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Method of artificial cooling

Method of artificial cooling

Cập nhật: 04-08-2021 10:10:57 | Project | Lượt xem: 63

Refrigeration/Artificial cooling is a cooling process performed by man-made devices or means. These include the following main methods:

1. Diffusion evaporation method

       It is the diffusion of an evaporating liquid into a gas. And the liquid will collect heat to cool the surroundings. For example, diffuse absorption refrigerators are commonly used in homes. The liquid ammonia in the evaporator in the cabinet will evaporate into hydrogen (which is a pressure-equilibrium gas). And collecting heat of the air in the cabinet makes the air in the cabinet reduce the temperature. Refrigeration method

In the case of spraying water into the air with the same temperature, the water will evaporate, collect heat and change the state of the air. The temperature of the air will be lower when the amount of water evaporates more or in other words, the lower the air humidity, the lower the evaporation temperature of the air after humidification.
Places where the air is hot and dry can apply this phenomenon to perform air cooling. But our country's air often has a high relative humidity, so this method does not bring a noticeable effect. Refrigeration method

2. Cold Mixing Method, Refrigeration Method
     The phenomenon of lowering temperature when salt and water are mixed in certain proportions. This effect depends on the solution concentration and the crystallization point.

For example, if 200g CaCl2 is mixed with 100g of water at 0oC, the solution temperature will drop to -42oC. With table salt (NaCl) this phenomenon occurs but to a lesser extent. But in reality people still use salted ice. To preserve fish when temperatures lower than 0°C are required on fishing vessels.

3. The method of using a dilator with external labor

     It is a cooling method based on the principle that when the gas expands, the pressure and temperature decrease. This system has 4 main devices: compressor, cooler, expander and cold chamber. The difference from the conventional refrigeration system is that the refrigerant does not change phase in the cycle, so there is no condenser and evaporator and the throttle valve is replaced by an expander.

     The process of compression and expansion is adiabatic (s=0). Endothermic and exothermic are isothermal but not isothermal processes. The scope of application of this method is relatively wide, often found in air conditioners and machines that produce nitrogen, liquid oxygen, liquefied gases, etc.

4. The throttling method does not produce external work,The cooling method

     Is the phenomenon where some refrigerants reduce pressure when passing through the throttling mechanism. From high pressure to lower pressure, there is no heat exchange with the outside.
     Throttling is a typical irreversible process. Although there is no heat exchange (q = 0), the pressure is reduced due to the vortex flow and strong friction. For ideal gas, after throttling the temperature stays the same, for real gas at ambient temperature only helium and hydrogen, increase the temperature. Most gases and vapors also drop in temperature, especially when venting moisture or liquids. Refrigeration method

5. Expansion method in rotating tube
     It is a method of air cooling thanks to a special effect that occurs when the following conditions are met:
     Air flow with a pressure of 6at at 20oC blows tangentially to the inner wall of the tube, perpendicular to the pipe axis. With a 12mm diameter pipe, the temperature at the wall will increase and the temperature at the center of the tube will decrease. If placed inside the pipe close to the air stream, enter a screen with a hole in the center. Refrigeration method

With a diameter much smaller than the pipe diameter, cold air will pass through the hole in the shield. No more hot air going in the opposite direction. The cold side temperature can reach -12oC, while the hot side temperature can reach 58oC (70oC difference).
This vortex tube effect has not yet been used in practice because the cooling coefficient is too small. Refrigeration method

6. Method of using the thermoelectric effect

     It is a phenomenon if direct current is passed through a closed loop of conductor. Composed of two different metals, one focal point will heat up and the other will cool (the Peltier effect).
     In practice, for a larger temperature difference between the two ends . Suitable material pairs are used to fabricate the closed loop. The commonly used material pairs are special semiconductors of bismuth, antimony, selenium and additives. Refrigeration method

     The temperature difference of thermoelectric air conditioners can be set to 60oC. But the power is relatively small, only 30W to 100W. Although simple, convenient and can change the function (hot-cold) when reversing the current ... but due to energy consumption and high cost, this method is not commonly used. Refrigeration method

7. Adiabatic demagnetizing cooling method

     This is a method that uses cryo technology to lower the temperature of the samples. From the boiling point of helium (4 – 4oK) down to near absolute temperature. The working principle is as follows: when placing a ferromagnetic salt between two strong magnetic poles. Refrigeration method

The salt crystals are ordered and give off a certain amount of heat. This heat is transferred to the outside to evaporate the liquid helium. When the process of exothermic and magnetization ends, the magnetic field is cut off, and the salt is demagnetized adiabatic. A sudden drop in temperature produces a cold yield q0, repeating this process many times can produce a low temperature. Refrigeration method

8. Method of liquefaction or sublimation of solids,Refrigeration method

     The principle of this method is based on the phenomenon of gaining heat when the solid changes state. The solid used here is the refrigerant. In practice, ice and dry ice are often used. Method of cooling

     Pure ice, when completely dissolved at 0oC, collects a heat of about 80Kcal/kg. When a lower temperature is required, use salt water. The melting point of salted ice depends on the concentration. With table salt (NaCl) this temperature is about -21oC when the concentration is 23%. Ice and salt water are widely used, especially in the fishing industry, due to their non-toxic, inexpensive nature. Refrigeration method

     Dry ice is CO2 in solid form, under normal conditions CO2 converts directly from solid to vapor. Leaves no liquid, so it is called dry ice. Its sublimation latent temperature at -78.5oC is about 137Kcal/kg. Dry ice is also widely used due to its large volumetric capacity and low cost. Refrigeration method

9. Liquid evaporation method

     Liquids when evaporating always gain heat, this amount of heat is called latent heat of vaporization. It is much larger than the latent heat of solidification, so the cooling efficiency is also high. Refrigeration method
     The endothermic evaporator can be either a refrigerant or a refrigerant. Commonly used refrigerants are Freons for rapid cooling. The refrigerant is usually nitrogen, at -198oC the latent heat of nitrogen vaporization is about 48Kcal/kg. When the temperature increases to 0oC, the nitrogen vapor increases by 48Kcal/kg. In addition to the cooling effect of nitrogen, there is also a preservation effect. Because it is an inert gas, it has the effect of inhibiting biochemical processes in the product. Refrigeration method

The above are some methods of artificial refrigeration. If you have a need to buy CO2 and nitrogen gas for food use, please contact VENMER for detailed and specific advice.

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