MANUFACTURED ICE AXD FREEZING MIXTURES 167 



cylinder of the compressor, B, Fig. 124. The piston then 

 forces it through the upper valve into the pipe leading to the 

 cooling coils, C. As the ammonia is compressed, it becomes 

 very much heated, and, although the gas is subjected to a 

 pressure of 140 Ib. or more per sq. in., it still retains its gaseous 

 form. The cooling coils are kept cool, however, by being 

 constantly drenched with cool water. As the heated am- 

 monia gas passes through these coils, it is cooled. Referring 

 again to the table given in the last article, we see that the 

 ammonia will liquefy under a pressure of 140 Ib. as soon as 

 the temperature drops to about 75F. Now the cooling coils 

 can be kept at a temperature as low as 75F. by drenching them 

 with water from the city mains or pumped from wells. 



It should be carefully noted that the cool water carries 

 away two distinctly different portions of heat from the am- 

 monia. First, the highly heated ammonia gas is cooled down 

 to the temperature of the water, that is, the water carries 

 away the SENSIBLE HEAT (Art. 149). Second, when the 

 ammonia changes from the gaseous form to the liquid form, it 

 gives up exactly as much heat as it absorbs- when it changes 

 from the liquid form to the gaseous form. The heat given off 

 when a gas liquefies is called HEAT OF CONDENSATION. This 

 heat of condensation exactly equals the heat of vaporization. 



The liquid ammonia accumulates in the reservoir (D, Fig. 

 124). It is now at about 70F. and under about 140 Ib. of 

 pressure. When the liquid ammonia has accumulated in 

 the reservoir to the desired amount, as shown by the glass 

 gauge, the valve at A is closed and no more ammonia is fed 

 into the system. The valve, E, is now opened and the am- 

 monia is permitted to flow into the coil of pipes submerged 

 in the brine. Being released from pressure, as soon as it 

 escapes through the valve, E, the ammonia begins to evaporate 

 rapidly, even boil. This means that it takes up heat of 

 vaporization. It receives this heat from the brine. The brine 

 consequently drops in temperature. When cans of pure 

 water are placed in this brine, as G, Fig. 124, the brine absorbs 



