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The harm and elimination method of air intake in refrigeration system

The harm and elimination method of air intake in refrigeration system 1 Overview In the refrigeration system, the so-called non-condensable gas means that when the refrigeration system is working, at a specific temperature and pressure in the condenser, the gas cannot be condensed into a liquid, but is always in a gas state. These gases mainly include nitrogen and oxygen , Hydrogen, carbon dioxide, hydrocarbon gas, inert gas, and mixtures of these gases. Due to the presence of non-condensable gas, the energy consumption of the compressor increases, while the cooling capacity of the refrigeration system decreases. 2 Causes of non-condensable gas 1. Insufficient emptying of the refrigeration system before charging the refrigerant Before charging the refrigerator, the compressor cylinder in the refrigeration system, the condenser, the evaporator, and the piping of the system were filled with air. In order to eliminate these air before charging the refrigerant, the interior of the refrigeration system Vacuuming, sometimes due to subjective and objective reasons, the vacuum inside the refrigeration system is insufficient, and the requirements are not met, leaving a small amount of air inside the system. 2. Bring in when charging refrigerant Before the refrigerant is charged in the refrigeration system, the tubes used for charging are filled with air. Due to man-made reasons, when the refrigerant is charged, the air in the sub is not exhausted, and it is directly connected to the refrigeration system. As the charged refrigerant enters the refrigeration system. 3. Non-condensable gas is mixed during the maintenance of the refrigeration system If the refrigeration system works for a long time, it is inevitable that it needs to be inspected, repaired, cleaned or replaced. In this case, it is sometimes necessary to disassemble the machinery or piping. Air often enters the refrigeration system during the process of disassembly and installation. 4. Infiltration into the system from the outside atmosphere In some refrigeration systems, if the working pressure is lower than atmospheric pressure, the air in the atmosphere will penetrate into the refrigeration system through various gaps. These gaps are distributed in various places such as various valves, compressors, non-welded places and so on. 5. Chemical reaction from refrigerant In an ammonia refrigeration system, the refrigerant ammonia can be decomposed into ammonia gas and hydrogen gas at a certain temperature and pressure. The degree of decomposition is positively related to temperature and pressure. The higher the temperature, the greater the pressure, and the easier the ammonia is to decompose. In the Freon refrigeration system, Freon may chemically react with impurities mixed into the system to produce non-condensable gas. For example, R12 reacts with water under certain conditions to produce carbon dioxide. 6. The decomposition of lubricating oil will also produce non-condensable gas Among the lubricating oils used in refrigeration systems, some lubricating oils, such as mineral lubricating oil, can decompose to produce a variety of hydrocarbon gases under complex working conditions, and these hydrocarbon gases will be mixed into the refrigerant in the system. 3 Distribution of non-condensable gases In refrigeration systems, when there are non-condensable gases on the low pressure side, these gases are quickly sucked by the compressor and enter the high pressure side. Therefore, usually the non-condensable gas mainly accumulates in the condenser and high-pressure accumulator on the high-pressure side of the system. Whether it is an evaporative condenser or a shell-and-tube condenser, non-condensable gases are attached to the heat exchange surface as much as possible, as shown in the following figure. The non-condensable gas in the reservoir is often concentrated in a space with a very low air velocity away from the air inlet. 4 Hazards of non-condensable gases 1. Reduce the system cooling capacity When the non-condensable gas is accumulated in the condenser, the non-condensable gas adheres to the inner wall of the condenser and occupies a certain space, so that the condensation area is reduced, and at the same time, the non-condensable gas forms a thermal resistance between the refrigerant and the inner wall of the condenser. The heat transfer efficiency is reduced, and the heat cannot be discharged out of the system in time, thereby reducing the cooling capacity of the refrigeration system. 2. Increased system energy consumption Due to the reduction in heat transfer efficiency, the condensation temperature and the condensation pressure in the condenser increase. Then, in the automatic control refrigeration system, in order to maintain the same degree of condensation, the flow of condensed water must be increased to reduce the condenser The temperature of the refrigerant and non-condensable air. This increases the energy consumption of the condensate pump. At the same time, the increase of the condensing pressure makes the pressure at the discharge port of the compressor larger than under normal operating conditions. The compressor needs to overcome a larger pressure during the exhaust process, so that the energy consumption for compression also becomes larger. 3. Damage to mechanical equipment The increased discharge pressure of the compressor makes the forces that react on the bearings, transmission devices and sliding surfaces also increase. In the long run, the wear and aging of equipment and the deterioration of lubricating oil have been accelerated, resulting in damage to mechanical equipment. At the same time, due to the severe wear of the sliding surface, the refrigerant leakage will also increase. Summary: The presence of non-condensable gas will increase the condensing pressure of the refrigeration system, the condensing temperature will increase, the compressor exhaust temperature will increase, the power consumption will increase, and the cooling efficiency will decrease. Carbonization affects the lubricating effect and burns the compressor motor of the refrigeration compressor in severe cases. 5 The main trace of non-condensable gas is the image 1. The discharge pressure and discharge temperature of the compressor increase, the pointer of the pressure gauge on the condenser (or reservoir) swings violently, the compressor cylinder is hot, and the condenser shell is very hot. 2. Uneven frost on the surface of the evaporator. 3. When there is a large amount of non-condensable gas, the temperature of the warehouse cannot be reduced due to the decrease of the cooling capacity of the device, the compressor runs for a long time, and even the compressor is stopped due to the action of the high-voltage relay. Case: The actual measured condensing pressure of the R22 system was 13.2kg / cm2 (gauge pressure), and the ambient temperature at that time was 35 degrees. Check the "R22 refrigerant temperature and pressure comparison table", the corresponding pressure at a temperature of 35 degrees is 12.81kg / cm2 (gauge pressure), which is lower than the measured condensation pressure, indicating that there is non-condensable gas in the system. The pressure content of its non-condensable gas is: 13.2-12.81 = 0.39kg / cm2 (gauge pressure). 6 Exclusion of non-condensable gases It is still necessary to manually remove non-condensable gases. In this method, the operator determines whether the refrigeration system contains more non-condensable gas according to the level of condensing pressure, and decides whether to discharge. This method depends largely on the experience of the operator, the operation is flexible, and the discharge of non-condensable gas is relatively thorough. Non-condensable gas is easy to separate naturally from the refrigerant at a low temperature and the system is at rest. Its specific gravity is smaller than that of the refrigerant. After separation, it accumulates in the high part of the system (above), so it should be selected at the lowest temperature period. The system has the longest downtime, and the discharge point at the highest point of the refrigeration system is discharged. It is also possible to open the valve directly above a container in the system for discharge, or to discharge each container separately. 1. Small Freon refrigeration system It is not necessary to set up special exhaust equipment, and the non-condensable gas in the system can be eliminated by using the system itself. The specific operation steps are: Step 1: Close the condenser discharge valve and the high-pressure accumulator discharge valve; Step 2: Start the compressor and pump the refrigerant in the low-pressure system to the condenser or high-pressure accumulator; Step 3: When the low-pressure part of the refrigeration system is maintained in a stable vacuum state, stop the compressor and close the suction valve, while the exhaust valve remains open, and at the same time open the cooling water shut-off valve to fully liquefy the high-pressure refrigerant gas ; Step 4: About 10 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; Step 5: Feel the airflow temperature by hand. When there is no coolness or feeling hot, it means that most of the discharged gas is non-condensable gas. Otherwise, it means that the Freon gas is discharged. At this time, the operation of discharging non-condensable gas should be suspended. However, the temperature difference between the saturation temperature corresponding to the pressure of the high-pressure system and the outlet temperature of the condenser should be checked. If the temperature difference is large, it indicates that there are more non-condensable gases, and the non-condensable gas should be intermittently released after the mixed gas is sufficiently cooled Sex gas Step 6: At the end of the discharge of non-condensable gas, tighten the multi-purpose channel of the compressor exhaust valve or close the air exhaust valve above the condenser to stop the condenser water supply. 3. Bleed valve of condenser Step 1: Close the outlet valve of the reservoir. Step 2: Turn on the compressor and press the refrigerant (and non-condensable gas) in the system into the condenser until the low-pressure relay is activated and stopped. Step 3: After stopping, let the condenser continue to circulate cooling water to make the refrigerant fully condense. Because the non-condensable gas is lighter than the refrigerant gas, it accumulates on the top of the condenser (some small-scale equipments have the condenser on the bottom of the compressor, in this case, they gather at the highest place in the high-pressure system). Step 4: Open the vent valve on the top of the condenser (or the multi-purpose through slot of the double-seat exhaust shut-off valve or the joint of the exhaust temperature meter socket, etc.) to release the non-condensable gas. The opening of the air release valve should not be too large. In order to judge the situation of deflation, you can use your hand to face the airflow. If you feel like the wind is blowing, it means that the gas is released; if there is oil on the hand and a cold feeling, it means that the refrigerant gas has been released. The bleed valve should be closed immediately. 4. Ammonia refrigerant bleed air from condenser bleed valve The method of automatically removing non-condensable gas is to control the discharge of non-condensable gas according to parameters such as temperature, and at the same time, the refrigerant recovery device recovers the refrigerant in the mixed gas as much as possible, leaving the non-condensable gas, Finally drain the system. Automatic elimination method is suitable for ammonia refrigerant system The following figure is a schematic diagram of the structure of an automatic air separator used in an ammonia refrigeration system: The working principle is similar to the manual separator, except that it is equipped with a pressure switch and a temperature controller, and the solenoid valve is used to replace the throttle valve. After the mixed gas of the sex gas enters the separator, most of the ammonia gas is condensed into liquid ammonia and accumulates at the bottom, while a small amount of ammonia gas and non-condensable gas accumulate in the separator, and the temperature keeps dropping. When the temperature reaches the set value, the solenoid valve opens, the mixed gas enters the ammonia water mixer, and only the non-condensable gas is discharged after the treatment. This air separator is simple to operate and highly automated. But in the course of work, it lacks flexibility and moves more mechanically

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