Knowledge! Understand the six mainstream refrigeration methods and their characteristics

I. Vapor Compression Refrigeration

Principle: In a vapor compression refrigeration cycle system, the compressor sucks in low-temperature and low-pressure refrigerant vapor from the evaporator. The refrigerant vapor is adiabatically compressed by the compressor into high-temperature and high-pressure superheated vapor, which is then pushed into the condenser for isobaric cooling. It releases heat to the cooling medium, then cools down to subcooled liquid refrigerant. The liquid refrigerant is adiabatically throttled by an expansion valve (or capillary tube) into a low-pressure liquid state. Inside the evaporator, it evaporates and absorbs heat from the air conditioning circulating water (air), thereby cooling the circulating water (air) to achieve the purpose of refrigeration. The low-pressure refrigerant is sucked into the compressor, and the cycle continues to operate.

Compressor Function:

The compressor compresses the refrigerant vapor from a low-pressure state to a high-pressure state, creating conditions for the refrigerant to liquefy at ambient temperature in the condenser. It is known as the "heart" of the entire device.

Condenser Function:

The condenser cools the superheated vapor discharged by the compressor and condenses it into refrigerant liquid. In the condenser, the heat of the refrigerant is released to the cooling medium.

Types: Water-cooled condenser, air-cooled condenser, evaporative condenser.

Air-cooled Condenser:

Convenient to use and install, it does not require cooling water, and the heat is discharged into the atmosphere by the unit. However, it has a low heat transfer coefficient, relatively heavier weight compared to other types, and its fins can accumulate dust reducing heat dissipation capacity, requiring timely cleaning.

Evaporator Function:

A heat exchange device that absorbs heat from the cooling medium through the evaporation of refrigerant liquid. Its task in the refrigeration system is to output cooling capacity externally.

Types: Flooded (immersion) evaporator, dry evaporator. Dry Evaporator: Immersion coil, shell and tube, plate, spray types, etc.

Throttling Device Function:

Throttling and pressure reduction: After the high-pressure, ambient temperature refrigerant passes through the expansion valve, it becomes a low-pressure, low-temperature refrigerant liquid.

Control refrigerant flow: The expansion valve controls the valve opening by sensing the change in the superheat of the refrigerant at the outlet of the evaporator with a sensing bulb, adjusting the flow of refrigerant entering the evaporator to match the evaporator's thermal load.

Control superheat: The expansion valve has the function of controlling the superheat of the refrigerant at the evaporator outlet, ensuring full utilization of the evaporator's heat transfer area and preventing compressor cylinder incidents.

Types: Manual throttle valve, thermostatic expansion valve, capillary tube, electronic expansion valve, float plate, fixed orifice plate, variable orifice plate.

II. Steam Absorption Refrigeration

Uses a refrigerant-absorbent working fluid pair, known as an absorption working pair.

Common working pairs: Lithium bromide-water (where water is the refrigerant), ammonia-water (where ammonia is the refrigerant) - The low-boiling-point substance is the refrigerant.

Device: The absorption refrigeration device consists of a generator, condenser, evaporator, absorber, circulation pump, throttling valve, and other components. The working medium includes the refrigerant that produces the cooling effect and the absorbent that absorbs/desorbs the refrigerant, forming a working pair.

Advantages:

Regions requiring air conditioning all year round, supplying cool air in summer and warm air in winter, are most suited for absorption systems.

Operates quietly, minimizes wear (except for the liquid pump operation), fewer faults, and simple maintenance. Does not depend on electricity. Capacity control is easy, only requiring control of the generator's heat source. High system safety, no explosion risk. The system's full load and light load performance are the same; when the load changes, it only requires adjustment of the generator's heat source and water circulation volume. When the evaporation temperature and pressure are reduced, the absorption capacity only decreases to a limited extent, ensuring stable operation.

Disadvantages:

When water is used as the refrigerant, low temperatures (below the freezing point of water at 0°C) cannot be achieved. Improper operation can cause lithium bromide to crystallize.

III. Steam Ejector Refrigeration

Principle: High-pressure steam supplied by the boiler (known as working steam) enters the main ejector, adiabatically expands in the Laval nozzle, using this high-speed steam flow to continuously extract steam from the evaporator, maintaining a high vacuum, i.e., a lower evaporation pressure. The cold water from the refrigeration device, after being throttled and depressurized, enters the evaporator, where part of it evaporates and absorbs the heat of the remaining water, thereby reducing its temperature. The cooled water is output by the pump, reused after supplying cooling capacity.

IV. Adsorption Refrigeration

Principle: A certain solid adsorbent has the ability to adsorb a specific refrigerant gas, and its adsorption capacity changes with the temperature of the adsorbent. By periodically cooling and heating the adsorbent, it alternately adsorbs and desorbs. During desorption, the refrigerant gas is released and condensed into a liquid; during adsorption, the refrigerant liquid evaporates, producing a cooling effect.

Classified by adsorption mechanism: Physical adsorption refrigeration, chemical adsorption refrigeration.

Principle: The basic structure of adsorption refrigeration consists of a solar collector, condenser, liquid storage, evaporator, and valves. The operation mechanism of the adsorption refrigeration system is: during the day, the collector temperature rises with the air temperature, causing the refrigerant pressure in the evaporator to increase, the gas enters the condenser and condenses into liquid; at night, the temperature decreases, the adsorbent absorbs the refrigerant vapor, the pressure in the evaporator decreases, leading to more liquid vaporization, evaporating and absorbing heat to cool down.

V. Thermoelectric Refrigeration

Thermoelectric refrigeration utilizes the thermoelectric effect (also known as the Peltier effect) - also called thermoelectric cooling or semiconductor refrigeration.

Principle: Thermoelectric refrigeration is a direct conversion from a temperature difference to voltage, referring to the phenomenon where electrons in a heated object move from a high-temperature area to a low-temperature area along with a temperature gradient, generating an electric current, and vice versa. When direct current passes through materials with thermoelectric energy conversion characteristics, it can produce a cooling function, known as thermoelectric cooling.

VI. Magnetic Refrigeration and Thermoacoustic Refrigeration

Magnetic Refrigeration: Based on the "Magnetocaloric Effect" (MCE), magnetic refrigeration is a promising alternative to traditional vapor cycle refrigeration technologies. In materials exhibiting this effect, the arrangement and randomization of magnetic moments caused by applying and removing an external magnetic field lead to temperature changes in the material, which can be transferred to the surrounding air.

Thermoacoustic Refrigeration: Based on the so-called thermoacoustic effect, the mechanism of thermoacoustic effect can be simply described as strengthening the sound wave by adding heat when the sound wave is dense and discharging heat when the sound wave is sparse; conversely, discharging heat when the sound wave is dense and absorbing heat when the sound wave is sparse weakens the sound wave. Of course, the actual thermoacoustic theory is much more complex than this.

VII. Other Refrigeration-Related Knowledge

Refrigerant Classification:

- Inorganic compounds: water, ammonia, carbon dioxide;

- Halocarbons: Freon;

- Hydrocarbons: methane, ethane, propane;

- Mixed refrigerants: azeotropic and non-azeotropic;

- Other hydrocarbons: ethylene, propylene.

Cooling Capacity: Refers to the total amount of heat removed from a closed space, room, or area by refrigeration equipment during refrigeration operation per unit time.

Ozone Depletion Potential (ODP): Indicates the degree of damage a substance does to the atmospheric ozone layer. The smaller, the better, with ODP=0 being harmless to the ozone layer.

Global Warming Potential (GWP): Indicates the degree of impact a substance has on causing the greenhouse effect. The smaller, the better, with GWP=0 not contributing to atmospheric warming.

Energy Efficiency Rating: A grading method that shows the difference in energy efficiency of household electrical appliances, divided into five levels according to national standards. Now it is divided into three levels.

Watt: Symbol: W, the power unit in the International System of Units. The definition of a watt is 1 joule per second (1J/s), i.e., the rate at which energy is converted, used, or dissipated per second, measured in joules.

BTU: 1 BTU is the amount of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit. 1 BTU is approximately equal to 251.9958 calories/1.055 kilojoules.

Ton of Refrigeration: The amount of refrigeration required to freeze 1 ton of saturated water at 0°C into ice in 24 hours. 1 US ton of refrigeration = 3024 kcal/hour = 3.517 kW. 1 Japanese ton of refrigeration = 3320 kcal/hour = 3.861 kW.

Dry Bulb Temperature: The temperature measured by a thermometer exposed to the air, i.e., the air temperature commonly mentioned in weather forecasts.

Wet Bulb Temperature: Refers to the air temperature when the moisture in the air reaches saturation at the same enthalpy level. On the enthalpy-humidity chart, it is the dry bulb temperature at the point where the air state line descends along the isenthalpic line to the relative humidity line.

Harm of Air: May cause refrigeration oil oxidation and blackening, generating oil contamination, and reacting with refrigerant, producing water and acids, corroding the refrigeration system. Air can also cause the condensing pressure, temperature, and pressure to rise, reducing refrigeration capacity and efficiency.

Harm of Moisture: During operation, due to the temperature drop at the throttling device, moisture condenses, causing ice blockages and system failure. Moisture can also react with refrigerant, causing adverse effects.

Harm of Impurities: Impurities include dust, metals, and metal oxides. These impurities can cause dirt blockages and mechanical or electrical faults. Oxides can promote the decomposition of Freon.