnace that is used for fuel-burning
- Feed Check Valve: It regulates the water supply, it is fitted slightly below the normal water level
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Boiler Accessories
These parts control the efficiency of the boiler, they include:
- Super Heater: This component superheats the steam before passing it further because saturated steam can cause the corrosion of machines
- Feed Pump: This pump ensures that the boiler receives a continuous supply of water.
- Air Preheater: This component preheats the air before passing it to the boiler
- Economizer: The fuel gasses from the boiler are passed through the economizer as it increases the boiler’s efficiency.
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Classification of a Boiler
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The various classifications of a Boiler are listed below based on various parameters,
Based on the Passage of Hot Water and Gases
- Water-tube boiler – the hot combustion gases surround the water which flows through the tubes
- Fire-tube boiler – here the opposite happens where the hot combustion gases are surrounded by water
Based on Water Circulation
- Natural circulation – the density difference in the water causes the water circulation
- Forced circulation – here the water is pumped to the desired route inside the boiler
Based on Usage
- Portable boiler – is mobile, used in temporary sites
- Marine boiler – used in ships
- Stationary boiler – used in power plants
- Locomotive – used in railway engines
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Based on the Boiler’s Position
- Horizontal
- Inclined
Based on Generated by the Steam
- Supercritical boiler – has more pressure than the critical pressure
- Low-pressure boiler – 15-20 bars
- High-pressure boiler – has greater pressure than 80 bars
- Sub-critical boiler – has subcritical pressure
- Medium pressure boiler – 20-80 bars
Based on the furnace change
- Supercharged fuel
- Pulverized fuel
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Uses of Boiler
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The various uses of the boiler are listed below for your reference,
- Reciprocating pumps
- Steam engines
- In cold areas
- Steam turbines
- Industrial processing in chemical engineering
- Thermal power stations
- Processing industries like textile mills, sugar mills, dairy industry, and chemical industries
- Paper industries
- Fast Moving Consumer Goods – FMCG
- Distilleries
- Health care industries
- Refineries
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Things to Remember
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- Steam boilers produce steam that powers turbines for power generation and other industrial heating uses.
- Hot water boilers heat water for domestic or commercial use.
- The increase in pressure in the boiler increases the boiling point of the substance.
- A boiler is used to produce steam energy
- Different types of boilers are used for different purposes
- Boilers are used on a large scale as they make use of water which is so easily available
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Sample Questions
Ques. What is the latent heat of fusion and latent heat of vaporization? (3 Marks)
Ans. The latent heat of fusion is defined as the heat per unit mass required to change a substance from solid to liquid at the same temperature and pressure. It is denoted by Lf. Latent heat of vaporization is defined as the heat per unit mass required to change a substance from liquid to vapour state without a change in temperature and pressure. It is denoted by Lv.
Ques. What is the normal boiling point and how does pressure affect the boiling point of a substance? (3 Marks)
Ans. The normal boiling point of a substance is defined as a temperature at which the liquid and vapour states of the substance coexist at atmospheric pressure. As per ideal gas law pressure is directly proportional to temperature. When pressure increases, the boiling point of the substance also increases. The increased pressure causes a more effective collision of molecules to happen, the rate at which the molecules move also increases, and vibration increases thus it naturally increases the temperature.
Ques. Explain Triple Point. (2 Marks)
Ans. While plotting a pressure vs temperature graph of a substance, we will have three different sections, separating the existence of different states of that substance at particular pressure and temperature. The graph is divided into a sublimation curve which marks the region where solid and gas coexist, a vaporization curve which marks the region where liquid and gas coexist, and a fusion curve which marks the region where solid and liquid coexists. The point where these three curves meet is known as the triple point. A triple point is defined as the point which marks a particular temperature and pressure where solid, liquid, and gaseous phases of a given substance are all at equilibrium with one another. It is unique for different substances.
Ques. 31.65 g of sodium chloride is added to 220.0 mL of water at 34 °C. How will this affect the boiling point of the water? Assume the sodium chloride completely dissociates in the water.
Given –
density of water at 35 °C = 0.994 g/mL
Kb water = 0.51 °C kg/mol (5 Marks)
Ans. To find the temperature change elevation of a solvent by a solute, use the equation:
ΔT = iKbm
where:
ΔT = Change in temperature in °C
i = van't Hoff factor
Kb = molal boiling point elevation constant in °C kg/mol
m = molality of the solute in mol solute/kg solvent
molality (m) of NaCl = moles of NaCl/kg water
From the periodic table –
atomic mass Na = 22.99
atomic mass Cl = 35.45
moles of NaCl = 31.65 g x 1 mol / (22.99 + 35.45)
moles of NaCl = 31.65 g x 1 mol / 58.44 g
moles of NaCl = 0.542 mol
kg water = density x volume
kg water = 0.994 g/mL x 220 mL x 1 kg/1000 g
kg water = 0.219 kg
mNaCl = moles of NaCl/kg water
mNaCl = 0.542 mol / 0.219 kg
mNaCl = 2.477 mol/kg
NaCl completely dissociates into the two ions, Na+ and Cl-. Therefore, here, i = 2.
ΔT = iKbm
ΔT = 2 x 0.51 °C kg/mol x 2.477 mol/kg
ΔT = 2.53 °C
Adding 31.65 g of NaCl to 220.0 mL of water will raise the boiling point by 2.53 °C.
Ques. What is the boiling point elevation when 147 g of lactic acid (C6H10O5) is dissolved in 647 g of cyclohexane (C6H12)? The boiling point constant for cyclohexane is 2.79 °C/m. (5 Marks)
Ans. The molality of the lactic acid solution is
m = (147 g / 162.14 g/mol) / 0.647 kg
m = 1.40127 mol/kg
ΔTb = Kb × m
ΔTb = (2.79 °C kg mol-1) × (1.40127 mol/kg)
ΔTb = 3.91 °C
Ques. Calculate the heat required to convert 3 kg of ice at –12 °C kept in a calorimeter to steam at 100 °C at atmospheric pressure.
Given-
specific heat capacity of ice = 2100 J kg-1 K-1,
specific heat capacity of water = 4186 J kg-1 K-1,
latent heat of fusion of ice = 3.35 105 J kg-1 K-1 and
latent heat of steam = 2.256 106 J kg-1. (5 Marks)
Ans. We have
Mass of the ice, m = 3 kg
specific heat capacity of ice, Sice = 2100 J kg-1 K-1
specific heat capacity of water, Swater = 4186 J kg-1 K-1
latent heat of fusion of ice, Lf(ice) = 3.35 105 J kg-1
latent heat of steam, Lsteam = 2.256 106 J kg-1
Now,
Q = heat required to convert 3 kg of ice at –12 °C to steam at 100 °C,
Q1 = heat required to convert ice at –12 °C to ice at 0 °C.
» m sice ΔT1 = (3 kg) (2100 J kg-1 K-1) [0 – (–12)] °C
= 75600 J
Q2 = heat required to melt ice at 0 °C to water at 0 °C
» m Lf ice = (3 kg) (3.35 105 J kg-1)
= 1005000 J
Q3 = heat required to convert water at 0 °C to water at 100 °C.
= m swater ΔT2 = (3kg) (4186J kg-1 K-1) (100 °C)
= 1255800 J
Q4 = heat required to convert water at 100 °C to steam at 100 °C.
» m Lsteam = (3 kg) (2.256106 J kg–1)
= 6768000 J
So,
Q = Q1 + Q2 + Q3 + Q4
= 75600J + 1005000 J + 1255800 J + 6768000 J
= 9.1106 J
Ques. A pan filled with hot food cools from 94 °C to 86 °C in 2 minutes when the room temperature is at 20 °C. How long will it take to cool from 71 °C to 69 °C? (3 Marks)
Ans. The average temperature of 94 °C and 86 °C is 90 °C, which is 70 °C above the room temperature. Under these conditions, the pan cools to 8 °C for 2 minutes.
We know the equation,
Change in temperature time = K × T
8 degrees2min= K × 700C
The average of 69 °C and 71 °C is 70 °C, which is 50 °C above room temperature. K is the same for this situation as for the original.
2 degreesTime= K × 500C
When we divide the above two equations, we have
8 degrees2 min2 degreesTime=K(70 degrees)K(50 degrees)
Time = 0.7 min
= 42 s
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