1、 1 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 p
2、resentsmethods 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
3、 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 sys
4、tem. 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 a
5、nd/or intermolecular forces. Potential energy (PE) is caused by attractive forces existingbetween molecules, or the elevation of the system. mgzPE (1) where m =mass g = local acceleration of gravity z = elevation above horizontal reference plane Kinetic energy (KE) is the energy caused by the veloci
6、ty of molecules and is expressed as 22mVKE (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 a
7、toms 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 th
8、e 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. 2 Work is positive when energy isremoved from the system (see
9、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
10、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 speci
11、fic 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, p
12、ressure p, and specific volume v ordensity . 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
13、as pvuh (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
14、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
15、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
16、, 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
17、 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 rati
18、o 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
19、 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 3 vapor. Pressure and temperature of asuperheated vapor are independent properties, bec
20、ause 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 equat
21、ion 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
22、 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 WQgzVpvumgzVpvum o u to u tinin )2()2( 22 s y s t e miiff gzVpvumgzVpvu
23、m )2()2( 22 (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, 0)2()2(22 W
24、QgzVhmgzVhml e a v i n gs t r e a ma l le n t e r i n gs t r e a ma l l (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 sy ste mif uumWQ )( (7) SECOND LAW OF THERMODYNAMICS The second law of the
25、rmodynamics 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.
26、 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
