22 CIRCULAR 7 4 0, U. S. DEPARTMENT OF AGRICULTURE 



Table 2. — Capacity and power data for typical 2-cylinder ammonia compressors 



Cylinder size 

 (inches) 



Displace- 

 ment per 

 revolution 



Speed 



Typical refrigerating capac- 

 ity and power require- 

 ments at 155 -pound 

 condenser pressure and 

 20-pound suction pressure 





Capacity 



Power 



3x3 



Cubic foot 

 0.024 

 .058 

 .113 

 .196 

 .249 

 .383 

 .465 

 .662 

 .909 



F. p. m. 

 400 

 375 

 360 

 360 

 360 

 360 

 360 

 300 

 300 



Tons 



2.1 

 4.7 

 8.9 

 15.6 

 20.0 

 31.0 

 39.0 

 48.0 

 67.0 



Hp. 



3 5 



4x4 - _._ ... 



7 1 



5x5 



6x6 



6V£x6M - 



7y 2 x7V 2 



8x8 



13.4 

 21.8 

 28.0 

 43.0 

 53.0 



9x9 



63.0 



10x10 



87.0 







EVAPORATOR 



The evaporator, or cooling coil, absorbs the heat from the room. 

 The ammonia, having had its load of heat removed in the condenser, 

 is expanded to a vapor. This expansion, or evaporation, under low 

 pressure, reduces the ammonia temperature to such a point that it is 

 ready to pick up more heat from the cold room. This is done by 

 direct expansion coils in the room or by air circulated from the room 

 to a bank of coils or finned surfaces. Here, as in the condenser, con- 

 ditions should be such as to permit the heat to flow with as little tem- 

 perature difference as possible between the ammonia and the air in the 

 room. If there is not sufficient cooling surface, if the surface is cov- 

 ered with frost, or if other factors retard the heat flow, the ammonia 

 would have to be extremely cold. This would mean a low suction 

 pressure, which reduces the capacity of the compressor. At low pres- 

 sures ammonia gas is less dense, and the smaller quantity of gas drawn 

 into the compressor at each stroke results in lower refrigerating 

 capacity. 



That the capacity of a typical compressor is increased markedly 

 as the suction pressure is raised is shown graphically in figure 3. 

 For example, at 140-pound head pressure and at a suction pressure of 

 24 pounds the compressor delivers 9 tons. An increase of 4 pounds 

 in suction pressure changes the capacity of the same machine to 10 

 tons. If by increased cooling surface or careful operation the pres- 

 sure could be increased to 36 pounds, about 12 tons of refrigeration 

 would be obtained, a gain of 33 percent. Similar changes in suction 

 pressure in an ammonia machine of any size would result in approx- 

 imately the same percentage increase in capacity. 



Another disadvantage of operating at low suction pressures is 

 that the coils are extremely cold and a large quantity of moisture 

 is condensed out of the air, resulting in low storage humidity. Ample 

 evaporator coil surface will permit the cooling to be done without 

 excessively cooling the air that touches the coils. The results of 

 cooling the air to low temperatures are shown in table 3. 



