Vapor and Gas Refrigeration Cycles

Vapor and Gas Refrigeration Cycles
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Published Date:02-08-2017
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Vapor and Gas Refrigeration Cycles 14.1 INTRODUKSJON (INTRODUCTION) This chapter is divided into two parts, one dealing with common vapor refrigeration cycles and one dealing with common gas refrigeration cycles. Most of these cycles are reversed power cycles, and their analysis amounts to a reapplication of the power cycle material presented in Chapter 13. Reversed vapor (Rankine) cycles are536 CHAPTER 14: Vapor and Gas Refrigeration Cycles commonly called vapor-compression refrigeration cycles and reversed gas cycles are normally referred to by the cycle name (e.g., a reversed Brayton refrigeration cycle). Like power cycle technology, refrigeration technology has had an enormous impact on our culture and the way we live. It changed our diet, the architecture of our buildings, the agriculture on our farms, and many other items that touch our everyday life. Just as we would find it very difficult to return to a time without the portable power produced by engines, we would also find life much less comfortable in a time without refrigeration and air conditioning. 14.2 PART I. VAPOR REFRIGERATION CYCLES The basic concepts of refrigeration, air conditioning, and heat pumps were introduced in Chapter 7. This technol- ogy is usually modeled as a backward-running heat engine. When a heat engine runs backward (or in reverse), it receives a net input of work W that causes an amount of heat Q to be removed from a low-temperature region L and an amount of heat Q to be added to a high-temperature region. So, it actually cools the low-temperature H region and heats the high-temperature region. A backward-running heat engine is a refrigeration machine, but its exact technical name depends on exactly how it is being used. For example, if food occupies the low-temperature region, then the device is indeed called 1 a refrigerator, but if people occupy the low-temperature region, then the device is called an air conditioner. On the other hand, if people occupy the high-temperature region and utilize Q for space-heating purposes, then H the device is called a heat pump. Though the details of their design and operation differ slightly, refrigerators, air conditioners, and heat pumps can all be modeled as backward-running heat engines. These distinctions are shown in Figure 14.1. HOW CAN AN ENGINE RUN BACKWARD? To get an engine to run backward, you need to put work into it where the work normally comes out. If the engine has an output shaft, you simply attach a motor or something to the shaft to turn it so that you are putting work into the engine instead of having the engine produce work. A heat engine converts some of the heat from a high-temperature source into work and rejects the remaining heat to a low-temperature sink. A “backward-running” heat engine draws heat from a low- temperature source, adds work energy to it, and rejects everything to a high-temperature sink (see Figure 14.1). Environment Environment People (warm region) (warm region) (warm region) Q Q Q H H H Backward- Backward- Backward- W W W R AC HP running running running heat engine heat engine heat engine Q Q Q L L L Food stuff People Environment (cold region) (cool region) (cold region) (a) Refrigerator (b) Air conditioner (c) Heat pump FIGURE 14.1 Characteristics of refrigerators, air conditioners, and heat pumps. 1 In addition to lowering room or building air temperature, air conditioners also usually filter the air and alter its humidity (see Chapter 8).14.3 Carnot Refrigeration Cycle 537 14.3 CARNOT REFRIGERATION CYCLE In Chapter 7, we discovered that refrigerators, air conditioners, and heat pumps usually have actual thermal efficiencies in excess of 100%. This is due simply to the mathematical way in which their thermal efficiencies are defined (the desired energy output divided by the required energy input) and does not imply the violation of any physical law. However, claims of thermal efficiency in excess of 100% cause obvious credibility problems in the public domain, so the term efficiency is not often used with this technology. Instead, we simply rename the thermal efficiency the coefficient of performance (COP), which is expressed as a pure number, usually between 1 and 10, rather than as a percentage. The COP definitions have been given in Eqs. (7.17) and (7.19) (recall that work into and heat out of a system are both negative quantities in our sign convention; as in previous chapters, we use their absolute values in these equations and assign algebraic signs to the symbols to avoid confusion): _ _ jQ j jQ j jQ j jQ j H H H H ðη Þ = COP = = = = (7.17) HP T heat _ _ _ jW j jQ j−Q HP jW j H L jQ j−Q HP H L hump and _ _ Q Q Q Q L L L L ðη Þ = COP = = = = (7.19) T R/AC refrigerator or _ _ _ jW j jQ j−Q R/AC jW j H L jQ j−Q R/AC H L air conditioner and it is easily shown that COP = COP +1 (14.1) HP R/AC From Figure 14.1 and Eq. (14.1), it is evident that Eqs. (7.17) and (7.19) can also be written as _ jQ j H COP = (7.17a) HP _ jW j in net and _ Q L COP = = COP −1 (7.19a) R/AC HP _ jW j in net Note that Eq. (7.17a) is simply the inverse of the general forward-running heat engine thermal efficiency equation (see Eq. (7.5)); that is, 1 COP  (14.2) HP ðη Þ T forward-running heat engine and Eq. (7.17a) then gives 1 COP  −1 (14.3) R/AC ðη Þ T forward-running heat engine Therefore,theCOPforanyoftheheatenginesdiscussedinChapter13operatingonareversed thermodynamiccycle 2 asa heat pump,refrigerator,orair conditioner can beeasilyobtained throughthe use ofEqs. (14.2) and (14.3). For example, Eq. (7.16) gives the Carnot thermal efficiency as T T −T L H L ðη Þ = 1− = T Carnot T T H L Then, Eqs. (14.2) and (14.3) can be used directly to give the COP of a Carnot engine running backward as a heat pump, refrigerator, or air conditioner as T H COP = (14.4) Carnot HP T −T H L T L COP = (14.5) Carnot R/AC T −T H L Also, it is easy to show that Eq. (14.1) remains valid for these systems. 2 However, it is difficult to imagine an internal combustion engine (like the Otto and Diesel cycle) running backward because it would require a heat-absorbing (endothermic) combustion reaction.538 CHAPTER 14: Vapor and Gas Refrigeration Cycles EXAMPLE 14.1 A new die-casting operation has a large amount of waste heat available at 200.°C in a location where the local environmen- tal temperature is 20.0°C. As chief engineer in charge of thermal energy management, investigate the possibility of recovering some of this waste heat by determining a. The thermal efficiency of a Carnot engine operating between these temperatures. b. The coefficient of performance of a Carnot heat pump operating between these temperatures. c. The coefficient of performance of a Carnot refrigerator or air conditioner operating between these temperatures. Solution First, draw a sketch of the system (Figure 14.2). Die-casting Die-casting Die-casting machine machine machine at 200.°C at 200.°C at 200.°C Waste heat Heat out Waste heat Carnot Carnot Carnot refrigerator heat Work heat Work Work or air engine pump conditioner Heat out Heat in Heat out Environment Environment Environment at 20.0°C at 20.0°C at 20.0°C (a) (b) (c) FIGURE 14.2 Example 14.1. a. Equation (13.2) gives the thermal efficiency of a Carnot engine operating between the temperature limits of 200. + 273.15 = 473.15 K and 20.0 + 273.15 = 293.15 K as T 293:15K L ðη Þ = 1− = 1− = 0:380 = 38:0% T Carnot T 473:15K H b. Equation (14.4) gives the coefficient of performance of the same Carnot engine running backward as a heat pump as T H 473:15K COP = = = 2:63 Carnot HP T −T 473:15−293:15K H L c. Equation (14.5) gives the coefficient of performance of the same Carnot engine running backward as a refrigerator or air conditioner as T 293:15K L COP = = = 1:63 Carnot R/AC T −T 473:15−293:15K H L Note that the COP = COP + 1 as Eq. (14.1) requires, and that Eqs. (14.2) and (14.3) are also Carnot HP Carnot R/AC satisfied here. Thermal energy management is a serious problem in the industrial environment. Lost thermal energy often reflects poor process design and lost money. It can be remedied by considering the waste heatas an energy source and applying a technology that can utilizeitin some fashion. Exercises 1. Suppose the waste heat in Example 14.1 is available at 35.0°C instead of 200.°C while the environmental temperature remains at 20.0°C. Determine the thermal efficiency of a Carnot engine operating between these temperatures and the coefficient of performance of a Carnot air conditioning unit. Answer:(η ) = 4.90%, and COP = 20.5. T Carnot Carnot AC 2. During winter, the environmental temperature in the die-casting facility in Example 14.1 drops to 0.00°C. Recompute the thermal efficiency and coefficient of performance, assuming the waste heat temperature remains at 200.°C. Answer: (η ) = 42.3%, COP = 2.37, and COP = 1.37. T Carnot Carnot HP Carnot R/AC14.4 In the Beginning There Was Ice 539 3. A salesperson from a waste heat recovery company visits you and claims to have a new engine that can convert 50.% of the waste heat in Example 14.1 into useful shaft work. How would you evaluate this claim? Answer:No engine can be more efficient than a (hypothetical) Carnot engine, and since a Carnot engine is only 38% efficient for converting this waste heat into useful work, the salesperson’s claim of a 50.% conversion is impossible to achieve. Development of refrigeration technology Development of refrigeration technology Natural refrigeration Natural refrigeration Artificial refrigeration Artificial refrigeration Surface Surface Radiation Radiation Vapor- Vapor- Expanding Expanding Ice and snow Ice and snow Absorption Absorption Miscellaneous Miscellaneous evaporation evaporation cooling cooling compression compression gas gas Used from aniquity. Used from aniquity. Used from aniquity. Used from aniquity. Used by the Used by the Originated by Jacob Originated by Jacob Originated by Originated by Originated and Originated and Refrigerating mixtures − Refrigerating mixtures − Harvested during Harvested during Evaporation from the Evaporation from the Egyptians. Shallow Egyptians. Shallow Perkins in 1834. Perkins in 1834. John Gorrie in John Gorrie in commericalized commericalized known from antiquity. known from antiquity. the winter and stored the winter and stored surface of porous surface of porous pans filled with water pans filled with water Commercialized by Commercialized by 1844. 1844. by Ferdinand by Ferdinand Reduced pressure − Reduced pressure − in pits and ravines in pits and ravines containers will keep containers will keep and exposed to the and exposed to the James Harrison in James Harrison in Commercialized Commercialized Carré in 1859. Carré in 1859. Originated by William Originated by William covered with straw. covered with straw. the contents cool. the contents cool. night sky will cause night sky will cause 1856. 1856. by Alexander by Alexander Cullen, 1755. Cullen, 1755. Extensive ice Extensive ice the water to freeze by the water to freeze by Kirk in 1862. Kirk in 1862. Thermoelectric − Originated Thermoelectric − Originated harvesting and harvesting and radiation heat transfer radiation heat transfer storage industries storage industries to the sky even when to the sky even when by Jean-Charles Peltier, by Jean-Charles Peltier, developed over the developed over the the surrounding air the surrounding air 1834. 1834. ages to provide ice ages to provide ice temperature is above temperature is above Joule−Thomson − Joule−Thomson − during the summer during the summer freezing. freezing. Originated by James Originated by James months and in months and in Prescott Joule and William Prescott Joule and William southern regions. southern regions. Thomson, 1850. Thomson, 1850. Vortex Tube − Originated by Vortex Tube − Originated by Georges Ranque, 1931. Georges Ranque, 1931. FIGURE 14.3 The development of natural and artificial refrigeration technologies. Two primary types of refrigeration are available, natural (e.g., ice) and artificial. Artificial refrigeration has been subdivided in this chapter into vapor cycles (specifically vapor-compression and absorption cycles) and gas expan- sion (specifically reversed Stirling and Brayton cycles). These refrigeration methods are illustrated in Figure 14.3 and discussed in detail in this chapter. 14.4 IN THE BEGINNING THERE WAS ICE The use of natural ice for refrigeration spread throughout the world in prehistoric times. China had ice houses for storing winter ice and snow by 1100 BC. The early Greeks and Romans are known to have used ice and snow for cooling drinks but not for preserving foods. In about 300 BC , the king of Macedon had several trenches dug and filled with snow to cool kegs of wine given to his troops on the eve of a major battle, hoping it would make them more courageous. Ice and snow were harvested by farm- ers during the winter throughout the United States, Europe, and Asia (see Figure 14.4). Ice was stored in special icehouses, underground, or in pits and ravines and covered with straw to insulate it from the daytime sun. Initially, natural ice refrigeration was merely a con- venience, providing a cool drink or preserving food a bit longer. However, the development and extensive use of ice as a refrigeration technol- ogyhadaverysignificantsocialimpact,inthatit allowed whole populations to change to a heal- FIGURE 14.4 thier diet. In the distant past, people used salting and drying as the main technology for preserving Ice harvesting in the 19th century.540 CHAPTER 14: Vapor and Gas Refrigeration Cycles IS IT CALLED AN ICEBOX OR A REFRIGERATOR? An icebox was a wooden box that contained both ice and food to be preserved (Figure 14.5). Originally, the ice was put on the bottom and the food on the top. But eventually it was realized that this was inefficient because the cold air is heavier than warm air. Thereafter, the ice was put in the top of the icebox and the food was always placed below it, so that the heavier cold air could circulate around the food. The icebox was invented in 1803 and manufactured in the United States until 1953. Ice needed to be added every day or two in the original iceboxes. But, by 1923, improved thermal insulation design required ice to be added only every five to seven days. Block of ice Food storage area Drain pan FIGURE 14.5 Domestic icebox. The term refrigerator is reserved for a device that does not use ice to produce cold temperatures, even though the device may be used to preserve food (Figure 14.6). There are vapor-compression refrigerators, gas expansion refrigerators, thermoelectric refrigerators, and so forth. Evaporator Compressor Condenser Throttle valve Room Refrigerator High-pressure vapor temperature temperature Low-pressure vapor Refrigerant 72°F (22°C) 35°F (1.5°C) High-pressure liquid storage Low-pressure liquid FIGURE 14.6 Domestic refrigerator.14.4 In the Beginning There Was Ice 541 meat and fish. By 1830, the use of ice to preserve food in American iceboxes was quite common. The stan- dard of living of average Americans improved between 1830 and 1860, as their diet changed from one of bread and salted or dried meat or fish to one that regularly included refrigerated fresh meat, fruits, and vegetables. Preserving food in a sterilized metal can was patented in 1825, but it did not become a commercial success until 1875, about the same time mechanical refrigeration systems were being marketed. As the food refrigeration industry grew, the demand for natural ice increased dramatically. Natural ice was har- vested from ponds, lakes, and rivers in rural communities around the world by farmers during the winter months. New York City used 12,000 tons of natural ice in 1843, 100,000 tons in 1856, and 1 million tons in 1879. Natural ice harvesting in the United States reached its peak in 1886 at 25 million tons. From 1845 to 1860, the mechanical refrigeration systems of Perkins, Gorrie, and Carré were used primarily for making ice to replace natural (winter) ice. A common unit of commercial and household refrigeration or air conditioning is the ton. The following example illustrates the use of this old unit of measurement. IS IT DANGEROUS TO STUFF A CHICKEN WITH SNOW? The great British philosopher and statesman Sir Francis Bacon (1561–1626) was keenly interested in the possibility of using snow to preserve meat. In March 1626, he stopped in the country on a trip to London and purchased a chicken. He had the chicken killed and cleaned on the spot, then he packed it with snow and took it with him to London (Figure 14.7). Unfortunately, the experiment only caused his own death a few weeks later. The 65-year-old statesman apparently caught a chill while stuffing the chicken with snow and came down with terminal bronchitis. Refrigeration was clearly not something to be taken lightly. FIGURE 14.7 The price of experimentation. WHAT IS A “TON” OF REFRIGERATION? A ton of refrigeration or air conditioning is the amount of heat that must be removed from 1 ton (2000 lbm) of water in one day (24 hours) to freeze it at 32°F at 1 atmosphere pressure. It is also the amount of heat absorbed by the melting of 1 ton of ice in 24 hours at 32°F at atmospheric pressure. Using more conventional units, 3 1tonof refrigerationorairconditioning = 200:Btu=min = 12:0×10 Btu=h = 214:kJ=min =12,600kJ/h542 CHAPTER 14: Vapor and Gas Refrigeration Cycles EXAMPLE 14.2 16 3 The earth’s polar ice caps contain about 2.50 × 10 m of ice. Determine the tons of refrigeration produced if all this ice 3 were to melt at 0.00°C in a 24.0 h period. The density of ice at 0.00°C is 917 kg/m . Solution 16 3 3 19 The mass of ice present in the polar ice caps is (2.50 × 10 m )(917 kg/m )(2.2046 lbm/kg) = 5.05 × 10 lbm = 2.53 × 16 10 tons. Since a ton of refrigeration is equal to the amount of heat absorbed by melting 1 ton of ice at 0.00°Catatmo- spheric pressure in one 24.0 h day, melting the earth’spolaricecapsat0.00°C in a 24.0 h period would produce 2.53 × 16 10 tons of refrigeration. Exercises 4. Suppose the polar ice caps in Example 14.2 melt over a period of 14.0 years. Then, how many tons of refrigeration 12 would be produced? Answer: 6.85 × 10 tons. 5. How many Btu per hour would be produced by the melting of the polar ice caps in Example 14.2? 20 Answer: 3.04 × 10 Btu/h. 15 6. How long would it take to melt the polar ice caps in Example 14.2 if the Earth receives an extra 10 kJ per year from the sun? Answer: 7.56 million years. 14.5 VAPOR-COMPRESSION REFRIGERATION CYCLE Like the steam engine, refrigeration technology had a significant impact on society and the way we live. First of all, it changed the way we process food; it created large new agricultural markets and provided a healthier diet for many people. Later, it was applied to making our living environment more comfortable and produc- tive. Initially, it was a spinoff technology from steam engine and gas power cycle prime movers that were simply made to operate thermodynamically backward. Then, it became a powerful force in shaping our culture. The first vapor-compression refrigeration system using a closed cycle process was patented in 1834 by the American Jacob Perkins (1766–1849). He chose ethyl ether (or, more accurately, diethyl ether, C H OC H ) 2 5 2 5 as the refrigerant, because at low pressures, its temperature was low enough to freeze water on the outside of the evaporator. The ether vapor was compressed in a piston-cylinder apparatus and condensed into a liquid at a higher saturation pressure and temperature. Finally, the liquid ether was throttled through a valve back into the low-pressure evaporator. This system is illustrated in Figure 14.8. Since this process occurs beneath the vapor dome of the working fluid (ether), it is clearly a reversed Rankine cycle device. All vapor-compression cycle refrigeration systems operate essentially on a reversed Rankine cycle, as shown in Figure 14.8b. In these systems, the boiler is normally called the evaporator and the prime mover is replaced by a compressor. Also, it would seem reasonable to replace the boiler feed pump of the forward-running Rankine cycle with some form of prime mover in the reversed Rankine or vapor-compression cycle, whose work output could be used to offset the work input to the compressor. Unfortunately, this is not economically feasible in most small- to medium-scale refrigeration systems, as the following example illustrates. Q H Condenser 3 2 Throttling 3 2 (expansion) W C T valve Compressor 4 4h 1 1s 1 Boiler (evaporator) s Q L (a) Equipment schematic (b) The thermodynamic cycle FIGURE 14.8 Jacob Perkins’s closed-loop vapor-compression refrigeration cycle.14.5 Vapor-Compression Refrigeration Cycle 543 WHERE DID “MECHANICAL REFRIGERATION” COME FROM? The first vapor-compression refrigeration system was patented by Jacob Perkins (1766–1849) in 1834 (Figure 14.9). Though Perkins was an American, his refrigerator was made in England and was not an economic success. A similar machine was made in the United Statesin 1856byAlexander Catlin Twinning (1801–1884), againwithlittlefinancialsuccess.Ineachcase,theeva- porator was immersed in a salt brine solution and the cold brine was used to make ice, but it attracted little attention for more than 20 years, after which natural refrigeration had begun to cause changes in people’s dietary habits. In 1855, James Harrison (1816–1893), a Scotsman who emigrated to Australia, produced a commercially successful refrigerator similar to Perkins’s for the manufacture ofice.Sincenatural iceis difficult tofind inAustralia,Harrison’s artificial ice machine was aninstant success. Supply duct Outdoor Heated air heat exchanger Indoor heat exchanger Heat Return duct Compressor FIGURE 14.9 Jacob Perkins’s 1834 refrigeration apparatus. EXAMPLE 14.3 A refrigeration system for a supermarket is to be designed using R-22 to maintain frozen food at−15.0°C while operating in an environment at 20.0°C. The refrigerant enters the condenser as a saturated vapor and exits as a saturated liquid. Deter- mine the COP for this refrigerator, using a. A reversed Carnot cycle operating between these temperature limits. b. An isentropic vapor-compression cycle with an isentropic expansion turbine installed between the high-pressure condenser and the low-pressure evaporator. c. An isentropic vapor-compression cycle with an aergonic, adiabatic, throttling expansion valve installed between the high- pressure condenser and the low-pressure evaporator. Solution First, draw a sketch of the system (Figure 14.10). Q H Condenser 2 3 3 2s T W W T C 4s 1 4 1 Evaporator s Q L FIGURE 14.10 Example 14.3, system sketch. a. Here, T = 20.0 + 273.15 = 293.15 K, and T =−15.0 + 273.15 = 258.15 K. Then, Eq. (14.5) gives H L T 258:15 L COP = = = 7:38 Carnot T −T 293:15−258:15 H L refrigerator (Continued)544 CHAPTER 14: Vapor and Gas Refrigeration Cycles EXAMPLE 14.3 (Continued) b. From Table C.9b in Thermodynamic Tables to accompany Modern Engineering Thermodynamics, the thermodynamic data at the monitoring stations shown in the schematic are Station1 Station2s T = −15:0°C T = 20:0°C 1 2s . s = s = 0:89973kJ/ðÞ kg K x = 1:00 1 2s 2s x = 0:9395 h = 256:5kJ/kg 1 2s . h = 231:0kJ/kg s = 0:89973kJ/ðÞ kg K 1 2s p = 909:9kPa 2s Station3 Station4s T = 20:0°C T = T = −15:0°C 3 4s 1 . x = 0:00 s = s = 0:25899kJ/ðkg KÞ 3 4s 3 h = 68:67kJ/kg x = 0:1765 3 4s . s = 0:25899kJ/ðkg KÞ h = 65:6kJ/kg 3 4s p = p = 909:9kPa 3 2 where we have calculated s −s s −s 1 f1 2s f1 0:89973−0:11075 x = = = = 0:9395 1 s s fgx1 fg1 0:83977 h = h +x ðh Þ = 27:33+ð0:9395Þð216:79Þ = 231:0kJ/kg 1 f1 1 fg1 s −s 3 f4 0:25899−0:11075 x = = = 0:1765 4s s fg4 0:83977 and h = h +x ðh Þ = 27:33+ð0:1765Þð216:79Þ = 65:59kJ/kg 4s f4 4s fg4 Then, _ Q h −h L 1 4s COP = = isentropic _ _ ðh −h Þ−ðh −h Þ W −W 2s 1 3 4s c t vapor compression - cycleðwithexpansion turbineÞ 231:0−65:59 = = 7:38 ð256:5−231:0Þ−ð68:67−65:59Þ which is identical to the Carnot efficiency of part a, as it should be, because the Rankine and Carnot cycles are identical in this case (see Figure 14.10). c. When the isentropic turbine is replaced by an adiabatic, aergonic throttling valve, the process from station 3 to station 4 becomes isenthalpic rather than isentropic, as shown in Figure 14.11. Q H Condenser 3 2 3 2s T Throttling W C valve 1 4h 4 1 Evaporator s Q L FIGURE 14.11 Example 14.3, Solution, part c.14.5 Vapor-Compression Refrigeration Cycle 545 The thermodynamic data for stations 1, 2s, and 3 remain unchanged from part b, but the isenthalpic throttling valve changes the data of station 4s to 4h as follows. Station4h T = T = −15:0°C 4h 1 h = h = 68:67kJ/kg 4h 3 x = 0:1910 4h . s = 0:27081kJ/ðkg KÞ 4h where we have calculated h −h 4h f4 68:67−27:34 x = = = 0:1906 4h h 216:79 fg4 and . s = s +x ðs Þ = 0:11075+ð0:1906Þð0:83977Þ = 0:27081kJ/ðÞ kg K 4h f4 4h fg4 Finally, _ Q h −h 231:0−68:67 L 1 4h COP = = = = 6:37 isentropie _ 256:5−231:0 h −h W 2s 1 c vapor-compressioncycle ðwiththrottlingvalveÞ Exercises 7. Determine the pressureofthe R-22in the evaporator in Example 14.3. Answer: p = p = p = p (R-22 at−15.0°C) = evaporator 1 4 sat 295.7 kPa. 8. If ammonia were used in the refrigeration system described in Example 14.3, determine the condenser pressure if all the other variables remain unchanged. Answer: p = p = p = p (ammonia at 20.0°C) = 857.12 kPa. condenser 2 3 sat 9. The head of your Engineering Department has decided to use R-134a instead of R-22 in the refrigeration system in Example 14.3. Assuming all the other variables remain unchanged, determine the new operating pressure in the evaporator. Answer: p = p = p = p (R-134a at−15.0°C) = 164 kPa. evaporator 1 4 sat The decrease in COP from 7.38 to 6.37 (13.7%) in the previous example is not normally sufficient to justify the increased expense of manufacturing, installing, and maintaining a turbine or other prime mover between the condenser and the evaporator in small- and medium-size systems. Also, the working fluid in this part of the cycle contains a mixture of liquid and vapor, and it is difficult to find any prime mover that operates efficiently and reliably with this type of two-phase fluid. Throttling expansion valves, on the other hand, are very inexpen- sive and reliable under these conditions. By introducing the isentropic efficiency of the compressor (η) , the general formula for the actual thermal effi- s c ciency (COP) of a reversed Rankine cycle can be written as _ Q h −h 1 L 4h COP = = (14.6) vapor compressioncycle - _ ðÞ h −h /ðÞ η W 2s 1 c s c R/AC and _ jQ j h −h H 2 3 COP = = (14.7) vapor-compressioncycle _ ðÞ h −h /ðÞ η W 2s 1 c s c HP where h = h +ðÞ h −h /ðÞ η : 2 1 2s 1 s c Because throttling processes are ideally isenthalpic, a pressure-enthalpy diagram is often used to describe vapor refrigeration cycles, as shown in Figure 14.12. Process 1 to 2s in this figure involves the compression of a liquid- vapor mixture. This is technically more difficult than compressing either a pure vapor or a pure liquid. A method of eliminating this problem is to superheat the vapor, as shown in Figure 14.12b.546 CHAPTER 14: Vapor and Gas Refrigeration Cycles 3 2s 3 2s T p 4h 1 4h 1 s h (a) Vapor-compression isentropic refrigeration cycle with isenthalpic throttling 2s 3 3 2s T p 1 4h 4h 1 s h (b) Same as (a) except with superheat FIGURE 14.12 T–s and p–h diagrams for an isentropic vapor-compression cycle. EXAMPLE 14.4 Repeat part c of Example 14.3 requiring that the evaporator outlet be a saturated vapor at−15.0°C and introduce a compres- sor isentropic efficiency of 75.0%. Solution First, draw a sketch of the system (Figure 14.13). Since the evaporator outlet is a saturated vapor, the compressor outlet is a superheated vapor, as shown in the figure. The thermodynamic data for the four monitoring stations are (see Example 14.3 for details) Station1 Station2s T = −15:0°C p = p = 909:9kPa 1 2s 2 . x = 1:00 s = s = 0:95052kJ/ðÞ kg K 1 2s 1 h = 244:13kJ/kg h = 271:92kJ/kgðÞ frominterpolationinTableC:10b 1 2s . s = 0:95052kJ/ðÞ kg K T = 39:3°C 1 2s Station3 Station4h T = 20:0°C T = T = −15:0°C 3 4h 1 x = 0:00 h = h = 68:67kJ/kg 3 4h 3 h = 68:67kJ/kg x = 0:1910 3 4h . . s = 0:25899kJ/ðÞ kg K s = 0:27088kJ/ðÞ kg K 3 4h Then, from Eq. (14.7), _ Q h −h L 1 4h COP = = vapor-compressioncycle _ ðÞ h −h /ðÞ η jWj 2s 1 s c c R/AC 244:13−68:67 = = 4:74 ðÞ 271:92−244:13 /0:75014.6 Refrigerants 547 2s 2s 3 2 3 2 T p 4h 1 1 4h s h FIGURE 14.13 Example 14.4. Exercises 10. After several years of use, the isentropic efficiency of the compressor in Example 14.4 decreases from 75.0% to 55.0% due to wear and a lack of maintenance. Determine the new COP for this system. Answer: COP = 3.47. _ 11. Determine the power required to drive the compressor W in Example 14.4 if the refrigeration system is to produce c _ _ Q = 20 tons of cooling. Recall that 1 ton of refrigeration is equal to 214. kJ/min. Answer: W = 19:8hp. L c _ 12. Determine the mass flow rate of refrigerant required in Example 14.4 if this system produces Q = 20 tons of cooling. L _ Recall that 1 ton of refrigeration is equal to 214. kJ/min. Answer: m = 23:9kg=min. Even if the compressor had an isentropic efficiency of 100%, the COP in this example would be only 4.74/0.750 = 6.32, which is still slightly less than the 6.37 of part c in Example 14.3. Thus, adding superheat to the cycle _ _ usually does not increase the COP because bothjWj/m _ and Q /m _ are increased. However, the required mass flow c L rate m _ is significantly reduced by the addition of superheat. Also, because condensers and evaporators are not 100% effective as heat exchangers, the temperature difference between the working fluid in these devices and their local environment is typicallyabout 15.0°F. 14.6 REFRIGERANTS Whereas the working fluid of the steam engine (water) was nearly ideal for vapor power cycles, it was totally unsuitable for the refrigeration cycles of commercial interest. The major problem faced by the early developers of refrigeration technology was not the design of the machinery per se but the search for a suitable nontoxic, safe, inexpensive working fluid with satisfactory low-temperature thermodynamic characteristics. Though water is the cheapest and safest refrigerant available, it is limited to high-temperature applications such as steam-jet refrigeration. Since most refrigeration needs are at temperatures near the freezing point of water, other refrigerants had to be found that boiled at lower temperatures. Perkins used ethyl ether as his refrigerant. It was a good refrigerant, but it was also toxic and flammable. Also, the entire ethyl ether refrigeration system operated below atmospheric pressure, making it difficult to prevent air from leaking into the system. The danger and complexity of ethyl ether refrigerators caused other inventors to search for alternative refrigeration technologies, which ultimately lead to the rapid development of gas expan- sion refrigeration cycles between 1860 and 1890. The French inventor Charles Tellier (1828–1913) introduced methyl ether (CH Cl) as a replacement for ethyl 3 ether in 1863. Though methyl ether was also toxic and flammable, it had a higher vapor pressure, and that allowed the entire refrigeration system to operate above atmospheric pressure, thus eliminating the problems caused by air leaking into the system. IS ETHER A REFRIGERANT OR AN ANESTHETIC? The di in diethyl ether is often dropped, and it is called either ethyl ether or simply ether. This is the same ether that was first successfully used as an anesthetic in 1846 by the Massachusetts dentist William T. G. Morton. Since the boiling point of ether at atmospheric pressure is 35°C(95°F), slightly below the temperature of the human body, it was common practice in the late 19th and early 20th centuries for physicians to use liquid ether as a local anesthetic by spraying it onto parts of the body where it would then freeze the tissue as it boiled away and consequently numb the local sensations. This is the source of the term freezing as a synonym for a local anesthetic (especially in dentistry) today.548 CHAPTER 14: Vapor and Gas Refrigeration Cycles HOW DID COMPRESSOR TECHNOLOGY DEVELOP? By the end of the World War I (1914–1918), reciprocating piston compressors still dominated refrigerant technology, and the primary refrigerants still in use in the Unoted States at that time were ammonia, carbon dioxide, and sulfur dioxide. In 1919, the French engineer Henri Corblin (1867–1947) patented a diaphragm refrigerant compressor in which the oscillat- ing motion of the center of a fixed diaphragm replaced the reciprocating motion of a piston in a cylinder. In 1918, the first hermetically sealed refrigeration compressor was developed by the Australian Douglas Henry Stokes, in which the motor and compressor were sealed together inside a container with the refrigerant. In 1933, Willis Carrier (1876–1950) developed his first centrifugal refrigerant compressor for use with R-11. During the last half of the 19th century, the development of refrigeration technology flourished in America, especially in the South. In 1866, Thaddeus S. C. Lowe (1832–1913) developed a high-pressure (80 atm) carbon dioxide compressor for manufacturing ice in Dallas, Texas, and Jackson, Mississippi; and in 1872, David Boyle (1837–1891) developed an ammonia compressor (10 atm) for manufacturing ice in Jefferson, Texas. This allowed CO and NH to enter the list of useful refrigerants. 2 3 The Swiss physicist Raoul Pierre Pictet (1846–1929) studied the various refrigerants then available and found that sulfur dioxide had suitable thermodynamic properties. In 1874, he developed an SO compressor and refrigerating 2 system that was quite successful. Sulfur dioxide has the advantages of being a natural lubricant for the compressor and it does not burn. Its chief disadvantage is that, on contact with moisture, it forms corrosive sulfuric acid. In the late 1920s, the American chemist and engineer, Thomas Midgley, Jr. (1889–1944), discovered that certain fluorine compounds were remarkably nontoxic and odorless while simultaneously having the proper thermody- namic properties of a good refrigerant. In the 1930s, the E. I. duPont de Nemours Company became commer- cially involved in the refrigeration industry by manufacturing and selling Midgley’s discovery as a refrigerant. DuPont marketed the product under the commercial trade name Freon. Midgley’s refrigerants were halogenated hydrocarbons in which halogen atoms (mainly chlorine and fluorine) were substituted for hydrogen atoms in simple hydrocarbon molecules. Midgley replaced the four hydrogen atoms in methane, CH , with two chlorine and two fluorine atoms to produce dichloro-difluoro-methane (or 4 dichlorodifluoromethane, CCl F ). Other common methane based refrigerants are monochlorodifluoromethane 2 2 CHClF and trichloromonofluoromethane CCl F. The complex chemical names of these compounds are logical 2 3 and technically correct, but they are difficult for the nonchemist to pronounce and remember. Consequently, a confusing variety of commercial trade names, such as Freon, Genetron, Isotron, and Frigen, came into popular 3 use during the 1940s. Shortly thereafter, the American Society of Refrigerating Engineers (ASRE) decided to adopt a standard method of refrigerant designation that was based only on the use of numbers. THE TEFLON CONNECTION A young DuPont chemist named Roy J. Plunkett discovered Teflon on April 6, 1938, while experimenting with a haloge- nated ethylene gas for use as a refrigerant. On this day, Plunkett received a pressurized tank of tetrafluoroethylene (C F ) 2 4 to study its properties as a nontoxic refrigerant. When he opened the tank nothing came out. After the valve was checked, the tank was weighted and found to be the same weight as when it was full. Something made no sense, so Plunkett had the tank cut open and found a waxy white powder. Being a chemist, Plunkett realized that the gas had somehow sponta- neously “polymerized” to form a new material, polytetrafluoroethylene. The waxy white powder had some remarkable physi- cal properties: it was not affected by strong acids or bases, was resistant to heat from−450°Fto725°F(−270°C to 385°C), and was very slippery. While these properties were interesting, it was decided that this new material had no particular com- mercial value. Then came World War II and the top-secret atomic bomb project (the Manhattan Project). A material was needed for gaskets that would resist the terribly corrosive properties of uranium hexafluoride gas. By a chance communica- tion, the director of the Manhattan Project became aware of the new polymeric material that Plunkett had discovered. It was then made into a test gasket and found to be very successful at containing the corrosive gas. After World War II, the new polymer material was not put to any practical use until it began to be used on nonstick cookware in France in 1954. Nonstick cooking utensils were first sold in the United States on December 15, 1960, at Macy’s Department Store in New York City. Taking letters from the complicated chemical name polytetrafluoroethylene, the new polymer was named Teflon. 3 The ASRE merged with the American Society of Heating and Air-Conditioning Engineers (ASHAE) to form the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) in 1959.14.7 Refrigerant Numbers 549 14.7 REFRIGERANT NUMBERS Most halogenated hydrocarbons used in refrigeration have a molecular structure of the form C H Cl F and the a b c d atomic valences require that c = 2(a+1)− b− d. These compounds are given refrigerant R numbers defined by C H Cl F isrefrigerantnumber: R-ða−1Þðb+1Þd, where c = 2ða+1Þ−b−d (14.8) a b c d When a = 1, then a− 1 = 0 (the methane series of halogenated hydrocarbons) and the zero is omitted in the R ,has a = 1, b = 0, c = 4, and d = 0. Consequently, its R num- number. For example, carbon tetrachloride, CCl 4 ber is R-(0)(0 + 1)0 = R-010 = R-10. Bromate compounds are indicated with a B after the R number followed by the number of bromine atoms. For example, CBrF = R-13B1. Also, ethane and higher hydrocarbon bases can have numerous isomers (compounds 3 containing the same number of atoms, but assembled in different ways). In these cases, the most symmetrical atomic arrangement is given the base R number R-(a − 1)(b+1)d, and the remaining arrangements are given the suffixes a, b, c, and so forth as the refrigerant molecule become less and less symmetrical. For example the differences between R-134 (CHF -CHF ) and R-134a (CH FCF ) are illustrated next: 2 2 2 3 FF H F HCCHFCCF FF H F R134 R134a Therefore, Midgley’s CCl F with a = 1, b = 0, c = 2, and d =2became Refrigerant-12 (abbreviated R-12), or 2 2 Freon-12 if manufactured by DuPont. Similarly, CHClF (a = b = c = 1, d = 2) became Refrigerant-22 or R-22, 2 F(a = 1, b = 0, c = 3, d = 1) became Refrigerant-11 or R-11, and so forth. Ethane-based refrigerants are the CCl 3 100 number series, and the ethane-based hexachloroethane C Cl (a = 2, b = 0, c = 6, d = 0) became Refriger- 2 6 ant-110 or R-110, and so forth. Propane-based refrigerants are the 200 number series, and butane-based refriger- ants are assigned the 600 number series. Inorganic (i.e., nonhydrocarbon based) refrigerants are assigned the 700 number series with the last two digits being the molecular mass of the refrigerant. For example, ammonia, NH,is Refrigerant-717 and water, H O, is Refrigerant-718. Table 14.1 lists the ASHRAE number, chemical 3 2 formula, and boiling point of some common refrigerants. Figure 14.14 presents typical saturation temperature- pressure curves for some common refrigerants plus a graphical representation of the refrigerant derivatives of methane, CH , and ethane, C H . 4 2 6 Table 14.1 The American Society of Heating, Refrigerating and Air-Conditioning Engineers Refrigerant Numbering System for Some Common Refrigerants Boiling Point at Atmospheric Pressure Refrigerant Number Chemical Formula °F °C R-10 CCl 170.2 76.8 4 R-11 CCl F 74.9 23.8 3 R-12 CCl F −21.6 −29.8 2 2 R-21 CHCl F 48.1 8.9 2 R-22 CHClF −41.4 −40.8 2 R-30 CH Cl 105.2 40.7 2 2 R-40 CH Cl −14.8 −23.8 3 R-50 CH (methane) −259.0 −161.7 4 R-110 C Cl 365.0 185.0 2 6 R-111 C Cl F 279.0 137.2 2 5 R-112 C Cl F 199.0 92.8 2 4 2 R-123 CHCl CF 81.7 27.6 2 3 FCF −15.7 −26.2 R-134a CH 2 3 R-170 C H (ethane) −127.8 −88.8 2 6 R-290 C H (propane) −43.7 −42.1 3 8 R-600 C H (butane) 33.1 0.6 4 10 R-717 NH (ammonia) −28.0 −33.3 3 R-718 H O (water) 212.0 100.0 2 Source: Reprinted by permission from the ASHRAE Handbook—1985 Fundamentals.550 CHAPTER 14: Vapor and Gas Refrigeration Cycles Ethyl ether Vacuum 110 100 90 80 70 60 50 40 30 20 10 0 −10 −20 −30 −40 30 20 10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 Gauge pressure (psig) Inches of Hg Vacuum (a) R-10 CCl 4 R-20 R-11 CHCl CFCl 3 3 R-30 R-21 R-12 CH Cl CHFCl CF Cl 2 2 2 2 R-40 R-31 R-22 R-13 CH Cl CH FCl CHF Cl CF Cl 3 2 2 3 R-50 R-41 R-32 R-23 R-14 CH CH F CH F CHF CF 4 3 2 2 3 4 (b) Refrigerant derivatives of methane (CH ) 4 R-502 Ammonia (R-717) R-500 R-22 R-12 Sulfur dioxide (R-764) R-11 Temperature (°F)14.7 Refrigerant Numbers 551 R-110 C Cl 2 6 R-120 R-111 C HCl C FCl 2 5 2 5 R-130 R-121 R-112 C H Cl C HFCl C F Cl 2 2 4 2 4 2 2 4 R-140 R-131 R-122 R-113 C H Cl C H FCl C HF Cl C F Cl 2 3 3 2 2 3 2 3 2 2 3 3 R-150 R-141 R-132 R-123 R-114 C H Cl C H FCl CH F Cl C HF Cl C F Cl 2 4 2 2 3 2 2 2 2 3 2 2 4 2 R-160 R-151 R-142b R-133 R-124 R-115 C H Cl C H FCl C H F Cl C H F Cl C HF Cl C F Cl 2 5 2 4 2 3 2 2 2 3 2 4 2 5 R-170 R-161 R-152a R-143a R-134a R-125 R-116 C H C H F C H F C H F C H F C HF C F 2 6 2 5 2 4 2 2 3 3 2 2 4 2 5 2 6 (c) Refrigerant derivatives of ethane (C H ) 2 6 Cl Fully halogenated (long atmospheric life) Toxic Flammable H F (d) FIGURE 14.14 (a) Typical saturation temperature–pressure curves for common refrigerants, (b) the refrigerant derivatives of methane, (c) the refrigerant derivatives of ethane, and (d) CFC behavior chart.552 CHAPTER 14: Vapor and Gas Refrigeration Cycles EXAMPLE 14.5 As a technical expert in a multibillion-dollar lawsuit, you are asked to determine the refrigerant numbers for the following refrigerants by the prosecuting attorney: a. Chloroform, CHCl . 3 b. Chlorotetrafluoroethane, CHClFCF . 3 c. Octafluoropropane, CF CF CF . 3 2 3 Solution Being totally unimpressed by the prosecuting attorney’s aggressive questioning, you calmly reply as follows: a. “Chloroformcontainsone carbon atom(a = 1), one hydrogenatom(b = 1), and three chlorineatoms (c = 3), and no fluorineatoms(d =0).”Making aquick calculationin yourheadusingEq.(14.8),youarrive at R-(a− 1)(b+1)d = R-(1− 1) (1+1)0 = R-020= R-20(dropping the leading 0). Then you reply, “So,the refrigerant number forchloroformis R-20.” b. “Chlorotetrafluoroethane, on the other hand, contains two carbon atoms (a = 2), one hydrogen atom (b = 1), one chlorine atom (c = 1), and four fluorine atoms (d = 4).” Using Eq. (14.8) again you find R-(2− 1)(1 + 1)4 = R-124, and you reply, “So, its refrigerant number is: R-124.” c. “Now octafluoropropane is a very interesting compound in that it contains three carbon atoms (a = 3), no hydrogen or chlorine (b = c = 0), and eight fluorine atoms (d = 8).” (Thinking, again using Eq. (14.8), R-(3− 1)(0 + 1)8 = R-218.) “Consequently its refrigerant number is: R-218.” Exercises 13. Suppose the prosecuting attorney in Example 14.5 asks you for the refrigerant number of carbontetrachloride CCl . What 4 would you say then? Answer: Your response: “The refrigerant number is: R-14.” 14. “Aha” the prosecutor in Example 14.5 exclaims, “You seem pretty confident of yourself, don’t you? Well, then, can you tell me what substance has refrigerant number 720?” (Recall that 700 series refrigerants are inorganic compounds and the last two digits of the R number correspond to the molecular mass of the compound.) Answer: Your response: “The compound is neon.” 15. The prosecuting attorney in Example 14.5 vociferates, “You don’t say, then give me the chemical formula and refrigerant number for trifluoromethane” Answer: Your response: “CHF which is R-23.” 3 14.8 CFCs AND THE OZONE LAYER Ozone (O ) in the upper atmosphere absorbs ultraviolet radiation from the sun and prevents much of it from 3 reaching the surface of the Earth. Exposure to ultraviolet radiation is a known source of skin cancer and other biological problems. All chlorofluorocarbons (CFCs) are combinations of chlorine, fluorine, and carbon atoms. After 1950, the use of chlorofluorocarbons dominated the domestic and automotive refrigeration and air conditioning markets. In the 1950s and 1960s, inexpensive chlorofluorocarbons found use as a propellant in aerosol spray cans for paint, deodorant, hair products, and so forth. In 1974, Professor Sherry Rowland at the University of California—Irvine and her postdoctorate student Mario Moline postulated that chlorofluorocarbons are so chemically stable that they can exist in the atmosphere for hun- dreds of years, eventually diffusing into the Earth’s stratosphere, where ultraviolet radiation decomposes them to release chlorine atoms. The chlorine atoms then catalyze the conversion of ozone into oxygen as follows: O +Cl O +ClO 3 2 ClO+O O +Cl 2 with the chlorine atom being regenerated. The overall reaction is then O+O 2O 3 2 The CFC production in 1974 was 1 million pounds per year, and the shear volume of CFCs released through spray cans and leaking refrigeration systems could possibly destroy the ozone layer faster than it is created by ultraviolet radiation acting on oxygen molecules. Rowland’s hypothesis alluded to a massive global problem, and it had profound impact on CFC use. But, what will replace the banned CFCs? It was not too difficult to find safe propellants (such as CO ) for use in 2 aerosol cans, but finding suitable replacements for refrigerants such as R-11 (used in large building air condi- tioning systems) and R-12 (used in domestic refrigerators and air conditioners and automotive air conditioners) was much less obvious.14.8 CFCs and the Ozone Layer 553 HOW WERE CFCs CONTROLLED? 1978: The U.S. Environmental Protection Agency (EPA) banned the use of CFCs in all nonessential aerosol cans. This action alone cut the U.S. consumption of CFCs by 50%. 1980: The European Community limited CFC production and use in aerosols. 1985: The Ozone Hole is discovered in the Antarctic. 1987: The Montreal Protocol is signed by 43 nations to decrease overall production of CFCs by 50% by 1999. 1990: Title VI of the Clean Air Act (Stratospheric Ozone Protection) is passed into law in the United States. 1992: The signers of the Montreal Protocol agree to a phase-out schedule for all HCFCs (including R-123) by the year 2030. The financial investment in existing refrigeration and air conditioning systems is massive, so the replacement refrigerants must have very similar thermodynamic properties to R-11 and R-12, so that they can be used in the 4 same operating equipment with minimal modifications. Today, R-123 is temporarily replacing R-11, and R-134a is replacing R-12. By the end of 1995, EPA banned most production and import of R-12. However, the use of R-12 is still permitted until supplies are depleted. Figures 14.15 and 14.16 show the p–h diagrams for these refrigerants and their replacements. R-123 is CHCl CF and is called a hydrochlorofluorocarbon (HCFC). While it still contains chlorine, it is 50 times 2 3 less detrimental to the ozone layer than R-11. Consequently, it is viewed as a temporary replacement for R-11, since it too must be phased out by the year 2030. The ultimate replacement for R-11 may be R-245fa (CF HCF CFH ), a propane-based halocarbon that does not contain chlorine. R-134a is CH FCF and is called a 2 2 2 2 3 hydrofluorocarbon (HFC). It contains no chlorine and will not damage the ozone layer. The other common refrig- erant in use in large-scale air conditioning and heat pump systems is R-22 (CHClF ). It is also an HCFC, and 2 1000 1000 R−134a R−11 R−12 500 500 R−123 200 200 100 100 50 50 20 20 10 10 5 5 2 2 1 1 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 Enthalpy (Btu/lbm) Enthalpy (Btu/lbm) FIGURE 14.15 FIGURE 14.16 Superimposed p–h diagrams for R-11 and R-123, showing the Superimposed p–h diagrams for R-12 and R-134a, showing the thermodynamic similarities between these two refrigerants. thermodynamic similarities between these two refrigerants. 4 R-123 is scheduled to be phased out in 2020 in new equipment. Pressure (psia) Pressure (psia)554 CHAPTER 14: Vapor and Gas Refrigeration Cycles CRITICAL THINKING Replacing refrigerants R-11 and R-12 with R-123 and R-134a in existing equipment is not a matter of simply draining out the old refrigerant and adding the new, because they have different physical and thermodynamic properties. This conver- sion is very expensive and the owners of the equipment must bear the costs. This is why the final phase-out of all CFCs is not scheduled until the year 2030, when the equipment existing today would be obsolete and need to be replaced anyway. If the CFCs are as dangerous as we think they are, then why are we waiting so long to eliminate them? Who else should share in the conversion costs? although it still contains chlorine, it is 20 times less detrimental to the ozone layer than R-11 or R-12. However, after January 1, 2010, no virgin R-22 can be used in existing systems, and after January 1, 2015, no recycled refrigerant R-22 can be used in existing systems. 14.9 CASCADE AND MULTISTAGE VAPOR-COMPRESSION SYSTEMS Refrigeration applications like the quick freezing of processed food or the production of liquefied gases such as liquefied natural gas (LNG, methane) and liquefied petroleum gas (LPG, propane and butane) require moder- ately cold refrigeration temperatures in the range of −30.°Cto −180°C(−22°Fto −290°F) with an outside ambient temperature near 20.°C(68°F). This temperature range is too large for a single vapor-compression refrigeration cycle, because it requires a very large pressure ratio across the compressor. To solve this problem, we can connect (or cascade) two or more cycles together to form a cascade vapor-compression refrigeration cycle with lower individual compressor pressure ratios, as shown in Figure 14.17. This figure shows a double-cascade Q H Condenser A 3A 2A Cycle A W C − A 4A 1A Evaporator A Condenser B 3B 2B Cycle B W C − B 4B 1B Condenser B Q L 2A 2 A s 3A 2B 2 B s 4 A h 1A 3B T 4 B 1B h s FIGURE 14.17 A dual-cascade, vapor-compression refrigeration system with the same refrigerant used in each cycle.

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