Water-Lithium Bromide Vapor Absorption Refrigeration System
In a water-lithium bromide vapor absorption refrigeration system, water is used as the refrigerant while lithium bromide (Li Br) is used as the absorbent. In the absorber, the lithium bromide absorbs the water refrigerant, creating a solution of water and lithium bromide. This solution is pumped by the pump to the generator where the solution is heated. The water refrigerant gets vaporized and moves to the condenser where it is cooled while the lithium bromide flows back to the absorber where it further absorbs water coming from the evaporator.
The water-lithium bromide vapor absorption system is used in a number of air conditioning applications. This system is useful for applications where the temperature required is more than 32 degree F.
Special Features of Water-Lithium Bromide Solution
Here are some special features of the water and lithium bromide in an absorption refrigeration system:
1) Lithium bromide has great affinity for water vapor, however, when the water-lithium bromide solution is formed, they are not completely soluble with each other under all the operating conditions of the absorption refrigeration system. Because of this, the designer must take care that such conditions would not be created where crystallization and precipitation of the lithium bromide would occur.
2) The water used as the refrigerant in the absorption refrigeration system means the operating pressures in the condenser and the evaporator must be very low. Even the difference of pressure between the condenser and the evaporator must be very low. This can be achieved even without installing the expansion valve in the system, since the drop in pressure occurs due to friction in the refrigeration piping and in the spray nozzles.
3) The capacity of any absorption refrigeration system depends on the ability of the absorbent to absorb the refrigerant, which in turn depends on the concentration of the absorbent. To increase the capacity of the system, the concentration of absorbent should be increased, which would enable absorption of more refrigerant. Some of the most common methods used to change the concentration of the absorbent are: controlling the flow of the steam or hot water to the generator, controlling the flow of water used for condensing in the condenser, and re-concentrating the absorbent leaving the generator and entering the absorber.
Lithium Bromide Absorption Refrigeration Chiller and Air Conditioner
Parts and How They Work
Below is a description of the main parts of the system. Please refer the figure above:
1) Evaporator: Water as the refrigerant enters the evaporator at a very low pressure and temperature. Since very low pressure is maintained inside the evaporator the water exists in a partial liquid state and partial vapor state. This water refrigerant absorbs the heat from the substance to be chilled and gets fully evaporated. It then enters the absorber.
2) Absorber: A concentrated solution of lithium bromide is available in the absorber. Since water is highly soluble in lithium bromide, solution of water-lithium bromide is formed. This solution is pumped to the generator.
3) Generator: Heat is supplied to the refrigerant water and absorbent lithium bromide solution in the generator from the steam or hot water. The water becomes vaporized and moves to the condenser, where it gets cooled. As water refrigerant moves further in the refrigeration piping and though nozzles, its pressure is reduced along with the temperature. This water refrigerant then enters the evaporator where it produces the cooling effect. This cycle is repeated continuously. Lithium bromide on the other hand, leaves the generator and re-enters the absorber for absorbing water refrigerant.
As seen in the image above, the condenser water is used to cool the water refrigerant in the condenser and the water-Li Br solution in the absorber. Steam is used for heating water-Li Br solution in the generator. To change the capacity of this water-Li Br absorption refrigeration system the concentration of Li Br can be changed.
Principles of Refrigeration by Roy J. Dossat, 4th edition, Prentice Hall