汽包废锅工作原理是什么？
参考答案：
高温气体经过废锅，使其水吸收高温气体之热量，迅速蒸发变为蒸汽，因为废锅始终充满水，因此蒸汽夹带着水或汽水混合物从废锅顶部连接着上升管进入逸出，汽包中的热水自上而下进入废锅，锅炉所产生的汽不断上升，上下管的水和汽水混合物不断地自然循环，汽包则不断地放出蒸汽，汽包上设有间断排污和连续排污，间断排污设在汽包的最低点，定期把锅炉水中的沉淀物排出，连续排污设在汽包的中上部，以排放锅炉水中的悬浮物及控制锅炉给水中的固体含量保证蒸汽的质量及锅炉的正常安全生产。
In the United States, 30 percent of the people have a “weight problem”。To many people the cause is obvious (明显的): they eat too much. But scientific evidence (证据) does little to support this idea. Going back to America of 1910, we find that people were thinner than today, yet they ate more food. In those days people worked harder physically, walked more, used machines much less, and didn’t watch television. Several modern studies, moreover, have shown that fatter people don’t eat more on average than thinner people. A 1979 study of 3,545 London office workers showed that fat people eat less than slim (苗条的) people. Studies also show that slim people are more active than fat people. A study by a research group at Stanford University found the following interesting facts: The more the man ran, the greater loss of the body fat The more they ran, the greater increase in food intake. Thus, those who ran the most ate the most, yet lost the greatest amount of body fat. 小题1: Nowadays many Americans have the problem that ______.
 ing to the passage, how many people in 900 Americans have a “weight problem”?
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