How do preheaters improve boiler efficiency?
By recovering waste heat from flue gases and reducing fuel required.
What is an economiser?
A heat exchanger device that recovers waste heat from flue gasses to preheat feedwater
Why is steam superheated?
To reduce condensation and to increase the work done by steam
Name the types of boilers?
Fire tube/ Internally fired (hot gasses pass through tubes to heat water)
Water tube/ Externally fired (Water flows through tubes surrounded by furnace gasses)
Name the methods of firing a boiler
- Coal firing (solid or pulverised coal)
- Oil firing (fuel is sprayed through a nozzle)
- Gas firing
- Nuclear firing
How does pressure affect boiling point?
A liquid boils when its vapour pressure equals the surrounding pressure.
When pressure is high, it takes more energy (a higher temperature) for liquid molecules to escape into vapour.
If pressure decreases, so will the boiling point temperature of the liquid
State 3 phases of steam generation?
- Sensible heat: water is heated at constant pressure and begins to boil
- Latent heat: Further heat added goes to breaking the intermolecular forces of the water molecules to form steam
- Heat is continuously added until all water has formed into steam
What is a calorimeter?
An instrument utilising the principle of constant enthalpy expansion for the measurement of the moisture content of steam
What are the common causes of the failure of boilers?
- Overpressurisation
- Insufficient water in the boiler causing overheating and vessel failure
- Pressure vessel failure due to inadequate construction or maintenance
What is the purpose of blowdown in boilers?
To control impurities and maintain water quality.
State the differences between tubular or regenerative air preheaters
Tubular types use stationary tubes for heat transfer.
Regenerative types store heat in a rotating element.
What are the main advantages of regenerative air preheaters?
High efficiency, compact size, greater heat recovery.
How does a rotating regenerative air preheater work?
A rotating heat storage element absorbs heat from flue gas and transfers it to incoming air.
How does a stationary regenerative air preheater work?
The elements (drum, etc) are stationary, airducts are rotated instead to expose sections to flowing cool air
How does a tubular air preheater work?
Hot flue gas passes through tubes and transfers heat through metal surfaces to incoming air.
State the key difference between an air preheater and economiser
Air preheater heats air.
Economiser heats feedwater.
What is the function of a bottom blowdown valve?
It removes sludge, sediments, and heavy impurities from the bottom of the boiler to maintain efficiency and safety. This prevents scale formation, corrosion, overheating, and poor heat transfer.
What is the function of a surface blow down line?
To remove dissolved salts and impurities near the water surface to control boiler water concentration and improve steam quality
What are the main differences between bottom and surface blowdown valves?
Bottom blowdown removes settled solids.
Surface blowdown removes dissolved impurities.
What is a safety valve?
A spring-loaded valve used to relieve pressure and to prevent possible explosions
What is a water column?
It allows the operator to see the fluid level in the boiler
What is a circulating pump?
A pump used to force water to move continuously through the system
What is a non-return stop or check valve?
A valve which allows water to enter but prevents backflow
Why are flue gases passed through preheaters before the stack?
To recover waste heat and improve boiler efficiency by heating air or feedwater.
