There are 3 physical methods by which temperature may be lowered and ice formed : - (!) By solution of solids; (2) by evaporation of liquids; (3) by expansion of gases. Refrigerating machines may be broadly divided into 2 classes: those which depend on the suitable compression and expansion of air, and those which depend on the condensation and evaporation of a liquid. These two classes correspond with hot-air engines and steam engines respectively, amongst the instruments which convert heat into mechanical work. Each has its peculiar difficulties to contend with, and its own adaptation to particular purposes.

The 3 methods before mentioned will now be described.

1. By Solution of Solid!. - Heat is absorbed in bringing solids to the liquid condition, and the cold thus produced may prove sufficient to convert water into ice. The salts commonly employed for this purpose are termed "freezing-mixtures." They are chiefly as given on the opposite page.

The best known of the numerous freezing-mixtures that have been hitherto described consists of 3 parts ice, and 1 of ordinary salt. Dissolving concurrently these two substances give a tempe-ratureof - 5-4° F.(21°C),the freezing-point of the solution. The melting of only a part of the mixture is sufficient to produce this temperature throughout the mass; and with constant admission of heat, and stirring, the low temperature is maintained till the whole is dissolved. The freezing apparatus of confectioners is well known: a tin pot containing cream, 1 wooden or metallic vessel enclosing the pot, and the interval filled with ice and salt, which is frequently stirred, that the ice may not sink to the bottom. In a Paris machine, for home use, the agitation of the freezing-mixture is maintained by rotation of the double cylinder containing it and the cream vessel round an axis at right angles to the cylinder's length. Meidinger has constructed a machine based on the observation that a solution of ordinary salt under 32° F. (0° C.) also fuses ice, and, so long as its concentration is maintained, produces the same low temperature as the mixture.

Cooling Air Part 10 40022

Fig.4

Mixtures

Thermometer sinks: o F.

Actual Reduction of Temperature; o F.

(1)

2 parts snow or pounded ice,l sodium chloride

from any temperature

o o to -5

o

• •

(2)

5 parts snow or pounded ice, 2 sodium" chloride, 1 ammonium chloride .. ..

.. to -12

• •

(3)

24 parts snow or pounded ice, 10 sodium chloride, 5 ammonium chloride, 5 potassium nitrate........................

.. to -18

• •

(4)

12 parts snow or pounded ice, 5 sodium chloride, 5 ammonium nitrate......

.. to -25

• •

(5)

3 parts sodium phosphate, 2 ammonium nitrate, 4 diluted mixed acids......

from

- 34 to -50

16

(6)

8 parts snow, 10 dilute sulphuric acid ..........

"

-68 to -91

23

(7)

1 part snow, 3 crystallised calcium chloride

"

-40 to -73

33

(8)

5 parts sodium phosphate, 3 ammonium nitrate, 4 dilute nitric acid.....................

"

0 to -34

34

(9)

1 part ammonium nitrate, 1 water ...........

"

40 to 4

36

(10)

5 parts ammonium chloride, 5 potassium nitrate, 16 water ..........................

"

50 to 10

40

(11)

1 part snow, 1 dilute sulphuric acid .............

"

-20 to -60

40

(12)

3 parts snow, 2 dilute nitric acid ..............

"

0 to -46

46

(13)

8 parts snow, 3 dilute sulphuric acid, 3 dilute nitric acid ,..............

"

-10 to -56

46

(14)

5 parts ammonium chloride, 5 potassium nitrate, 8 sodium sulphate, 16 water..........

"

50 to 4

46

(15)

5 parts sodium sulphate, 4 dilute sulphuric acid ..........................

"

50 to 3

47

(16)

3 parts sodium nitrate, 2 dilute nitric acid

"

50 to -3

53

(17)

2 parts snow, 3 calcium chloride

"

15 to -68

53

(18)

3 parts snow, 2 dilute sulphuric acid

"

32 to -23

55

(19)

1 part ammonium nitrate, 1 sodium carbonate, 1 water.................

"

50 to -7

57

(20)

8 parts snow, 5 hydrochloric acid..............

"

32 to -27

59

(21)

6 parts sodium sulphate, 4 ammonium chloride, 2 potassium nitrate, 4 dilute nitric acid.................

"

50 to -10

60

(22)

9 parts sodium phosphate, 4 dilute nitric acid....................

"

50 to -12

62

(23)

7 parts snow, 4 dilute nitric acid

"

32 to -30

62

(24)

1 part snow, 2 crystallised calcium chloride

"

0 to -66

66

(25)

3 parts snow, 4 calcium chloride

"

20 to -48

68

(26)

4 parts snow, 5 calcium chloride

"

32 to -40

72

(27)

2 parts snow, 3 crystallised calcium chloride

"

32 to -50

82

(28)

3 parts snow, 4 potash ..........................

"

32 to -51

83

(29)

6 parts sodium sulphate, 5 ammonium nitrate, 4 dilute nitric acid .................

"

50 to -40

90

Mixture.

Decrease of temperature.

Specific heat of the solution.

Volume weight of solution.

Loss of heat units.

To use for 120 c

1 kilo. Mixture.

1 litre Mixture.

Salt kilo.

Water kilo.

Costs.

1 ordinary salt 3 ice .. ..

C. 21°

0.83

1.18

125

100

0.5

1.5

0.34 to 0.12

3 cryst. Glauber salt, 2 coned. muriatic acid

37°

0 74

1.31

55

74

2.7

1.8

1.0 to 0.6

2 ammonia ni-trate, 1 sal-ammoniac, 3 water ..

30°

0.70

1.20

42

51

3

3

7.6 to 6.8

3 sal-ammoniac 2 saltpetre, 10 water .

26°

0.76

1.15

40

46

2.1

4.2

2.6to2'2

3 sal-ammoniac, 2 saltpetre, 4 cryst. Glau-ber salt, 9 water .. ..

32°

0.72

1.22

50

61

2.5

2.5

1.8 to 1.6

Salt mixtures give much greater lowering of temperature than simple salts, as they dissolve in much less water. Thus, 1 part sal-ammoniac is dissolved in 3 parts water, and lowers the temperature about 19° C.; saltpetre dissolves in 6 parts water, and lowers the temperature about 11° C. (Compare the fourth and fifth on the of salt and ice. He provides a sieve-like vessel, containing salt, to maintain the concentration as the ice melts. The lowering of temperature is uniform throughout the vessel, and no stirring is required. The machine has come largely into use in perfumery.

On the basis of his own experiments, Meidinger has formed a table showing the respective merits of various freezing-mixtures. The following extract contains the most serviceable: list.) It will be seen that the salt-ice mixture proves considerably more energetic and cheaper than any of the others so far as use of the materials only once is concerned. The second mixture, too, cannot be restored; nor can the last, easily, on account of the crystallised Glauber salt. Both are comparatively cheap, however. The mixture, in which, by vaporisation of the solution, the salt is easily renewed in its original condition, ammonia nitrate and sal-ammoniac, is so costly at the first, that it would not do to use it only once. This was the mixture employed in an apparatus first exhibited by Charles at the Paris Exhibition in 1B67. The tin vessel containing the substance to be frozen is enclosed in a large wooden vessel containing the freezing-mixture, and is furnished with screw wings, which stir the mixture as the vessel is rotated. Another form is that of Toselli's glaciere Italiennc roulante. The in a conical-shaped vessel suspended in the freezing-mixture, and the outer vessel, enveloped in cloth, is rolled to and fro on the table.

None of these machines has found very extensive use-Large masses have to be operated with to obtain even small results; and the sum of operations must generally prove too troublesome in a private house.

As to the question of manufacturing Ice on a large scale by means of solution of salt, Meidinger comes to the conclusion that by means of 1 lb. of coal (for restitution of salt used) not more than 2 lb. of ice can be prepared; not to speak of the machine force required for transport of the large quantity of liquid. This is very unfavourable; an ammonia machine will give * or 5 times better results. Much improvement is, in the circumstances, hardly to be looked for. It would be necessary to find a salt that, in dissolving, gave a much greater lowering temperature than the mixtures known, and this cannot be expected, since all the known salts have been examined in reference to this point. The real cause of the small productions of such apparatus lies in the fact that restitution of the salt is effected only by change of aggregation (vaporisation), and this involves large expenditure of heat. It may be mentioned that according to experiments by Rudorff, on cold produced by solution of 20 different salts, the two which gave the greatest lowering of temperature were sulphuretted ammonium cyanide, and sulphuretted potassium cyanide - 105 parts of the former, dissolved in 100 parts water, produce a lowering of tempera-tore of 31.2° C; and 130 parts of the latter, in 100 parts of water, as much as 34.5° C.

In making ice cream without machinery, it is always found necessary, after the freezing begins, to beat the cream with a paddle by hand. This facilitates freezing, and at the same time secures a smooth and uniform congelation. In machinery for freezing cream on a large scale, it is desirable that this beating be done Automatically, and the closer the action of the paddle imitates the movement imparted by the hand, the better. In the apparatus shown in Fig. 18, this is accomplished by simple mechanism; at the sume time, there is improved machinery for rotating, and scraping the interior of the freezing can, the whole being so constructed that a large quantity of ice cream of excellent quality may be quickly produced by a small expenditure of power. The machine consists of ice tub, can, scrapers to remove the cream from the sides as it freezes, the paddle, and the lid. The tin scrapers, attached at a, are bent to conform to the shape of the can, so as not to bear hard on the metal and thus scrape off the tin. The paddle 6 is a bar of galvanised iron, having a tin blade protected by a wooden point. The lid is of iron or tin, with apertures at the flange, so that it may be placed over the scraper supports.