Introduction
In a thermal power plant, the boiler is called the heart of the system. Without the boiler, no steam is produced — and without steam, the turbine cannot rotate, meaning no electricity generation.
For mechanical engineers working in power plants, understanding boiler working is extremely important for:
Interviews
Competitive exams
Commissioning
Maintenance
Troubleshooting
In this complete mechanical guide, I will explain:
Boiler mountings & accessories
Draft system
Common boiler problems
Interview questions
Everything explained in simple, practical language.
🔥 What is a Boiler?
A boiler is a closed pressure vessel in which water is heated using fuel combustion to generate steam at high pressure and temperature.
Simple Definition:
A boiler converts water into steam by using heat energy.
In thermal power plants, coal is generally used as fuel.
⚙️ Boiler Working Principle
The working principle of a boiler is based on heat transfer.
When coal burns inside the furnace:
Heat is generated.
Heat transfers to water through tubes.
Water absorbs heat and converts into steam.
Steam is collected in the steam drum.
Steam is further superheated before going to turbine.
🔄 Step-by-Step Boiler Working Process
Let’s understand it practically.
Step 1: Fuel Combustion
Coal is pulverized into fine powder and burned in the furnace.
Step 2: Water Circulation
Feed water enters the boiler drum.
Step 3: Steam Formation
Water absorbs heat and converts into wet steam.
Step 4: Steam Separation
Steam separates from water in the drum.
Step 5: Superheating
Steam passes through superheater tubes and becomes dry steam.
Step 6: Steam to Turbine
High-pressure steam goes to the steam turbine.
🏭 Boiler Diagram (Main Parts Explained)
🔧 Main Parts of a Power Plant Boiler
Let’s discuss each component in detail.
1️⃣ Furnace
The furnace is where coal combustion takes place.
Temperature: 1000°C – 1700°C
Fuel: Pulverized coal
Produces hot flue gases
Common Issues:
Clinker formation
Flame instability
Slagging
2️⃣ Boiler Drum
The boiler drum separates steam from water.
Functions:
Steam-water separation
Water storage
Chemical dosing point
Drum Level Control is very critical.
Low level → Tube damage
High level → Carryover
3️⃣ Water Wall Tubes
These tubes surround the furnace.
Function:
Absorb maximum heat
Protect furnace wall
Water flows inside these tubes and converts into steam.
4️⃣ Superheater
The superheater increases steam temperature.
Why needed?
Wet steam damages turbine blades.
Superheater converts:
Wet steam → Dry superheated steam
5️⃣ Economizer
The economizer increases boiler efficiency.
Function:
Preheats feed water using flue gases
Reduces fuel consumption
6️⃣ Air Preheater (APH)
Air preheater heats combustion air using flue gases.
Benefits:
Better combustion
Fuel saving
Increased efficiency
🔥 Types of Boilers Used in Power Plants
1️⃣ Fire Tube Boiler
Hot gases flow inside tubes
Water surrounds tubes
Low pressure applications
2️⃣ Water Tube Boiler
Water flows inside tubes
Hot gases outside
Used in thermal power plants
Water tube boilers are used because:
High pressure capacity
High steam generation rate
Better safety
🌀 Draft System in Boiler
Draft means flow of air and flue gas.
Three main fans:
FD Fan (Forced Draft Fan) → Supplies air
PA Fan (Primary Air Fan) → Dries and carries coal
ID Fan (Induced Draft Fan) → Removes flue gases
Balanced draft system maintains proper pressure inside furnace.
📊 Boiler Efficiency
Boiler efficiency is the ratio of steam heat output to fuel heat input.
Typical Efficiency:
80% – 90%
Factors affecting efficiency:
Excess air
Coal quality
Tube cleanliness
Heat losses
Formula (Simple Form):
Efficiency = (Heat Output / Heat Input) × 100
🛠 Boiler Mountings
These are safety devices.
Important mountings:
Safety valve
Pressure gauge
Water level indicator
Blow-off valve
Fusible plug
⚙️ Boiler Accessories
These improve efficiency.
Accessories include:
Economizer
Air preheater
Superheater
Feed pump
🚨 Common Boiler Problems & Troubleshooting
1. Tube Leakage
Causes:
Overheating
Poor water chemistry
Scale formation
2. Drum Level Fluctuation
Causes:
Sudden load change
Control valve problem
3. Flame Failure
Causes:
Low coal supply
Improper air-fuel ratio
4. High Stack Temperature
Causes:
Fouled tubes
Air leakage
🏗 Role of Mechanical Engineer in Boiler Section
Responsibilities include:
Tube inspection
Soot blower monitoring
Fan alignment
Vibration monitoring
Shutdown planning
Efficiency improvement
During shutdown:
Tube thickness measurement
Hydro test
Valve overhaul
Expansion joint inspection
🎯 Interview Questions (Very Important)
Q1: What is boiler working principle?
Answer: Heat transfer converts water into steam inside a closed pressure vessel.
Q2: Why superheater is used?
To increase steam temperature and prevent turbine blade damage.
Q3: Difference between fire tube and water tube boiler?
Water tube boiler handles high pressure and used in power plants.
Q4: What is boiler efficiency?
Ratio of useful steam heat to fuel heat input.
🌍 Modern Boiler Technologies
Supercritical boilers
Ultra-supercritical boilers
CFB boilers
Low NOx burners
Automated control systems
These increase efficiency and reduce pollution.
📌 Key Points to Remember
Boiler is heart of thermal power plant
Converts water into high-pressure steam
Water tube boiler is used in power plants
Superheater protects turbine
Efficiency depends on heat recovery
Conclusion
A boiler is the most critical equipment in a thermal power plant. Its working is based on heat transfer and steam generation. For mechanical engineers, deep understanding of boiler components, working principle, and maintenance practices is essential for career growth in the power sector.
If you master boiler fundamentals, you can confidently handle:
Interviews
Maintenance jobs
Commissioning projects
Efficiency improvement tasks