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REFRIGERATING
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REFRIGERATING and See also:ICE-MAKING. " Refrigeration " (from See also:Lat. frigus, See also:frost) is the cooling of a See also:body by the See also:transfer of a portion of its See also:heat to another and therefore a cooler body. For See also:ordinary temperatures it is performed directly with See also:water as the cooling See also:agent, especially when well water, which usually has a temperature of from 52° to 55° F., can be obtained. There are, however, an increasingly large number of cases in which temperatures below that of any available natural cooling agent are required, and in these it is necessary to resort to See also:machines which are capable of producing the required cooling effect by taking in heat at See also:low temperatures and rejecting it at temperatures somewhat above that of the natural cooling agent, which for obvious reasons is generally water. The See also:function of a refrigerating See also:machine, therefore, is to take in heat at a low temperature and reject it at a higher one. This involves the See also:expenditure of a quantity of See also:work W, the amount in any particular See also:case being found by the See also:equation W = Q2-Q1,where W is the work, expressed by its See also:equivalent in See also:British thermal See also:units; Q2 the quantity of heat, also in B.Ther.U., given out at the higher temperature T2; and Qi the heat taken in at the See also:lower temperature Tr. It is evident that the discharged heat Q2 is equal to the abstracted heat Qi, plus the work expended, seeing that the work W, which causes the rise in temperature from Tr to T2, is the thermal equivalent of the See also:energy actually expended in raising the temperature to the level at which it is rejected. The relation then between the work expended and the actual cooling work performed denotes the efficiency of the See also:process, and this is expressed by Qi/(Q2-QI); but as in a perfect refrigerating machine it is understood that the whole of the heat Qr is taken in at the See also:absolute temperature and the whole of the heat Q2, is rejected at the absolute temperature T2, the heat quantities are proportional to the temperatures, and the expression TI/(T2-TI) gives the ideal coefficient of performance for any stated temperature range, whatever working substance is used. These coefficients for a number of cases met with in practice are given in the following table. They T,. Tz Temperature ee Temperature at which Heat is rejected in r Degrees Fahr. which Heat is extracted in Degrees Fahr. 50° 6o° 70° 80° 90° See also:IOO° -10° 7.5 6.4 5.6 5.0 4.5 4•I o° 9.2 7.7 6.6 5.8 5.1 4.6 10° II.7 9.4 7.8 6.7 5.9 5.2 2o° 16•o 12.0 9.6 8•o 6.8 6•o 30° 24.5 16.3 12.2 9.8 8.2 7.0 40° 50.0 25.0 16.7 12.5 Io•0 8.3 show that in all cases the heat abstracted exceeds by many times the heat expended. As an instance, when heat is taken in at o° and rejected at 7o°, a perfect refrigerating machine would abstract 6.6 times as much heat as the equivalent of the energy to be applied. If, however, the heat is to be rejected at too°, then the coefficient is reduced to 4.6. By examining Table I. it will be seen how important it is to reduce the temperature range as much as possible, in See also:order to obtain the most economical results. No actual refrigerating machine does, in fact, take in heat at the exact temperature of the body to be cooled, and reject it at the exact temperature of the cooling water, but, for See also:economy in working, it is of See also:great importance that the See also:differences should be as small as possible. There are two distinct classes of machines used for refrigerating and ice-making. In the first refrigeration is produced by the expansion of atmospheric See also:air, and in the second by the evaporation of a more or less volatile liquid. Compressed-air Machines.-A compressed-air refrigerating machine consists in its simplest See also:form of three essential parts -a See also:compressor, a compressed-air cooler, and an expansion See also:cylinder. It is shown diagrammatically in fig. I in connexion with a chamber which it is keeping cool. The compressor draws in air from the See also:room and compresses it, the work expended in See also:compression being almost entirely converted into heat. The compressed air, leaving the compressor at the temperature T2, passes through the cooler, where it is cooled by means of water, and is then admitted to the expansion cylinder, where it is See also:expanded to atmospheric pressure, performing work on the See also:piston. The heat equivalent of the See also:mechanical work per- formed on the piston is abstracted from the air, which is dis- charged at the temperature TI. This temperature TI is neces- Compression Cylinder Expansion Cylinder sarily very much below the temperature to be maintained in the room, because the cooling effect is produced by transferring heat from the room or its contents to the air, which is thereby heated. The rise in temperature of the air is, in fact, the measure of the cooling effect produced. If such a machine could be constructed with reasonable mechanical efficiency to compress the air to a temperature but slightly above that of the cooling water, and to expand the air to a temperature but slightly below that required to be maintained in the room, we should of course get a result approximating in efficiency somewhat nearly to the figures given in Table I. Unfortunately, however, such results cannot be obtained in practice, because the extreme lightness of the air and its very small heat capacity (which at See also:constant pressure is •2379) would necessitate the employment of a great See also:volume, with extremely large and mechanically in-efficient cylinders and apparatus. A See also:pound of air, representing about 12 cub. ft., if raised to° F. will only take up about 2.4 B.T.U. Consequently, to make such a machine mechanically successful a comparatively small See also:weight of air must be used, and the temperature difference increased; in other words, the air must be discharged at a temperature very much below that to be maintained in the room. This' theory of working is founded on the See also:Carnot See also:cycle for a perfect heat motor, a perfect refrigerating machine being simply a reversed heat motor. Another theory involves the use of the See also:Stirling regenerator, which was proposed in connexion with the Stirling heat See also:engine (see See also:Ant ENGINES). The air machine invented by Dr. A. See also:Kirk in 1862, and described by him in a See also:paper on the " Mechanical See also:Production of See also:Cold " (Proc. Inst. C.E., See also:xxxvii., 1874, 244), is simply a reversed Stirling air engine, the air working in a closed cycle instead of being actually discharged into the room to be cooled, as is the usual practice with ordinary compressed-air machines. Kirk's machine was used commercially with success on a fairly large See also:scale, chiefly for ice-making, and it is recorded that it produced about 4 lb of Ice for 1 lb of See also:coal. In 1868 J. See also:Davy Postle read a paper before the Royal Society of See also:Victoria, suggesting the See also:conveyance of See also:meat on See also:board See also:ship in a frozen See also:state by means of refrigerated air, and in 1869 he showed by experiment how it could be done; but his apparatus was not commercially See also:developed. In 1877 a compressed-air machine was designed by J. J. Coleman of See also:Glasgow, and in the See also:early See also:part of 1879 one of his machines was fitted on board the See also:Anchor See also:liner " See also:Circassia," which successfully brought a See also:cargo of chilled See also:beef from See also:America-the first imported by the aid of refrigerating machinery, ice having been previously used. The first successful cargo of frozen mutton from See also:Australia was also brought by a See also:Bell-Coleman machine in 1879. In the Bell-Coleman machine the air was cooled during compression by means of an injection of water, and further by being brought into contact with a shower of water. Another, perhaps the See also:principal, feature was the interchanger, an apparatus whereby the compressed air was further cooled before expansion by means of the comparatively cold air from the room in its passage to the compressor, the same air being used over and over again. The See also:object of this interchanger was not only to cool the compressed air before expansion, but to condense part of the moisture in it, so reducing the quantity of ice or See also:snow produced during expansion. A full description of the machine may be found in a paper on " Air-Refrigerating Machinery " by J. J. Coleman (Proc. Inst. C.E. lxviii., 1882). At the See also:present See also:time the Bell-Coleman machine has practically ceased to exist. In such compressed-air machines as are now made there is no injection of water during compression, and the compressed air is cooled in a See also:surface cooler, not by actual mixture with a shower of cold water. Further, though the inter-changer is still used by some makers, it has been found by experience that, with properly constructed valves and passages in the expansion cylinder, there is no trouble from the formation of snow, when, as is the See also:general practice, the same air is used over and over again, the compressor taking its See also:supply from the insulated room. So far as the air discharged from the expansion cylinder is concerned, its humidity is precisely the same so See also:long as its temperature and pressure are the same, inasmuch as when discharged from the expansion cylinder it is always in a saturated See also:condition for that temperature and pressure. The ideal coefficient of performance is about 1, but the actual coefficient will be about 8i after allowing for the losses incidental to working. In practice the air is compressed to about 50 lb per square See also:inch above the See also:atmosphere, its temperature rising to about 3000 F. The compressed air then passes through coolers in which it is cooled to within about 5° of the initial temperature of the cooling water, and is deprived of a portion of its moisture, after which it is admitted into the expansion cylinder and expanded nearly to atmospheric pressure. The thermal equivalent of the See also:power exerted on the piston is taken from the air, which, with cooling water at 6o° F. and after allowing for See also:friction and other losses, is discharged at a temperature of 6o° to 8o° below zero F. according to the See also:size of the machine. The pistons of the compression and expansion cylinders are connected to the same crankshaft, and the difference between the power expended in compression and that restored in expansion, plus the friction of the machine, is supplied by means of a See also:steam engine coupled to the crankshaft, or by any other source of power. For marine purposes two See also:complete machines are frequently mounted on one See also:bed-See also:plate and worked either together or separately.
In some machines used in the See also:United States the cold air is not discharged into the rooms but is worked in a closed cycle, the rooms being cooled by means of overhead pipes through which the cold expanded air passes on its way back to the compressor.
Liquid Machines.—Machines of the second class may conveniently be divided into three types: (a) Those in which there is no recovery of the refrigerating agent, water being the agent employed; they will be dealt with as " Vacuum machines."
(b) Those in which the agent is recovered by means of mechanical compression; they are termed " Compression machines."
(c) Those in which the agent is recovered by means of absorption by a liquid; they are known as " Absorption machines."
In the first class, since the refrigerating liquid is itself rejected, the only agent cheap enough to be employed is water. The vacuum boiling point of water varies with pressure; thus at machines. one atmosphere or 14.7 lb per square inch it is 212° F., whereas at a pressure of •o85 lb per square inch it is 32°, and at lower pressures there is a still further fall in temperature. This See also:property is made use of in vacuum machines. Water at ordinary temperature, say 60 , is placed in an air-tight See also:glass or insulated See also:vessel, and when the pressure is reduced by means of a vacuum See also:pump it begins to See also:boil, the heat necessary for evaporation being taken from the water itself. The pressure being still further reduced, the temperature is gradually lowered until the freezing-point is reached and ice formed, when about one-See also:sixth of the See also:original volume has been evaporated.
The earliest machine of this See also:kind appears to have been made in 1755 by Dr. See also: E. C. See also:Cane later adopted the same principle. In 1878 F. Windhausen patented a vacuum machine for producing ice in large quantities, and in 1881 one of these machines, said to be capable of making about 12 tons of ice per See also:day, was put to work in See also:London. The See also:installation was fully described by Carl Pieper (Trans. See also:Soc. of See also:Engineers, 1882, p. 145) and by Dr. John See also:Hopkinson (See also:Journal of Soc. of Arts, 1882, vol. xxxi. p. 20). The process, however, not being successful from a commercial point of view, was abandoned. At the present time vacuum machines are only employed for domestic purposes. The See also:hand apparatus invented by H. A. Fleuss consists of a vacuumpump capable of reducing the air pressure to a fraction of a milli-See also:metre, the suction See also:pipe of which is connected first with a vessel containing sulphuric acid, and second with the vessel containing the water to be frozen. Both these vessels are mounted on a rocking See also:base, so that the acid can be thoroughly agitated while the machine is being worked. As soon as the pump has sufficiently exhausted the air from the vessel containing the water, vapour is rapidly given off and is absorbed by the acid until sufficient heat has been abstracted to bring about the desired reduction in temperature, the acid becoming heated by the absorption of water vapour, while the water freezes. The small Fleuss machine will produce about It lb of ice in one operation of 20 minutes. Iced water in a carafe for drinking purposes can be produced in about three minutes. The acid vessel holds 9 lb of acid, and nearly 3 lb of ice can be made for each 1 lb of acid before the acid has become too weak to do further See also:duty. Another machine, which can be easily worked by a boy, will produce 20 to 30 lb of ice in one See also:hour, and is perhaps the largest size practicable with this method of freezing. The temperature attainable depends on the strength and condition of the sulphuric acid; ordinarily it can be reduced to zero F., and temperatures 20° lower have frequently been obtained. Though See also:prior to 1834 several suggestions had been made with regard to the production of ice and the cooling of liquids by the evaporation of a more volatile liquid than water, the compress first machine actually constructed and put to work See also:sion was made by John See also:Hague in that See also:year from the designs machines. of See also:Jacob See also:Perkins (Journal of Soc. of Arts, 1882, vol. xxxi. p. 77). This machine, though never used commercially, is the See also:parent of all See also:modern compression machines. Perkins in his patent See also:specification states that the volatile fluid is by preference See also:ether. In 1856 and 1857 See also: 2.-Vapour Compression Machine. A See also:diagram of a vapour compression machine is shown in fig. 2. There are three principal parts, a refrigerator or evaporator, a compression pump, and a condenser. The refrigerator, which consists of a coil or See also:series of coils, is connected to the suction See also:side of the pump, and the delivery from the pump is connected to the condenser, which is generally of somewhat similar construction to the refrigerator. The condenser and refrigerator are connected by a pipe in which is a valve named the regulator. Outside the refrigerator coils is the air, brine or other substance to be cooled, and outside the condenser is the cooling See also:medium, which, as previously stated, is generally water. The refrigerating liquid (ether, See also:sulphur dioxide, anhydrous ammonia, or carbonic acid) passes from the bottom of the condenser through the regulating valve into the refrigerator in a continuous stream. The pressure in the refrigerator being reduced by the pump and maintained at such a degree as to give the required boiling-point, which is of course always lower than the temperature outside the coils, heat passes from the substance outside, through the coil surfaces, and is taken up by the entering liquid, which is converted into vapour at the temperature Ti. The vapours thus generated are See also:drawn into the pump, compressed, and discharged into the condenser at the temperature T2, which is some-what above that of the cooling water. Heat is transferred from the compressed vapour to the cooling water and the vapour is converted into a liquid, which collects at the bottom and returns by the regulating valve into the refrigerator. As heat is both taken in and discharged at constant temperature during the See also:change in physical state of the agent, a vapour compression machine must approach the ideal much more nearly than a compressed-air machine, in which there is no such change. This will be seen by taking as an example a case in which the cold room is to be kept at 10° F., the cooling water being at 60°. Under these conditions, the actual evaporating temperature T1, in a well-constructed ammonia compression machine, after allowing for the differences necessary for the See also:exchange of heat, would be about 5° below zero, and the See also:discharge temperature T would be about 75° An ideal machine, working between 5° below zero and 75° above, has a coefficient of about 5.7, or nearly six times that of an ideal compressed-air machine of usual construction performing the same useful cooling work. A vapour compression machine does not, however, work precisely in the reversed Carnot cycle, inasmuch as the fall in temperature between the condenser and the refrigerator is not produced, nor is it attempted to be produced, by the adiabatic expansion of the agent, but results from the evaporation of a portion of the liquid itself. In other words, the liquid-refrigerating agent enters the refrigerator at the condenser temperature and introduces heat which has to be taken up by the evaporating liquid before any useful refrigerating effect can be performed. The extent of this loss is determined by the relation between the liquid heat and the latent heat of See also:vaporization at the refrigerator temperature. If r represents the latent heat of the vapour, and q2 and qi the amounts of heat contained in the liquid at the respective temperatures of T2 and Ti, then the loss from the heat carried from the condenser into the refrigerator is shown by (q2-gi)/r and the useful refrigerating effect produced in the refrigerator is r-(q2-qi). Assuming, as in the previous example, that T2 is 75° F., and that Ti is 5° below zero, the results for various refrigerating agents are as follows:- Latent Liquid See also:Net Proportion Heat. Heat. Refrigeration. of Loss. r g2-gi r-(q2-qi) (g2-si)/r Anhydrous ammonia 590'33 72'556 517.774 0.1225 Sulphurous acid . 173'13 29.062 144.068 o•i68 Carbonic acid . . 119.85 47.35 72.50 0.395 The results show that the loss is least in the case of anhydrous ammonia and greatest in the case of carbonic acid. At higher See also:con-denser temperatures the results are even much more favourable to ammonia. As the See also:critical temperature (88.4° F.) of carbonic acid is approached, the value of r becomes less and less and the refrigerating effect is much reduced. When the critical point is reached the value of r disappears altogether, and a carbonic-acid machine is then dependent for its refrigerating effect on the reduction in temperature produced by the See also:internal work performed in expanding the gaseous carbonic acid from the condenser pressure to that in the refrigerator. The See also:abstraction of heat does not then take See also:place at constant temperature. The expanded vapour enters the refrigerator at a temperature below that of the substance to be cooled, and whatever cooling effect is produced is brought about by the superheating of the vapour, the result being that above the critical point of carbonic acid the difference T2-T2 is in-creased and the efficiency of the machine is reduced. The critical temperature of anhydrous ammonia is about 266° F., which is never approached in the ordinary working of refrigerating machines. Some of the principal physical properties of sulphurous acid, anhydrous ammonia, and carbonic acid are given in Tables III., IV. and V. t Vapour-tension q r u Volume of Temp. of in Pounds per Heat of Liquid Latent Heat of one Pound Ebullition. sq. in. from 32° Fahr. Evaporation. of Saturated Degs. Fahr. Absolute. B.T.U. B.T.U. Vapour. Cub. ft. -22 5.546 -19.55 176.98 13.168 -13 7'252 -16'31 174'94 10.268 - 4 9.303 - 13.05 172.91 8.122 5 1I.8o3 - 9.79 170.82 6.504 14 14.789 - 6.85 168.75 5.254 23 18.544 - 3.26 166.63 4'293 32 22.468 0.00 164'47 3'540 41 27'445 3'27 162.39 2.931 50 33'275 6.55 160.24 2'451 59 39.958 9.83 158.08 2.066 68 47637 13.10 155.89 1'746 . 77 56.311 16.38 153.67 1'490 86 66.407 19.69 151'49 1.266 95 77.641 22.99 149.27 1.089 104 90.297 26.28 147.02 0.913 Vapour (NH3). t Vapour-tension q r u Volume of Temp. of in Pounds per Heat of Liquid Latent Heat of one Pound Ebullition. sq. in. from 32° Fahr. Evaporation. of Saturated Degs. Fahr. Absolute. B.T.U. B.T.U. Vapour. Cub. ft. -40 10.238 -60.048 600.00 25.630 -31 13'324 -53.064 597'24 20.120 - 22 16.920 - 45.918 595.08 15.971 -13 21.472 -38.646 593'00 12'783 - 4 27.000 -31.212 590.00 10.316 5 33.701 -23.634 586.82 8.394 14 41.522 - 15.894 581.00 6.888 23 50.908 - 8.028 576.00 5.703 32 61.857 0.000 571.00 4'742 41 74'513 8.172 562.50 3'973 50 89.159 16.506 555.48 3.364 59 105.939 24.966 550.00 2.851 68 124'994 33.588 541'00 2.435 77 146.908 42'354 531.00 2.098 86 170.782 51.282 523.00 1.810 95 197.800 60.336 512.50 1.570 104 227.662 69.552 501.50 1.361 Vapour (See also:CO2) . Temp. of Vapour-tension q r u in Pounds per Heat of Liquid Latent Heat of Volume of one Pound Ebullition. sq. in. from 32° Fahr. Evaporation. of Saturated Degs. Fahr. Absolute. B.T.U. B.T.U. Vapour. Cub ft. -22 213.345 -24.80 126.72 '4330 -13 248.903 -21.06 123.25 .3670 - 4 288.727 -17.19 11943 .3130 5 334'240 -13.17 115.25 •2680 14 385'443 - 9.00 110.65 '2295 23 440'913 - 4'63 105'53 '1955 32 503'497 0.00 99.81 .1670 41 573.187 4'93 93.35 '1430 50 649'991 10.28 85.93 •1202 59 733'906 16.22 7740 •1020 68 826.356 23.08 66.47 '0833 77 930.184 31.63 51.80 '0673 86 1039.701 45'45 27.00 .0481 87.8 1062i458 51.61 15.12 0416 88'43 1070.991 59.24 0.00 •0J52 The See also:action of a vapour compression machine is shown in fig. 3. Liquid at the condenser temperature being introduced into the refrigerator through the regulating valve, a small portion evaporates and reduces the remaining liquid to the temperature T1. This is shown by the See also:curve AB, and is the useless work represented by the expression (q2-qi)/r. Evaporation then continues at the constant temperature T, abstracting heat from the substance outside the refrigerator as shown by the See also:line BC. The vapour is then compressed along the line CD,to the temperature T2, when, by the action of the cooling water in the condenser, heat is abstracted at constant temperature and the vapour condensed along the line DA. In a compression machine the refrigerator is usually a series of See also:iron or See also:steel coils surrounded by the air, brine or other substance it 23 is desired to cool. One end (generally the bottom) of the coils is connected to the liquid pipe from the condenser and the other end a r--ten to the suction of the compressor. Liquid from the condenser is ad- mitted to the coils through an ad- justable regulating valve, and by taking heat from the substance out- side is evaporated, the vapour being continually drawn off by the See also:corn-FIG. 3.—Action of Vapour pressor and discharged under increased Compression Machine. pressure into the condenser. The condenser is constructed of coils like the refrigerator, the cooling water being contained in a tank; frequently, however, a series of open coils is employed, the cooling water falling over the coils into a See also:collecting See also:tray below, and this form is perhaps the most convenient for ordinary use as it affords great facilities for inspection and See also:painting. The compressor may be driven by a steam engine or in any other convenient manner. The pressure in the condenser varies according to the temperature of the cooling water, and that in the refrigerator is dependent upon the temperature to which the outside substance is cooled. In an ammonia machine See also:copper and copper See also:alloys must be avoided, but for carbonic acid they are not objectionable. The compression of ammonia is sometimes carried out on what is known as the Linde or " wet " See also:system, and sometimes on the " dry " system. When wet compression is used the regulating valve is opened to such an extent that a little more liquid is passed than can be evaporated in the refrigerator. This liquid enters the compressor with the vapour, and is evaporated there, the heat taken up preventing the rise in temperature during compression which would otherwise take place. The compressed vapour is discharged at a temperature but little above that of the cooling water. With dry compression, vapour alone is drawn into the compressor, and the temperature rises to as much as 18o or 200 degrees. Wet compression theoretically is not quite so efficient as dry compression, but it possesses practical advantages in keeping the working parts of the compressor cool, and it also greatly facilitates the regulation of the liquid, and ensures the full duty of the machine being,continuously performed. Very exact See also:comparative trials have been made by Professor M. Schroeter and others with compression machines using sulphur dioxide and ammonia. The results are published in Vergleichende Versuche an Kditemaschinen, by Schroeter, Munich, 1890, and in Nos. 32 and 51 of Bayerisches Industrie and Gewerbeblatt, 1892. Some of the results obtained by Schroeter in 1893 with an ordinary brine cooling machine on the Linde ammonia system are given in Table VI.: Temperature reduction in refriger- 42.8 to 37.4 28.4 to 23 14 to 8.6 -0.4 to -5'8 ator. Degs. Fahr I.H.P. in steam cylinder . . 15'79 16.48 15.29 14.28 I.H.P. in compressor. 14'32 14'3 13'54 I='98 Pressure in refrigerator in pounds 43.2 32.6 19.8 9.9 per sq. in. above atmosphere . . Pressure in condenser in pounds '16 •0 115.0 IIo O Io8'o per sq. in. above atmosphere . Heat abstracted in refrigerator. 342192 263400 171515 121218 B.T.U. per hour Heat rejected in condenser. 377567 301200 214347 158594 B.T.U. per hour The principle of the absorption process is chemical or physical rather than mechanical; it depends on the fact that many Absorp- vapours of low boiling-point are readily absorbed in 'See also:ion water, and can be separated again by the application machines• of heat. In its simplest form an absorption machine consists of two iron vessels connected together by a See also:bent pipe. One of these contains a mixture of ammonia and water, which on the application of heat gives off a mixed vapour containing a large proportion of ammonia, a liquid containing but little ammonia being See also:left behind. In the second vessel, which is placed in cold water, the vapour See also:rich in ammonia is condensed under pressure. To produce refrigeration the operation is reversed. On allowing the weak liquor to cool to normal temperature, it becomes greedy of ammonia (at 60° F. at atmospheric pressure water will absorb about 16o times its own volume of ammonia vapour), and this produces an evaporation from the liquid in the vessel previously used as a condenser. This liquid, containing a large proportion of ammonia, gives off vapour at a low temperature, and therefore becomes a refrigerator abstracting heat from water or any surrounding body. When the ammonia is evaporated the operation as described must be again commenced. Such an apparatus is not much used now. Larger and more elaborate machines were made by F. P. E. Additional information and CommentsThere are no comments yet for this article.
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