The action of a vapour compression machine is shown

consists of a coil or series of coils, is connected to the suction 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 medium, which, as previously stated, is generally water. The refrigerating liquid (ether, 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 Tl. The vapours thus generated are drawn into the pump, compressed, and discharged into the condenser at the temperature T2, which is somewhat 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 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 exchange of heat, would be about 5° below zero, and the dischar e temperature T would be about 75°. An ideal machine, working between 5° below zero and 75° above, has a coefficients 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 vaporization at the refrigerator temperature. If r represents the latent heat of the vapour, and qg and ql the amounts of heat contained in the liquid at the respective temperatures of T2 and Tl, then the loss from the heat carried from the condenser into the refrigerator is shown by (q¢-gl)/r and the useful refrigerating effect produced in the refrigerator is r-(qz-gl). Assuming, as in the previous example, that T2 is 75° F., and that T1 is 5° below zero, the results for various refrigerating agents are as follows:- TABLE II.

A table should appear at this position in the text. See Help:Table for formatting instructions.

Latent Liquid Net ' Proportion

Heat. Heat. Refrigeration. -of Loss. f az-qi 1-(qz-ai) (ar qi) /1

Anhydrous ammonia 590-33 72-556 517-774 0-1225 Sulphurous acid 173- 13 29-062 144-068 0- 168 Carbonic acid 3 119-85 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 condenser temperatures the results are even much more favourable to ammonia. As the 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 internal work performed in expanding the gaseous carbonic acid from the condenser pressure to that in the refrigerator. The abstraction of heat does not then take 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 super heating of the vapour, the result bein that above the critical point of carbonic acid the difference T2-T2 is increased 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.

32 REFRIGERATING

TABLE III.-Ledoux's Table for Saturated Sulphur Dioxide Vapour (SO2).

7

1 Vapour-tension '1 ' Voluzfne 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 '-I3'05 172-91 8-122

5 11-803 - 9-79 170-82 6-504

14 14'789 6'85 15375 5'254

23 18-544 - 3-26 166'63 4-293

32 22-468 0-00 164-47 3-540

41 27°445 3'27 152'39 2'931

50 33'275 6'55 160'24 2 451

59 39-958 9-83 158'O8 2-066

68 47-637 I3'I0' 155-89 1 746

77 56'311 1533 153'57 1 490

86 66-407 19-69 151-49 1-266

95 77'541 22'99 149'27, P039

104 90-297 26-28 147-02 0-913

TABLE IV.-MolIier's Table for Saturated Anhydrous Ammonia Vapour (NH3).

l

t Vapour-tension 4 " Voluzie of

Temp of in Pounds per Heat of Liquid Latent Heat of one Pound Ebuuitiom sq. in. from 32° Fahr. Evaporation. of Saturated DegS Fahn Absolute. B.T.U. B.T.U. é3%0L;I'. u r.

-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 -38646 593'00 12733

- 4 27'00O -31-212 590-00 10-316

5 33'701 -23'634 53582 3'394

I4 41-522 -15-894 581-00 6-888

23 50-908 - 8-028 576-oo 5-703

32 61-857 0-000 571-00 4-742

41 74'513 8'172 562~5<> 3'973

50 89'159 15'506 55543 3'364

59 105'939 24966 550'00 2°851

68 124'994 33533 541'00 2'435

77 145908 42°354 531'00 2093

86 170-782 51-282 523-00 1-810

95 197-800 60-336 512-50 I-570

104 227-662 69-552 501-50 1 361

TABLE V.¥-Mollier's Table for Saturated Carbon Dioxide Vapour (C02).

1 Vapour-tension 9 l V Volugle of

Temp. of in Pounds per Heat of Liquid Latent Heat of one Pound Ebullitxon. 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 '433O

-13 248-903 -21-06 123-25 -3670

4 288-727 -17'19 119'43 '313°

5 334-240 -I3-I7 115-25 '268O

14 335443 “ 9'00 110'65 '2295

23 440'913 " 4'53 105'53 '1955

32 503-497 0-00 99-81 -1670

41 573437 4'93 93'3S '1430

50 649-991 - 10-28 85-93 '1202

59 733-906 16-22 77-40 'IOIO

68 826-356 V. 23-08 66-47 -0833

77 930'184 31'63 51'80 '0673

| 86 1039-701 45-45 27-00 -0481

87-8 IO62n458 51-61 15-12 -0416

AA 88-43 1070-991 59-24 0-00 -0352

in fig.

Liquid at the condenser temperature being introduced into the re frig era tor through the regulating valve, a small portion evaporates and reduces the remaining liquid to the temperature Tl. This is shown by the curve AB, and is the useless work represented by the expression (gg-gl)/r. Evaporation then continues at the constant temperature T, abstracting heat from the substance outside the refrigerator as shown by the 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 iron of steel coils surrounded by the air, brine or other substance it 23