过程装备与控制工程专业英语2230.docx

1、过程装备与控制工程专业英语2230过程装备与控制工程专业英语 学院: 化学化工学院 专业: 过程装备与控制工程 班级: 2007级1班 学号: 0704310112 姓名: 许贵发 Reading Material 22Reciprocating Compressors and Their Applications1. IntroductionThe purpose of compressors is to move air and other gases from place to place. Gases, unlike liquids, are compressible and requ

2、ire compression devices, which although similar to pumps, operate on somewhat different principles. Compressors, blowers, and fans are such compression devices.Compressors. Move air or gas in higher differential pressure ranges from 35 psi to as high as 65000 psi in extreme cases.Blowers. Move large

3、 volumes of air or gas at pressures up to 50 pounds per square inch. Fans. Move air or gas at a sufficient pressure to overcome static forces. Discharge pressures range from a few inches of water to about 1 pound per square inch.2. What is a Compressor?Basic Gas LawsBefore discussing the types of co

4、mpressors and how they work, it will be helpful to consider some of the basic gas laws and the manner in which they affect compressors.By definition, a gas is a fluid having neither independent shape nor form, which tends to expand indefinitely.Gases may be compose of only one specific gas maintaini

5、ng its own identity in the gas mixture. Air, for example, is a mixture of several gases, primarily nitrogen (78% by volume), oxygen (21%), argon (about 1%), and some water vapor. Air may also, due to local conditions, contain varying small percentages of industrial gases not normally a part of air.T

6、he First Law of ThermodynamicsThis law states that energy cannot be created or destroyed or destroyed during a process, such as compression and delivery of a gas. In other words, whenever a quantity of one kind of energy disappears, an exactly equivalent total of other kinds of energy must be produc

7、ed.The Second Law of ThermodynamicsThis law is more abstract, but can be stated in several ways:(1) Heat cannot, of itself, pass from a colder to a hotter body.(2) Heat can be transferred from a body at a lower temperature to one at a higher temperature only if external work is performed.(3) The ava

8、ilable energy of the isolated system decreases in all real processes. (4) By itself, heat or energy (like water), will flow only downhill (i. e., from hot to cold).Basically, then, these statements say that energy which exists at various levels is available for use only if it can move from a higher

9、to a lower level.Ideal or Perfect Gas LawsAn ideal or perfect gas is one to which the laws of Boyle, Charles, and Amonton apply.Such perfect gases do not really exist, but these three laws of thermodynamics can be used if corrected by compressibility factors based on experimental data.Boyles Law sta

10、tes that at a constant temperature, the volume of an ideal gas decreases with an increase in pressure.For example, if a given amount of gas is compressed at a constant temperature to half its volume, its pressure will be doubled. 或constantCharles Law state that at constant6 pressure, the volume of a

11、n ideal gas will increase as the temperature increases.If heat is applied to a gas it will expand, and the pressure will remain the same. This law assumes the absence of friction or the presence of an applied force. 或Amontons Law states that at constant volume, the pressure of an ideal gas will incr

12、ease as the temperature increases. 或Gas and Vapor By definition, a gas is that fluid form of substance in which the substance can expand indefinitely and completely fill its container. A vapor is a gasified liquid or solid-a substance in gaseous form.The term gas and vapor are generally used interch

13、angeably.3. How Compressors Work?To understand how gases and gas mixtures behave, it is necessary to recognize that gases consist of individual molecules of the various gas components, widely separated compared to their size. These molecules are always traveling at high speed; they strike against th

14、e walls of the enclosing vessel and produce what we know as pressure. Refer to Fig. 5. 7.Temperature affects average molecule speed. When heat is added to a fixed volume of gas, the molecules travel faster, and hit the containing walls of the vessel more often and with greater force. See Fig. 5. 8.

15、This then produces a greater pressure. This is consistent with Amontons Low.If the enclosed vessel is fitted with a piston so that the gas can be3 squeezed into a smaller space, the molecule travel is now restricted. The molecules now hit the walls with a greater frequency, increasing the pressure,

16、consistent with Boyles Law, See Fin. 5. 9.However, moving the piston also delivers energy to the molecules, causing them to move with increasing velocity. As with heating, this results in a temperature increase. Furthermore, all the molecules have been forced into a smaller space, which results in a

17、n increased number of collisions on a unit area of the wall. This, together with the increased velocity, results in increased pressure.The compression of gases to higher pressures results in higher temperatures, creating problems in compressor design. All basic compressor elements, regardless of typ

18、e, have certain design-limiting operating conditions. When any limitation is involved, it becomes necessary to perform the work in more the one step of the compression process. This is termed multistaging and uses one basic machine element designed to operate in series with other elements of the mac

19、hine. This limitation varies with the type of compressor, but the most important limitations include:(1) Discharge pressure-all types.(2) Pressure rise or differential-dynamic units and most displacement types.(3) Compression ratio-dynamic units.(4) Effect of clearance-reciprocating units (this is r

20、elated to the compression ratio).(5) Desirability of saving power.Methods of compressionFour methods are used to compress gas. Two are in the intermittent class, and two are in the continuous flow class. (these are descriptive, not thermodynamic or duty classification terms.)(1) Trap consecutive qua

21、ntities of gas in some type of enclosure, reduce the volume (thus increasing the pressure), then push the compressed gas out of the enclosure.(2) Trap consecutive quantities of gas in some type of enclosure, carry it without volume change to the discharge opening, compress the gas by backflow from t

22、he discharge system, then push the compressed gas out of the enclosure.(3) Compress the gas by the mechanical action of rapidly rotating impellers or bladed rotors that impart velocity and pressure to the flowing gas. (Velocity is further converted into pressure in stationary diffusers or blades.)(4

23、) Entrain the gas in a hi9gh velocity jet of the same or another gas (usually, but not necessarily, steam) and convert the high velocity of the mixture into pressure in a diffuser.阅读材料22往复式压缩机及其应用1. 简介 压缩机的作用是进行气体的输送，气体与液体不同，气体具有可压缩性，输送时需要压缩装置，该装置虽与水泵类似，但其工作原理仍有区别，压缩机，鼓风机，风机均为压缩设备。压缩机，可在35 Psi到高达650

24、00 Psi的极端压强下输送气体。鼓风机，可在压强高达50psi下大流量输送气体。风机，在足够的压力下输送气体，克服静阻力，压力范围在几英寸水柱到1psi。2. 什么是压缩机？气体基本定律在介绍压缩机类型以及工作原理之前，首先讨论下气体基本定律极其对压缩机的影响方式。依据定义，气体是一种没有特定形状的流体，可以无限制地膨胀。混合气体中各气体保持其各自的性质，比如说，空气。空气是几种气体的混合物，主要是氮气（78%体积含量），氧气（21%），氩气（约1），以及一些水蒸气。当然，不同条件下空气的成分也有区别，空气也可能含有一些如工业煤气等非正常空气所含有的成分。热力学第一定律这个定律阐述了在一个过

25、程中，能量不会创生，也不会消灭。在压缩输送气体的过程也一样。换句话说，当一种形式的能量被消耗掉的时候，另一种形式的能量就产生了。热力学第二定律此法则比较抽象，可用几种描述方法：1) 热量不能自发地由低温物体传到高温物体。2) 在外界对系统做工的时候，热量可以从低温物体传到高温物体。3) 孤立系统的有用能在所有过程中都是减少的。4) 热量或能量会自发地由大变小（如水，由热自发到冷）。这阐述了不同存在方式的能量的利用过程必须是一个由高品位到低品位的过程。理想气体定律理想气体服从Boyle、Charles、以及Amonton定律，理想气体并不存在，但解决实际气体问题时，如果使用由实验得出的修正因数来

26、修正，仍可以应用理想气体定律。Boyle定律表明，如果温度恒定，一定量气体体积缩小，必然导致其压强增大。例如：定量气体被等温压缩到原体积的一半，它的压强将变为原来的两倍。 或 常数Charles定律表明，压力一定时，气体体积随温度升高而增大。气体在受热膨胀时，压强可以保持不变，此定律建立在忽略摩擦等外阻的前提。 或 Amonton定律表明，恒定容积时，理想气体的压力会随着温度的升高而增大。 或 气体及水蒸气从定义上说来，气体是流体状物质，可以无限膨胀，完全充满容器。蒸气是某种液态或固态物质的气态形式。水蒸气以及气体经常交替使用。压缩机如何工作？为了解气体及气体混合物，有必要认识到气体是由单个分

27、子组成，分子与分子之间的距离与分子直径相比较大，分子保持高速运动，它们撞击容器壁，产生压强，可参照图5.7。温度会影响分子运动的平均速度，在定容状态下气体被加热会使得分子运动加快，撞击器壁的频率加快，撞击力也随之增大，见图5.8。这使得器壁产生了更大的压强，这种描述是符合Amonton定律的。如果封闭容器装配有活塞，就可以使气体被压缩到一个很小的空间内，限制气体分子的运动，气体撞击器壁的频率变大，压力也变大，这是符合Boyle定律的，见图5.9。然而，活塞在运动过程会对气体做工，使得分子运动加剧。而加热过程，则使得温度升高，此外，所有气体分子被限制于小范围空间内，单位容器面积上受到的气体分子撞

28、击增多，再加上撞击速度的增大，使得压强升高。气体被压缩程度越高，温度升高也越高，这给压缩机的设计带来了困难。压缩机的所有因素，并且无论是何种类型的压缩机，设计时都要受到工作条件的限制。考虑到某些因素，应该让压缩机压缩过程分多步进行，此称为多级压缩，过程使用的是串联操作。随压缩机种类不同，限制因素也不一样，其中最主要的因素包括：排气压力（所有类型压缩机均要考虑）。 压强升高，过程的动态变化以及排气类型。 动态压缩比。 压缩余隙的影响（与压缩比有关） 节能。压缩方式 气体的压缩有四种方法，其中两种属于间断操作，另外两种属于连续操作。（这些都是描述性的，不是热力学或工作类型的条款。）（1） 将定量气

29、体封闭于密闭容器中，然后减小容器体积（使得压强增大），最后将压缩气体排出。（2） 将气体限制于容器中，过程不引起卸料口体积变化，而是通过卸料系统的逆流来压缩气体。（3） 通过机械旋转的叶轮或装有叶片的转子将速度和压力传给气体，达到压缩气体的目的（速度进一步转化为静压强）。（4） 自身先变为高速气流或由另一种高速喷射状气流带动（通常见到的为蒸气），然后将速度转变为静压强。Reading Material 24Four Types of ValveCocks Undoubtedly the cock was the very first fluid closure device conceived

30、 by man. It has sufficed for well over 2000years and will continue to do so since it is the essence of simplicity, normally fully opened and closed in one quarter of a revolution of the plug, as featured in Fig. 5.13.By rotating the plug the port is alternately made to line up with its counterpart i

31、n the body and to the blank wall of the body to give open and shut conditions respectively.Unquestionably its origin is attributed to the Roman workers in metal and many of their early examples have survived to intrigue engineers and others alike. The idea of a conical plug rotating in a conical she

32、ll or body and retained in intimatebut not too intimate contact with it was a distinct break-through on the part of its originator, whoever that might have been. It must have become of pressing necessity when the open conduit gave way to the metal pipe originally made of rolled sheet lead, folded into near circular form, and sealed longitudinally in the manner practiced by present day plumbers.Coc