MECHANICAL REFRIGERATION 199 



with the proper amount of ammonia. Before the ammonia 

 is put in, however, all air and moisture must be removed; 

 otherwise the efficiency of the system will be seriously 

 interfered with. Special valves are usually provided for 

 discharging the air, which is removed from the system 

 by starting the compressor and pumping the air out, the 

 operation of gas cylinder being just the reverse of that when 

 it is working ammonia gas. It is practically impossible 

 to get all the air out of the entire system by this means, 

 so that some other course must be taken to remove any 

 remaining air after the compressor has been started at 

 regular work. This can be accomplished by admitting 

 the ammonia a little at a time, permitting the air to escape 

 through a purge valve, the air being thus expelled by dis- 

 placement. The cylinder containing the anhydrous am- 

 monia is connected to the charging valve by a suitable 

 pipe, and the valve opened. The compressor is then kept 

 running slowly with the suction and discharge valves wide 

 open and the expansion valve closed. When one cylinder 

 is emptied put another in its place, being careful to close 

 the charging valve before attempting to remove the empty 

 cylinder, opening it when the fresh cylinder is connected 

 up. 



" From sixty to seventy-five per cent of the full charge 

 is sufficient to start with so that the air may have an op- 

 portunity of escaping with as little loss of ammonia as 

 possible. An additional quantity of ammonia may then 

 be put in each day until the full charge has been introduced. 

 When the ammonia cylinders have been emptied and a 

 charge of, say, seventy-five per cent of the full amount 

 has been introduced, the charging valve is closed and the 

 expansion valve opened. The glass gauge on the ammonia 

 receiver will indicate the depth of ammonia. The appear- 

 ance of frost on the pipe leading to the coils and the cooling 

 of the brine in the tank will indicate that enough ammonia 

 has been introduced to start with. It is sometimes difficult 

 to completely empty an ammonia cylinder without first 

 applying heat. The process of cooling being the same 

 when the ammonia expands from the cylinder into the 



