The liquid vaporization refrigeration method uses the heat absorption effect during liquid vaporization to achieve refrigeration. Vapor compression, absorption, vapor jet and adsorption refrigeration are all liquid vaporized refrigeration.
First, the principle of liquid gasification refrigeration
1. Liquid gasification refrigeration
Definition: Liquid gasification is a process of endothermic absorption, the use of liquid gasification to absorb the heat of the surrounding medium, so that the temperature of the surrounding medium is reduced below the ambient temperature, called liquid gasification refrigeration.
Refrigeration cycle: gas condensation is an exothermic process, the use of condensation, the heat absorbed in the gasification process is then dissipated, and the resulting condensate is then gasified to absorb heat, so that the cycle, constitutes a refrigeration circulation system.
2. Vapor compression refrigeration
Liquid gasification (that is, evaporation process) to be carried out at low pressure, gas condensation to be carried out under high pressure, in order to maintain the pressure difference between the evaporation part and the condensing part, the use of compressors and throttle devices, in order to make the fluid (liquid, gas) can flow in a closed system, the use of compressors to drive, so that the formation of steam compression refrigeration refrigeration cycle process. This is shown in the figure
The role of the compressor in the figure:
Drives refrigerant flow and maintains pressure differences between the evaporator and condenser
Second, the working medium pair of absorption refrigeration
Working mass pairs are usually composed of a mixture of a high boiling point absorbent and a low boiling point refrigerant.
Requirements for absorbents:
(1) Has the ability to strongly absorb refrigerant:
(2) At the same pressure, its boiling temperature should be much higher than the boiling temperature of the refrigerant;
(3) There should be no risk of explosion or combustion, and there should be no toxicity to the human body;
(4) The corrosiveness of metal materials is small;
(5) The price is low and easy to obtain.
Commonly used working pairs in absorption refrigeration processes:
(1) Water - lithium bromide
Water as a refrigerant, lithium bromide as an absorbent, mainly used in air conditioning systems.
(2) Ammonia monohydrate
Ammonia as a refrigerant and water as an absorbent, mainly used in industrial cryogenic systems.
Third, the principle of absorption refrigeration _ take the ammonia-water working fluid pair as an example
(1) The working heat source is used to heat the aqueous ammonia solution with a certain concentration delivered by the pump from the absorber in the generator, and most of the low boiling point refrigerant ammonia in the solution is evaporated.
(2) The refrigerant ammonia enters the condenser, is condensed into liquid ammonia by the cooling medium, and then lowers the pressure to the evaporation pressure through the throttling mechanism.
(3) After the refrigerant liquid ammonia is throttled, it enters the evaporator, absorbs the heat in the cooling system and vaporizes into ammonia under evaporation pressure.
(4) The remaining low-concentration aqueous ammonia solution in the generator is reduced to the evaporation pressure by the absorbent throttling mechanism under the action of pressure difference, and mixed with the low-pressure ammonia gas coming out of the evaporator, absorbing the ammonia and restoring to the original concentration.
(5) The solution that recovers the concentration in the absorber is then pressurized by the circulation pump and sent to the generator to continue circulation
Fourth, the advantages and disadvantages of the absorption system
merit
Quiet operation, only liquid pump operation, less failure, simple maintenance.
It can use low-pressure steam, hot water, and even waste steam and waste heat.
By controlling the heat source temperature and flow rate of the generator, the cooling capacity can be controlled.
Simple structure, easy to operate.
shortcoming
When water is used as refrigerant, low temperatures cannot be obtained (the freezing point of water is 0 °C).
Improper operation, lithium bromide is easy to crystallize.
The cooling load is large