Six refrigeration methods, how many do you know?

Vapor compression refrigeration

Principle: In the vapor compression refrigeration cycle system, the compressor inhales low-temperature and low-pressure refrigerant vapor from the evaporator, adiabatically compresses it into a high-temperature and high-pressure superheated vapor, and then presses it into the condenser for constant pressure cooling, and releases it to the cooling medium. The heat is then cooled into a subcooled liquid refrigerant. The liquid refrigerant is adiabatic throttling through the expansion valve (or capillary tube) to become a low-pressure liquid refrigerant, and evaporates in the evaporator to absorb the heat in the air-conditioning circulating water (air), thereby cooling the air-conditioning cycle. Water (air) achieves the purpose of refrigeration, and the refrigerant flowing out of low pressure is sucked into the compressor, and this cycle works.

1. Compressor function:

The refrigerant vapor is compressed from a low pressure state to a high pressure state, creating the conditions for the refrigerant to liquefy at room temperature in the condenser. It is called the "heart" of the whole device.

2. Condenser function:

The superheated vapor of the refrigerant discharged from the compressor is cooled and condensed into a refrigerant liquid, and the heat of the refrigerant in the condenser is discharged to the cooling medium.

Classification: water-cooled condenser, air-cooled condenser, evaporative condenser.

Air-cooled condenser:

Easy to use and install, no cooling water is needed, and the heat is brought into the atmosphere by the extension. However, the heat transfer coefficient is also low, and the weight is too large compared to other types. The surface of the fins will accumulate dust, which will reduce the heat dissipation capacity and must be cleaned in time.

3. Evaporator function:

The heat exchange equipment that relies on the evaporation of the refrigerant liquid to absorb the heat of the cooling medium, its task in the refrigeration system is to output the cooling capacity to the outside.

Classification: flooded (immersed) evaporator, dry evaporator. Dry evaporator: immersion coil, shell and tube, plate, spray, etc.

4. Function of throttling device:

Cut-off and pressure reduction: After the high-pressure and normal-temperature refrigerant flows through the expansion valve, it becomes a low-pressure, low-temperature refrigerant liquid.

Control the refrigerant flow: The expansion valve senses the change of the refrigerant superheat at the outlet of the evaporator through the temperature sensor to control the opening of the valve, and adjusts the refrigerant flow into the evaporator to match the heat load of the evaporator.

Control superheat degree: The expansion valve has the function of controlling the superheat degree of the refrigerant at the outlet of the evaporator, that is, to keep the heat transfer area of the evaporator fully utilized, and to prevent the occurrence of the compressor cylinder flushing accident.

Classification: Manual throttle valve, thermal expansion valve, capillary, electronic expansion valve, float plate, fixed orifice plate, variable orifice plate.

Vapor absorption refrigeration

Taking refrigerant-absorbent as working fluid, it is called absorbing working fluid pair.

Common working fluid pairs: lithium bromide-water (refrigerant is water), ammonia-water (refrigerant is ammonia)-low boiling point working fluid is refrigerant.

Device: The absorption refrigeration device is composed of generator, condenser, evaporator, absorber, circulating pump, throttle valve and other components. The working medium includes refrigerant for producing cold capacity and absorbent for absorbing and desorbing refrigerant. form a working-substance pair.

Advantage:

Absorption systems are most suitable for year-round air-conditioned areas that require cooling in summer and heating in winter.

Quiet operation reduces wear to a minimum (except for liquid pump operation), with fewer breakdowns and simple maintenance. Does not rely on electricity. Capacity control is easy, only the heat source of the generator needs to be controlled. The system has high security and no explosion. The full load of the system has the same effect as the light load. When the load changes, it is only necessary to adjust the heat source of the generator and the amount of water circulation. When the evaporation temperature and pressure are reduced, the absorption capacity is only reduced to a limited extent, and the operation is stable.

Shortcoming:

When water is used as a refrigerant, a low temperature (the freezing point of water is 0°C) cannot be obtained. Lithium bromide tends to crystallize when not handled properly.

Steam jet refrigeration

Principle: The high-pressure steam (called working steam) supplied by the boiler enters the main ejector, expands adiabatically in the Laval nozzle, and uses this high-speed steam flow to continuously extract steam from the evaporator, and maintains a relatively high temperature in it. High vacuum, i.e. lower evaporation pressure. The cold water from the refrigeration device enters the evaporator after being throttled and decompressed, and a part of it evaporates and absorbs the heat of the remaining water to reduce its temperature. The cooled cold water is output by the pump and used repeatedly after supplying the cooling capacity.

Adsorption refrigeration

Principle: A certain solid adsorbent has an adsorption effect on a certain refrigerant gas, and the adsorption capacity varies with the change of the adsorbent temperature. By periodically cooling and heating the adsorbent, it is alternately adsorbed and desorbed. During desorption, the refrigerant gas is released and condensed into liquid; during adsorption, the refrigerant liquid evaporates to produce refrigeration.

Classification by adsorption mechanism: physical adsorption refrigeration, chemical adsorption refrigeration.

Principle: The basic structure of adsorption refrigeration consists of five modules: solar collector, condenser, liquid accumulator, evaporator and valve. The operation mechanism of the adsorption refrigeration system is: during the day, the temperature of the collector increases with the increase of the air temperature, the pressure in the refrigerant evaporation collector increases, and the gas enters the condenser and condenses to make a liquid; at night , the temperature decreases, the adsorbent will absorb the refrigerant vapor, the pressure in the evaporator will decrease, so there will be more liquid vaporization, and the heat will be absorbed in the evaporation to cool down.

Thermoelectric refrigeration

Thermoelectric refrigeration is a refrigeration method using the thermoelectric effect (ie the Peltier effect) - also known as thermoelectric refrigeration, semiconductor refrigeration.

Principle: Thermoelectric refrigeration is a direct conversion of voltage generated by temperature difference, which means that when the electrons in the heated object move from the high temperature area to the low temperature area with the temperature gradient, a current phenomenon is generated, and vice versa, when direct current is passed, Materials with thermoelectric energy conversion properties can produce refrigeration, which is called thermoelectric refrigeration.

Magnetic refrigeration, acoustic refrigeration

Magnetic refrigeration: Magnetic refrigeration based on the “magnetocaloric effect” (MCE) is a promising alternative to conventional vapor cycle refrigeration technology. In materials with this effect, the alignment and randomization of magnetic momentum when an external magnetic field is applied and removed causes temperature changes in the material that can be transmitted to the ambient air.

Acoustic cooling: Based on the so-called thermoacoustic effect, the mechanism of thermoacoustic effect can be simply described as adding heat when the sound wave is dense, and expelling heat when the sound wave is sparse, then the sound wave is strengthened; on the contrary, when the sound wave is dense, the heat is discharged and inhaled when the sound wave is sparse. heat, the sound waves are weakened. Of course, the actual thermoacoustic theory is far more complicated than this.

Other refrigeration related knowledge

Refrigerant classification:

Inorganic compounds: water, ammonia, carbon dioxide;

Halogenated hydrocarbons: Freon;

Hydrocarbons: methane, ethane, propane;

Mixed refrigerants: azeotropic and non-azeotropic;

Other hydrocarbons: ethylene, propylene.

Refrigeration capacity: refers to the total amount of heat removed from the confined space, room or area per unit time by the refrigeration equipment during the refrigeration operation of the refrigerator.

Ozone Decay Index ODP: Indicates the degree of damage to the atmospheric ozone layer by a substance. It should be as small as possible, and ODP=0 is harmless to the atmospheric ozone layer.

Greenhouse effect index GWP: Indicates the degree of influence of the substance causing the greenhouse effect. Should be as small as possible, GWP=0 will not cause atmospheric warming.

Energy efficiency grade: It means that the energy efficiency grade is a grading method to indicate the difference between the energy efficiency of household electrical appliances. According to the relevant provisions of the national standard, the energy efficiency label in my country used to divide the energy efficiency into five grades. There are now three levels.

Watt: Symbol: W, the SI unit of power. A watt is defined as 1 joule per second (1J/s), the rate at which energy (measured in joules) is converted, used or dissipated per second.

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

Refrigeration ton: The cooling capacity required to freeze 1 ton of saturated water at 0°C to ice at 0°C in 24 hours. 1 US cold ton = 3024 kcal/hour = 3.517 kW. 1 Japanese cold ton = 3320 kcal/hour = 3.861 kilowatts.

Dry bulb temperature: It is the temperature measured by a thermometer in ordinary air, that is, the temperature often mentioned in our general weather forecast.

Wet bulb temperature: refers to the air temperature when the water vapor in the air reaches saturation under the same enthalpy value of the air state. On the air enthalpy hygrometer, it is from the air state point along the isenthalpy line to the 100% relative humidity line. dry bulb temperature.

Air hazards: It may be that the refrigerating machine oil is oxidized and blackened, resulting in oil pollution, and reacts with the refrigerant to precipitate water and acid, which corrodes the refrigeration system. The air will also increase the condensing pressure temperature and pressure, reduce the cooling capacity and reduce the cooling efficiency

Hazards of moisture: During work, the temperature of the throttling device is lowered, causing the moisture to condense, resulting in ice blockage, making the system unable to work. Moisture can also react with the refrigerant, causing adverse effects.

Hazards of impurities: impurities include dust, metals and metal oxides, etc. These impurities can cause fouling and mechanical circuit failure. At the same time, oxides can promote the decomposition of Freon.

To sum up, by increasing the vacuum degree of the refrigeration system, the above part will be reduced to a certain extent, so that the refrigeration system can work according to the design requirements