制冷技术(英文版)Ch6-090602

1、Chapter 6 Cycle Analyses of Vapor Compression Refrigeration6-1) Single stage Vapor Compression Refrigeration Systems(1) Introduction of mechanical powered vapor compression refrigeration cycle (2) The pressure-enthalpy diagram Fig.6.1, Diagrammatic representation of a pressure versus specific enthal

2、py diagram with a logarithmic pressure scaleA log p-h diagram for R134a is shown in Fig.5-2.Fig.6.2, log p-h diagram for R134a 1 (3) The Basic vapor compression cycle Fig 6.3. Basic Vapor-Compression Refrigeration Cycle Fig 6.4.T-s and p-h Diagram for basic Vapor-Compression Refrigeration CycleAs th

3、e refrigerant passes through the evaporator, heat transfer from the refrigerated space results in the vaporization of the refrigerant. Considering a control volume enclosing the refrigerant side of the evaporator， the rate of heat transfer per unit mass of refrigerant flow in the evaporator is as fo

4、llows: (6.1) whereis the mass flow rate of the refrigerant and is the refrigeration capacity. The refrigeration capacity is usually expressed in kW or in tons of refrigeration. One ton of refrigeration being equals 3.517 kW.Next consider the compressor. (6.2)Where is the work done per unit mass of r

5、efrigerant.Next, the refrigerant passes through the condenser, where the refrigerant condenses and there is heat transfer from the refrigerant to the cooler surroundings. For a control volume enclosing the refrigerant side of the condenser, the rate of heat transfer from the refrigerant per unit mas

6、s of refrigerant is: (6.3)Finally, the refrigerant at state 3 enters the expansion valve and expands to the evaporator pressure. This process is usually modeled as a throttling process in which there is no heat transfer, i.e., for which (6.4) (6.5) Example 6.1The property data can be found in Table

7、6.1.Fig.6.5. Schematic p-h diagram for the Example 6.1Specific refrigerating effect: Specific heat transfer in condenser: Specific work of isentropic compression: Volumetric refrigerating effect: Volumetric work of isentropic compression: The mass flow of refrigerant is obtained from an energy balan

8、ce on the evaporator. Thus,so, Table 6.1, Thermodynamic property data for the Example 6.1(4) Comparison of Simple Vapor Compression Cycle with Reversed Carnot Cycle (6.7)Fig 6.6 Comparison of simple vapor compression cycle with Carnot cycle (5) Liquid subcooing Fig.6.7. Subcooling of the liquid and

9、superheating of the vapor (6) Vapor superheating (7) Actual vapor compression cycleFig.6.8 shows an actual vapor compression cycle compared with a basic cycle. There are several differences between them.Fig.6.8 p-h diagram of actual vapor compression cycleExample 6.3An air-cooled, direct-expansion,

10、single-stage mechanical vapor-compression refrigerator uses R-22 as refrigerant and operates under steady conditions. Pressure drops occur in all pipe works, and heat gains or losses occur as indicated in Fig.6.9. Power consumption includes the compressor and the fans.Refrigeration load ; Ambient ai

11、r temperature : t0= 30°C ; Refrigerated space temperature: tR = 10°C; Compressor power input: Condenser fan power input: Evaporator fan power input: Fig.6.9 Schematic of real, direct-expansion, single-stage mechanical vapor-compression refrigeration system7 Table 6.3 Measured and computed

12、thermodynamic properties of R22 for Example 6.3Fig.6.10 Pressure-enthalpy diagram of actual vapor compression system and the Carnot cycle operating between same inlet air temperatures tR and t0These results are summarized in Table 6.4.Table 6.4 Energy transfers and irreversibility rates for refriger

13、ation system in the Example 6.3.The analysis demonstrated in this example can be applied to any actual vapor compression refrigeration system. The only required information for the second-law analysis is the refrigerant thermodynamic state points and mass flow rates and the temperatures at which the

14、 heat is transferred. In this example, the extra compressor power required to overcome the irreversibility in each component is determined. The component with the largest loss is the compressor. This loss is due to motor inefficiency, friction losses, and irreversibilities caused by pressure drops,

15、mixing, and heat transfer between the compressor and the surroundings. The unrestrained (不受约束) expansion in the expansion device is also a large, but could be reduced by using an expander rather than a throttling process. An expander may be economical on large machines.(8) The effect of evaporating

16、and condensing temperatures on the cycle quantitiesa) Change of evaporating temperatureFig.6.11.Effects of increased evaporating temperatureTable.6.6 Results comparison of this two example at constant condensing temperature-20144.84181.3536.52982.70247.780.3453.97-10150.94178.8827.941515.46280.520.3

17、315.40b) Change of the condensing temperatureFig.6.12.Effect of decreased condensing temperature6-2) Multi-stage Vapor Compression SystemsThe reasons for using a multistage vapor compression system instead of a single-stage system are as follows:1. The compression ratio of each stage in a multistage

18、 system is smaller than that in a single stage one, so compressor efficiency is increased. 2. Liquid refrigerant enters the evaporator at a lower enthalpy and increases the refrigeration effect. Discharge gas from the high-stage compressor can have a lower temperature than the one-stage system at th

19、e same pressure difference between condensing and evaporating pressure. Multistage system can accommodate the variation of refrigeration load. (1) Two-stage expansion system with a flash cooler (同一制冷剂，两级压缩，两级节流， 有闪蒸冷却器)A two-stage system is a refrigeration system working with a two-stage compression

20、 and mostly also with a two-stage expansion. A schematic system layout and the corresponding process in a log p-h diagram are shown in Fig.6.13 10. Fig.6.13 Two stage expansion system with a flash cooler(2) The System combined with a vertical intercooler(同一制冷剂，两级压缩，一级节流中间完全冷却。 有中间冷却器)Fig.6.14 shows

21、the schematic diagram and the refrigeration cycle of a two-stage expansion system with a vertical coil intercooler. In this system, the subcooled liquid refrigerant from the receiver at point 5 is divided into two streams. One stream enters the coil inside the intercooler. The other enters the shell

22、 side after throttling to point 7, the inter-stage pressure11.Fig.6.14 Two stage compound system with a vertical intercooler (3) Comparison between the flash cooler and the vertical coil intercooler Hot gas discharged from the low-stage compressor is always cooled to a nearly saturated vapor state a

23、t the inter-stage pressure in the vertical coil intercooler. This process is more appropriate for refrigerant like ammonia, which has a high discharge temperature. In flash coolers, the cooling is caused by the mixing of flashed vapor and hot gas, and this will not result in a dry saturated state. T

24、herefore, flash coolers are usually used in refrigeration systems using HCFCs or HFCs. (4) Multi-stage solid carbon dioxide production systemFig 6.16 Phase boundary lines for carbon dioxide on the pressure-enthalpy diagram Fig.6.17. A refrigeration system for dry ice production6-3) Cascade Vapor Com

25、pression Systems （复叠式， 不同制冷剂） (1) Two circuit cascade systemA typical cascade system using ammonia and CO2 as refrigerants is shown in Fig.6.18. In this cascade system, each refrigerant circuit is separate. CO2 will be used as a refrigerant for the low temperature circuit and ammonia will be used fo

26、r the high temperature circuit. The condenser of the CO2 circuit will act as the evaporator of the NH3 circuit Fig.6.18 Schematic arrangement for NH3/ CO2 cascade system13A typical simple NH3/ CO2 cascade system p - h (pressure - enthalpy) diagram is shown in Fig.6.19.Fig.6.19 p-h diagram for NH3/CO

27、2 cascade system6-4) Auto Cascade Vapor Compression Systems （自动复叠式， 非共沸混合制冷剂）Auto-cascade refrigeration system was proposed by Ruhemann in 1946, in which only one compressor is used. When Multicomponent zeotropic refrigerant mixtures are used in single-compressor cascade systems, it is known as auto

28、cascade systems or auto refrigerating cascades, to achieve temperature to -180.The autocascade system has many advantages in comparison with other double or multi-cascades types of refrigeration. These advantages are the compact design of the systems elements; its reliability, safety, and flexibilit

29、y in operation; the relative simplicity of its maintenance; and its reasonable price. And autocascade system avoids many limitations about lubricant circumfluence.Fig.6.20 Vapor pressure versus temperature for R23 and R22 to illustrate the working of the auto-cascadeReferences1 http:/me.kaist.ac.kr/

30、upload/course/MAE554_2007/transcritical_2007.ppt2.Refrigeration cycle, http:/highered.mcgraw-3.Notes on vapor-compression refrigeration, http:/me.queensu.ca/courses/MECH398/RefrigerationLabSLDS.pdf4 ASHRAE, Handbook Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engine

31、ers，Atlanta, Ga, USA, 20055 Basic refrigeration cycle, http:/www.freeze-6 Simple Refrigeration Cycle Lab, /engt3020/met3100/labs/lab6/lab6.htm7 Zheng, X.D, Refrigeration Principles and Device, China Machine Press8 Pita, E G., Refrigeration Principles and Systems, John Wiley & Sons, Inc.19849 Domenech. R.L; Lopez, R.C; 2007.9, Experimental energetic analysis of the liquid injection effect in a two-stage refrigeration facility using a compound compressor, Heating, Ventilating, Air-Cond