1. The chilled water pump of the water storage tank keeps the ice maker circulating through the plate or grid evaporator;
2. After the compressor is in operation, it is suctioned-compressed-exhausted-condensed (liquefied)-throttled-and then evaporated in the evaporator at a low temperature of -10 ℃ to -18 ℃. The frozen water is continuously condensed into an ice layer on the surface of the evaporator at a lower temperature at a water temperature of 0°C. The technique and principle of the ice machine. When the ice layer condenses to a certain thickness, the refrigerant evaporation temperature reaches the set temperature of the temperature control, that is, the defrost solenoid valve is turned on, and the heat pump is often used to remove ice, and then the next cycle is realized. There are two types of refrigeration: natural refrigeration and artificial refrigeration. The artificial refrigeration sentence in engineering technology is to use a certain device (refrigeration device), consume a certain amount of energy, forcibly make the temperature of an object lower than the temperature of the surrounding environmental medium, and maintain this low temperature process.
There are many methods of artificial refrigeration, and vapor compression refrigeration is the most widely used refrigeration method. To make a refrigeration system operate in the best state, not only must the design be scientific and reasonable, and the installation is correct, but also the timely maintenance and maintenance during operation is also crucial. It is an effective measure to ensure long-term normal operation of the system, extend the service life and save energy consumption.
The refrigeration device is an independent closed system, and the working fluid circulating in the system does not allow any impurities to enter. The entry of impurities, especially the entry of impurities outside the system, will prevent the system from operating properly, reduce efficiency, and increase energy consumption. An accident occurs in serious cases.
Several common impurities in refrigeration equipment are air, moisture, lubricating oil and mechanical impurities. Let’s take the Freon refrigeration system as an example to talk about the hazards of several impurities and how to eliminate them:
Non-condensable gases in the system
In addition to refrigerants, there are often some mixed gases in the system, and they do not condense under condensing pressure and temperature. They are collectively called non-condensable gases, and are simply referred to as air in engineering. Its composition is mainly air, and there may be polymer decomposition products such as refrigerants and lubricants. These gases are an important factor that affects the efficient operation of the equipment. These gases mainly come from: A. The equipment or pipelines are not completely evacuated during installation or maintenance; B. When charging refrigerant or refrigeration oil, air enters due to careless operation System; C. When the working pressure in the pressure- relief system is lower than the outside atmospheric pressure, air may infiltrate from the valve, shaft seal, etc.; D. The polymer such as refrigerant and living oil decomposes. The air in the system is mainly collected in the condenser and collected in a small amount in the upper part of the high-pressure liquid storage tank.
When there is air in the system, it will increase the condensing pressure of A and the system, which will lead to an increase in the compression of the refrigeration cycle, a reduction in the air delivery of the compressor, and an increase in power consumption; B, an increase in the temperature of the exhaust gas causes the compressor to operate Conditions deteriorate, and at the same time, the mixture of high-temperature refrigerant vapor and air may explode when it encounters a vapor or an open flame; C. The heat transfer efficiency of the condenser is low because the accumulation of air in the condenser indicates that the additional heat increases Resistance; D. The corrosion of the system increases. The moisture and oxygen in the air will aggravate the corrosion of the metal materials, and the aging and oxidation of polymers such as cold and cold training oil.
In view of the multiple hazards of air to the system, it is necessary to prevent air from invading the system as much as possible. The following phenomena may occur when there is air in the system: A. The exhaust temperature rises; B. The pressure in the condenser is higher than the saturation pressure corresponding to the condensation temperature, or the condensation temperature is lower than the pressure in the condenser Corresponding saturation temperature; C, exhaust pressure gauge shakes violently. Because the air in the system is harmful to the operation of the system and inevitably penetrates, the refrigeration system should be operated with air release. However, for the Freon refrigeration system, because the specific gravity of air is less than Freon, small and medium-sized Freon refrigeration systems generally do not use a dedicated air separator, but use simple manual operation: A. Close the condenser discharge valve (if there is a high-pressure storage Liquid tank, you only need to close the high-pressure storage tank outlet valve); B, start the compressor, pump the refrigerant in the low-pressure system to the condenser or high-pressure storage tank; C, when the low-pressure part is pumped to a stable vacuum state , Stop the compressor and close the compressor suction valve. However, the exhaust valve is not closed, and the cooling water is opened enough to fully liquefy the high-pressure gaseous refrigerant; D. After about ten minutes, loosen the multi-channel bolts of the compressor exhaust valve, or open the air vent valve on the top of the condenser to exhaust air ; E. Feel the temperature of the airflow by hand. When there is no coolness or feeling hot, it means that most of the exhaust is air. Otherwise, it means that the Freon gas is exhausted. At this time, the air release operation should be suspended. At this time, the high pressure system should be checked. The temperature difference between the saturation temperature corresponding to the pressure and the outlet temperature of the condenser. If the temperature difference is large, it means that there is still more air, and it should be intermittently released after the mixed gas is fully cooled; F. At the end of the air release, it should be tightened Compression is the multi-purpose channel of the exhaust valve or the air valve on the condenser is closed to stop the condenser water supply. For large Freon refrigeration systems, of course, air vents should be installed, and there are many factors that affect the air discharge effect, especially when there are multiple condensers and liquid receivers in the refrigeration system, but in the end, it is based on the specific refrigeration system piping. Design and the ambient temperature of the system, reasonably determine the location of air discharge. In the condenser and the reservoir, the air is always collected in the piping system with the lowest temperature and the lowest gas velocity. Then, the ratio of working medium to air must be determined. Timely air emissions are an important part of ensuring efficient and energy-saving operation of refrigeration systems.
Lubricating oil in the system
In the compression refrigeration system, the compressor must lubricate the moving parts, and the lubricating oil in the machine is continuously moved by the working medium more or less with the air flow, and enters other equipment of the system.
After the condenser and the evaporator, it will cause harm to the system. To make the system operate efficiently and energy- savingly, corresponding measures must be taken. There are two main reasons why lubricating oil can enter the system: one is the discharge speed of the compressor. According to the moving star law, the higher the speed, the larger the oil droplets that can be carried; the second is the discharge temperature and temperature of the compressor The increase of the oil accelerates the evaporation of oil. In fact, the influence of oil on the heat exchange equipment in the refrigeration system is related to the mutual solubility of the refrigerant and the oil, and the dissolution relationship between the freon refrigerant and the oil varies with the type and temperature of the freon. The more fluorine atoms in Freon, the lower the solubility in lubricating rain. Commonly used refrigerants R11 and R12 are completely dissolved with oil, but can be artificially independent of temperature, while R22 is related to temperature. It is generally completely dissolved in condensation, and partially dissolved in the evaporator, and is divided into an oil-rich layer (floating in liquid refrigeration Agent above) and lean oil layer (in refrigerant). In the working medium, when the two kinds of mutual solubility increase, the relative impact on the system is relatively small, otherwise, it is greater.
The characteristic that the working fluid in the Freon refrigeration system is easily dissolved in the lubricating oil makes the lubricating oil of the system have to adopt a reflux cycle. During the operation of the system, it is necessary to ensure the normal circulation of lubricating oil and maintain a stable oil level in the compressor crankcase. This requires the balance of lubricating oil circulation when the system is running, that is, the amount of oil brought out by the exhaust gas should be equal to the amount of oil returned to the compressor, such as the crankcase of the compressor. The return flow of lubricating oil is to return to the compressor after passing through the oil separator; second, there is no technical measure to ensure the return flow on the return gas pipeline. For the evaporative exhaust pipes and chillers whose liquid supply method is up and down, when the thermal expansion valve is used to directly supply liquid, the higher return air speed can be used to bring the oil back. The piping design in the Freon refrigeration system should calculate the optimal diameter of the return air pipe according to the specific situation and design it into a corresponding form. For some of the upper and lower evaporation tubes, shell and tube evaporators, etc., there is more refrigerant in the equipment, and the return gas speed cannot return the oil. At this time, the liquid must be pumped.
Similar to the air infiltration system, the entry of oil will also increase the cold screwing pressure and increase the power consumption of the system. Therefore, the system should be equipped with an oil separator and a reliable oil return line as much as possible to ensure the reliability of the system operation.
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