问题 问答题 论述题

直流锅炉的水冷壁形式及特点。

答案

参考答案:

直流锅炉出现的初期,水冷壁有三种相互独立的结构形式:即本生型、苏尔寿型和拉姆辛型。随着锅炉向高参数、大容量化的发展,按照采用膜式水冷壁和实现变压运行的要求,现代直流锅炉的水冷壁结构形式演变为两种形式:一种是垂直管屏,另一种是螺旋管圈。

(1)垂直管屏光管水冷壁:这种水冷壁结构管单,造价成本低,抵抗膜态沸腾的能力差,随着机组的大容量化,为了保证炉膛下辐射区水冷壁内的质量流速,下辐射区水冷壁的流路一般设计成二次垂直上升,两个流路之间用不受热的下降管相连接,习惯上将采用2~3次垂直上升水冷壁的锅炉称为UP型锅炉。垂直管屏光管水冷壁不适合变压运行。

(2)螺旋管圈水冷壁:这种水冷壁有以下优点:①工作在炉膛下辐射区的水冷壁同步经过炉膛内受热最强的区域和受热最弱的区域;②水冷壁中的工质在下辐射区一次性沿着螺旋管上升,没有中间联箱,工质在比体积变化最大的阶段避免了再分配;③不受炉膛周界的限制,可灵活选择并联工作的水冷壁管根数和管径,保证较大的质量流速。这些优点,使得水冷壁能够工作在热偏差最小和流量偏差最小的良好状态,因此,水动力稳定性较高,最适合变压运行。这种水冷壁的主要缺点是结构复杂,制造安装及检修工作量大,流动阻力比较大;另外,随着锅炉容量提高,管带宽度增大,盘旋圈数减少,热偏差增大。

阅读理解

请仔细阅读下面小题中的个人说明,从A 到F的六个选项中,选出符合个人要求的最佳选项,其中一项为多余选项。

小题1:Jeff likes sports and travel. He would like something that can help him

spend boring time on his journey.

小题2: Mrs. Green doesn’t like to exercise, but she likes cooking and reading

books about foreign places. 

小题3:Mr. Dean wants to improve his garden during summer. So he would like something that could help him to do that.

小题4: Lisa , a  22-year-old girl, worries about her weight. She is wondering how to lose weight.

小题5:Sophia likes to look smart. When she is free, she enjoys listening to the music. Now she plans to learn something about music.

A.Learn how to play the guitar. This popular video helps the beginner to become

a guitar player in just 200 hours.

B.Road trips. A program loved by young people, especially those fat girls. Join in it and make yourself become thin.

C.The new pocket MP4. This kind of MP4 fits right into your pocket. It lets you listen to your favorite artist, even when you are on a journey.

D.Singapore. The story of a city. This book is about the history. the culture and the favorite dishes of this small country.E. Spanish in 10 Minutes a Day. A fun, 132-page complete language learning work book. Includes 150 sticky labels: flash cards, a crossword puzzle and more. It is one of the best-selling language series in print.

F. Special scissors(剪刀). This kind of tools will make it easy to deal with flowers and trees. Anyone can learn to use it quickly.

单项选择题