26 



BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 



sulphur dioxid is below the atmospheric, any leakage of air will be 

 into the system and will cause corrosion of the metal by forming 

 sulphurous acid. The low pressures required in using sulphur di- 

 oxid as a refrigerant in connection with its large specific volume 

 makes a large and cumbersome machine necessary. The ratios of 

 the volumes of the cylinders necessary for a given capacity of ma- 

 chine, taking that of carbon dioxid as one, are approximately as 

 follows: Carbon dioxid 1, ammonia 4.4, sulphur dioxid 13. 



If either liquid ammonia, carbon dioxid, or sulphur dioxid is placed 

 in a test tube, as shown in figure 10, it will boil under atmospheric 



pressure below the orcli- 

 s.Z* nary temperature of the 

 smTounding air, and the 

 heat of evaporation will 

 pass from the surround- 

 ing air directly into the 

 refrigerant. The air, 

 therefore, will be refrig- 

 erated or cooled, due to 

 the fact that its heat is 

 taken up by the boiling 

 liquid. In other words, 

 the heat required for 

 the evaporation of the 

 liquid is extracted from 

 the air. At the boiling 

 point the refrigerant will 

 absorb a definite amount 

 of heat from the air to 

 effect the vaporization 

 of a definite amount of 

 the liquid, the heat 

 being absorbed directly 

 through the walls of the 

 test tube from the outside air. There will be a circulation of air 

 around the test tube, as indicated by the arrows in the figure, due to 

 the greater weight of the cooled film of air lying next the surface of 

 the tube which flows down and away from the bottom of the vessel, 

 allowing the warmer and therefore lighter air to rise and take its 

 place at the top. Figure 10 embodies the principle of the direct- 

 expansion system of refrigeration explained in detail later. 



If the test tube containing the refrigerant is immersed in a second 

 vessel containing a solution of brine, as shown in figure 11, the 

 evaporating liquid will absorb heat from the surrounding medium 

 just as in the preceding case, but in this case the surrounding medium 

 is brine instead of air and the heat required to effect the vaporization 



Fig. 10. 



