Natural circulation and forced circulation are two kinds of circulation states during the operation of the boiler, and the two are essentially different. Fang Kuo Xiaobian will compare and compare these two cycle states, so that you can better understand the boiler operation knowledge.

First, the natural circulation.
       Working principle: The operation mode of actively promoting the water circulation by relying on the density difference caused by the heating of the working medium between the falling pipe outside the furnace and the rising pipe in the furnace. The circulation circuit is composed of a drum, a down pipe, a header, a riser pipe, a steam water introduction pipe and the like. The pot water descends from the outer down pipe of the furnace into the lower header of the water wall, and then is heated into a steam-water mixture (containing steam rate of 5-25%) through the furnace water wall (ie, the riser pipe), and then enters the drum through the soda water introduction pipe to perform steam-water separation. The saturated steam is sent to the superheater, and the pot water still returns to the down tube to continue the circulation.
       The main features of natural circulation boilers are:
       1. Work only under sub-critical pressure. Under the subcritical pressure, the natural circulation boiler with the flue gas parameter of 16.6~18.2 MPa and the working pressure of the drum reaches 18.6~20.6 MPa, and the natural circulation still has sufficient water circulation driving force to achieve safe and reliable operation.
          2. Has a fixed evaporation section end point. The drum separates the two sections of evaporation and superheating, which serve as the heat accumulator and water reservoir of the boiler respectively.
        3. Under the normal operating conditions, the design is correct, can maintain the appropriate circulating water flow rate or cycle rate (the ratio of the circulating water volume entering the rising pipe to its steam production), and has good self-compensation ability, that is, the heat absorption of the circuit increases. The circulating water flow rate also increases.
       4. Under the subcritical pressure, the water-cooled wall often adopts an internally threaded pipe to ensure the ability to effectively prevent the working fluid in the pipe from deviating from the nuclear boiling (DNB).
         5. The salt water concentration and steam quality of the pot water can be maintained by continuous sewage discharge in the drum. The quality of the feed water can be lower than that of the DC boiler, which can reduce the cost of the chemical water treatment part.
        6. The resistance of the steam-water system is small, and the water supply pressure is not high, which can reduce the power consumption of the feed pump.
        7. The drum is thick-walled, the cost is high, and there is a problem of temperature difference between the upper and lower sides and the inner and outer walls, and the starting and stopping time is extended.

Second, the mandatory cycle.
      Working principle: Under the action of the water pump head, the working medium passes through the preheating, evaporation and overheating heating surfaces, and is preheated, evaporated and superheated to the required temperature.
       The water or steam in the forced circulation boiler is not only powered by the difference in density between water and steam, but also requires a certain pump work to circulate in the heating pipe. Due to the good water circulation, the forced circulation furnace has a very low requirement for the water supply speed; the working pressure is generally large, and the material and the wall of the boiler tube are required to be high. The drum of the forced circulation boiler is no longer necessary. It can have a drum and can have no drum. However, the pump head has higher requirements and consumes more electricity. When the pump stops running, the flow of water in the pot also stops, and a reliable method is needed to prevent the water from vaporizing the water.

The difference between the forced circulation boiler and the natural circulation boiler is that the circulation between the downcomer of the natural circulation boiler and the heating surface is generated by the natural indenter between the two, and the forced circulation boiler is forced by the furnace water circulation pump. The forced circulation boiler on the equipment has a furnace water circulation pump (compulsory circulation pump), while the natural circulation boiler does not.
The horizontal boiler evaporation section is generally forced circulation, and the vertical boiler evaporation section is not necessarily a natural circulation, but may also be a forced circulation.