In this article we will discuss about:- 1. Meaning of High Pressure Boilers 2. Types of High Pressure Boilers 3. Characteristics 4. Advantages.
Meaning of High Pressure Boilers:
These are used to increase the efficiency of the plant. These are developed because of rising cost of fuel and restrictions on air pollution. Modern power generation plants generally use high pressure boilers. These high pressure boilers have pressures above 140 bar and the temperature may be up to 540 – 610°C.
High pressure boilers use both natural and forced circulations. Forced circulation is achieved with the help of centrifugal pumps. By using this forced circulation, because of high velocity of water scale formation in the tubes and boiler drums is reduced.
Types of High Pressure Boilers:
1. La Mont Boiler:
ADVERTISEMENTS:
This is a modern high pressure, water tube boiler employs forced circulation. This was first introduced by La Mont in 1925. This is generally used in Europe and USA.
Working:
This boiler consists of an economiser, boiler drum, superheater, evaporator, air preheater etc. The feed water from the hot well is pumped by the feed pump through the economiser and preheated water is supplied to the boiler drum.
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The water is heated up by the hot gases flowing in the upward direction, which are later exhausted through the chimney high in the atmosphere to reduce air pollution.
This boiler drum also acts as steam separating drum. The circulating pump, which is operating at a pressure of about 2.5 bar above the boiler pressure, pumps the water to the distributor header.
This distributor header has nozzles, which distribute the feed water into the evaporator.
The water in the evaporator tubes gets evaporated. And the mixture of water and steam is forced into the boiler drum, where the steam is separated. This steam is passed through the superheater, so as to have superheated steam and the superheated steam goes to the steam turbine for power generation.
ADVERTISEMENTS:
The air preheater is also provided in the flue gas passage for the economic combustion of fuel in the furnace. These boilers can generate steam up to critical pressure.
2. Benson Boiler:
The main problem experienced with the La Mont boiler is the formation and attachment of bubbles on the inner surfaces of the tubes. These attached bubbles to the tube surfaces reduce the heat flow and steam generation, as it offers high thermal resistance than water film.
In order to overcome this difficulty, Benson developed the boiler in 1927. In this boiler the above difficulty is overcome by raising the boiler pressure to critical pressure of steam. As at critical pressure steam and water co-exist at the same density and no bubble formation occurs. Further at critical pressure the enthalpy of evaporation (Latent heat) is zero. The water thus gets converted into steam directly without passing through evaporation process.
ADVERTISEMENTS:
It is a high pressure, water tube, forced circulation boiler. It has unique characteristic of absence of drum and entire process of heating, steam formation and superheating takes place in a single continuous tube.
Working:
Feed water pump feeds water to the economiser, from where it goes to the radiant heating section. Here most of the heat is transmitted and water gets heated almost to the critical temperature. In the evaporative section i.e., in radiant evaporator and connective evaporator, evaporation is completed and superheating begins. In the superheater the final desired temperature is obtained.
Advantages:
ADVERTISEMENTS:
1. Due to the absence of drum, initial cost is low.
2. Circulation pump is absent.
3. The boiler is light in weight because of absence of drum.
4. The boiler can be started within 15 minutes and can be put in service.
5. The boiler can be operated at supercritical pressure also.
Disadvantages:
1. It requires specific means of washing the tubes to avoid solid deposition.
2. The tubes are likely to get overheated when the flow is not sufficient.
3. It requires skilled operators to co-ordinate the operations.
3. Schmidt-Hartmann Boiler:
This boiler uses distilled water for its operation for generating high pressure steam. This steam is recirculated without any wastage in the circuit.
In this case high pressure steam generated from pure water is used for generating low pressure steam from impure water or normal water and is used for power generation.
As shown, distilled water from the water drum flows into the primary evaporating tubes by natural circulation. The hot gases from the combustion chamber passes over the evaporator tubes and the steam upto a pressure of 100 bar is generated from distilled water. This steam then goes to the primary separator. And then this steam passes through the tubes provided in the steam generator, which are submerged in impure water in the steam generator. Here heat will be transferred from steam to impure water and the impure water is converted into steam at about 60 bar pressure.
This low pressure steam generated from the impure water is then passed through the superheater for superheating. Finally the superheated steam is supplied to the steam turbines for power generation.
Note that the condensate of high pressure steam from below the boiler drum passes into the feed water heater and then to the water drum.
The impure water from the hot well will be pumped by the feed pump and is made to pass through the tubes submerged. Where primary heating of this impure water takes place and preheated water is supplied to the steam generator.
As shown air preheater is also provided, which supplies preheated air for combustion.
Advantages:
1. It can use impure water for steam generation.
2. Water softening plant is not required.
3. There is rare chance of overheating of components of primary circuit (distilled water circuit) as there is no danger of salt deposition. Hence long life of these parts.
4. Loeffler Boiler:
It is a modern water boiler utilizing forced circulation. It differs from other boilers as it has two superheaters viz., Radiant superheater and convective superheater.
Radiant superheater is located above the combustion chamber, so that it receives intense radiations heat. The feed pump supplies feed water to the economiser, where the water is heated by the hot gases and the hot water will be supplied to evaporating/boiler drum. Note that a part of superheated steam (@ 65%) generated is passed to the evaporating drum through the nozzles where it mixes with feed water and steam is generated.
The saturated steam from the boiler or evaporating drum is drawn by steam circulating pump and is passed through the radiant superheater and then convective superheater. About 35% of steam coming out from the superheater is supplied to high pressure turbine. The steam coming out from the turbine is then passed through steam reheater before supplying to low pressure turbine.
About 35% of total superheated steam is used for power generation and remaining 65% is used in the evaporating drum for generating steam.
It has a steam-generating capacity of 90-95 T/hr and steam pressure upto 150-155 bar.
5. Velox Boiler:
It is known fact that, the heat transfer rate from the flue gasses can be increased by increasing the velocity of gases.
Thus to have high transfer rate, with reduced surface area, is by using high velocity (approximately or above sonic velocity) of flue gases. This concept is used in Vilox boiler. And in order to get high velocity of flue gases, pressurised combustion is used.
Air is compressed at about of 3 bar pressure with the help of a compressor driven by the gas turbine before supplying to the combustion chamber. In the combustion chamber, combustion of high pressure air and fuel takes place to produce supersonic velocity of hot gases. These gases are then passed through the annulus of tubes as shown.
The heat is transferred from gases to water while passing through the annulus to generate the steam. The mixture of water and steam thus formed then enters the steam separator in a tangential manner. It forms a vortex in the separator and due to centrifugal action, the moisture content of the steam is separated. This moisture separated is sent back to feed water line.
The dry steam so obtained in the steam separator is sent to the superheater, where it is further heated by means of hot flue gases coming out from combustion chamber.
The gases coming out of superheater are used to run the gas turbine, which drives the compressor. The exhaust gases coming out from the gas turbine are passed through the economiser to preheat the water and finally they are exhausted through the chimney.
Advantages:
(i) If can start quickly.
(ii) The boiler design is compact and has flexibility towards varying load.
Disadvantages:
Solid fuel cannot be used. It operates only with liquid or gaseous fuel.
Characteristics of Modern High Pressure Boilers:
Even though the modern boilers are termed as high pressure boilers, it does not mean them fully. Because high pressure operation is only one of the characteristics of modern boilers used for power generation.
Other aspects/characteristics are as follows:
(1) Modern power boilers are water tube.
(2) They use forced type of water circulation.
(3) They use feed water heaters.
(4) They use modern methods of firing viz. pulverised firing; cyclone furnace; fluidised bed combustion.
(5) They use F.D. and I.D. fans for creating the necessary draught.
(6) They use water treatment plant, demineralisation plant to purify boiler feed water to minimise scale formation and salt deposits in the boiler tubes.
(7) They use various controls for pressure temperature water level etc.
(8) They use complex assembly of evaporator superheaters, reheaters, economisers and air heaters etc.
Advantages of High Pressure Boilers:
1. Because of high velocities, tendency of scale formation in the tubes is reduced.
2. Small light weight tubes of better heating surface arrangements can be used. This makes the unit compact and reduces erection time and cost of boiler.
3. Due to forced circulation, for heating of all the parts is uniform, which avoids overheating and development of thermal stresses.
4. There is more flexibility in the arrangement of furnace, tubes and other boiler components.
5. The steam can be raised quickly to meet the variable load requirements.
6. The efficiency of the plant is increased.
7. A very rapid start from cold is possible if an external power supply is available. Hence the boiler can be used for carrying peak loads or standby purposes with hydraulic power stations.