火电厂自动化电气自动化毕业设计外文翻译Word文档格式.docx

1、蒸发汽化的基础和使用1.1 为何需要了解蒸汽关于目前为止最大的发电工业部门来说，蒸汽动力是最为基础性的。假设没有蒸汽动力，社会的模样将会变得和现在大为不同。我们将不得已的去依靠水力发电厂、风车、电池、太阳能蓄电池和燃料电池，这些方法只能为我们平日用电提供专门小的一部分。蒸汽是专门重要的，产生和使用蒸汽的安全与效率取决于如何样操纵和应用外表，在术语中通常被简写成C&I操纵和外表。此书旨在在发电厂的工程规程和电子学、仪器外表以及操纵工程之间架设一座桥梁。作为开篇，我将在本章大体描述由水到蒸汽的形状变化，然后将表达蒸汽产生和使用的差不多原那么的概述。这看似简单的课题实际上却极为复杂。那个地点，我们有

2、必要做一个概述：这本书不是内容详尽的论文，有的时候甚至会掩盖一些细节，而这些细节将会使热力学家和燃烧物理学家都为之一震。但我们应该了解，这本书的目的是为了使操纵外表工程师充分明白得这一课题，从而能够安全的处理有用操纵系统设计、运作、爱护等方面的问题。，1.2沸腾：水到蒸汽的状态变化当水被加热时，其温度变化能通过某种途径被察觉例如用温度计。通过这种方式得到的热量因为在某时水开始沸腾时其成效可被察觉，因而被称为感热。然而，我们还需要更深的了解。沸腾怎么说是什么含义？在深入了解之前，我们必须考虑到物质的三种状态：固态，液态，气态。当气体中的原子被电离时所产生的等离子气体经常被认为是物质的第四种状态，

3、但在实际应用中，只需考虑以上三种状态固态，物质由分子通过分子间的吸引力紧紧地靠在一起。当物质吸取热量，分子的能量升级同时使得分子之间的间隙增大。当越来越多的能量被吸取，这种成效就会加剧，粒子之间相互脱离。这种由固态到液态的状态变化通常被称之为熔化。当液体吸取了更多的热量时，一些分子获得了足够多的能量而从表面脱离，那个过程被称为蒸发凭此洒在地面的水会逐步的消逝在蒸发的过程中，一些分子是在相当低的温度下脱离的，然而随着温度的上升，分子更加迅速的脱离，同时在某一温度上液体内部变得专门剧烈，大量的气泡向液体表面升起。在这时我们称液体开始沸腾。那个过程是变为蒸汽的过程，也确实是液体处于汽化状态。让我们试

4、想大量的水装在一个放开的容器内。液体表面的空气对液体施加了一定的压力，随着液体温度的上升，便会有足够的能量使得表面的分子挣脱出去，水这时开始改变自身的状态，变成蒸汽。在此条件下获得更多的热量将可不能引起温度上的明显变化。所增加的能量只是被用来改变液体的状态。它的效用不能用温度计测量出来，然而它仍旧发生着。正因为如此，它被称为是潜在的，而不是可认知的热量。使这一现象发生的温度被称为是沸点。在常温常压下，水的沸点为100摄氏度。假如液体表面的压力上升，需要更多的能量才能够使得水变为蒸汽的状态。换句话说，必须使得温度更高才能够使它沸腾。总而言之，假如大气压力比正常值升高百分之十，水必须被加热到一百零

5、二度才能够使之沸腾。沸腾的水表面的蒸汽据说为饱和的，在特定的压力下，沸腾发生时的温度被成为饱和温度。关于蒸汽在任何混合的温度和压强及其他因素下的信息都能够在蒸汽表格中查到，现在我们能够通过软件查询而不是用传统的表格。这些秩序表最初是在1915年由英国的物理学家Hugh Longbourne Callendar出版发行的。因为知识以及测量技术的进步，作为测量单位改变的结果，现在显现了许多版本的蒸汽表，然而它们都只能查出一种结果，在任何压强下，饱和温度，每单位液体的热量，具体的体积等等。在发电厂操纵系统的设计过程中，了解蒸汽和蒸汽表是必不可少的。例如，假如一个设计师需要补偿蒸汽流量的压力变化，或者

6、排除在水位测量中的密度误差，参考这些表是至关重要的。另一个与蒸汽有关的词是界定汽水混合物中的蒸汽含量。在英国，即是所谓的蒸汽干度在美国使用的术语是蒸汽品质。这意味着，假如每公斤的混合物含有0.9公斤蒸汽和0.1公斤的水，干燥分数是0.9。在相同大气压下，当它的温度超过了它的饱和温度时，水蒸气就成为过热蒸气。当它沸腾之后收集起来，通过一个管道将它远离流体，然后加入更多的热量给它，这一过程中进一步给过热蒸汽补充能量，从而提高热量转换为电能的效率。如前所述,热量补充给已开始沸腾的水可不能引起温度的进一步变化。相反，它却改变流体的状态。一旦形成了蒸汽，焓降有助于蒸汽的总热量的增加。这些显热再加上潜热用

7、于增加每公斤流体过热程度。电厂的一个要紧目标是将投入使用的燃料能量转化为可用的热或发电。在利益经济和环境效益同等重要的情形下，重要的是在这一转换过程获得最高水平的经济和环境效益。当从蒸汽中获得尽可能多的能量后，液体变成冷却水，然后进行再热，终于回到了锅炉重新使用。1.3蒸汽的性质：正如前言，这本书介绍给用户的锅炉及蒸汽发生器，以及他们的工厂或住房和其他复合物，或驱动涡轮，这些差不多上发电机的原动力。此书将这种过程统称为发电厂。在所有这些工程中，蒸汽差不多上由加热水使其沸腾得到的，我们在开始研究发电厂C I之前，必须了解参与这一进程的机理和蒸汽本身。第一，我们必须先考虑一些差不多的热力过程。其中

8、两个是卡诺和朗肯循环，尽管C I工程师可能无法直截了当利用它，但如何运用它仍旧是一个专门必要的了解。1.3.1卡诺循环电厂的要紧功能是将某种形式的燃料资源转换成电力能源。尽管许多尝试，但并没有证明在未经中间媒介的情形下，能够直截了当将化石燃料或原子核燃料的能量转换为电能。假设太阳能电池和燃料电池在以后的大规模使用得以实现，将足以对化石燃料使用产生阻碍，但目前这种电厂只限于小规模的应用。水涡轮机的水力发电厂能够产生大量的电力，但这种电厂有一定限制的地点，他们必须有满足使用这些机器的足够高的水位。因此，假如期望从化石燃料或从核反应中获得大量的电能，第一必须从可用资源中开释能量，然后传送到发电机，那

9、个过程从头到尾需要使用一种介质来传递能量。此外，有必要采纳能够使其相对安全和提高效率的介质。对地球来讲，水至少在一样情形下是一种丰富和廉价的介质。随着技术的进展，在二十世纪，使用其他媒介的可能性也已被考虑，如使用水银，但除了应用程序（如全新航天器的限制和适用条件）,这些差不多达到了积极的使用,和蒸汽一样普遍适用于电站。卡诺循环的两个热力学定律。第一，焦耳定律，与机械能做功有关：卡诺定律定义了在热能转换成机械能的工程中的温度关系。他认为，假如该进程是可逆的，热能够转化成机械能，然后提取和重复使用，并使其闭环。如图1.1，活塞没有遇到任何摩擦，内气缸完全由绝缘材料制成。活塞是由工作流体驱动。气缸的

10、一端，能够自由的从理想导体切换为绝缘体。外汽缸有两部分组成，其中之一能够提供热量而其本身的温度（T1）下降, 另一个是一个无底冷水槽温度（T2）是不变的。如图1.2所示 ，显示了压力/容积关系的流体在汽缸内的整个循环周期。由于这一进程是一个反复循环的过程，因此研究能够从任何方便的起点开始，我们将在A点开始，在气缸盖在那个时候假定为是一个理想导体，使热量从热源进入气缸。结果是，中期开始扩大，假如它被承诺自由扩大，玻意耳定律其中指出，在任何温度之间关系的压力和容量是常数中规定的温度可不能上升，但将留在其初始温度T1 。这确实是所谓的等温膨胀。当介质的压力和容积已达到B点时，气缸盖由理想导体转换成一

11、个绝缘体，而介质承诺连续扩大，而没有热的增减，这确实是所谓的绝热膨胀。当介质的压力和容积已达到C点时，气缸盖转变成理想导体，但外部热源被散热器取而代之。活塞开始驱动，然后压缩介质。热流经头部的散热片，当温度达到中等，在散热片点D，缸盖再次切换到理想绝缘体，戒指被压缩直至到达初始条件的压力和温度，那个周期便完成了，在绝热情形下对外做功。1.3.2朗肯循环卡诺循环设定一个汽缸绝缘墙和能够随意由导体转换成绝缘体的气缸盖，它可能仍旧是一个科学的概念并没有实际应用中得到运用。在20世纪初，一名苏格兰的工程教授叫威廉林肯,他对卡诺循环提出了修改，在那个基础上进展形成的理论在火力发电厂被广泛使用。即使现在的

12、联合循环电厂仍旧使用他的两个时期的操作。朗肯循环示意图如图1.3。从A点开始，在恒压条件下，通过热源使介质膨胀到B点，然后绝热膨胀发生，直至达到曲线图状态点C，从那个地点开始，在恒温条件下，介质的体积减小直至到达D点，最后将其压缩回其初始条件。The basics of steam generation and use1.1 Why an understanding of steam is neededSteam power is fundamental to what is by far the largest sector of the electricity-generating ind

13、ustry and without it the face of contemporary society would be dramatically different from its present one. We would be forced to rely on hydro-electric power plant, windmills, batteries, solar cells and fuel cells, all of which are capable of producing only a fraction of the electricity we use.Stea

14、m is important, and the safety and efficiency of its generation and use depend on the application of control and instrumentation, often simply referred to as C&I. The objective of this book is to provide a bridge between the discipline of power-plant process engineering and those of electronics, ins

15、trumentation and control engineering.I shall start by outlining in this chapter the change of state of water to steam, followed by an overview of the basic principles of steam generation and use. This seemingly simple subject is extremely complex. This will necessarily be an overview: it does not pr

16、etend to be a detailed treatise and at times it will simplify matters and gloss over some details which may even cause the thermodynamicist or combustion physicist to shudder, but it should be understood that the aim is to provide the C&I engineer with enough understanding of the subject to deal saf

17、ely with practical control-system design, operational and maintenance problems.1.2 Boiling: the change of state from water to steamWhen water is heated its temperature rises in a way that can be detected （for example by a thermometer）. The heat gained in this way is called sensible because its effec

18、ts can be sensed, but at some point the water starts to boil. But here we need to look even deeper into the subject. Exactly what is meant by the expression boiling? To study this we must consider the three basic states of matter: solids, liquids and gases. （A plasma, produced when the atoms in a ga

19、s become ionised, is often referred to as the fourth state of matter, but for most practical purposes it is sufficient to consider only the three basic states.） In its solid state, matter consists of many molecules tightly bound together by attractive forces between them. When the matter absorbs hea

20、t the energy levels of its molecules increase and the mean distance between the molecules increases. As more and more heat is applied these effects increase until the attractive force between the molecules is eventually overcome and the particles become capable of moving about independently of each

21、other. This change of state from solid to liquid is commonly recognised as melting.As more heat is applied to the liquid, some of the molecules gain enough energy to escape from the surface, a process called evaporation （whereby a pool of liquid spilled on a surface will gradually disappear）. What i

22、s happening during the process of evaporation is that some of the molecules are escaping at fairly low temperatures, but as the temperature rises these escapes occur more rapidly and at a certain point the liquid becomes very agitated, with large quantities of bubbles rising to the surface. It is at

23、 this time that the liquid is said to start . It is in the process of changing state to a vapour, which is a fluid in a gaseous state.Let us consider a quantity of water that is contained in an open vessel. Here, the air that blankets the surface exerts a pressure on the surface of the fluid and, as

24、 the temperature of the water is raised, enough energy is eventually gained to overcome the blanketing effect of that pressure and the water starts to change its state into that of a vapour （steam）. Further heat added at this stage will not cause any further detectable change in temperature: the ene

25、rgy added is used to change the state of the fluid. Its effect can no longer be sensed by a thermometer, but it is still there. For this reason it is called latent, rather then sensible, heat. The temperature at which this happens is called the boiling point. At normal atmospheric pressure the boili

26、ng point of water is 100 C.If the pressure of the air blanket on top of the water were to be increased, more energy would have to be introduced it to break free. In other words, the temperature must be raised further to make it boil. To illustrate this point, if the pressure is increased by 10% abov

27、e its normal atmospheric value, the temperature of the water must be raised to just above 102 C before boiling occurs.The steam emerging from the boiling liquid is said to be saturated and, for any given pressure, the temperature at which boiling occurs is called the saturation temperature.The infor

28、mation relating to steam at any combination of temperature, pressure and other factors may be found in steam tables, which are nowadays available in software as well as in the more traditional paper form. These tables were originally published in 1915 by Hugh Longbourne Callendar （1863-1930）, a Brit

29、ish physicist. Because of advances in knowledge and measurement technology, and as a result of changing units of measurement, many different variants of steam tables are today in existence, but they all enable one to look up, for any pressure, the saturation temperature, the heat per unit mass of fl

30、uid, the specific volume etc.Understanding steam and the steam tables is essential in many stages of the design of power-plant control systems. For example, if a designer needs to compensate a steam-flow measurement for changes in pressure, or to correct for density errors in a water-level measureme

31、nt, reference to these tables is essential.Another term relating to steam defines the quantity of liquid mixed in with the vapour. In the UK this is called the dryness fraction （in the USA the term used is steam quality）. What this means is that if each kilogram of the mixture contains 0.9 kg of vap

32、our and 0.1 kg of water, the dryness fraction is 0.9.Steam becomes superheated when its temperature is raised above the saturation temperature corresponding to its pressure. This is achieved by collecting it from the vessel in which the boiling is occurring, leading it away from the liquid through a pipe, and then adding more heat to it. This process adds further energy to the fluid,