Condensing steam turbine refers to a steam turbine in which steam expands and does work in the steam wheel, except for a small part of the shaft seal leakage, all of which enter the condenser and condense into water.
In fact, in order to improve the thermal efficiency of the steam turbine and reduce the diameter size of the steam turbine exhaust cylinder, the steam that has done part of the work is pumped out of the steam turbine and sent to the reheat heater to heat the boiler feed water. Steam turbines commonly used in thermal power plants for power generation. Condensing equipment is mainly composed of condenser, circulating water pump, condensate pump and pump. The exhaust steam of the steam turbine enters the condenser, is cooled and condensed into water by the circulating water, is pumped out by the condensate pump, and is heated by the heater at all levels and sent to the boiler as feed water.
The exhaust steam of the steam turbine is condensed into water in the condenser, and the volume is suddenly reduced, so that the originally filled sealed space forms a vacuum, which reduces the exhaust pressure of the steam turbine and increases the ideal enthalpy drop of the steam, thereby improving the thermal efficiency of the device. The non-condensing gases (mainly air) in the turbine exhaust are extracted by the extractor to maintain the necessary vacuum. Brief introduction The most commonly used condenser for steam turbines is the surface type. The cooling water is discharged into the cooling water pool or cooling water tower after cooling and then recycled. Power plants close to rivers, rivers and lakes, if the amount of water is sufficient, can directly discharge the cooling water discharged by the condenser into the river, river and lake, which is called runoff cooling. But this approach can cause thermal pollution to rivers and lakes. Power plants in areas with severe water shortages can use air-cooled condensers. However, it has a large structure, a large consumption of metal materials, and is rarely used in general power plants except for train power stations. In the old power plants, some use hybrid condensers, and the steam turbine exhaust steam is directly mixed with cooling water for cooling. However, because the exhaust condensate is polluted by cooling water, it needs to be treated before it can be used as boiler feed water, and it is rarely used.
How it works
It is mainly composed of the steam turbine body, condensate pump, condenser and circulating water pump, which means that the steam enters the condenser after the steam turbine is worked and cooled into water by gas, and then sent back to the boiler by the condensate pump. Among them, the condenser plays a crucial role, its main purpose is to improve the thermal efficiency of the steam turbine, which is the use of steam recooling into water, its volume will be greatly reduced, so that the remaining space into a vacuum, increasing the ideal enthalpy of steam.
The function of the extractor is to make the steam turbine and condenser establish the necessary vacuum before the steam turbine starts, and in the operation of the condensing steam turbine, the air and other non-condensing gases are continuously extracted from the condensing equipment in time to ensure the heat exchange efficiency of the condenser heat exchange tube and maintain the vacuum degree. The performance of the vacuum equipment directly determines the exhaust pressure of the condensing steam turbine, which in turn affects the size of the enthalpy drop and the level of steam consumption. Different evacuation methods
, will affect the equipment investment cost, operation mode and system complexity of the steam turbine unit, so the vacuum equipment is quite important for condensing steam turbines.
Operational characteristics
The exhaust pressure of condensing steam turbines has a significant impact on operating economy. The main factors affecting the vacuum degree of the condenser are the cooling water inlet temperature and the cooling ratio. The former is related to the region, season and water supply method of the power plant; The latter represents the ratio of the design flow rate of the cooling water to the steam exhaust volume of the turbine. The cooling rate is large, and a high vacuum degree can be obtained. However, the increase in cooling ratio increases the power consumption and equipment investment of the circulating water pump. Generally, the cooling ratio of surface condenser is designed to be 60~120. Due to the large demand for circulating water of condensing steam turbines, water source conditions have become one of the important conditions for site selection of power plants.
Ideally, the condensate temperature of the surface condenser should be the same as the exhaust steam temperature, and the heat taken away by the cooling water is only the latent heat of vaporization of the exhaust steam. However, in actual operation, due to the exhaust steam flow resistance and the presence of non-condensing gas, the condensate temperature is lower than the exhaust gas temperature, and the temperature difference between the two is called supercooling. Improper arrangement of the cooling water pipe, too high condensate level during operation and soaking the cooling water pipe will increase the degree of supercooling. Under normal circumstances, the supercooling degree should not be greater than 1~2 °C.
Genset Power
Although reducing the exhaust pressure of the condensing steam turbine can improve the thermal efficiency, due to the increase of the specific volume of the exhaust steam, the flow area and blades of the final stage of the turbine need to increase accordingly, which increases the manufacturing cost and makes processing difficult. Therefore, the optimal exhaust pressure needs to be determined by a comprehensive technical and economic analysis. Generally, the exhaust pressure of condensing steam turbine is 0.004~0.006 MPa.
Turbine power is determined by steam flow. The maximum flow rate that can be passed through a condensing steam turbine is determined by the blade length of the final stage. Since the larger the blade, the greater the centrifugal force, which makes it limited by the strength of the material. The maximum length of the last stage blade can reach 1000~1200 mm, the maximum allowable circumferential speed at the tip of the blade is 550~650 m/s, and the limit power of the single exhaust steam port is about 100~120 MW. The low-pressure cylinder adopts a split-flow structure, which can increase the power of the single machine. By the end of the 80s, the maximum condensing stand-alone power of conventional thermal power plants was 1300 MW for twin-shaft units and 800 MW for single-shaft units.
Condensing turbines designed at low speeds (1500 or 1800 rpm) increase the ultimate power, but this increases turbine size and material consumption, as the total turbine weight is inversely proportional to the third of the rotational speed. Therefore, in addition to nuclear power plants that often use low-speed steam turbines to adapt to the characteristics of low parameters and large flow, China's thermal power plants all use 3000 rpm steam turbines.
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