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Absorption Refrigeration (Absorption Chillers)

If we pour some distilled water in a glass balloon then with proper plug and connections vacuum it and also use a precise manometer (mercuric) to show the vacuum amount. According to the ambient temperature, we can observe that in a certain degree of pressure (relative vacuum), water begins to boil inside the glass balloon (Without using a heater for heating the container). And finally after a few seconds, container’s wall will be cold.

Now, based on this experiment we can see some physical principles, and finally the refrigeration process. In the first place we must explain how water is boiling without being heated by flame or heater? Fusion or phase change from liquid to vapor depends on relations between temperature and pressure of the fluid and its molecular structure.

For example, Water or H2O is boiling at 100 ° C in 1 atm pressure, if pressure term is changing and we increase water’s pressure to 2 atm in a closed container, water will boil at 120 ° C (Like what happens in boilers). The reverse action is also true, it means that if we are vacuuming the air inside a container by vacuum pumps, which decrease pressure from its natural condition. For example, at half of atm, water will boil at 81 ° C. And if we increased the vacuum to 6 mmHg (about one hundredth of atmospheric pressure) water will boil at about 6 ° C. These properties vary in different fluids, for example liquid ammonia or alcohol or different liquid frions, each one in a specific pressure will change phases and evaporate. Like what happens in refrigerators at home, so we were aware of the role of pressure and temperature of the liquid in evaporation.

Now we will explain why by evaporation, temperature is reduced and why container walls are cold. Based on what was described, under normal conditions water evaporates when its temperature reaches 100 ° C. If during evaporation, heating element (lights or heaters) is turned off, boiling or evaporation operation is stopped, so we understand that evaporation needs to take energy (evaporation is a endothermic process), And this process may occur at a higher pressure than atmospheric pressure (like cooker boilers) or below atmospheric pressure as in the experienced balloon or the absorption chiller. But we should know that the body is warmer than -273° C could be a heat generating for bodies that are cooler than it. For example, water at 10 ° C that is entering absorption chillers through cold water pipes, can supply heat for vaporization of distilled water that is in the absorption chiller and ready to vaporize due to low pressure And the effect of this heat transfer, temperature of cold water is reduced to 6 ° C, like what happening in water and lithium bromide absorption chillers And this is what we use to cool the air in the air-condition units and fan coil or industrial processes.

The example above is foundation of water and lithium bromide absorption chillers.

In evaporators of absorption chillers that cool, water is used for refrigerating purposes, vacuum or real pressure is about 4 to 6 mmHg and water just under this pressure evaporates as refrigerant and receive latent vaporization heat from water flowing in evaporator’s pipes and consequently make it cold.

The steam produced by lithium bromide is absorbed in the absorber part and is prevented to increase pressure in evaporator. This solution (LiBr) that absorb water vapor and diluted, derived to generators and condensed by vapor or hot water that flows in generator pipes. Then vapor derived to absorber for re-absorption and separated vapor is condensed and returned to the evaporator.

Balloon that was described in the above example acts like evaporator in absorption chiller.

Operation of main parts

  1. Solution Pump: A dilute lithium bromide solution (63% concentration) is collected in the bottom of the absorber shell. From there, a hermetic solution pump moves the solution through shell and tube heat exchangers for preheating.
  2. Generator: After exiting the heat exchanger, the dilute solution moves into the upper shell. The solution surrounds a bundle of tubes which carries either steam or hot water. The steam or hot water transfers heat into the pool of dilute lithium bromide solution. The solution boils, sending refrigerant vapor upward into the condenser and leaving behind concentrated lithium bromide. The concentrated lithium bromide solution moves down to the heat exchanger, where it is cooled by the weak solution being pumped up to the generator.
  3. Condenser: The refrigerant vapor migrates through mist eliminators to the condenser tube bundle. The refrigerant vapor condenses on the tubes. The heat is removed by the cooling water which moves through the inside of the tubes. As the refrigerant condenses, it collects in a trough at the bottom of the condenser.
  4. Evaporator: The refrigerant liquid moves from the condenser in the upper shell down to the evaporator in the lower shell and is sprayed over the evaporator tube bundle. Due to the extreme vacuum of the lower shell [6 mm Hg (0.8 kPa) absolute pressure], the refrigerant liquid boils at approximately 39 °F (4 °C), creating the refrigerant effect. (This vacuum is created by hygroscopic action – the strong affinity lithium bromide has for water – in the Absorber directly below.)
  5. Absorber: As the refrigerant vapor migrates to the absorber from the evaporator, the strong lithium bromide solution from the generator is sprayed over the top of the absorber tube bundle. The strong lithium bromide solution actually pulls the refrigerant vapor into solution, creating the extreme vacuum in the evaporator. The absorption of the refrigerant vapor into the lithium bromide solution also generates heat which is removed by the cooling water. Now the dilute lithium bromide solution collects in the bottom of the lower shell, where it flows down to the solution pump. The chilling cycle is now completed and the process begins once again.

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