How does an Absorption Chiller Work?
There are many different types of Absorption chillers but they all work on a similar principle. In a low pressure system an absorption fluid is evaporated, removing heat from the chilled water. A heat source such as steam, exhaust gas or hot water is used to regenerate the Absorption solution.
The various configurations are explained below
In a Single Lift Hot water driven absorption chiller, as the name suggests, the chilled water is cooled down once by a refrigerant from a double tray in the evaporator. The vaporized refrigerant is absorbed into a concentrated solution (Normally potassium bromide) This concentrated solution comes from the Generator. The concentrated solution becomes diluted as it absorbs the evaporated Refrigerant whilst the heat is absorbed into the cooling water. The diluted solution in the absorber then flows to the Generator through a Heat exchanger. The hot 95˚C water heats up the diluted solution and the refrigerant is vaporized. The vaporized refrigerant is condensed and returned to the refrigerant circuit. The Diluted absorbent is now regenerated and can be recycled.
Double Effect Hot Water Driven
In a 2-Lift Hot water driven absorption chiller there is a main cycle and an auxiliary cycle. The chilled water is cooled down twice by the refrigerant from a double tray in the evaporator and the vaporized refrigerant is absorbed into concentrated solution which is coming from the 2nd generator. The quantity of Vapour that can be absorbed in absorber is increased by the double tray system. The concentrated solution becomes diluted solution and the heat is absorbed into the cooling water. The diluted solution in the absorber flows to the 1st generator through a low temp. heat exchanger and a high temp. heat exchanger, and 95˚C hot water heats up the diluted solution and the refrigerant is vaporized. The absorbent solution becomes an intermediate solution in the 1st generator and it flows to 2nd generator through the high temp. heat exchanger. The intermediate solution in the 2nd generator is heated by the hot water and more refrigerant is vaporized in the 2nd generator. The vapour is absorbed into absorbent solution in the aux. absorber to become aux. diluted solution. The aux. diluted solution is delivered to aux. generator through aux. heat exchanger, and the solution is heated by hot water coming from 1st generator and becomes aux. concentrated solution. The aux. concentrated solution is delivered to aux. absorber through aux. heat exchanger. The refrigerant vapours which are generated in the 1st generator and aux. generator are condensed in the condenser and then flow into evaporator. The heat in the condenser is absorbed by cooling water.
Direct Fired Absorption Chiller
The direct fired system is similar to the Single Lift Hot water series except that instead of using hot water to regenerate the absorption solution, the solution is heated directly by the gas flame to regenerate the solution and the refrigerant.
Double Effect Exhaust Gas Driven Absorption Chiller
Water boils at a low temperature approximately at 4.4˚C because it is under vacuum conditions in the evaporator. This means that the chilled water is cooled down through the tubes in the evaporator by the evaporative latent heat. This reduces the outlet temperature to approximately 7oC.
A refrigerant pump is used to spray the refrigerant(distilled water) over the evaporator tubes to improve the heat transfer. The refrigerant (water) vapour flows into the absorber and it is absorbed in lithium bromide solution. As this process continues, the lithium bromide becomes a diluted solution and reduces its absorption capacity. A solution pump then transfers this diluted solution to the generators where it is re-concentrated in two stages (double-effect) to boil off the previously absorbed water. The diluted solution is pumped to the high-temperature generator where it is heated and re-concentrated to a medium concentration solution by the exhaust heat from the reciprocating engine exhaust gas. The intermediate solution from the high-temperature generator flows to the low-temperature generator where it is heated to become a concentrated solution by the high temperature water vapour released from the solution in the high temperature generator. Since the low-temperature generator acts as the condenser for the high-temperature generator, the heat energy applied in the high-temperature generator is used again in the low-temperature generator. The reduced heat input is approximately 45% compared to an single-stage chiller. Vapour released in the shell side of the low-temperature generator enters the condenser to be cooled and returned to a liquid state. The refrigerant water then returns to the evaporator to begin a new cycle.
To remove the heat from the Chiller, cooling water from a cooling tower is firstly circulated through the tubes of the absorber to remove the heat of vaporization.