Why should the refrigeration system be vacuumed?

Update:17 May 2021
Summary:

Why does the refrigeration system emphasize the need to […]

Why does the refrigeration system emphasize the need to vacuum? Let’s take a look at the composition of air. As shown in the figure below, nitrogen is 78%, oxygen is 21%, and other gases are 1%. After entering the refrigeration system, what effect does the composition of these gases have on the refrigeration system? ?
1. The influence of air on the system
   The influence of nitrogen in the air on the system:
   First of all, we must know that nitrogen is a non-condensable gas. The so-called non-condensable gas means that the gas circulates with the refrigerant in the system, does not condense with the refrigerant, and does not produce a refrigeration effect.
   The existence of non-condensable gas is very harmful to the refrigeration system, which is mainly manifested in the increase of the system’s condensing pressure, condensing temperature, compressor discharge temperature, and power consumption. Nitrogen enters the evaporator and cannot evaporate with the refrigerant; it also occupies the heat exchange area of ​​the evaporator, so that the refrigerant cannot be fully evaporated, and the cooling efficiency is reduced; at the same time, the excessively high exhaust temperature may cause the lubricating oil to be carbonized and affect Lubrication effect, burn down the refrigeration compressor motor in severe cases.
   The influence of oxygen in the air on the system:
  Oxygen and nitrogen are also non-condensable gases. We have already analyzed the hazards of non-condensable gases above, and we will not repeat them here. However, it is worth noting that, compared with nitrogen, if oxygen enters the refrigeration system, there are these hazards:
   The oxygen in the air will chemically react with the refrigerating oil in the refrigeration system to produce organic matter, and finally form impurities, which enter the refrigeration system and cause bad consequences such as dirty blockage.
Oxygen, refrigerant, water vapor, etc. are prone to chemical reactions that form acids and oxidize the refrigerating oil. These acids will damage the components of the refrigeration system and damage the insulation layer of the motor; at the same time, these acid products will always stay in the refrigeration system. There is no problem. As time goes by, the compressor will eventually be damaged. The following picture illustrates these problems.
  The influence of other gases on the refrigeration system:
   Water vapor affects the normal operation of the refrigeration system. The solubility in Freon liquid is the smallest, and the solubility gradually decreases as the temperature decreases. The most intuitive impact of water vapor on the refrigeration system has the following three points, let's illustrate with pictures and texts:
There is water in the refrigeration system. The first effect is the throttling structure. When the water vapor enters the throttling mechanism, the temperature quickly drops and the water reaches the freezing point, resulting in icing, blocking the small through holes of the throttling structure, resulting in The failure of the ice block.
   Corrosive pipeline water vapor enters the refrigeration system, and the water content of the system increases, causing corrosion and blockage of the pipeline and equipment.
   Produce sludge deposits. In the process of compressor compression, water vapor encounters high temperature and refrigerating oil, refrigerant, organic matter, etc., and produces a series of chemical reactions, resulting in damage to motor windings, metal corrosion, and formation of sludge deposits.
  To sum up, in order to ensure the effect of the refrigeration equipment and extend the life of the refrigeration equipment, it is necessary to ensure that the refrigeration system is free of air. Therefore, the correct method must be used to exclude the air from the system. In the actual application of the refrigeration system, deposits and corrosion will cause the expansion valve, filter dryer and filter screen to block and fail. The only reliable way to remove the water vapor in the air from the refrigeration system is to take the correct operation steps and use a deep vacuum pump.
   For newly installed units, a vacuum pump must be used to vacuum the entire refrigeration system, and the compressor of this unit is not allowed to vacuum the system, otherwise it may cause irreparable damage to the compressor.
2. the cold storage system is filled with refrigerant
   The working procedure of charging refrigerant to the refrigeration system of the unit, the refrigerant charging operation should be carried out after the vacuuming of the refrigeration system is completed, to prevent the infiltration of air due to the internal vacuum of the system.
   Before adding refrigerant to the refrigeration system of the unit, the circuit of the solenoid valve in the system should be connected to make it in the open state. When adding refrigerant for the first time, add liquid refrigerant from the shut-off valve of the unit's high pressure valve or the liquid filling port on the accumulator. If possible, a filter drier can be installed on the filling pipe between the unit and the freon cylinder to prevent moisture and impurities in the refrigerant from entering the system.
   The first filling amount of refrigerant can be filled at 80% of the rated value of the refrigeration system demand, and the first filling amount should not be too much.
For large-scale refrigeration systems, after filling, the unit is stationary for about 30 minutes, and after the internal pressure of the refrigeration system of the standby group is stabilized, the compressor can be started (for water-cooled units, the cooling water system should be turned on first), and the operation of the refrigerant should be observed Status. Generally, you can observe the refrigerant flow state in the system sight glass to judge whether the refrigerant filling amount is appropriate. If the refrigerant is insufficient, you can fill in the gaseous refrigerant from the low pressure detection port until the bubbles disappear in the sight glass. , The evaporator is covered with uniform frost, and the compressor return pipe is covered with thin frost, it can be judged that the refrigeration system is filled with the right amount of refrigerant.
  In most cases of cold storage refrigeration systems, gas or liquid refrigerant can be charged from the low-pressure side. When the refrigerant charge is large, the high-pressure side can also be charged with liquid refrigerant. requires attention! If the liquid refrigerant is charged from the low pressure side, it must be added slowly to avoid liquid shock! If you are afraid of liquid shock, you can also charge in gaseous state.
  The method of adding refrigerant from the low pressure side:
When charging gaseous refrigerant from the low-pressure side, first turn the low-pressure three-way shut-off valve counterclockwise to the end, and then connect the fluorine cylinder's fluoride pipe to the bypass wire seat of the low-pressure three-way shut-off valve, and then open it For the bottle valve of the R22 fluorine bottle, loosen the joint of the bypass wire seat of the three-way stop valve slightly, and use R22 gas to drive out the air in the fluorine pipe. When you hear the sound of "sizzling" at the joint Immediately lock it, and refrigerant charging begins.
  When the injection volume reaches the specified quality, close the fluorine bottle shut-off valve, turn the low-pressure three-way shut-off valve counterclockwise to the end, close the bottle valve of the R22 fluorine bottle, remove the fluorine pipe, and the fluorine addition is complete.