问题 问答题 简答题

国产300MW机组启动旁路系统有哪些特点?

答案

参考答案:

国产300MW机组普遍采用总容量为锅炉蒸发量的37%或47%的两级旁路并联系统。大旁路(又称全机旁路)分两路从主蒸汽联络管上接出,每路容量为锅炉额定蒸发量的10%或15%,经装在锅炉房的快速减压减温器,再经装在汽轮机凝汽器喉部外侧的扩容式减温减压器,直通凝汽器。小旁路(又称高压缸旁路)从主蒸汽联络管上接出,容量为锅炉蒸发量的17%,经装在锅炉房的快速减压减温器,接入再热器冷段。

主蒸汽大、小旁路减温水来自给水泵出水母管。扩容式减温减压器减温水来自凝结水升压泵出水母管。

选择题
单项选择题

Electronic mail has become an extremely important and popular means of communication.
The convenience and efficiency of electronic mail are threatened by the extremely rapid growth in the volume of unsolicited commercial electronic mail. Unsolicited commercial electronic mail is currently estimated to account for over half of all electronic mail traffic, up from an estimated 7 percent in 2001, and the volume continues to rise. Most of these messages are fraudulent or deceptive in one or more respects.
The receipt of unsolicited commercial electronic mail may result in costs to recipients who cannot refuse to accept such mail and who incur costs for the storage of such mail, or for the time spent accessing, reviewing, and discarding such mail, or for both. The receipt of a large number of unwanted messages also decreases the convenience of electronic mail and creates a risk that wanted electronic mail messages, both commercial and noncommercial, will be lost, overlooked, or discarded amidst the larger volume of unwanted messages, thus reducing the reliability and usefulness of electronic mail to the recipient. Some commercial electronic mail contains material that many recipients may consider vulgar or pornographic in nature.
The growth in unsolicited commercial electronic mail imposes significant monetary costs on providers of Internet access services, businesses, and educational and nonprofit institutions that carry and receive such mail, as there is a finite volume of mail that such providers, businesses, and institutions can handle without further investment in infrastructure. Many senders of unsolicited commercial electronic mail purposefully disguise the source of such mail.
Many senders of unsolicited commercial electronic mail purposefully include misleading information in the messages’ subject lines in order to induce the recipients to view the messages. While some senders of commercial electronic mail messages provide simple and reliable ways for recipients to reject (or ’opt-out’ of) receipt of commercial electronic mail from such senders in the future, other senders provide no such ’opt-out’ mechanism, or refuse to honor the requests of recipients not to receive electronic mail from such senders in the future, or both.
Many senders of bulk unsolicited commercial electronic mail use computer programs to gather large numbers of electronic mail addresses on an automated basis from Internet websites or online services where users must post their addresses in order to make full use of the website or service.
The problems associated with the rapid growth and abuse of unsolicited commercial electronic mail cannot be solved by the government alone. The development and adoption of technological approaches and the pursuit of cooperative efforts with other countries will be necessary as well.

According to the passage, efficiency of e-mail is threatened by ______.

A.heavy e-mail traffic

B.fraudulent e-mail messages

C.large volume of messages

D.increasing amount of unwanted e-mail