The circulation of the medium in the evaporation system of the power station boiler has four modes: natural circulation, auxiliary circulation, direct current and compound circulation.
Natural cycle
The circulation is established by the density difference of the working fluid in the descending pipe and the ascending pipe of the evaporation system. Boilers below ultra-high pressure generally use natural circulation. Subcritical pressure boilers can also use natural circulation, but the pressure in the drum is generally limited to below 20 MPa.
Auxiliary cycle
The main difference from natural circulation is that a circulation pump is installed between the descending pipe and the ascending pipe of the evaporation system. The circulating driving force depends not only on the density difference of the working medium, but also on the pressure of the circulating pump. Therefore, the arrangement of the evaporation surface is freer, and the diameter of the drum can also be smaller. This type of circulation is mainly used in subcritical pressure boilers.
Once-through boiler
There is no drum in the once-through boiler, and the feed water relies on the pressure of the feed pump to pass through the heating surfaces at all levels and turn into superheated steam for output. Once-through boilers are widely used in units above high pressure, and they can use supercritical pressure parameters. Because the once-through boiler has no drum and uses small diameter tubes, the heat storage of soda and metal in the boiler is relatively small, and the salt that enters the boiler with the feed water cannot be removed by blowdown, so the requirements for automatic control and water treatment are relatively high.
Compound cycle
A circulating pump is added to the once-through boiler steam-water system to combine the once-through boiler and the auxiliary circulation. There are various arrangements for the steam-water system of the composite cycle boiler. At high load, the circulating pump acts as a booster pump, and the system operates as a once-through boiler. When the recirculation is put into operation below a certain load, the flow through the water wall is the sum of the feed water flow and the recirculation flow. The feature of this system is that the difference between the flow velocity in the water wall under high and low loads is reduced, which is beneficial to low-load operation, and the flow resistance at high loads is not too large.