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- A boiler is called a high-pressure boiler when it operates with a steam pressure above 80 bars. The high-pressure boilers are widely used for power generation in thermal power plants.
- In a high-pressure boiler, if the feed-water pressure increases, the saturation temperature of water rises and the latent heat of vaporization decreases. The feed water can be heated to saturation of temperature in the economizer with the help of waste heat. Recovery from the exhaust gases is escaping to the chimney. Then the boiler supplies only latent heat of vaporization and superheat. Thus, a boiler operation at high pressure will require less heat addition for steam generation.
High-pressure boilers have the following functions:
- Forced circulation of water – in all modern high-pressure boilers, the force circulation of water is maintained with the help of pumps. It increases the help of pumps. It raises the mean temperature of heat addition and evaporation capacity if the boiler.
- Large number of small diameter tubes – the surface-area-to-volume-ratio (area density) increases with the use of small-diameter tubes. It helps in a high rate of the heat transfer to water flowing inside the tubes. Therefore, a large number of small density tubes in a zigzag manner are used for water circulation in forced circulation. Further, use of a short tube reduces the pressure loss and gives the better control the quality of steam.
- Higher steam pressure and temperature – the steam is generated at a pressure of 80 to 300 bars and temperature of 450°C to 585°C with two superheaters in series. The use of such stream is very suitable for power generation. It increases the thermal efficiency of the plant and reduces the moisture contents in low-pressure stages of expansion in the turbine.
- Improved mode of heat transfer – modern high-pressure boilers use the heat transfer by radiation along with conduction and convection. The total heat-receiving equipment is divided into several parts. So they can easily be located in various zones of the furnace for most efficient heat transfer to the water circuit.
- Improved mode of heating – the high-pressure boilers use the following methods for improved heat transfer rate:
- Evaporative of water above the critical pressure of steam.
- Heating of water by mixing superheated steam for high heat transfer rate.
- Increasing the combustion of air velocity over the tube.
- Pressurized combustion – for increasing the combustion rate and thus heat-release rate, pressurized air is used in the furnace. It gives a large amount of heat in a small space.
- Compactness – the high rate of heat transfer inside the boiler reduces the overall size of the boiler, and the boiler becomes compact.
- High efficiency – High-pressure boilers have better firing methods, monitoring, furnace conditions, control flue gases and water velocity.
- Intensive heating – the furnace temperature in high-pressure boilers is high enough and therefore, 70% of heat is transferred to water by radiation. This is a faster way of heat transfer.
- Once through construction – in high-pressure boiler operating at and above the critical pressure, the water directly flashes into steam in the tube itself. It eliminates the need of a boiler drum.
- High-pressure boilers use the forced circulation of water which ensures the positive circulation of water and increased evaporative capacity.
- They require less heat of vaporization.
- They are compact and thus require less floor space.
- Due to the high velocity of water, the tendency of scale formation is minimized.
- All parts are uniformly heated and the danger of overheating is minimized.
- The steam can be raised quickly to meet the variable load requirements without the use of complicated control devices.
- The plant efficiency is increased.
- With the use of high-pressure boilers, the steam generation is economical.