38 
BULLETIN 
U. S. DEPARTMENT OF AGRICULTURE. 
To start. — (1) Turn the water on the condenser and water jacket, 
and open the discharge valve on the compressor; (2) start the 
machine slowly at first and after the machine is in motion and the 
pressures are at proper point open the suction valve on the com- 
pressor gradually; (3) open the expansion valve sufficiently to give 
the required back pressure and the machine is in full operation. 
To stop. — (1) Close the king valve which is between the receiver 
and the expansion valve and pump the low pressure to zero. This 
is considered better practice than closing the expansion valve each 
time the machine is stopped, thereby necessitating readjustment of 
the expansion valve upon starting up again; (2) open all drips and 
Fig. 16.— Element ary diagram of absorption system. 
drains and shut down the machine; (3) close the suction and dis- 
charge valves, and all oil cups and water valves; (4) clean the ma- 
chine thoroughly and examine all bolts and nuts to see that they 
are tight and see that the bearings are properly adjusted. 
VAPOR ABSORPTION SYSTEM. 
The absorption and expulsion of ammonia gas by water forms the 
basic principle of the absorption process of refrigeration. 
By referring to the elementary diagram, figure 16, the principles 
of this system of refrigeration will be understood. The generator G 
is an inclosed vessel which is partly filled with a solution of ammonia 
gas in water, under high pressure. Heat is applied to the generator, 
