附录B英文翻译
THERMODYNAMICS AND REFRIGERATION CYCLES
THERMODYNAMICS is the study of energy, its transformations, and its relation to states of matter. This chapter covers theapplication of thermodynamics to refrigeration cycles. The first partreviews the first and second laws of thermodynamics and presentsmethods for calculating thermodynamic properties. The second andthird parts address compression and absorption refrigeration cycles,two common methods of thermal energy transfer.
THERMODYNAMICS
A thermodynamic system is a region in space or a quantity ofmatter bounded by a closed surface. The surroundings includeeverything external to the system, and the system is separated from
the surroundings by the system boundaries. These boundaries canbe movable or fixed, real or imaginary.Entropy and energy are important in any thermodynamic system.Entropy measures the molecular disorder of a system. The moremixed a system, the greater its entropy; an orderly or unmixed configuration is one of low entropy. Energy has the capacity for producing an effect and can be categorized into either stored ortransient forms.
Stored Energy
Thermal (internal) energy is caused by the motion of molecules and/or intermolecular forces.
Potential energy (PE) is caused by attractive forces existingbetween molecules, or the elevation of the system.
word/media/image1.gif (1)
where
m =mass
g = local acceleration of gravity
z = elevation above horizontal reference plane
Kinetic energy (KE) is the energy caused by the velocity of molecules and is expressed as
word/media/image2.gif (2)
where
V is the velocity of a fluid stream crossing the system boundary.
Chemical energy is caused by the arrangement of atoms composing the molecules.
Nuclear (atomic) energy derives from the cohesive forces holding protons and neutronstogether as the atom’s nucleus.
Energy in Transition
Heat Q is the mechanism that transfers energy across the boundaries of systems with differing temperatures, always toward thelower temperature. Heat is positive when energy is added to the system (see Figure 1).
Work is the mechanism that transfers energy across the boundaries of systems with differing pressures (or force of any kind),always toward the lower pressure. If the total effect produced in thesystem can be reduced to the raising of a weight, then nothing butwork has crossed the boundary. Work is positive when energy isremoved from the system (see Figure 1).
Mechanical or shaft work W is the energy delivered or absorbed by a mechanism, such as a turbine, air compressor, or internal combustion engine.
Flow work is energy carried into or transmitted across thesystem boundary because a pumping process occurs somewhereoutside the system, causing fluid to enter the system. It can be
more easily understood as the work done by the fluid just outsidethe system on the adjacent fluid entering the system to force orpush it into the system. Flow work also occurs as fluid leaves the
system.
Flow work =pv(3)
where p is the pressure and v is the specific volume, or the volumedisplaced per unit mass evaluated at the inlet or exit.
A property of a system is any observable characteristic of thesystem. The state of a system is defined by specifying the minimumset of independent properties. The most common thermodynamicproperties are temperature T, pressure p, and specific volume v ordensity ρ. Additional thermodynamic properties include entropy,stored forms of energy, and enthalpy.
Frequently, thermodynamic properties combine to form otherproperties. Enthalpy h is an important property that includes internal energy and flow work and is defined as
word/media/image3.gif (4)
where u is the internal energy per unit mass.
Each property in a given state has only one definite value, andany property always has the same value for a given state, regardlessof how the substance arrived at that state.
A process is a change in state that can be defined as any changein the properties of a system. A process is described by specifyingthe initial and final equilibrium states, the path (if identifiable), andthe interactions that take place across system boundaries during the
process.
A cycle is a process or a series of processes wherein the initialand final states of the system are identical. Therefore, at the conclusion of a cycle, all the properties have the same value they had at thebeginning. Refrigerant circulating in a closed system undergoes a
cycle.
A pure substance has a homogeneous and invariable chemicalcomposition. It can exist in more than one phase, but the chemicalcomposition is the same in all phases.
If a substance is liquid at the saturation temperature and pressure,it is called a saturated liquid. If the temperature of the liquid islower than the saturation temperature for the existing pressure, it iscalled either a subcooled liquid (the temperature is lower than thesaturation temperature for the given pressure) or a compressed liquid (the pressure is greater than the saturation pressure for the giventemperature).
When a substance exists as part liquid and part vapor at the saturation temperature, its quality is defined as the ratio of the mass ofvapor to the total mass. Quality has meaning only when the substance is saturated (i.e., at saturation pressure and temperature).Pressure and temperature of saturated substances are not independent properties.
If a substance exists as a vapor at saturation temperature andpressure, it is called a saturated vapor. (Sometimes the term drysaturated vapor is used to emphasize that the quality is 100%.)
When the vapor is at a temperature greater than the saturation temperature, it is a superheated vapor. Pressure and temperature of asuperheated vapor are independent properties, because the temperature can increase while pressure remains constant. Gases such asair at room temperature and pressure are highly superheated vapors.
FIRST LAW OF THERMODYNAMICS
The first law of thermodynamics is often called the law of conservation of energy. The following form of the first-law equation isvalid only in the absence of a nuclear or chemical reaction.
Based on the first law or the law of conservation of energy for anysystem, open or closed, there is an energy balance as
Net amount of energy Net increase of stored
=
added to systemenergy in system
or
[Energy in] – [Energy out] = [Increase of stored energy in system]
Figure 1 illustrates energy flows into and out of a thermodynamic system. For the general case of multiple mass flows with uniform properties in and out of the system, the energy balance can bewritten
word/media/image4.gif
word/media/image5.gif
word/media/image6.gif (5)
where subscripts i and f refer to the initial and final states,respectively.
Nearly all important engineering processes are commonly modeled as steady-flow processes. Steady flow signifies that all quantities associated with the system do not vary with time. Consequently,
word/media/image7.gif (6)
where h = u + pv as described in Equation (4).
A second common application is the closed stationary system forwhich the first law equation reduces to
word/media/image8.gif (7)
SECOND LAW OF THERMODYNAMICS
The second law of thermodynamics differentiates and quantifiesprocesses that only proceed in a certain direction (irreversible) fromthose that are reversible. The second law may be described in several ways. One method uses the concept of entropy flow in an opensystem and the irreversibility associated with the process. The concept of irreversibility provides added insight into the operation ofcycles. For example, the larger the irreversibility in a refrigerationcycle operating with a given refrigeration load between two fixedtemperature levels, the larger the amount of work required to operate the cycle. Irreversibilities include pressure drops in lines and
heat exchangers, heat transfer between fluids of different temperature, and mechanical friction. Reducing total irreversibility in acycle improves cycle performance. In the limit of no irreversibilities, a cycle attains its maximum ideal efficiency.In an open system, the second law of thermodynamics can bedescribed in terms of entropy as
word/media/image9.gif (8)
where
dS = total change within system in time dt during processsystem
δm s = entropy increase caused by mass entering (incoming)
δm s = entropy decrease caused by mass leaving (exiting)
δQ/T = entropy change caused by reversible heat transfer between system and surroundings at temperature T
dI = entropy caused by irreversibilities (always positive)
Equation (8) accounts for all entropy changes in the system. Rearranged, this equation becomes
word/media/image10.gif (9)
In integrated form, if inlet and outlet properties, mass flow, andinteractions with the surroundings do not vary with time, the generalequation for the second law is
word/media/image11.gif (10)
In many applications, the process can be considered to operatesteadily with no change in time. The change in entropy of the systemis therefore zero. The irreversibility rate, which is the rate ofentropy production caused by irreversibilities in the process, can bedetermined by rearranging Equation (10):
word/media/image12.gif (11)
Equation (6) can be used to replace the heat transfer quantity.Note that the absolute temperature of the surroundings with whichthe system is exchanging heat is used in the last term. If the temper-
ature of the surroundings is equal to the system temperature, heat istransferred reversibly and the last term in Equation (11) equals zero.
Equation (11) is commonly applied to a system with one massflow in, the same mass flow out, no work, and negligible kinetic orpotential energy flows. Combining Equations (6) and (11) yields
word/media/image13.gif
word/media/image5.gif (12)
In a cycle, thereduction of work produced by a power cycle (orthe increase in work required by a refrigeration cycle) equals theabsolute ambient temperature multiplied by the sum of irreversibilities in all processes in the cycle. Thus, the difference in reversibleand actual work for any refrigeration cycle, theoretical or real, operating under the same conditions, becomes
word/media/image14.gif (13)
THERMODYNAMIC ANALYSIS OF
REFRIGERATION CYCLES
Refrigeration cycles transfer thermal energy from a region of lowtemperature T to one of higher temperature. Usually the higher-TRtemperature heat sink is the ambient air or cooling water, at temperature T0, the temperature of the surroundings.
The first and second laws of thermodynamics can be applied toindividual components to determine mass and energy balances andthe irreversibility of the components. This procedure is illustrated inlater sections in this chapter.
Performance of a refrigeration cycle is usually described by acoefficient of performance (COP), defined as the benefit of thecycle (amount of heat removed) divided by the required energyinput to operate the cycle:
Useful refrigerating effect
COP≡Useful refrigeration effect/Net energy supplied from external sources (14)
Net energy supplied from external sourcesFor a mechanical vapor compression system, the net energy supplied is usually in the form of work, mechanical or electrical, andmay include work to the compressor and fans or pumps. Thus,
word/media/image15.gif (15)
In an absorption refrigeration cycle, the net energy supplied isusually in the form of heat into the generator and work into thepumps and fans, or
word/media/image16.gif (16)
In many cases, work supplied to an absorption system is verysmall compared to the amount of heat supplied to the generator, sothe work term is often neglected.
Applying the second law to an entire refrigeration cycle showsthat a completely reversible cycle operating under the same conditions has the maximum possible COP. Departure of the actualcycle from an ideal reversible cycle is given by the refrigeratingefficiency:
word/media/image17.gif (17)
The Carnot cycle usually serves as the ideal reversible refrigeration cycle. For multistage cycles, each stage is described by a reversible cycle.
工程热力学和制冷循环
工程热力学是研究能量及其转换和能量与物质状态之间的关系。这个章节讲述了工程热力学在制冷循环中的应用。第一部分回顾了热力学第一定律、第二定律以及计算热力学参数的方法。第二部分和第三部分讲述了压缩和吸收式两种制冷循环，两种最寻常的能量转换形式。
工程热力学
热力学系统是被一个封闭曲面包围的一个空间区域或者一定量的物质。对于这个系统而言，周围的环境都是外界物质。也就是说，这个系统的界面把系统与环境分开。边界是可移动的也可以是固定的，可以是真实的也可以是假定的。熵是系统分子无序性的量度。系统越复杂，熵就越大；一个有序简单系统的熵就会很小。能量可以产生作用，并且可以分为储存形式和短暂形式两种。
1、储存能
热能(内能)是分子的运动或者分子间的相互作用产生的。
势能是由分子间的吸引或者是系统位置被提升而产生的。
word/media/image1.gif (1)
式中：m——质量；g——重力加速度；z——距水平基准面的高度
动能的产生是由于分子具有速度。其表达式如下：
word/media/image18.gif (2)
式中：V——流体流过边界面的速度
化学能是由组成分子的原子的排列产生的。
原子能是起源于把质子与中子聚在一起组成原子的那种聚合力
2、不稳定能
热量Q的工作原理是用不同的温度把能量传出系统的边界，通常是高温传到低温。当热量被加入到系统中时，热量的符号为正(可看图1)。机械功或者轴功是由机械装置传出或者传入的能量。例如：这些装置有汽轮机、空气压缩机、内燃机。
流动功是由在系统外部产生的流动流经过系统界面而带入的能量，从而把流体带入这个系统。也可以这样理解，系统的外部空间有两股相邻的流体，后面的一股推动前面的一股流进系统，这种作用的来源就是流动功。当流体流出系统时，流动功同样产生。
流动功(每单位)=pv (3)
式中：p代表压力，v代表比容，即：物质流在流进或流出的每单位质量的体积。
一个系统的参数是该系统非常明显的特征，系统的状态由指定的独立的参数来定义。最常用的热力学参数是温度T、压力P、比容v和密度ρ。其他的热力学参数包括熵、内能和焓。
一般情况下，最基本的热力学参数组合到一起组成其它的参数。焓h是一个重要的参数，它包括内能和流动功。其定义如下：
word/media/image19.gif (4)
其中：u是单位质量的内能。
每一个给定状态的参数有唯一的确定的值，并且不论物质处于什么样的状态，任何一个参数只要处于给定的状态下，就会有同样的值。
系统中任何一个参数变化了，就可以确定整个系统发生了变化。一个过程可以由系统的初状态和处于平衡态的末状态来描述。这个过程中路径和相互作用超出了系统的边界。
一个循环是经过一个过程或几个过程，系统的初状态与末状态是相同的。因此，由循环可以得到一个结论，所有的参数值与初状态相同。一个闭式的制冷过程就是一个循环。
一种纯净的物质含有均一的、不变的化学组成成分。这种物质可以处在多个相态，但是在所有的相态中它的化学成分不变。
如果一种物质在其饱和压力和饱和温度下是液态，这时液体被称为饱和液体。如果液体的温度在给定的压力下低于其饱和温度，被称为过冷液体，如果液体的压力在给定的温度下高于其饱和压力，被称为压缩液体。
当一种物质在其饱和温度下，一部分是液体一部分是气体，规定饱和干度为气体的质量与总质量之比。干度只有在饱和状态(饱和温度与饱和压力)下才有意义。饱和物质的压力和温度不是相互独立的参数。
如果物质在饱和温度与压力下是处于液态，那么它被称为饱和蒸气(有时候干饱和蒸气的说法是为了强调干度是100%)。
当蒸气的温度高于它的饱和温度时，此时的蒸气被称为过饱和蒸气。过饱和蒸气的压力和温度是相互独立的参数，因为当压力保持稳定时，温度可以上升。在室内的温度和压力下，气体一般都是过饱和蒸气。
热力学第一定律
热力学第一定律常常又被称为能量守恒定律。热力学守恒定律的以下公式仅在没有原子变化和化学反应时成立。
进入系统的净能量=系统储存能的净增量
或者
进入的能量—流出的能量=系统储存能的增量
图1表明一个热力学系统能量的流进与流出。在一般的情况下，对于多种物质以不同的参数流进与流出，能量的平衡公式可以写为：
word/media/image20.gif
word/media/image5.gif
word/media/image21.gif (5)
式中：下脚标i 和f分别指的是系统的处状态和末状态。
几乎所有的热力学过程都是以稳流为模型的。稳流指的是与系统有关的流体量不随着时间而变化。因此：
word/media/image22.gif (6)
式中：h = u + pv的含义与公式(4)代表的含义相同。
热力学第一定律的另一种应用是用于闭式的固定系统。热力学第一定律的表达式可以写为：
word/media/image8.gif (7)
热力学第二定律
热力学第二定律做出了与可逆过程的区别和量化了只在不可逆中发生的过程。热力学第二定律可以有多种叙述方法。一种方法可以用在开式系统里熵流的概念和过程的不可逆性来描述。不可逆性的概念为系统循环的运作提供了更深入的研究。例如，在给定的两个温度之间，有给定的制冷负荷，这个制冷循环的不可逆性越大，它的运行就需要更大功。不可逆产生的原因包括压力的线性下降，在热交换过程中热交换器的热量损失，以及各种不可避免的机械摩擦。循环系统中减低总的不可逆性可以提高系统的循环特性。在没有不可逆性时，这个系统达到最大理想效率。在一个开式系统里，热力学第二定律用熵表达为：
word/media/image23.gif (8)
式中：word/media/image24.gif在这个系统的热力学过程中dt时间内总的交换量。
word/media/image25.gif由质量的流进引起的熵增。
word/media/image26.gif由质量的流出引起的熵减。
word/media/image27.gif在一定的温度下由系统与环境的热交换产生的可逆引起的熵的变化。
word/media/image28.gif由于不可逆引起的熵(总是正的)
公式(8)说明了在系统中所有的熵变。重新整理以下，这个公式可以写为：
word/media/image29.gif (9)
整体上，如果系统中流进与流出的参数，质量流量和同环境的交换量不随时间而变化。热力学第二定律的一般公式可以写为：
word/media/image30.gif (10)
在很多应用中，这个过程被认为是一个稳态过程。所以，系统的熵增为零。不可逆的比率指的是由不可逆性产生的熵占总熵的比率。这个比率可以由公式(10)计算出来。
word/media/image31.gif (11)
公式(6)可以用来代替热交换量。注意的一点是：环境与系统的绝对温度是热交换要用到的最后一个条件。如果环境的温度与系统的温度是相同的，那么热传递是可逆的，并且在公式(11)的最后一个条件为零。
公式(11)通常用于同质量流进同质量流出的系统中，没有功，可以忽略动能、势能。把公式(6)和公式(11)联立可以得到
word/media/image32.gif
word/media/image5.gif (12)
在一个循环中，一个能量循环产生功的降低，(或者一个制冷循环所需要的功的增加)等于周围环境的绝对温度乘以在循环中各个不可逆的总量。因此，在同等的条件下，任何制冷循环中不管是理论还是实际中，可逆过程的功与实际的功会变为：
word/media/image33.gif (13)
制冷循环的热力学分析
制冷循环是把热能从一个低温的区域传递到另一个高温的区域。通常较高温度TR是周围环境中的空气或者冷却水的温度，T0是环境的温度。
热力学第一和第二定律可以应用到单个成分中去决定质量和能量的平衡，同时也可以来分析其的不可逆能力。这个过程在以下的几个章节中会讲到。
制冷循环的性能通常用性能参数来衡量，性能参数定义为循环可以移走的热量除以系统
输入的所必需的能量。
COP≡word/media/image34.gif (14)
对于蒸气压缩系统来说，提供的净能量通常是以功、机械能或电能的形式出现，并且还包括压缩机、风机、水泵所需要的能量。因此：
word/media/image15.gif (15)
在吸收式制冷循环中，提供的净能量是以热量的形式传送到发生器和功传送到泵与风机中去。所以：
word/media/image35.gif (16)
在很多情况下，给吸收式系统提供的功相对于给发生器提供的热量是非常小的。所以功常常可以被忽略。
热力学第二定律在制冷循环中的应用，显示了在相同条件下，一个完整的可逆循环可能拥有的最大COP。实际制冷循环与可逆的理想循环的区别是制冷效率：
word/media/image17.gif (17)
卡诺循环通常用于理想可逆的制冷循环。对于多级循环，每一个阶段都可以由一个可逆循环来描述。
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