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Chapter: Mechanical and Electrical - Power Plant Engineering - Power Plants and Boilers

High Pressure Boilers

A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.

HIGH PRESSURE BOILERS

 

A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.

 

In all modern power plants, high pressure boilers (> 100 bar) are universally used as they offer the following advantages. In order to obtain efficient operation and high capacity, forced circulation of water through boiler tubes is found helpful.

 

1. The efficiency and the capacity of the plant can be increased as reduced quantity of steam is required for the same power generation if high pressure steam is used.

 

2. The forced circulation of water through boiler tubes provides freedom in the arrangement of furnace and water walls, in addition to the reduction in the heat exchange area.

 

3. The tendency of scale formation is reduced due to high velocity of water.

 

4. The danger of overheating is reduced as all the parts are uniformly heated.

 

5.  The differential expansion is reduced due to uniform temperature and this reduces the possibility of gas and air leakages.

 

Superheater operation is similar to that of the coils on an air conditioning unit, although for a different purpose. The steam piping is directed through the flue gas path in the boiler furnace. The temperature in this area is typically between 1,3001,600 degrees Celsius. While the temperature of the steam in the superheater rises, the pressure of the steam does not. Almost all the steam superheater systems are designed to remove droplets entrained in the steam to prevent damage to the turbine blading and associated piping.

 


1LA-MONT BOILER

 

 

It is a forced circulation- water tube boiler which was first introduced in 1925 by La

 

Mont.

 

The feed water from hot well is supplied to a storage and separating drum (boiler) through the economizer. Most of the sensible heat is supplied to the feed water passing through the economizer


The feed water from hot well is supplied to a storage and separating drum (boiler) through the economizer. Most of the sensible heat is supplied to the feed water passing through the economizer. A pump circulates the water at a rate 8 to 10 times the mass of steam evaporated. This water is circulated through the evaporator tubes and the part of the vapour is separated in the separator drum. The large quantity of water circulated (10 times that of evaporation) prevents the tubes from being overheated.

 

The centrifugal pump delivers the water to the headers at a pressure of 2.5 bar above the drum pressure. The distribution headers distribute the water through the nozzle into the evaporator. The steam separated in the boiler is further passed through the super-heater.

 

To secure a uniform flow of feed water through each of the parallel boiler circuits a choke is fitted entrance to each circuit. These boilers have been built to generate 45 to 50 tons of superheated steam at a pressure of 120 bar and temperature of 500°C.

 

 

Important Components

 

1.     Steam separating drum The feed water from the hot well is stored in the drum. The steam is separated from water in the drum and the steam is usually collected at the top

of the drum.

 

2.     Circulating pump Water from the steam separating drum is drawn by a circulating pump and it circulates water through the evaporator tubes. Pump circulates water at a rate of 8-10 times the mass of steam evaporated. Forced circulation is necessary to prevent the overheating of tubes.

 

3.     Distribution header The distribution header distributes the water through the nozzle into the evaporator.

4.     Radiant evaporator Water from the drum first enters the radiant evaporator through the pump and header. The water is heated by the radiation heat from the combustion chamber. In radiant evaporator, the hot flue gases do not pass over the water tubes.

 

5.     Convective evaporator The mixture of water and steam coming out from the radiant evaporator enters the convective evaporator tubes. The hot flue gases passing over the evaporator tubes transfer a large portion of heat to the water by convection. Thus, water becomes steam and the steam enters to the steam separating drum.

 

6.     Superheater The steam from the steam separating drum enters the superheater tubes where it is superheated by the hot flue gases passing over them. The superheated steam then enters the steam turbine to develop power.

 

7.     Economiser The waste hot flue gases pass through the economiser where feed water is pre-heated. By pre-heating the feed water, the amount of fuel required to convert water into steam is reduced.

8.     Air pre-heater The hot flue gases then passes through the air pre-heater where the air required for combustion is pre-heated.

 

Advantages

 

1.     La-Mont boilers can generate 45 to 50 tons of superheated steam at a pressure of 120 bar and temperature of 500°C.

 

2.     Drum is of small size.

 

3.     Tendency of scale formation is eliminated due to forced circulation of water.

 

Disadvantages

 

1.     Bubbles are formed on the inside of the water tubes and this bubbles reduce the heat transfer rate.

 

2.     Initial and operating costs are high.

 

3.     Maintenance costs are very high.

 


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