【正文】 reinforced with support straps which are welded to the tube wall with support blocks. High steam parameters also lead to higher material loading in the evaporator. The previously existing design reserves are no longer available, with the result that a detailed stress analysis is required for the design of the evaporator tubing in each case. As a result of the requisite 超超臨界直流鍋爐變壓運(yùn)行特性分析 第 31 頁(yè) 共 40 頁(yè) large wall thicknesses, the design of highly loaded heating surface areas is in part no longer determined by the primary stresses due to internal pressure but rather by the secondary stresses due to restrained thermal expansion. The higher evaporator temperatures also result in increasing temperature differences between the tubes and support straps on startup and shutdown. This in turn leads to longer startup times, especially on cold start. For steam temperatures up to approx 550℃ , all heating surfaces can be constructed of ferritic or martensitic materials, while at 600℃ austenitic materials are necessary for the final superheater heating surfaces for both the HP section of the boiler as well as the reheater. In addition to the strength parameters, corrosion behavior on the fluegas and oxidation behavior on the steam sides is especially determinative for material selection. With regard to strength parameters, construction of superheater heating surfaces for steam temperatures up to 650℃ is currently already feasible with austenitic steel materials. The corrosion resistance of the available materials however reduces the design limits to about 630℃ . Superheaters and reheaters are specially designed inline tube bundles that increase the temperature of saturated steam. In general terms, they are simplephase heat exchangers with steam flowing inside the tubes and the flue gas passing outside, general in crossflow. These critical ponents are manufactured from steel alloy material because of higher operating temperature. They are typically configured to help control steam outlet 超超臨界直流鍋爐變壓運(yùn)行特性分析 第 32 頁(yè) 共 40 頁(yè) temperature, keep metal temperature below acceptable limits and control steam flow pressure loss. The main difference between superheaters and reheaters is the steam pressure. The physical design and location of the surfaces depend upon the desired outlet temperatures, heat absorption, fuel ash characteristics and cleaning equipment. These surfaces can be either horizontal or vertical. The superheater and sometimes reheater are often divided into multiple sections to help control steam temperature and optimise heat recovery. Economizers and air heaters perform a key function in providing high overall boiler thermal efficiency by recovering the low level, . low temperature, energy from the flue gas before it is exhausted to the atmosphere. Economizers recover the energy by heating the boiler feedwater while air heaters heat the bustion air. Air heating also enhances the bustion of many fuels and ensuring ignition. The economizer is a counterflow heat exchanger for recovering energy from the gas beyond the superheater and, if used, the reheater. It increases the temperature of the feedwater。 The tube bundle is typically an arrangement of parallel horizontal serpentine tubes with the water flowing inside but in the opposition direction (counterflow) to the flue gas. Tube spacing is as tight as possible to promote heat transfer while permitting adequate tube surface cleaning and limiting flue gas side pressure loss. By design, steam is usually not generated inside these tubes. The air heaters utilizes the heat in the boiler flue 超超臨界直流鍋爐變壓運(yùn)行特性分析 第 33 頁(yè) 共 40 頁(yè) gases leaving the economizer to heat the bustion air and provide hot air for drying coal. An improvement of 1% in boiler efficiency is achieved for 22℃ rise in the coal bustion air temperature. The air outlet temperature limit in coal fired plant is dictated by the coal mill exit temperature and capacity of the tempering air system with the gas outlet temperature limited by considerations of fouling of the heat transfer surface and corrosion of downstream equipment. Most central station boilers are equipped with air pollution control systems. These often include an efficient precipitator and sometimes an SO2 removal systems. In addition, sufficient stack height is frequently used to ensure an acceptable level of pollution concentration in the plants surroundings. The selection of bustion equipment depends on the type of fuel used. For solid fuels such as coal, the bustion systems (mechanical stoker, pulverizerburner and cyclonefurnace) are generally suitable. Mechanical stoker were first developed in the history of the boiler. Almost any coal can be burned on some type of stoker. Other advantages of stokers include low power requirement and large operating range. They are usually used for the boiler capacity form 75,000 to 400,000lb of steam per hour. Because of small capacity, they are seldom used for today’ s central electric power station. The pulverizerburner system was introduced in the decade of last century. This system overes the size limitation of mechanical stoker. Modern pulverizing systems 超超臨界直流鍋爐變壓運(yùn)行特性分析 第 34 頁(yè) 共 40 頁(yè) are so well developed that they can burn almost any type of coal, particularly those in the higher grades and ranks. In addition, the system has improved response to the load change, higher bustion efficiency, and less manpower required in operation. The cyclone furnace is the most recent advancement. This system is primarily designed to burn a variety of coals, particularly those in the lower grades and ranks. The cyclone furnace is applicable to coals having a higher ash content and ashsoftening temperature at 2600F or lower. Compared with the pulverizer