Following is a description of the cooling cycle in a direct-fired, two-stage, absorption liquid chiller, powered by natural gas. Absorption cooling relies on three principles: 1) Liquids, such as water, boil (evaporate) at a lower temperature when under low pressure; 2) Lithium bromide has a strong affinity for water and readily absorbs it, and; 3) in a sealed (hermetic) system a vacuum is created when water in the system is absorbed by lithium bromide. While the description of the process is broken down into stages to make the process easier to understand, in actuality the process is continuous.     1. Solution Pump/Heat Exchangers A dilute solution of lithium bromide and water descends from the Absorber to the Solution Pump which moves the dilute solution to a Heat Exchanger (there are actually two Heat Exchangers, but for simplicity, only one is shown). The flow of dilute solution is split into two streams. One stream is pumped to the First-Stage Generator, the other is pumped to the Second-Stage Generator. 2. First-Stage Generator The dilute solution in the First-Stage Generator is heated by a gas burner. Heating the dilute solution boils off much of the water, leaving a very concentrated lithium bromide solution. The concentrated solution is returned to the Heat Exchanger, while the hot water vapour goes to the Second-Stage Generator. 3. Second-Stage Generator The dilute solution in the Second-Stage Generator is heated by the hot water vapour from the First-Stage Generator. As the water vapour gives up its heat it condenses to liquid. As in the First-Stage Generator, heating the dilute solution produces hot water vapour and concentrated solution. The concentrated solution goes to the heat exchanger while both the water vapour and the liquid water go to the Condenser. 4. Condenser As it enters the Condenser, the liquid water vapourizes due to the low pressure inside the condenser. This water vapour combines with the water vapour coming from the Second-Stage Generator. The combined water vapour condenses to liquid as it is cooled by the condenser water. The liquid then flows down to the Evaporator. 5. Evaporator The Evaporator is where the system water used for cooling the facility's supply air is chilled. The liquid water coming from the Condenser is sprayed over the Evaporator tubes which carry the system chilled water. Due to the extreme vacuum in the Evaporator some of the water vapourizes. As the water vapourizes it draws heat from the returning system water. The chilled water is then resupplied to the system. 6. Absorber The vacuum needed to vapourize the water in both the Condenser and the Evaporator is created by the absorption of water by the lithium bromide inside the Absorber. As the water liquid/vapour mix descends to the Absorber from the Evaporator, concentrated lithium bromide solution from the First and Second-Stage Generators is sprayed into the flow of the descending liquid/vapour mix and the water is absorbed by the lithium bromide. As the water is absorbed a vacuum is created. As the water also gives off heat as it is absorbed, which is removed by the condenser water. The resulting dilute lithium bromide solution collects in the Absorber where it flows down to the Solution Pump to begin the cycle again. back to PilotLight online