ADVERTISEMENTS:
In this article we will discuss about the design of once through boiler. Also learn about the advantages and benefits of once through boiler.
Owing to the sharply increasing costs of construction and fuel it becomes essential for the designers to economise on the installation cost and to increase fuel efficiency in the new stations by using modern sophisticated technology.
Higher size units with higher steam parameters seem a natural choice for economical installation and operation of thermal power plants. The 800 MW units would be designed on supercritical steam pressure with a drumless boiler on “once through principle”.
ADVERTISEMENTS:
As the steam pressure increases, the differential between the specific weight of saturated water in down-comers and specific weight of steam-water mixture in furnace wall tubes, which causes natural circulation in boiler, goes on decreasing.
Sluggish circulation causes film boiling. In film boiling, the tube metal remains in contact with steam bubbles which provide high thermal resistance for heat flow and therefore tube metal sharply deteriorates due to high metal temperature leading to failure of the boiler tubes.
It is not possible to prevent “film boiling” in the upper furnace tubes for pressures above 180 bar with natural circulation. Therefore, generally above 160 bar pressure, controlled circulation in water walls is used by providing boiler circulating pump between down- comers and lower water distributing heaters and the water walls.
In such controlled circulation boilers, it is possible to utilize high steam pressure up to 200 bar but beyond this, the effectiveness of boiler drum in separating the saturated steam from water is reduced. Therefore, beyond 200 bar, drumless boiler is envisaged.
In sub-critical range (<225.65 bar), a separator vessel is utilised to separate out salts from steam water mixture, but in supercritical range (> 225.65 bar), the separate vessel cannot function and only once through (monotube) is adopted.
i. Fig. 12.24 shows a once through or monotype boiler (a design adopted by Sulzers Brother Ltd.) In this design there is a separator vessel with sub-critical pressures; the once through circulation is provided by a “feed pump”.
ii. Fig. 12.25 shows a combined circulatory boiler (a design adopted by M/S Combustion Engineering Co.). In this design a “Mixing vessel” provides suction to boiler circulating pumps at sub-critical pressures and inlet saturated steam to super-heater and serves as a receiving header for steam-water mixture from evaporator suction.
The boiler circulating pumps are required to function in the start-up or low pressure conditions but when the pressure goes above critical pressure then these are stopped and once through circulation is provided by boiler feed pump.
Advantages of Once through boilers:
The following are the advantages of once through boilers for large thermal units:
1. No higher limit for the higher steam pressure.
2. Full steam temperature can be maintained over a wider range of load.
3. Starting and cooling down of the boiler is fast.
4. No circulation disturbance due to rapid pressure fluctuations.
5. A once through boiler is smaller in size and weighs less in comparison to a natural circulation boiler.
6. Easy control of steam temperature during start-up and shut down (which is very advantageous for start-up of boiler and turbine).
7. Greater freedom in arrangement and location of heating surfaces.
ADVERTISEMENTS:
8. Easy to adopt variable pressure operation for better performance at part load operation.
9. Elimination of heavy walled drum decreases the metallurgical sensitivity of boiler against changes in pressure.