Cycle way

The circulation of the medium in the power plant boiler evaporation system has four modes: natural circulation,auxiliary circulation, direct current and compound circulation.

Natural circulation

The circulation is established by the difference in the density of the working fluid in the downcomer and the ascending tube 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 less than 20 MPa.

Auxiliary loop

The main difference with natural circulation is that a circulation pump is installed between the downcomer and the ascending pipe of the evaporation system. In addition to the difference in working fluid density, the circulation driving force also adds the pressure of the circulation pump. Therefore, the layout of the evaporation surface is relatively free, and the diameter of the drum can also be smaller. This circulation method is mainly used for boilers with subcritical pressure.

Once-through boiler

There is no drum in the once-through boiler, and the feedwater depends on the pressure of the feedwater pump to pass through the heating surfaces at all levels and finally all become superheated steam output. Once-through boilers are widely used in units above high pressure, which can use supercritical pressure parameters. Since the once-through boiler does not have a drum and uses a small-diameter tube, the heat storage of the steam 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 loop

A circulating pump is added to the steam-water system of the once-through boiler to combine the once-through boiler and auxiliary circulation. There are many layout schemes for the steam and water system of the combined circulation boiler. Figure 3 is a schematic diagram of a typical supercritical pressure compound cycle. When the load is high, the circulating pump acts as a booster pump, and the system operates as a once-through boiler. When the recirculation is lower than 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 to reduce the difference in flow velocity in the water wall under high and low loads, which is conducive to low load operation, and the flow resistance under high load is not too large. Figure 4 shows a subcritical pressure compound circulation system, also called low rate circulation. In this system, a steam-water separator is installed at the outlet of the evaporation heating surface. The cycle magnification at full load is between 1.2 and 2.0. Compared with pure once-through boilers, the evaporation system of low-rate circulating boilers has less resistance and is more suitable for variable pressure operation, and the diameter of the separator used is much smaller than that of ordinary drums.