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畢 業(yè) 設 計（論 文） 外 文 文 獻 翻 譯 學 號： 姓 名： 所在院系： 專業(yè)班級： 指導教師： 原文標題： The basics of steam generation and use 2009年 4月 29日 蒸發(fā)汽化的基礎和使用 The basics of steam generation and use 1.1 為何需要了解蒸汽 對于目前為止最大的發(fā)電工業(yè)部門來說，蒸汽動力是最為基礎性的。若沒有蒸汽動力，社會的樣子將會變得和現(xiàn)在大為不同。我們將不得已的去依靠水力發(fā)電廠、風車、電池、太陽能蓄電池和燃料電池，這些方法只能為我們平日用電提供很小的一部分。 蒸汽是很重要的，產生和使用蒸汽的安全與效率取決于怎樣控制和應用儀表，在術語中通常被簡寫成C&I（控制和儀表）。此書旨在在發(fā)電廠的工程規(guī)程和電子學、儀器儀表以及控制工程之間架設一座橋梁。 作為開篇，我將在本章大體描述由水到蒸汽的形態(tài)變化，然后將敘述蒸汽產生和使用的基本原則的概述。這看似簡單的課題實際上卻極為復雜。這里，我們有必要做一個概述：這本書不是內容詳盡的論文，有的時候甚至會掩蓋一些細節(jié)，而這些細節(jié)將會使熱力學家和燃燒物理學家都為之一震。但我們應該了解，這本書的目的是為了使控制儀表工程師充分理解這一課題，從而可以安全的處理實用控制系統(tǒng)設計、運作、維護等方面的問題。， 1.2沸騰：水到蒸汽的狀態(tài)變化 當水被加熱時，其溫度變化能通過某種途徑被察覺（例如用溫度計）。通過這種方式得到的熱量因為在某時水開始沸騰時其效果可被察覺，因而被稱為感熱。 然而，我們還需要更深的了解?！胺序v”究竟是什么含義？在深入了解之前，我們必須考慮到物質的三種狀態(tài)：固態(tài)，液態(tài)，氣態(tài)。（當氣體中的原子被電離時所產生的等離子氣體經常被認為是物質的第四種狀態(tài)，但在實際應用中，只需考慮以上三種狀態(tài)）固態(tài)，物質由分子通過分子間的吸引力緊緊地靠在一起。當物質吸收熱量，分子的能量升級并且使得分子之間的間隙增大。當越來越多的能量被吸收，這種效果就會加劇，粒子之間相互脫離。這種由固態(tài)到液態(tài)的狀態(tài)變化通常被稱之為熔化。 當液體吸收了更多的熱量時，一些分子獲得了足夠多的能量而從表面脫離，這個過程被稱為蒸發(fā)（憑此灑在地面的水會逐漸的消失）在蒸發(fā)的過程中，一些分子是在相當?shù)偷臏囟认旅撾x的，然而隨著溫度的上升，分子更加迅速的脫離，并且在某一溫度上液體內部變得非常劇烈，大量的氣泡向液體表面升起。在這時我們稱液體開始沸騰。這個過程是變?yōu)檎羝倪^程，也就是液體處于汽化狀態(tài)。 讓我們試想大量的水裝在一個敞開的容器內。液體表面的空氣對液體施加了一定的壓力，隨著液體溫度的上升，便會有足夠的能量使得表面的分子掙脫出去，水這時開始改變自身的狀態(tài)，變成蒸汽。在此條件下獲得更多的熱量將不會引起溫度上的明顯變化。所增加的能量只是被用來改變液體的狀態(tài)。它的效用不能用溫度計測量出來，但是它仍然發(fā)生著。正因為如此，它被稱為是潛在的，而不是可認知的熱量。使這一現(xiàn)象發(fā)生的溫度被稱為是沸點。在常溫常壓下，水的沸點為100攝氏度。 如果液體表面的壓力上升，需要更多的能量才可以使得水變?yōu)檎羝臓顟B(tài)。換句話說，必須使得溫度更高才可以使它沸騰?？偠灾绻髿鈮毫Ρ日Ｖ瞪甙俜种?，水必須被加熱到一百零二度才可以使之沸騰。 沸騰的水表面的蒸汽據說為飽和的，在特定的壓力下，沸騰發(fā)生時的溫度被成為飽和溫度。 關于蒸汽在任何混合的溫度和壓強及其他因素下的信息都可以在蒸汽表格中查到，如今我們可以通過軟件查詢而不是用傳統(tǒng)的表格。這些秩序表最初是在1915年由英國的物理學家Hugh Longbourne Callendar出版發(fā)行的。因為知識以及測量技術的進步，作為測量單位改變的結果，如今出現(xiàn)了許多版本的蒸汽表，但是它們都只能查出一種結果，在任何壓強下，飽和溫度，每單位液體的熱量，具體的體積等等。 在發(fā)電廠控制系統(tǒng)的設計過程中，了解蒸汽和蒸汽表是必不可少的。例如，如果一個設計師需要補償蒸汽流量的壓力變化，或者消除在水位測量中的密度誤差，參考這些表是至關重要的。 另一個與蒸汽有關的詞是界定汽水混合物中的蒸汽含量。在英國，即是所謂的蒸汽干度（在美國使用的術語是蒸汽品質）。這意味著，如果每公斤的混合物含有0.9公斤蒸汽和0.1公斤的水，干燥分數(shù)是0.9。 在相同大氣壓下，當它的溫度超過了它的飽和溫度時，水蒸氣就成為過熱蒸氣。當它沸騰之后收集起來，通過一個管道將它遠離流體，然后加入更多的熱量給它，這一過程中進一步給過熱蒸汽補充能量，從而提高熱量轉換為電能的效率。 如前所述,熱量補充給已開始沸騰的水不會引起溫度的進一步變化。相反，它卻改變流體的狀態(tài)。一旦形成了蒸汽，焓降有助于蒸汽的總熱量的增加。這些顯熱再加上潛熱用于增加每公斤流體過熱程度。 電廠的一個主要目標是將投入使用的燃料能量轉化為可用的熱或發(fā)電。在利益經濟和環(huán)境效益同等重要的情況下，重要的是在這一轉換過程獲得最高水平的經濟和環(huán)境效益。當從蒸汽中獲得盡可能多的能量后，液體變成冷卻水，然后進行再熱，終于回到了鍋爐重新使用。 1.3蒸汽的性質： 正如前言，這本書介紹給用戶的鍋爐及蒸汽發(fā)生器，以及他們的工廠或住房和其他復合物，或驅動渦輪，這些都是發(fā)電機的原動力。此書將這種過程統(tǒng)稱為‘發(fā)電廠’。在所有這些工程中，蒸汽都是由加熱水使其沸騰得到的，我們在開始研究發(fā)電廠C ＆ I之前，必須了解參與這一進程的機理和蒸汽本身。 首先，我們必須先考慮一些基本的熱力過程。其中兩個是卡諾和朗肯循環(huán)，雖然C ＆ I工程師可能無法直接利用它，但如何運用它仍然是一個非常必要的了解。 1.3.1卡諾循環(huán) 電廠的主要功能是將某種形式的燃料資源轉換成電力能源。盡管許多嘗試，但并沒有證明在未經中間媒介的情況下，可以直接將化石燃料（或原子核燃料）的能量轉換為電能。若太陽能電池和燃料電池在未來的大規(guī)模使用得以實現(xiàn)，將足以對化石燃料使用產生影響，但目前這種電廠只限于小規(guī)模的應用。水渦輪機的水力發(fā)電廠能夠產生大量的電力，但這種電廠有一定限制的地方，他們必須有滿足使用這些機器的足夠高的水位。 因此，如果希望從化石燃料或從核反應中獲得大量的電能，首先必須從可用資源中釋放能量，然后傳送到發(fā)電機，這個過程從頭到尾需要使用一種介質來傳遞能量。此外，有必要采用可以使其相對安全和提高效率的介質。對地球來講，水至少在一般情況下是一種豐富和廉價的介質。隨著技術的發(fā)展，在二十世紀，使用其他媒介的可能性也已被考慮，如使用水銀，但除了應用程序(如全新航天器的限制和適用條件),這些已經達到了積極的使用,和蒸汽一樣普遍適用于電站。 卡諾循環(huán)的兩個熱力學定律。第一，焦耳定律，與機械能做功有關：卡諾定律定義了在熱能轉換成機械能的工程中的溫度關系。他認為，如果該進程是可逆的，熱可以轉化成機械能，然后提取和重復使用，并使其閉環(huán)。如圖1.1，活塞沒有遇到任何摩擦，內氣缸完全由絕緣材料制成?；钊怯伞肮ぷ髁黧w”驅動。氣缸的一端，可以自由的從理想導體切換為絕緣體。外汽缸有兩部分組成，其中之一可以提供熱量而其本身的溫度(T1)下降, 另一個是一個無底冷水槽溫度(T2)是不變的。 如圖1.2所示 ，顯示了壓力/容積關系的流體在汽缸內的整個循環(huán)周期。由于這一進程是一個反復循環(huán)的過程，所以研究可以從任何方便的起點開始，我們將在A點開始，在氣缸蓋（在這個時候假定為是一個理想導體），使熱量從熱源進入氣缸。結果是，中期開始擴大，如果它被允許自由擴大，玻意耳定律（其中指出，在任何溫度之間關系的壓力和容量是常數(shù)）中規(guī)定的溫度不會上升，但將留在其初始溫度（T1） 。這就是所謂的等溫膨脹。 當介質的壓力和容積已達到B點時，氣缸蓋由理想導體轉換成一個絕緣體，而介質允許繼續(xù)擴大，而沒有熱的增減，這就是所謂的絕熱膨脹。當介質的壓力和容積已達到C點時，氣缸蓋轉變成理想導體，但外部熱源被散熱器取而代之。活塞開始驅動，然后壓縮介質。熱流經頭部的散熱片，當溫度達到中等，在散熱片（點D），缸蓋再次切換到理想絕緣體，戒指被壓縮直至到達初始條件的壓力和溫度，這個周期便完成了，在絕熱情況下對外做功。 1.3.2朗肯循環(huán) 卡諾循環(huán)設定一個汽缸絕緣墻和可以隨意由導體轉換成絕緣體的氣缸蓋，它可能仍然是一個科學的概念并沒有實際應用中得到運用。在20世紀初，一名蘇格蘭的工程教授叫威廉林肯,他對卡諾循環(huán)提出了修改，在這個基礎上發(fā)展形成的理論在火力發(fā)電廠被廣泛使用。即使現(xiàn)在的聯(lián)合循環(huán)電廠仍然使用他的兩個階段的操作。 朗肯循環(huán)示意圖如圖1.3。從A點開始，在恒壓條件下，通過熱源使介質膨脹到B點，然后絕熱膨脹發(fā)生，直至達到曲線圖狀態(tài)點C，從這里開始，在恒溫條件下，介質的體積減小直至到達D點，最后將其壓縮回其初始條件。 The basics of steam generation and use 1.1 Why an understanding of steam is needed Steam power is fundamental to what is by far the largest sector 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 fraction 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 engineering 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 necessarily 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 understanding 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 steam When 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 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 plasma, 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 between 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 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 surface 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 bubbles 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 pressure 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 further 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 point. 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 point, 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 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 Hugh 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 saturation 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 for 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 kilogram 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, 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 detectable 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 number 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 possible 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 reverts 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 steam As stated in the Preface, the boilers and steam-generators that are the subject of this book provide 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 by 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, 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 cycle The primary function of a power plant is to convert into electricity the energy locked up in some form of fuel resource. 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 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 where 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 transfer 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 general, 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 apply, 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 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 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 being 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 from the hot source to enter the cylinder. The result is that the medium begins to expand, and if it is allowed to expand freely, Boyles law (which states that at any temperature the relationship between pressure and volume is constant) dictates that the temperature will not rise, but will stay at its initial temperature (Tl). This is called isothermal expansion. When the pressure and volume of the medium have reached the values at point B, the cylinder head is switched from being a perfect conductor to being a perfect insulator and the medium allowed to continue its expansion with no heat being gained or lost. This is known as adiabatic expansion. When the pressure and volume of the medium reach the values at point C, the cylinder head is switched back to being a perfect conductor, but the external heat source is removed and replaced by the heat sink. The piston is driven towards the head, compressing the medium. Heat flows through the head to the heat sink and when the temperature of the medium reaches that of the heat sink (at point D), the cylinder head is once again switched to become a perfect insulator and the medium is compressed until it reaches its starting conditions of pressure and temperature.The cycle is then complete, having taken in and rejected heat while doing external work. 1.3.2 The Rankine cycle The Carnot cycle postulates a cylinder with perfectly insulating walls and a head which can be switched at will from Being a conductor to being an insulator. Even with modifications to enable it to operate in a world where such things are not obtainable, it would have probably remained a scientific concept with no practical application, had not a Scottish professor of engineering, William Rankine, proposed a modification to it at the beginning of the twentieth century [I]. The concepts that Rankine developed form the basis of all thermal power plants in use today. Even todays combined-cycle power plants use his cycle for one of the two phases of their operation. Figure 1.3 illustrates the principle of the Rankine cycle. Starting at point A again, the source of heat is applied to expand the medium, this time at a constant pressure, to point B, after which adiabatic expansion is again made to occur until the medium reaches the conditions at point C. From here, the volume of the medium is reduced, at a constant pressure, until it reaches point D, when it is compressed back to its initial conditions.