Another class of direct thermally driven cycles require no electricity because the entire cycle operates at a single pressure thus eliminating the need for a mechanical pump. Along with the above advantages of conventional absorption cycles, they also have several other advantages:
These much maligned machines use three working fluids at a single pressure. They achieve cooling by lowering the partial pressure of the refrigerant thus allowing it to evaporate. In this cycle, ammonia is the refrigerant (see Figure 1-1). The ammonia is driven from mixture with water in the generator by the application of heat. The vapor ammonia travels up through a rectifier which removes almost all of the remaining water. On its naturally upward path, the ammonia is condensed and then flows down into the evaporator.
The evaporator is perhaps the most interesting component of the system. Here, the now liquid ammonia is exposed to gaseous hydrogen which lowers the partial pressure on the liquid ammonia allowing it to evaporate and take in heat from the surroundings. The cool vapor mixture of ammonia and hydrogen falls into the absorber. Here, water from the generator absorbs the ammonia allowing the light hydrogen to rise back to the evaporator. The water is transported from the generator down to the absorber via a thermally operated bubble pump which moves the water up.
Since the 1920's, this cycle has been frequently used and perfected experimentally despite its bad reputation. However, no thermodynamic system analysis is available. This cycle has been used as a refrigerator in homes, campers, RV's, hotel rooms, bars, etc., and have even been built on a scale large enough to provide space cooling for a house.
In the Einstein cycle, butane acts as the refrigerant, ammonia as an inert gas, and water as an absorbent (see Figure 1-2). Since there are three fluids, there are actually three cycles within the overall cycle: a butane cycle, an ammonia cycle, and a water cycle. Starting in the evaporator, liquid butane arrives from the condenser (see point 11). Once in the evaporator (component 1), the partial pressure above the butane is greatly reduced by ammonia vapor flowing from the generator (point 31). With its partial pressure reduced, the butane evaporates and cools itself, the ammonia, and the surroundings. The ammonia-butane vapor mixture leaves the evaporator (point 5) and enters the pre-cooler where it exchanges heat with the vapor ammonia arriving from the generator.
The further heated mixture flows out of the pre-cooler into the condenser/ absorber (6) which is being continuously cooled by the environment. Meanwhile, liquid water from the generator is sprayed into the condenser/absorber (35). With its great affinity for ammonia vapor, this sprayed water absorbs the ammonia from the ammonia-butane mixture and the two are condensed. The absorption of the ammonia vapor increases the partial pressure on the butane vapor causing it to condense.
The butane and the ammonia-water mixture fall to the bottom and are separated by their respective density differences. Since the butane is lighter, it ends up on top and is siphoned back to the evaporator completing the butane cycle. Meanwhile, the ammonia-water mixture leaves the condenser/absorber and enters the solution heat exchanger (27). Here, the mixture is heated (28) before entering the generator (29).
Inside the generator, heat is applied to the strong ammonia-water solution driving off ammonia vapor where it rises under the influence of pressure created by an elevation, h1, and is carried back to the evaporator (1). This completes the ammonia cycle. The remaining weak ammonia-water solution is pumped up to a reservoir via a bubble pump (36). In the reservoir, any residual ammonia vapor is sent to the condenser via tube 34 since it is of too minute a quantity to be sent to the evaporator. The weak ammonia water solution falls to the solution heat exchanger where it gives up its heat to the strong ammonia-water solution leaving the condenser. Finally, the water cycle is completed as the water is sprayed into the condenser (35).
While the overall pressure of the cycle is constant, there are slight pressure variations within the cycle necessary for fluid motion. These are due only to height variations and are not large enough to affect property evaluation.
The U.K. version was patented in 1928 on November 15 nearly two years before the U.S. version. It highlights four different single pressure refrigerators one of which appears in the U.S. Patent. Two of these use methyl-bromide as the refrigerant while the other two use butane. All four use ammonia and water for returning the refrigerant to the evaporator at a constant pressure.
In the U.K. patent, the first two cycles utilize methyl-bromide as the refrigerant. Since methyl-bromide is heavier than the ammonia-water mixture, the fluid arrangement in the condenser/absorber is different with the ammonia-water being on top. Otherwise these two cycles operate similarly to the butane cycles, but offer alternative generator configurations.
The U.S. patent contains only the third of the four cycles presented in the U.K. patent. The fourth cycle in the U.K. patent is different from the U.S. patent: the pre-cooler has been removed, and the ammonia-butane vapor mixture is bubbled into the weak ammonia-water mixture in the condenser rather than the weak ammonia-water mixture being sprayed into the ammonia-butane vapor. Because of this latter change, the weak ammonia-water mixture is not sprayed into the condenser but simply flows in.