Absorption Refrigeration Systems 

A consequence of many industrial processes is the generation of waste heat. Energy in the form of heat is a versatile resource to have on hand if the means exist to convert that energy into another more useable form. Heat that is not reutilised by a system is most often lost to atmosphere.

It is equally true that many facilities with surplus heat also have need of cooling for other processes; ones that are not necessarily related to those that generate the excess heat. Absorption chillers offer a method by which we are able to make use of this waste heat to produce something of greater value, such as chilled water for machine or room cooling.

Absorption chillers aim to replicate the outcomes of conventional chillers, but without the significant energy requirements of compressors. This is achieved using a solution containing, most commonly, water, as the refrigerant, and Lithium-Bromide (LiBr) as the absorbant.

Lithium bromide is an extremely hygroscopic substance that is non-toxic. In solution, water may be easily separated from the LiBr by evaporation. It is these two characteristics which make the solution ideal for absorption chillers.

A simple absorption cycle is described in the diagram shown in Figure 1. The most important note to make of this system is the lack of a compressor which would normally account for more than two-thirds of the total refrigeration electrical requirements.

  • 1. From the Absorber chamber, LiBr solution is pumped into the Generator chamber.

  • 2. In the Generator chamber, using waste heat from any available process, water is vaporised from the solution to create two zones within the Generator chamber; one at the top containing water vapour, and one at the bottom containing LiBr.

  • The LiBr is returned to the Absorber chamber

  • The water vapour is transported to a condenser

  • 3. The water vapour is passed through the condenser to be transformed back into a liquid state. The condensing effect is achieved through the use of a separate cooling water loop which rejects the heat using a regular cooling tower.

Figure 1-Simplified Absorption Cycle

 

  • 4. Liquid water is passed to the evaporator chamber via an expansion device which acts to lower the pressure of the fluid. By decreasing the water pressure, we lower it’s vaporising temperature. The vaporisation of water absorbs a large amount of energy, and it is in this way we cool the required process fluid. The process fluid is passed through an immersed serpentine coil with in the chamber sump. The hot process liquid gives off it’s energy to the water, allowing evaporation to occur, the result being a cooling of the process liquid.

  • 5. Once the water has been vaporised it passes back to the absorber chamber. In the absorber, LiBr which has been returned from the generator is again ready to absorb the water from the air. By virtue of being hygroscopic, LiBr very effectively absorbs water vapour from the air in the chamber to once again form Lithium-Bromide solution and restart the cycle.

There are two main benefits to the use of an absorption chiller on a site. First and foremost is the elimination of a refrigeration compressor and its associated motor/electrical infrastructure. Not only does this represent a substantial reduction in capital cost, but also a large drop in ongoing expenses such as power use and machine servicing.

Another primary benefit of these systems is a reduction in quantity of hazardous chemicals being used on the site. Lithium-bromide is a non-flammable substance, greatly simplifying the risk assessment of the plant. All chemicals are also contained within the chiller, further minimising risk to personnel and the environment.

The use of an absorption chiller is limited by the availability of a suitable heat source, but also by the chemical composition of the brine contained within. Through this article, LiBr has been used as an example however this is not the only option. Depending on the cooling outcomes desired, an alternative solution may be required. In a water-based refrigerant system, the freeze point of the water limits the temperatures at which we can produce cooling. Temperatures down to approximately 6°C represent the lower limit of such systems.

For lower process outlet temperatures, ammonia is an excellent option to be considered for use as the refrigerant; with water in this instance acting as the absorbant. The introduction of ammonia into the system allows much lower temperatures to be achieved, down to -30°C or lower. Ammonia is a common, inexpensive , and natural, industrial refrigerant with highly favourable thermal and environmental properties.

The construction of an absorption chiller requires thorough planning and a strong understanding of the site infrastructure as a whole in order to achieve the best outcome; a result of this is that most often this style of chiller is a bespoke unit rather than “off the shelf” as many other chillers are. Diligent preparations in the implementation of absorption chillers has the immediate benefit of increased plant COPs and, over a longer term, substantially reduced energy consumption compared to a regular vapour-compression chiller system.

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