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1、电厂蒸汽 动力的基础和使用1.1 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 使用 的基本原 则的概述。
2、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
3、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, 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
4、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
5、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。
6、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
7、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
8、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
9、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
10、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, 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
11、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
12、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
13、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 Power and use 1.1 Why anu nderstandingof steam is neededSteam power is fundamental to what is by far the largestsectorof the electric
14、itygeneratingindustry and without it the face of contemporarysociety would be dramatically different from its present one. Wewould be forced to relyon hydro-electric power plant, windmills, batteries,solar cells and fuel cells, all of which are capableof producing only a fraction of the electricity
15、we use.Steam is important, and thesafety and efficiency of its generationand use depend on the application of control and instrumentation, often simply referred to asC&I. The objective of this book is to provide a bridge betweenthe discipline of power-plant processengineeringand those of electronics
16、, instrumentation and control engineering.I shall start by outlining in this chapterthe changeof state of water to steam, followed by an overview of the basicprinciples of steam generation and use. This seemingly simple subject is extremely complex. This will necessarilybe an overview: it does not p
17、retend to bea detailed treatise andat times it will simplify matters and gloss over somedetails which may even causethe thermodynamicist or combustion physicist to shudder,but it should be understood that the aimis to provide the C&I engineerwith enough understandingof the subject to dealsafely with
18、 practical control-system design, operational and maintenanceproblems.1.2 Boiling: the change of statefrom water to steamWhen water is heatedits temperaturerises in a way that can be detected (for example by a thermometer. The heat gained in thisway is called sensiblebecause its effects canbe sensed
19、, butat some point the water starts to boil. But here we needto look even deeperi nto the subject. Exactly what is meantby the expressionboiling? To study this we must consider the threebasic statesof matter: solids, liquids and gases.(A plasma, produced when the atomsin a gasbecome ionised,is often
20、 referred to asthe fourth state of matter,but for most practical purposesit is sufficient to consider only the three basic states.In its solid state, matter consists of many molecules tightly bound togetherby attractive forces between them. When the matter absorbs heathe energy levels of its molecul
21、es increase and them ean distancebetweenthemoleculesincreases.As more and more hea tis applied theseeffects increaseuntil the attractive force betweent he molecules is eventuallyovercome and theparticles becomecapable of movingabout independentlyof each other.This change of statefrom solid to liquid
22、 is commonly recognised as melting.As more heat is applied to the liquid, some of the molecules gain enough energyto escape fromthe surface,a process calledevaporation (whereby a pool of liquid spilled on a surfacewill gradually disappear.What is happening during the process of evaporationis that so
23、me of the molecules are escaping at fairly low temperatures,but as the temperature rises theseescapesoccur more rapidly and at a certain point the liquid becomesvery agitated,with large quantities of bubbles rising to the surface. It isat this time that the liquid is said to start boiling. It is in
24、the processof changing stateto a vapour, which isa fluid in a gaseouss tate.Let usconsidera quantity of water that is containedin an open vessel. Here,the air that blanketsthe surfaceexerts a pressureon the surfaceof the fluid and, asthe temperatureo f the water is raised ,enough energy is eventuall
25、y gained to overcome the blanketing effect of that pressureand the water starts to changeits state into that of a vapour (steam.Further heat addedat this stage will not causeany further detectable changein temperature:the energy added is used tochangethe state of thefluid. Its effect canno longer be
26、 sensed by at hermometer,but it is still there.For this reason it is called latent, rather then sensible,heat. The temperaturea t which this happensis called the boiling point. At normal atmosphericpressure the boilingpoint of water is 100 C.If the pressureof the air blanket on topof the water were
27、to be increased ,more energy would have to be introduced it to break free. In other words, the temperaturemust be raisedfurther to make it boil. To illustrate this point, if the pressureis increasedby 10% above its normal atmosphericvalue, the temperature of the water must be raisedto just above 102
28、 Cbefore boiling occurs.The steamemerging from theboiling liquid is said to be saturatedand,for any given pressure,the temperatureat which boiling occursis called the saturationtemperature.The information relating to steam at any combination of temperature,pressureandother factors may be found in st
29、eamtables, which are nowadays available in software as well asin the more traditional paper form. Thesetableswere originally published in 1915 by Hugh Longbourne Callendar (1863-1930, aBritish physicist. Becauseof advancesi n knowledge and measurementtechnology, and as a result of changing units of
30、measurement,many different variants of steam tablesare today in existence,but they all enableone to look up,for any pressure,the saturation temperature,the heatper unit massof fluid, the specific volume etc.Understandingsteam and the steam tables is essentialin many stages of thedesignof power-plant
31、 control systems. For example, if a designer needs to compensatea steamflow measurementf or changes in pressureo, r to correctfor density errors in a water-level measurement,r eference to thesetables is essential.Another term relating to steamdefines the quantity of liquid mixed in with the vapour.
32、In the UK this is called the drynessfraction (in the USA the term used is steam quality. What this means ist hat if each kilogram of the mixture contains0.9 kg of vapour and 0.1 kg of water,the dryness fraction is 0.9.Steam becomessuperheated when itstemperaturei s raised above the saturation temper
33、ature corresponding to its pressure.This is achievedby collecting it from the vesselin which the boiling is occurring, leading it away from theliquid through a pipe, and then adding moreheat to it. This process addsfurther energy to thefluid, which improves the efficiency of the conversion of heat t
34、o electricity.As statedearlier, heat addedonce the water has startedto boil does notcause any further detectablechange intemperature.Instead it changesthe state of the fluid. Once the steam hasformed, heat addedto it contributes to the total heatof the vapour. This is thesensibleheatplus the latent
35、heatplustheheatusedin increasing the temperatureof each kilogram of the fluid through the number of degrees of superheato which it hasbeenraised. 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 interestsof eco
36、nomics and the environment it is important to obtain the highest to the water to enablepossible level of efficiency in this conversion process. As we have alreadyseen,the greatest efficiency is obtained by maximising theenergylevel of the steamat the point of delivery to the next stage of theprocess
37、. When as much energy as possible hasbeen abstractedfrom the steam,t he fluid reverts to the form of cold water, which is then warmed andtreated to removeany air which may have become entrained in it before it is finally returnedto the boiler for re-use.1.3 The nature of steamAs statedin the Preface
38、,the boilers and steam-generatorsthat are thesubject of this book provide steam to users such as industrialplant, or housing and other complexes,or to drive turbines that are the prime movers for electrical generators. For thepurposesof this book, such processesare grouped togetherunder the generic
39、name power plant.In all these applicationsthe steamis produced by applying heat to water until it boils, and before we embark on our study of power-plant C&I we must understandthe mechanisms involved in this process and thenature of steam itself.First, we must pauset o consider some basic thermodyna
40、mic processes.Two of these are the Carnot and Rankinecycles, and although the C&I engineermay not make use of thesedirectly, it is nevertheless usefulto have abasic understandingof what they are how they operate.1.3.1 TheCarnot cycleThe primary function of a power plant is to convert into electricit
41、y the energy locked up in some form of fuel resource. In spite of many attempts, it has not proved possible to generateelectricity in large quantities from the direct conversionof the energy contained in a fossil fuel (or even a nuclear fuel without the use of amedium that actsasan intermediary. Sol
42、ar cells and fuel cells may one day achievethis aim ona scalelarge enough tomake animpact on fossil-fuel utilisation, but at presentsuch plants are confined to small-scale applications. The waterturbines of hydro-electric plants are capableof generatinglarge quantities of electricity, but such plant
43、s are necessarily restricted to areaswhere they are plentiful supplies of water at heights sufficient for use by thesemachines.Therefore, if one wishesto obtain large quantities of electricity from a fossil fuel or from a nuclearreaction it is necessaryt o first release the energythat is available w
44、ithin that resourceand then to transfer it toa generator,and this process necessitatetshe use of a medium to convey the energy from source to destination. Furthermore, it is necessaryto employ a medium that isreadily available and which canbe used with relative safety andefficiency. On plant Earth,
45、water is, at least ingeneral,a plentiful and cheapmedium for effecting such transfers. With the development of technology during the twentieth century other possibilities have beenconsidered,such asthe use of mercury,but except for applications such asspacecraft whereentirely newsetsof limitations a
46、nd conditions apply, none of thesehasreached active use, and steam is universally usedin power stations.Carnot framed one of thetwo laws of thermodynamics. The first, Jouleslaw, had related mechanicalenergy to work: Carnotslaw defined the temperaturerelations applying to the conversion of heat energ
47、yinto mechanicalenergy.He sawthat if this processwere to be madereversible, heatcould be convertedinto work and then extracted and re-usedto make a closed loop. In his concept(Figure 1.1, a piston moves freely without encountering any friction inside acylinder made of some perfectlyinsulating materi
48、al. The piston is driven by a working fluid. The cylinder hasa headat one end that can be switched at will from being a perfectconductor to being a perfect insulator. Outside the cylinder aretwo bodies, one of which can deliver heat without its own temperature ( T1 falling, the other being a bottoml
49、ess cold sink at a temperature(T2 which is also constant. The operation of the system isshown graphically in figure 1.2, which shows the pressure/volume relationship of the fluid inthe cylinder over the whole cycle. Asthe processis a repeatingcycle its operation can be studied from any convenient starting point, and we shall begin at the point A, where t
