溫度變送器——外文翻譯中英文翻譯-資料下載頁

　　

【正文】 arranged inline rather than staggered. The tubes are usually cylindrical and have 5 or outside diameter. There is no extended surface()attached to the tubes. The material selection depends on the steam temperature and pressure. Carbon steel has an allowable temperature up to 430℃ and is frequently used for loetemperature superheaters. Chromemoly, stainless steel, or same similar heat resistant alloy can withstand the temperature up to 650℃. Therefore they are selected for theSuperheater in a hightemperature zone. Temperature regulation and control are importation for both superheaters andreheaters. Steam temperature adjustments are frequently made at the time of themissioning of a boiler. The principal methods are an addition or regulatingthe hot gas temperature and mass flow rate. These are generally acplished by changing the excess air or the effectiveness of the evaporation section.During a boiler operation, there are many factors affecting the temperature of steam leaving the superheater and reheater. These include a boiler load, excessair, feedwater temperature, and cleanliness of heating surfaces. Control of steamtemperature during operation must be done without changing the arrangement of equipment. The most effective approaches are gas bypass, burner control, attemperation, gas recirculation, excess air, divided furnace. A gas bypass is to control the gas flow rate to superheater. The main disadvantages of this approach are the operating difficulties experienced by the movable dampers located in the hightemperature zone and the slow response to load change.Burner control is used to control the flame location and bustion rate. Tilting burners can direct the flame toward or away from the superheater. These will result in a change of heat absorption in the furnace and change of gas temperature in the superheater. As the boiler load is reduced, burners are removed one by one from service. This will change the bustion rate and, thus, change the gas flow rate to the superheater.Attemperation is one of approaches frequently used. The attemperator is usually located at the point between the primary and secondary superheaters.There are two basic types of attemperator. The first is the tubular type in which some of superheated steam is passed through the tubes of a heat exchanger and has heat transferred to the boiler water(either boiler feedwater or water in the boiler drum).Subsequent to attemperation, the divided streams from the primary superheater will bine and enter the secondary superheater.The second type of attemperator involves a spray of feedwater into the atream ofsuperheated steam. The feedwater evaporates and reduces the steam temperature. Controlling the amount of feedwater will result in control of the ateam temperature. Care must be exercised to ensure that the spray water has sufficient purity. The spray water should mix well with the superheated steam so that there are no water droplets in the inlet of the secondary superjeater.Gas recirculation is used to control the steam temperature by changing the heat absorption rates both in the furnace and in the superheater. When the ateam temperature needs to be raised, some of the furnace. Therefore, the furnace temperature will bee lower, resulting in a lower heat absorption in the furnace and thereby a higher flue gas temperature in the furnace exit. This high gas temperature, bined with an increase in the gas floe rate, will increase the heat transfer rate in the superheater and thus increase the steam outlet temperature.Temperature control can be affected bu using different amounts of excess air. the more the excess air, the higher the steam outlet temperature would be. The reasons for this are similar to those for the gas recirculation method. It must be pointed out, however, that too much excess air will result in a reduction of boiler bustion efficiency. A dividedfurnace boiler is usually arranged with a generation of saturated steam in one section and a superheating of steam in another section. The temperature of the superheated steam is regulated by controlling the firing rates in the two furnaces. This method is not economical and is seldom applied in a central electric power station.17