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1、蒸发汽化的基础和使用 The basics of steam generation and use1.1 为何需要了解蒸汽对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。若没有蒸汽动力，社会的样子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很小的一部分。蒸汽是很重要的，产生和使用蒸汽的安全与效率取决于怎样控制和应用仪表，在术语中通常被简写成C&I（控制和仪表）。此书旨在在发电厂的工程规程和电子学、仪器仪表以及控制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态变化，然后将叙述蒸汽产生和使
2、用的基本原则的概述。这看似简单的课题实际上却极为复杂。这里，我们有必要做一个概述：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控制仪表工程师充分理解这一课题，从而可以安全的处理实用控制系统设计、运作、维护等方面的问题。，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效果可被察觉，因而被称为感热。然而，我们还需要更深的了解。“沸腾”究竟是什么含义？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。
3、（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，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。当物质吸收热量，分子的能量升级并且使得分子之间的间隙增大。当越来越多的能量被吸收，这种效果就会加剧，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。当液体吸收了更多的热量时，一些分子获得了足够多的能量而从表面脱离，这个过程被称为蒸发（凭此洒在地面的水会逐渐的消失）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，并且在某一温度上液体内部变得非常剧烈，大量的气泡向液体表面升起。在这时我
4、们称液体开始沸腾。这个过程是变为蒸汽的过程，也就是液体处于汽化状态。让我们试想大量的水装在一个敞开的容器内。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不会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，但是它仍然发生着。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。如果液体表面的压力上升，需要更多的能量才可以使得水变为蒸汽的状态。换句话说，必须使得温度更高才可以
5、使它沸腾。总而言之，如果大气压力比正常值升高百分之十，水必须被加热到一百零二度才可以使之沸腾。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可以在蒸汽表格中查到，如今我们可以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，如今出现了许多版本的蒸汽表，但是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。在发电厂控制系统的设计过程中，了解蒸汽
6、和蒸汽表是必不可少的。例如，如果一个设计师需要补偿蒸汽流量的压力变化，或者消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，如果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不会引起温度的进一步变化。相反，它却改
7、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于增加每公斤流体过热程度。电厂的一个主要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况下，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，这些都是发电机的原动力。此书将这种过程统称为发电厂。在所有这些工程中，蒸汽都是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
8、这一进程的机理和蒸汽本身。首先，我们必须先考虑一些基本的热力过程。其中两个是卡诺和朗肯循环，虽然C I工程师可能无法直接利用它，但如何运用它仍然是一个非常必要的了解。1.3.1卡诺循环电厂的主要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况下，可以直接将化石燃料（或原子核燃料）的能量转换为电能。若太阳能电池和燃料电池在未来的大规模使用得以实现，将足以对化石燃料使用产生影响，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方，他们必须有满足使用这些机器的足够高的水位。因此，如果希望从化石燃料或从核反应中获
9、得大量的电能，首先必须从可用资源中释放能量，然后传送到发电机，这个过程从头到尾需要使用一种介质来传递能量。此外，有必要采用可以使其相对安全和提高效率的介质。对地球来讲，水至少在一般情况下是一种丰富和廉价的介质。随着技术的发展，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),这些已经达到了积极的使用,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，如果该进程是可逆的，热可以转化成机械能，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
10、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。气缸的一端，可以自由的从理想导体切换为绝缘体。外汽缸有两部分组成，其中之一可以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，所以研究可以从任何方便的起点开始，我们将在A点开始，在气缸盖（在这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，如果它被允许自由扩大，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）中规定的温度不会上升，但将留在其初始温度（T1） 。这就
11、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个绝缘体，而介质允许继续扩大，而没有热的增减，这就是所谓的绝热膨胀。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，这个周期便完成了，在绝热情况下对外做功。1.3.2朗肯循环卡诺循环设定一个汽缸绝缘墙和可以随意由导体转换成绝缘体的气缸盖，它可能仍然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
12、循环提出了修改，在这个基础上发展形成的理论在火力发电厂被广泛使用。即使现在的联合循环电厂仍然使用他的两个阶段的操作。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，从这里开始，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 sect
13、or of the electricity-generating industry 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 fra
14、ction of the electricity we use.Steam 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 engin
15、eering and those of electronics, instrumentation 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 necess
16、arily be an overview: it does not pretend 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 unde
17、rstanding of the subject to deal safely 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
18、is called sensible because its effects 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 pla
19、sma, produced when the atoms in a gas 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 betwe
20、en them. When the matter absorbs heat 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
21、 moving about independently of each 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 surfa
22、ce will gradually disappear). What is 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 bub
23、bles rising to the surface. It is at this time that the liquid is said to start boiling. 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 p
24、ressure on the surface of the fluid and, as 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 furthe
25、r detectable change in temperature: the energy 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 poin
26、t. At normal atmospheric pressure the boiling 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 poin
27、t, if the pressure is increased by 10% above 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
28、called the saturation temperature.The information 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 Hu
29、gh Longbourne Callendar (1863-1930), a British 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 saturatio
30、n temperature, the heat per unit mass of fluid, 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 fo
31、r density errors in a water-level measurement, 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 kil
32、ogram of the mixture contains 0.9 kg of vapour 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
33、, leading it away from the liquid through a pipe, and then adding more heat to it. This process adds further energy to the fluid, which improves the efficiency of the conversion of heat to electricity.As stated earlier, heat added once the water has started to boil does not cause any further detecta
34、ble change in temperature. Instead it changes the state of the fluid. Once the steam has formed, heat added to it contributes to the total heat of the vapour. This is the sensible heat plus the latent heat plus the heat used in increasing the temperature of each kilogram of the fluid through the num
35、ber of degrees of superheat to which it has been raised.In a power plant, a major objective is the conversion of energy locked up in the input fuel into either usable heat or electricity. In the interests of economics and the environment it is important to obtain the highest to the water to enable p
36、ossible level of efficiency in this conversion process. As we have already seen, the greatest efficiency is obtained by maximising the energy level of the steam at the point of delivery to the next stage of the process. When as much energy as possible has been abstracted from the steam, the fluid re
37、verts to the form of cold water, which is then warmed and treated to remove any air which may have become entrained in it before it is finally returned to the boiler for re-use.1.3 The nature of steamAs stated in the Preface, the boilers and steam-generators that are the subject of this book provide
38、 steam to users such as industrial plant, or housing and other complexes, or to drive turbines that are the prime movers for electrical generators. For the purposes of this book, such processes are grouped together under the generic name power plant. In all these applications the steam is produced b
39、y applying heat to water until it boils, and before we embark on our study of power-plant C&I we must understand the mechanisms involved in this process and the nature of steam itself.First, we must pause to consider some basic thermodynamic processes. Two of these are the Carnot and Rankine cycles,
40、 and although the C&I engineer may not make use of these directly, it is nevertheless useful to have a basic understanding of what they are how they operate.1.3.1 The Carnot cycleThe primary function of a power plant is to convert into electricity the energy locked up in some form of fuel resource.
41、In spite of many attempts, it has not proved possible to generate electricity in large quantities from the direct conversion of the energy contained in a fossil fuel (or even a nuclear fuel) without the use of a medium that acts as an intermediary. Solar cells and fuel cells may one day achieve this
42、 aim on a scale large enough to make an impact on fossil-fuel utilisation, but at present such plants are confined to small-scale applications. The water turbines of hydro-electric plants are capable of generating large quantities of electricity, but such plants are necessarily restricted to areas w
43、here they are plentiful supplies of water at heights sufficient for use by these machines.Therefore, if one wishes to obtain large quantities of electricity from a fossil fuel or from a nuclear reaction it is necessary to first release the energy that is available within that resource and then to tr
44、ansfer it to a generator, and this process necessitates the use of a medium to convey the energy from source to destination. Furthermore, it is necessary to employ a medium that is readily available and which can be used with relative safety and efficiency. On plant Earth, water is, at least in gene
45、ral, a plentiful and cheap medium for effecting such transfers. With the development of technology during the twentieth century other possibilities have been considered, such as the use of mercury, but except for applications such as spacecraft where entirely new sets of limitations and conditions a
46、pply, none of these has reached active use, and steam is universally used in power stations.Carnot framed one of the two laws of thermodynamics. The first, Joules law, had related mechanical energy to work: Carnots law defined the temperature relations applying to the conversion of heat energy into
47、mechanical energy. He saw that if this process were to be made reversible, heat could be converted into work and then extracted and re-used to make a closed loop. In his concept (Figure 1.1), a piston moves freely without encountering any friction inside a cylinder made of some perfectly insulating
48、material. The piston is driven by a working fluid. The cylinder has a head at one end that can be switched at will from being a perfect conductor to being a perfect insulator. Outside the cylinder are two bodies, one of which can deliver heat without its own temperature ( T1 ) falling, the other bei
49、ng a bottomless cold sink at a temperature (T2) which is also constant.The operation of the system is shown graphically in figure 1.2, which shows the pressure/volume relationship of the fluid in the cylinder over the whole cycle. As the process is a repeating cycle its operation can be studied from any convenient starting point, and we shall begin at the point A, where the cylinder head (at this time assumed to be a perfect conductor of heat), allows heat
