=Compressor.=--The “Compressor” is a pump, a cylinder fitted with a piston which withdraws the ammonia from the tank in which it is expanded. The piston is tightly fitted and when it travels in one direction, the gas flows in, filling the space, like any ordinary pump whether it be a water or gas pump. Upon the return stroke, the gas is compressed in the cylinder until the pressure in the piston is sufficient to equal or overcome that exerted against it accumulated in the condenser, when it is discharged thereto. There are spring actuated valves interposed in the line of gas travel which close and retain in the condenser that gas which has been discharged, allowing the piston and compressor to repeat the just described performance many times per minute. =Condenser.=--The use of this element is to liquify the ammonia, really to extract the heat absorbed by the ammonia in the cooler and the heat generated in the compression. This condensing operation changes the form of the substance from gaseous to liquid. The most simple style of condenser is a series of pipes stacked together with ammonia on the inside of the several pipes, and water flowing over the outside. The gas on leaving the compressor is hot and at a high temperature, frequently as high as 250° F. or more, and the pressure from 140 to 220 lbs., depending upon the water supply, its temperature and quantity, and the area of the surface of the condenser. The water flowing over the condenser absorbs the heat from within, the ammonia becoming cooled by contact with the comparatively cool walls of the pipe while flowing from one end to the other of the condensing coil, gradually changing from a gaseous to a liquid form. =Receiver.=--From the condenser the liquid ammonia is collected in the receiver so as to have a quantity stored for use and in reserve for the fluctuating requirements. =Expansion Valve.=--A small but important item in the system is the expansion valve which is a valve with a controllable opening and comparatively small. This is interposed in the line between the liquid receiver and the cooler or expansion tank. =Cooler or Expansion Tank.=--The next element is the cooler in whatever form it may exist, whether it be ammonia coils submerged in brine tank, shell and tube cooler built like a boiler in which the ammonia surrounds the tube through which the brine solution is pumped, or ammonia “direct” expansion coils in the air within a room, it matters not. At this point the heat given off by the substance to be cooled is absorbed by the ammonia and taken up for discharge to the water flowing over the condenser.