Feb. 24, 1888.] 



SCIENTIFIC NEV\^S. 



187 



CIVIL AND MECHANICAL ENGINEERS' SOCIETY. 



At the meeting held on the ist inst., Mr. T. B. Lightfoot 

 read the following communication on " The Use of 

 Ammonia as a Refrigerating Agent." 



In the United States and in Germany, as well as to 

 some extent elsewhere, ammonia has been very gener- 

 ally employed for refrigerating purposes during the last 

 ten years. In this country its application has been 

 ■extremely limited, and even at the present time there 

 are but few ammonia machines successfully at work in 

 Great Britain. Up to a comparatively recent date the 

 ■only machine manufactured on a commercial scale was 

 the original Harrison's ether machine, first produced by 

 Siebe in the year 1857 — a machine costly both to make 

 and to work. In 1878 the desirability of supplementing 

 our meat supply by means of the large stocks in our 

 colonies led to the rapid development of the special class 

 of refrigerating apparatus commonly known as the dry 

 ■air refrigerator, specially designed for use on board ship, 

 where it was considered undesirable to employ chemical 

 refrigerants. Owing to their simplicity, and perhaps 

 also to their novelty, these cold air machines have very 

 frequently been applied on land, under circumstances in 

 which the same result could have been obtained with 

 much greater economy by the use of ammonia or some 

 ■other chemical agent. Recently, however, more atten- 

 tion has been directed to the question of economy, and 

 consideration is being given to the applicability of certain 

 machines to special purposes, with the result that am- 

 monia promises to become largely adopted. 



In all cases where a liquid is employed the refrigerat- 

 ing action is produced by the change in physical state 

 from the liquid to the vaporous form. It is well known 

 that such a change can only be brought about by the 

 acquirement of heat ; and for the purpose of refrigeration 

 that liquid is the best which has the highest heat of 

 vaporisation. In fact, however, liquids vary, not only 

 in the amount of heat required to vaporise them, but 

 also in the conditions under which such change can be 

 effected. The boiling point of anhydrous ammonia is 

 372 degs. below zero Fahr. at atmospheric pressure, and 

 therefore for all ordinary cooling purposes its evapora- 

 tion can take place at pressures considerably above that 

 of our atmosphere. 



The considerations which chiefly influence the selec- 

 tion of a liquid refrigerating agent are : — 



1. The amount of heat required to effect the change 

 from the liquid to the vapourous state, commonly called 

 the latent heat of vapourisation. 



2. The temperatures and pressures at which such 

 change can be effected. 



This latter attribute is of twofold importance : for, in 

 order to avoid the renewal of the agent, it is necessary to 

 deprive it of the heat acquired during vapourisation 

 under such conditions as will cause it to assume the 

 liquid form, and thus become again available for re- 

 frigeration. As this rejection of heat can only take place 

 if the temperature of the vapour is somewhat above that 

 of the cooling body which receives the heat, and which, 

 for obvious reasons, is in all cases water, the liquefying 

 pressure at the temperature of the cooling water, and the 

 facility with which this pressure can be reached and 

 maintained, are of great importance in the practical 

 working of any refrigerating apparatus. 



Ammonia in its anhydrous form is a liquid having at 

 normal pressure a latent heat of vapourisation of 900, 



and a boiling point at the same pressure of 37I 

 degs. below zero Fahr. In the use of ammonia two 

 distinct systems are employed. So far as the mere 

 evaporating or refrigerating part of the process is con- 

 cerned, it is the same in both. The object is to 

 evaporate the liquid anhydrous ammonia at such tension 

 and in such quantity as will produce the required cooling 

 effect. 



The Absorption Process. — The principle employed in 

 this process is physical, rather than mechanical. Ordinary 

 ammonia liquor of commerce, containing about 38 per 

 cent, by weight of pure ammonia and 62 per cent, of 

 v/ater, is introduced into a vessel named the generator. 

 This vessel is heated by means of steam circulating 

 through coils of iron piping, and a mixed vapour of 

 ammonia and water is driven off. This mixed vapour is 

 then passed into a second vessel, in order to be subjected 

 to the cooling action of water ; and here, owing to the 

 difference between, the boiling points of water and 

 ammonia, fractional condensation takes place, the bulk of 

 the water, which condenses first, being caught and run 

 back to the generator, while the ammonia in a nearly 

 anhydrous state is condensed and collected in the lower 

 part of the vessel. 



This process of fractional condensation is due to Rees 

 Reece, and forms an important feature in the modern 

 absorption machine. In the improved form of apparatus, 

 ammonia is obtained in a nearly anhydrous condition, 

 and in this state passes on to the refrigerator. In this 

 vessel, which is in communication with another vessel 

 called the absorber, containing cold water or very weak 

 ammonia liquor, evaporation takes place, owing to the 

 readiness with which cold water or weak liquor absorbs 

 the ammonia, water at 59 degs. Fahr. absorbing 727 

 times its volume of ammonia vapour. The heat necessary 

 to effect this vapourisation is abstracted from brine or 

 other liquid, which is circulated through the refrigerator 

 by means of a pump. Owing to the absorption of 

 ammonia, the weak Hquor in the absorber becomes 

 strengthened, and it is then pumped back into the 

 generating vessel to be again dealt with as above described. 



In the refrigerator, the liquid ammonia in becoming 

 vapourised will take up the precise quantity of heat 

 given off during its cooling and liquefaction in the con- 

 denser, plus the amount due to the difference in heat of 

 vapourisation, owing to the lower pressure at which 

 the change of state takes place in the refrigerator, and 

 less the small amount due to the difference in temper- 

 ature between the vapour entering the condenser and that 

 leaving the refrigerator, less also the amount necessary 

 to cool the liquid ammonia to the refrigerator tempe- 

 ature. When the vapour enters into solution with the 

 weak liquor in the absorber, the heat taken up in the 

 refrigerator is imparted to the cooling water, subject 

 also to corrections for differences of pressure and tem- 

 perature. 



With regard to the amount of heat used, it will have 

 been seen that the whole of that required to vapourise 

 the ammonia, and whatever water vapour passes off 

 from the generator, has to, be supplied from without. ' 



Compression Process. — In this process ammonia is 

 used in its anhydrous form. So far as the action of the 

 refrigerator is concerned, it is precisely the same as in 

 the case of the absorption apparatus, but instead of the 

 vapour being hquefied by absorption by water, it is 

 drawn off from the refrigerator by a pump, by means of 

 which it is compressed and delivered into the condenser 



