Drying filter is one of the most commonly used accessories in refrigerant pipeline, which is mainly used to clean harmful substances in the system, thus protecting important components such as expansion valve and compressor.
In the refrigerant system, many harmful substances will enter the pipeline through various channels, and the common ones are as follows:
1. Production or maintenance process: At this time, the refrigerant system will be open to the outside world, and no matter how long it takes, water vapor and dust will enter the pipeline of the system. If nitrogen is not introduced for protection during welding, copper oxygen film and even welding slag may be generated;
2. Charging refrigerant or refrigerating oil: If there are many solid impurities and moisture in these things, they will be brought into the system and corrode the metal parts in the system;
3. In the operation of refrigeration system: In operation, when the refrigerant contains water, it will hydrolyze to generate acidic substances, which will corrode metals. The mixture of refrigerant and lubricating oil can dissolve copper and bring it to the compressor to form a copper film on its metal surface, thus affecting the work of the compressor.
4. Refrigerated oil: When the temperature is low enough, the frozen oil will precipitate stone frogs, and the POE oil used as an environment-friendly refrigerant has strong hygroscopicity, so it is easier to hydrolyze out acidic characteristics.
The existence of the above substances will seriously threaten or destroy the performance of moving parts in the system and cylinders and refrigeration systems that need compressed gas, so they should be removed in time.
The effective materials in the drying filter mainly include:
1. Metal filter screen: it filters larger solid particles (mainly used in R22 and small and medium-sized systems that are insensitive to water, such as mineral oil).
2, polyester filter: filter tiny particles.
3, glass wool: filter tiny particles.
4. Molecular sieve: it filters solid impurities with larger particles, mainly absorbing water, which is 3-4 times that of other substances, and can also absorb nearby acidic substances.
5. Activated alumina: absorbs moisture, mainly acidic substances.
6. Activated carbon: absorbs paraffin, but it is not needed in the system running at medium and high temperature because the paraffin will only precipitate when the temperature of refrigeration oil is lower than -35 degrees.
Dry filter uses are divided into:
Filtration capacity: filter harmful substances in the system, thus protecting important components such as expansion valves and compressors.
Moisture absorption capacity: It is difficult to determine how much moisture absorption capacity dryer is needed for the refrigeration products produced according to the standard process, so the general dry filter production is nominal according to the standard requirements.
Adsorption capacity of acid and ester substances: Experiments show that the ability of drying filter to remove acid and ester substances is directly proportional to the quality of adsorbed substances, while the external size and external surface shape have little influence on this ability, and there is no better industry-recognized method to measure this ability.
Ability to adsorb paraffin: For the refrigeration system at low temperature, because the lubricating oil will precipitate paraffin, which will cause damage to the valve parts, so it should be removed in time, and activated carbon is often used for adsorption.
Refrigerant flow rate: Because excessive pressure drop will affect the system capacity, the smaller the pressure drop through the drying filter, the better.
Classification of dry filters: dry filters are used for liquid pipes and suction pipes, which can remove substances that may damage the system, but the emphasis is different.
Dry filter for liquid pipe: it mainly protects valves such as stop valve and expansion valve, and avoids the blockage of throttling elements such as capillary tube. It is a standard configuration in refrigeration system. The liquid tube drying filter is usually followed by a liquid indicator.
Suction pipe drying filter: mainly protects the compressor. There are generally two situations in which the dry filter for suction pipe is used in the industry.
1. On-site installation system: Because the on-site installation is different from the factory, the on-site environment is complex and the cleanliness is poor, so it is best to install a dry filter in the suction pipe.
2. Large-scale system: Because the cost of the compressor is relatively high, it is worthwhile to install a detachable drying filter in the suction pipe to protect the compressor.
In order to ensure the pressure drop as small as possible, the suction pipe drying filter is mainly used when the compressor is burnt down, so it can absorb a lot of moisture and acidic substances or solid impurities in a short time, which will reduce the circulation capacity of the drying filter, thus affecting its performance. If the measured pressure difference between the inlet and outlet exceeds the lower value, it is necessary to replace the drying filter.
Generally, the position of the drying filter in the liquid pipe is behind the liquid reservoir, but in front of the sight glass, stop valve and thermal expansion valve; There are two positions in the suction pipe:
1. Between the gas-liquid separator and the compressor;
2. Between the four-way valve and the gas-liquid separator.
But it should be between the four-way valve and the gas-liquid separator:
1. If impurities enter the suction pipe, it may block the filter screen of the oil return hole of the gas-liquid separator, which can be avoided if there is a filter in front;
2. The gas-liquid separator is required to be installed as close as possible to the compressor.
For the drying filter in the liquid pipe of the heat pump system, there are some special designs in installation because it needs two-way flow. In practical application, there are the following common designs:
1, only installed on the refrigeration cycle;
2. Set the accumulator, the drying filter and the expansion valve on the common pipeline with four one-way valves, so both cooling and heating can be used;
3, using two check valves and two drying filters to work in the refrigeration and heating cycle respectively;
4. Use a bidirectional drying filter.
The drying filter not only undertakes the drying and filtering work of the normal refrigeration system, but also has an important function of cleaning the system after the compressor is burned out.
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