问题 问答题

提高水环式真空泵真空度的方法?

答案

参考答案:

(1)打开阀门增加供水量。(2)检直漏气密封不严密的部分。拧紧密封处的连接螺栓及丝堵,更换填料,增加封水量,更换新垫片。(3)修复水环式真空泵侧盖,调整间隙至规定值,检查调整轴承。(4)增加冷水供给,降低水温。(5)检查清洗水环式真空泵叶轮、管道,除去阻塞物。

选择题
单项选择题

It is clear that some chemicals can damage the health of animals and humans. (1) , this is not the only problem that (2) be caused by the careless use of chemicals. Chemicals can also (3) the ecological balance of the environment. If the ecological balance is disturbed, the results can be (4) serious.

The (5) of DDT illustrates the problem. DDT, a chemical which kills insects, at first seemed to be a perfect answer (6) many problems. It would control insects that caused dangerous diseases, (7) insects that caused billions of dollars of damage to crops every year. Governments permitted and even (8) the use of DDT. Farmers in many countries (9) to spray it on their crops. The (10) results were good: Damage to crops (11) down, and profits went up. However, the chemical had effects which the scientists didn’t see in (12) . First, it also killed insects which were the (13) enemies of the harmful insects and which were therefore beneficial to farmers. Second, and perhaps worse, DDT did not kill (14) harmful insect. A few insects had natural resistance to the chemical. They (15) and reproduced in large numbers. In a few years there were large numbers of insects which were not (16) by DDT, and there were (17) insects which could act as natural (18) on these new "super-insects". Finally, it became clear that DDT was not solving the insect problem. In fact, it was making the problem worse. It (19) became necessary to find a second (20) for the effects of the first.

7()

A.as well as 

B.as well 

C.but 

D.also