undefined | unit 3 boilers

Unit-3

APPLIED THERMODYNAMICS

Q 1) What is vapour pressure and its effect on the condensation process?

A 1) Vapor pressure is the pressure exerted by water vapor molecules alone/individually. Vapor pressure is one way to keep track of the amount of gas water vapor. The higher the vapor pressure, the greater the amount of water vapors in the air.

Condensation occurs when a water vapor molecule collides with a liquid water surface, and chemically binds to liquid water molecules. The higher the vapor pressure, the faster the rate of condensation.

Q 2) Write Advantages and disadvantages of Water Tube Boiler?

A 2) There are many advantages of water tube boiler due to which these types of boiler are essentially used in large thermal power plant.

Larger heating surface can be achieved by using more numbers of water tubes.

Due to convectional flow, movement of water is much faster than that of fire tube boiler, hence rate of heat transfer is high which results into higher efficiency.

Very high pressure in order of 140 kg/cm2 can be obtained smoothly.

Disadvantages of Water Tube Boiler

The main disadvantage of water tube boiler is that it is not compact in construction.

Its cost is not cheap.

Size is a difficulty for transportation and construction.

Q 3) Write the advantages of free circulation?

A 3) The advantages of free circulation are

Free circulation of water helps to maintain a uniform temperature true everywhere within the boiler so that unequal expansion of various parts of the boiler is prevented.

Free circulation of water facilities the escape of steam from the heating surface as soon as it formed. If steam does not escape quickly after its formation the boilerplates do not remain constantly in touch with water and as a result, these plates may be overheated.

Q 4) Advantages of forced circulation?

A 4) The advantages of forced circulation are:

The rate of heat transfer from the flue gases to the water higher.

Tubes having comparatively smaller diameters can be used. This reduces the overall weight of the boiler.

The number of boiler drums required may be reduced.

Less scale formation in the boilers is required.

Steam can be quickly generated.

The fluctuation of load can be easily met without taking the help of any complicated controlled device.

Chance of overheating of the boilerplates in minimum.

Weight per unit mass of steam generated is less.

Q 5) Explain Stationary boiler and Portable boiler?

A 5)

a) Stationary boilers: The boilers which cannot be transported easily from one place to another are called stationary boilers.

Example: Lancashire boiler, Babcock and Wilcox boiler

(b) Portable boiler: The boilers which can be easily transported (moved) from one place to another are called portable boilers.

Example: Locomotive boiler.

Q 6) What are externally tired and internally tired boilers?

A 6)

a) Externally fired boiler: - In the boiler if the fire is outside the shell, that boiler is known as externally fired boiler. Example: Babcock & Wilcox boiler

(b) Internally fire boiler:- In the boiler in which the furnace is located inside the boiler shell it is known as internally fired boiler. Example: Cochran boiler, Lancashire boiler etc.

Q 7) Write the name of different boiler mountings?

A 7) Boiler mountings are fittings which are mounted on the boiler for its proper functioning. Mountings are water level indicator, safety valve, pressure gauge, etc. It may be noted that a boiler cannot function safely without the mountings

Following are the important boiler mountings:

Water level indicator

Pressure gauge

Safety valve

Stop valve

Blow off

Feed check valve

Fusible plug

Q 8) Write the name of different Boiler Accessories?

A 8) The boiler accessories are required to improve the efficiency of the steam power plant and to enable for the proper working of the boiler. The boiler accessories are the devices, which form an integral part of a boiler but are not mounted on it. They include superheaters, economiser, feed pump etc. It may be noted the accessories help in controlling and running the boiler efficiently. The boiler accessories aren’t mounted directly on the boiler.

The essential boiler accessories are:

Economiser

Air pre-heater

Superheaters

Feed pump

Steam Separator

Steam trap

Q 9) What is Air pre-heater?

A 9) The air preheater is an accessory that recovers the heat in the exhaust gas by heating the air supplied to the furnace of the boiler. Supplying preheated air into the furnace produces a high furnace temperature and accelerates the combustion of the fuel. Thus the thermal efficiency of the plant will be increased.

Air Pre-heater - Boiler Mountings and Accessories

The advantages of air pre-heater are,

Increase in the steam generation rate.

Better combustion with less soot, smoke and ash, and

Low-grade fuels can be used.

Q 10) Explain difference between air cooled and water-cooled condenser?

A 10)

Air-cooled condenser	Water-cooled condenser
Condensing medium used - air	Condensing medium used - water
Simple construction. Initial cost as well as the maintenance cost is very low	Complicated construction. Initial cost and maintenance cost is high
The hot air can easily dispose	Problem of disposal used water ( unless there is a recirculation system)
No handling problem	Water cooled condenser are difficult to handle
Air process low thermal conductivity and condenser capacityof air cooled system is very low	High thermal conductivity of water favours high heat transfer capacity
No corrosion due to air.	Higher chance of fouling effect. Water cause corrosion of condenser tubes.
Do not require sophisticated piping arrangement for carrying the air	Piping arrangement is required for carrying water.
Air cooled condenser are used low capacity operation	The water cooled condensing used for large capacity plants.
It requires more power for circulation of air and high power fans are noisy.	It requires less power to the circulation on water

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