rate depends upon the partial pressure of the vapor above the liquid and goes up with lower partial pressure. While having the same total pressure throughout the system, the refrigerator maintains a low partial pressure of the refrigerant (therefore high evaporation rate) in the part of the system that draws heat out of the low-temperature interior of the refrigerator, but maintains the refrigerant at high partial pressure (therefore low evaporation rate) in the part of the system that expels heat to the ambient-temperature air outside the refrigerator. The refrigerator uses three substances: ammonia, hydrogen gas, and water. The cycle is closed, with all hydrogen, water and ammonia collected and endlessly reused. The system is pressurized to the pressure where the boiling point of ammonia is higher than the temperature of the condenser coil (the coil which transfers heat to the air outside the refrigerator, by being hotter than the outside air.) This pressure is typically at which pressure the dew point of ammonia will be about . The cooling cycle starts with liquid ammonia at room temperature entering the evaporator. The volume of the evaporator is greater than the volume of the liquid, with the excess space occupied by a mixture of gaseous ammonia and hydrogen. The presence of hydrogen lowers the partial pressure of the ammonia gas, thus lowering the evaporation point of the liquid below the temperature of the refrigerator's interior. Ammonia evaporates, taking a small amount of heat from the liquid and lowering the liquid's temperature. It continues to evaporate, while the large enthalpy of vaporization (heat) flows from the warmer refrigerator interior to the cooler liquid ammonia and then to more ammonia gas. In the next two steps, the ammonia gas is separated from the hydrogen so it can be reused. The ammonia (gas) and hydrogen (gas) mixture flows through a pipe from the evaporator into the absorber. In the absorber, this mixture of gases contacts water (technically, a weak solution of ammonia in water). The gaseous ammonia dissolves in the water, while the hydrogen, which doesn't, collects at the top of the absorber, leaving the now-strong ammonia-and-water solution at the bottom. The hydrogen is now separate while the ammonia is now dissolved in the water. The next step separates the ammonia and water. The ammonia/water solution flows to the generator (boiler), where heat is applied to boil off the ammonia, leaving most of the water