Mabch 2, 1891.] 



KNOWLEDGE. 



57 



refrigeration has rapidly increased in importance as new 

 ajjplications have been perceived, and as further improve- 

 ments in machinery have lieen effected. The subject has 

 engaged the attention of many able engineers, and some 

 three hundred jjatents have been taken out in connection 

 with it. At the present time the production of low tem- 

 peratures plays an important part, not only in the meat 

 trade, but for the preservation of other perishable articles 

 of food, as fish, eggs, and butter, in the brewing industry, 

 and in the production of ice. New applications are being 

 found every day, among which may be instanced the 

 preparation of preserved fruits and similar processes, where 

 crystallization from solution has to be effected. The im- 

 portant problem of the cooling of theatres is engaging 

 attention at the present time, and will no doubt soon 

 receive a satisfactory solution. 



The principles involved in refrigeration present many 

 interesting features which are scarcely touched upon in the 

 ordinary works on heat. For the production of hiifh 

 temperatures it is usual to employ the force of chemical 

 affinity ; chemical combination in the process of combus- 

 tion being attended with an evolution of heat. On the 

 other hand, many chemical compounds are formed with 

 alisurption of heat ; but these can only be produced in an 

 indirect manner, and their formation cannot be employed 

 for the production of low temperatures. 



It is a curious circumstance that in oixler to understand 

 the rationidc of the refrigerating process it is necessary to 

 consider, in the first place, the means of attaining high 

 temperatures and the working of heat engines. By the 

 burning of fuel in the furnace steam is produced in the 

 boiler of a steam-engine, and by the changes of volume of 

 the irorkimi xuhsttinci' (steam) a portion of the energy of 

 heat is transformed into mechanical power. In other 

 words, the energy which formerly consisted in that motion 

 of minute particles which constitutes heat, is in the steam- 

 engine converted into the form of energy which consists 

 in the motion o[ a large mass of matter (<'.//• of the piston 

 or the fly-wheel of the engine). Briefly, in heat engines, of 

 which steam-engines form one class, a calorific efi'ect is 

 converted into mechanical power. In refrigerating 

 machinery, on the other hand, mechanical power is so 

 employed as to yield a calorific effect ; but in this case the 

 calorific efiect is negatire, and the final result is the pro- 

 duction of a low temperature. The refrigerating machine 

 is not in itself a complete apparatus, since it requires to 

 be diiirn by a steam-engine. In order, therefore, to 

 attain logical precision in our view of the process of the 

 artificial production of cold, it is necessary to consider as 

 one complete system the combination of the steam-engine 

 and the freezing machine. In this dual arrangement we 

 start with the production of a higli temperature in a fur- 

 nace, and finally attain a very low temperature in the 

 freezing chamber. 



The working parts of the freezing machine are very 

 similar to those of the steam-engine. In both tliero is a 

 system of cylinders, pistons, and valves, and a working 

 substance which undergoes alternately compression and 

 expansion. In the Bell-Coleman machines the working 

 substance is air. The process begins with the compression 

 of air by the stroke of a piston m the roiiijire.ssidii ri/lindcr. 

 The power which drives this piston is obtained directly 

 from the piston of the steam-engine. The compression 

 cylinder is surrounded by a jacket in which cold water 

 constantly circulates. The heat generated by the com- 

 pression of the air is almost entirely taken up by the cold 

 water. Thus we obtain air very little above the ordinary 

 temperature, but under a high pressure. When the 

 pressure is released the air expands. If the expansion be 



allowed to take place into a vacuum, then — as Joule first 

 proved — no change of temperatui-e takes place. But if 

 the expansion takes place under such conditions that 

 mechanical power is developed, the mechanical work is 

 done at the expense of the heat of the expanding air, 

 which consequently is chilled. This is what actually takes 

 place in the exjumxion cylinder. The air, in expanding 

 drives a piston which is connected with the cylinder of the 

 steam-engine in such a way that it aids the back stroke 

 of the piston in the steam cylinder. Thus the frigorific 

 efiect is obtained in the refrigerating machine by an action 

 wliich lightens the work of the driving engine. By means 

 of this expansion the air is readily cooled to — ■50'^ Fahr., or, 

 if desired, to a still lower temperature. It was here that 

 a great practical chfficulty came in. Atmospheric air con- 

 tains water- vapour, and at such low temperatures this was 

 deposited m the form of hoar-frost. This fi'ost or snow 

 choked the valves and otherwise hindered the working of 

 the machine. It was not found practicable to remove the 

 moisture entirely before the admission of the ak to the 

 machine ; and till the invention of Mr. Coleman the snow 

 diflSculty appeared to condemn the use of air as the work- 

 ing substance. This difficulty was overcome by the device 

 of allowing a partial expansion of the air before it entered 

 the expansion cylinder. This preliminary partial expan- 

 sion is effected in sloping tubes placed in the reft-igerating 

 chamber itself. Under these conditions, the aqueous 

 vapour deposits not as snow but in a mist or ram, and the 

 moisture is run oft' by suitable taps placed at the bottom 

 of the sloping tubes. The air thus freed from moistm-e 

 enters the expansion cylinder to undergo the second and 

 greater expansion by which the principal part of the 

 frigorific efi'ect is obtained. 



The cold air fi-eezing machines are those employed on 

 board ship for the transport of meat from Australia, New 

 Zealand, and America, The meat is placed in large 

 chambers, the walls of which are double, the interspace 

 being filled with wood charcoal as a non-conducting 

 material. A jet of intensely cold air is delivered into the 

 chamber at each stroke of the piston of the expansion 

 cylinder, and the temperature of the chamber is thus kept 

 at or near the freezing point during the whole voyage. 



There is another important class of freezing machines, 

 of which the ammonia machines are the most important 

 type. In this second class the working substance is not 

 a permanent gas such as air, but a substance (such as 

 ammonia) capable of being condensed to a liquid by pres- 

 sure, even at the ordinary temperature of the atmosphere. 

 In these machines the frigorific effect is due in the first 

 place to the heat absorbed by the vaporization of the 

 liquefied substance ; and secondly, as in the air machines, 

 to expansion of the vapour, ^'olume for volume, the 

 working substance exercises a much greater cooling effect 

 in the ammonia machines than in the air machines. 

 Consequently the machinery is more compact and more 

 economical of fuel. Au important point of difterence 

 between these two types is that whereas the air machines 

 work with an oiien ci/clc, drawing in a fresh supply of 

 material at each stroke of the piston, the ammonia ma- 

 chines work in cloned ci/clc, the same working material 

 going through the same round of changes over and over 

 again. It will readily be perceived that this circumstance 

 necessitates very different arrangements in the freezing 

 chamber to those which have been described above, where 

 the working substance itself is delivered from the machine 

 and is the direct cooling agent. The refrigerating cham- 

 ber connected with an ammonia machine is generally 

 cooled by the circulation of a cold liquid in pipes, on a 

 system similar to that employed in heating by means of 



