The simplest refrigeration consists of four major components: â‘ compressor; â‘¡ condenser; â‘¢ throttle valve; â‘£ evaporator;
First talk about what is called refrigeration. The word refrigeration can only be said to be a technical term. Strictly speaking, it is wrong. No scientist in that country can create "cold". What exactly is cold? Let me give an example to illustrate: In the cold winter months, the temperature drops to -5 ℃. We said that the weather is really cold today, but the people in the Northeast say it is not cold. It's hot, but the temperature suddenly drops to + 25 ° C, we would say it's too cold; this cold is determined by people's common sense, and there is no definition of cold in physics. In the project, cold is determined by the production needs. As the manager asked, is the cold storage cold? You said it was cold. This cold refers to -18 ℃; the manager asked, is the temperature of the fruit store stable? You say it is very stable. The meaning of this answer is that the temperature of the fruit store is stable at ± 0 ℃. This is the definition of cold in our industry. But we still call this method of using mechanical equipment to reduce the temperature of the cooling object to the required temperature, which is the term. What is refrigeration? For example, we put a kettle containing one kilogram of 20 ° C cold water on an iron plate that has been heated to 500 ° C. The water will be opened in a short time. If the kettle is not opened, the water will dry in a short time. Everyone said that the steel plate is heating the water, and conversely it can be said that the water is cooling the steel plate. Moreover, it can be calculated how many degrees it drops, because a kilogram of water rises from 20 ° C to 100 ° C, it needs 80 calories of heat from the outside, and water needs to provide 539 calories of heat from 100 ° C to dry. That is to say, one kilogram of 20 ℃ cold water is boiled to dryness, and the outside is required to provide 619 calories of heat. If it extracts 619 calories of heat from the outside world or steel plate and turns it into water vapor according to the angle of cooling, which makes the steel plate cool down, this is refrigeration, which uses water to cool the steel plate. If you pour water on a steel plate, it will be more intuitive.
In the above refrigeration process, if the size of the steel plate is fixed and the cooling factor of the outside air is excluded, then how much the steel plate has fallen can be accurately calculated. The physical quantities such as 'calorie', 'temperature', 'calorie', and '° C' mentioned here can be understood by anyone who wants to learn physics. In junior middle school physics, heat is always transferred from objects with high temperature to objects with low temperature through conduction, convection, and radiation. After an object loses some heat, its temperature will also decrease. Our goal is to transfer the heat in the product and the air to the refrigerant that is cooler than the product through the refrigeration system, so as to reduce the temperature of the product.
Our refrigeration system and boiler heating system are identical in terms of thermodynamics, and their heat conduction formulas are also exactly the same. Let us first compare with the boiler, and further talk about the role of refrigerant in cooling. The kettle mentioned above can also be regarded as a boiler, but when the water is boiled, we will fill the thermos. If we put a root tube on the spout and connect it to the bathroom, then we can wash the sauna and the kettle will become small. Boiler. It should be noted that the water in the kettle is always 100 ° C, and the steam temperature at the outlet of the kettle is also 100 ° C. Why is it not 110 ° C, not 90 ° C? This is because the boiling temperature of water under an atmospheric pressure is 100 ° C, which is determined by the physical properties of water. In the Qinghai-Tibet Plateau, the atmospheric pressure is low, and the water is opened at about 70 ° C. Without a pressure cooker, you can only eat raw rice. In the pressure cooker, the temperature can reach 110 ° C, because the weight of the pressure valve of the pressure cooker just keeps the pressure in the pot At 1Kg / CM2 gauge pressure (actually 2 atmospheres). Generally small boilers can burn 4Kg / CM2 gauge pressure steam, the steam temperature is also close to 140 ℃, and the water temperature in the boiler is also 140 ℃ as the steam temperature. The temperature of the burner of the gas stove can reach about 1000 ° C. The burner transfers the heat to the water, so that the temperature of the water rises to the boiling point. One kilogram of water from the boiling point to dry (all become steam), the heat and the heat from the gas burner As mentioned above, the kettle is the same for cooling the steel plate, and the flame near the bottom of the kettle is a cooling process. The combustion temperature of coal in the boiler is around 1200 ℃. Without the cooling of the water in the boiler, the exhaust pipe in the boiler will be burned down. From our perspective, the water here is the refrigerant. In turn, water vapor enters the bathroom and immediately condenses into small water droplets (fog), which releases heat to increase the temperature in the bathroom. The same kilogram of steam fired by one kilogram of water, after the steam releases 539 calories in the bathroom, all the water becomes water When the steam turns into water, the temperature of the small water drop is 100 ℃, which is a condensation process. Of course, the small water drops will continue to release heat and reduce the temperature. When the water drops become water drops and fall on the ground or attached to the wall, it is only about 30 ℃. This is not a condensation process, but only an ordinary cooling process. The boiler steam is also passed into the indoor hot water tank (indoor heating exhaust pipe). The hot water tank becomes a condenser for the steam. If the supplied steam pressure is 1Kg / CM2 gauge pressure (actually 2 atmospheres) The surface temperature of the hot water heater is 110 ° C. The hot water heater emits heat to the indoor air, causing the indoor temperature to rise, and the steam condenses into water in the hot water heater. If 539 calories are emitted into the room, the hot water heater 1 kg of water condenses. In terms of refrigeration, the whole process is that the heat of coal combustion is absorbed by water and boils, becoming steam. The steam comes to the hot water with the absorbed heat. The surface of the hot water emits heat to the air, and the steam loses heat. After that, it is condensed into distilled water again, and this water can be returned to the boiler through the equipment to continue to use.
Now back to the four major requirements of refrigeration:
â‘ The compressor is the same as the air compressor;
â‘¡Condenser can be understood as hot water tin or distiller (tin pot) for wine industry;
â‘¢Evaporator can be understood as the kettle or boiler mentioned above;
â‘£Throttle valve can be understood as a valve from the high-temperature hot water upstairs to the boiler, open a small valve, let the distilled water flow into the boiler to continue to use, so as not to let the water in the hot water flow out. It makes the steam in the boiler back to the hot water tank, which is different from our refrigeration, because the whole system is pressure equalized, and the condensing part of the refrigeration system is high pressure, the throttle valve is to control the reasonable distribution of refrigerant to the evaporator, let The evaporator is in normal cooling working state.
The four main requirements for cooling on a refrigerator are:
â‘ Compressor, a guy with a round head and a head hidden behind the refrigerator;
â‘¡The condenser is the heat sink behind the refrigerator;
â‘¢ The evaporator, the freezing frame in the initial single-door refrigerator, can be seen clearly, and the finned tube like the cooler can be seen when the lining of the frost-free refrigerator is disassembled;
â‘£The throttle valve, there is a thin copper tube spirally wound behind the refrigerator, that is the capillary. The outer shell of the refrigerator is quite the same as the outer body of the cold storage.
In the refrigeration industry, the refrigerant can be water, ammonia, F12, F22, F502, liquid nitrogen, etc. Lithium bromide absorption chillers for air conditioning use water as a refrigerant. F12 is used in the refrigerator, and liquid ammonia (not ammonia water) is used in the refrigeration system of the large refrigerator. The properties of liquid ammonia can be found on the ammonia physical performance table. Its evaporation temperature at one atmospheric pressure is- 33.3 ° C. If liquid ammonia is released from a steel cylinder at normal temperature, it will immediately become liquid ammonia at -33.3 ° C (because the outside is an atmospheric pressure) as soon as it exits the cylinder. If it flows onto the cement floor, the temperature of the cement floor immediately makes it boil, which is cement The heat of the ground is transferred to the liquid ammonia, which evaporates the liquid ammonia into vaporous ammonia, and the local part of the cement floor quickly drops to about -30 ℃. If the liquid ammonia flowing on the cement floor is exactly one kilogram, all the liquid ammonia To evaporate light, he must absorb 326 calories of heat from the cement floor. As much as he absorbs and evaporates, if there is less than 326 calories, there must be liquid ammonia remaining. If the liquid ammonia is placed in a metal basin, and then the bottom of the metal basin is in contact with the water surface, the heat of the water is immediately transferred to the liquid ammonia, the liquid ammonia is heated and boils, and the water freezes quickly; if the basin is suspended in the air, around the bottom of the basin The air is immediately cooled by the heat transferred to the liquid ammonia and loses heat. The cooled air is falling, and the surrounding hot air is immediately replenished. Below the basin, you can see the cold air with fog slowly descending. This basin is the 'evaporator'. As for the distiller, some people have seen it, some people haven't seen it, but everyone has seen the phenomenon of condensed water on the lid of the tea tank, or the tin pot for hanging wine in the countryside. The principle is the same. Inside the cylinder head is hot steam, and outside the cylinder head is cold air. The water vapor transfers heat to the cold air through the cylinder head. The steam that has lost a certain amount of heat condenses into water on the surface of the cylinder head, which is condensation The principle of the device is the same as the hot water mentioned above. Now let's talk about the process of refrigeration in the warehouse: if liquid ammonia is in the evaporator (pipe) at a pressure of 0.3Kg / CM2 gauge (it should be 0.03Mpa gauge pressure, for convenience of habit, Kg / CM2 is used), it The boiling temperature should be ï¼28 ℃; and outside the evaporator is a cold storage of -18 ℃. If there is a commodity higher than -18 ℃, the heat in the commodity is quickly transferred to the air, causing the air temperature to rise to- 15 ℃, -15 ℃ air transfers the heat from the commodity to -28 ℃ liquid ammonia, the liquid ammonia absorbs the heat and the temperature does not rise, but boils and evaporates into a gas (ammonia vapor), so the air With back-and-forth transmission, the heat in the goods gradually decreases, the temperature gradually decreases, and finally it drops to -18 ° C. The refrigeration can be ended. This is the job of the evaporator. How much heat is transferred from the air in the store to the evaporator. The liquid ammonia inside evaporates the corresponding weight. Of course, in addition to the heat in the commodity, there is also the heat from the outside air temperature through the enclosure structure. The heat brought by the air when the door is opened makes the temperature of the warehouse rise from time to time, so it needs to be turned on regularly to cool down.
However, without the participation of the compressor, the work of the evaporator cannot be sustained, because the liquid ammonia is heated to evaporate into ammonia vapor, and the ammonia vapor gradually squeezes the space of the evaporator, and the pressure in the evaporator gradually increases, and the pressure increases. The boiling temperature of liquid ammonia will rise. When the final pressure rises to 1Kg / CM2 gauge pressure, the temperature will also rise to about -18 ℃. The temperature of liquid ammonia is the same as that of the cold storage. Because of the temperature balance, heat cannot be transferred to liquid ammonia. The cooling will stop. The task of the compressor is to evacuate the ammonia vapor generated in the evaporator, so that the pressure in the evaporator has been maintained at the 0.3Kg / CM2 gauge pressure required by our production. At this time, the pressure in the evaporator is called the evaporation pressure, and the temperature of the liquid ammonia in the evaporator is called the evaporation temperature. The ammonia vapor extracted by the compressor is not discharged to the atmosphere, but to the condenser. After the ammonia vapor is compressed into the condenser, the pressure of the condenser will gradually increase, and then the task of the condenser. We know that ammonia vapor carries the heat from the cold storage. The ammonia vapor is extracted from the evaporator by the compressor and then compressed into the condenser. Then the compressor has completed the task of transferring heat. Ammonia vapor is now collected in the condenser (with a lot of heat in the cold storage), the pressure continues to increase, and the temperature also increases with the pressure. For example, the pressure rises to 14Kg / CM2 gauge pressure, and the temperature also Corresponding to rising to +39 ℃, if +34 ℃ cooling water is supplied outside the condenser tube, the ammonia vapor in the condenser will transfer heat to the water, and every time 264 calories of heat is sent to the cooling water, there will be one in the condenser A kilogram of ammonia vapor condenses into liquid ammonia and gives up most of the space occupied by the original ammonia vapor. If there is no way out for heat, the pressure in the condenser will continue to rise until the condenser explodes or trips to the safety valve. But in fact, the discharge temperature of the compressor is not +39 ℃ at a gauge pressure of 14 kg / cm ^, but more than +100 ℃. This is because the work that the electric motor drives the piston of the compressor to compress the ammonia vapor is converted into heat, that is, the thermal work equivalent. This can make the bottom of the pump and the leather tube hot when we inflate the bicycle. The principle is the same. The compressor does 1KW of work on ammonia steam, which adds 860 calories of heat to the ammonia steam. This part of heat is sensible heat. It heats the ammonia steam and causes the temperature of the ammonia steam to rise. This heat is transferred to the cooling water. , Will not be condensed by the cooling water into liquid ammonia, only lower the temperature. Only when the ammonia vapor temperature drops to +39 ℃, the real condensation work will be carried out. When the condensation work continues, as long as the pressure remains unchanged, the temperature will not change. The temperature at this time is called the condensation temperature, and the pressure at this time is called the condensation pressure. This is the job of the condenser. The liquid ammonia condensed in the condenser can be sent to the evaporator for continued use, but it must be controlled by a throttle valve, or if the ammonia vapor in the condenser that will not be condensed will pass into the evaporator, it is messy. The throttle valve must be adjusted to the correct liquid ammonia supplement in the evaporator, which is the task of the throttle valve.
To sum up: first, liquid ammonia absorbs the heat of the cooling object in the evaporator and evaporates into ammonia vapor; the ammonia vapor contains the absorbed heat and is pumped to the condenser by the compressor and compressed into high-pressure, high-temperature ammonia vapor. Ammonia steam adds heat added by the thermal equivalent of the motor; the ammonia steam in the condenser transfers the heat to the cooling water at a lower temperature, and the ammonia steam that has lost heat is condensed into liquid ammonia; the throttle valve will The condensed liquid ammonia is supplemented to the evaporator in a controlled manner, so that the evaporator can work continuously; the whole working process is to force the heat in the refrigeration object below -18 ℃ to be forced to +30 ℃ cooling water. The cooling object loses heat and the temperature drops to -18 ° C as we need it. After the cooling water absorbs the heat, the heat is transferred to the atmosphere through the evaporation of water vapor, or the wind blows away the heat
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