The causes and solutions of capillary "ice blockage" in refrigeration equipment

May 23, 2024

The causes and solutions of capillary "ice blockage" in refrigeration equipment


In actual operation of refrigeration systems, capillary tubes may experience "ice blockage" faults. This article will delve into the problem of capillary "ice blockage" in refrigeration equipment from three aspects: causes, fault phenomena, and troubleshooting methods.


The main reasons for ice blockage are as follows:


1. The refrigerant may contain too much water during production, transportation, storage, and use. If the water content of the refrigerant exceeds a certain limit, water molecules will condense into ice crystals in the low-temperature zone of the capillary, blocking the capillary. The general requirement is that the water content of fluorocarbons should not exceed 20ppm.


2. During the assembly and maintenance process of the refrigeration system, solid impurities such as welding slag, copper shavings, and dust may be introduced into the pipelines and components due to system impurities and oil pollution. Impurity particles accumulate on the capillary inlet mesh, which can affect refrigerant flow and exacerbate icing. In addition, if too much lubricating oil is carried inside the compressor case, it can also deposit and coking in the capillaries, blocking the channels.


3. Improper selection of capillaries requires precise calculation and selection of the inner diameter and length of capillaries based on parameters such as system cooling capacity and compressor displacement. If the inner diameter of the capillary is too small and the throttling pressure drop is too large, it will cause the evaporation temperature to be too low and exacerbate icing. Improper selection of pipe length may cause imbalance between gas and liquid phases, resulting in increased liquid resistance and icing inside the pipe.


4. If the condensation temperature and evaporation temperature deviate too much from the condensation temperature or the evaporation temperature is too high, it will reduce the subcooling of the refrigerant at the capillary inlet, making it easier to produce bubbles, exacerbating gas-liquid two-phase separation and icing. The condensation temperature should generally be controlled between 40-50 ℃, and the evaporation temperature depends on specific operating conditions, such as 5-12 ℃ for household air conditioning.


5. If the thermal expansion valve fails or the filter is blocked, it will reduce the refrigerant entering the evaporator, decrease the evaporation pressure and temperature, and more easily produce subcooled droplets in the capillary, which will condense into ice.


Fault phenomenon of capillary ice blockage


Once the capillary tube is blocked by ice, it can cause a series of abnormal phenomena.


By observing and analyzing these phenomena, it can be preliminarily determined whether it is an ice blockage fault:


1. The abnormally low evaporation temperature is hindered by ice blockage, which hinders the flow of refrigerant to the evaporator, resulting in insufficient refrigerant in the evaporator. The evaporation pressure and temperature are significantly lower than normal values. In the case of air conditioning units, the outlet temperature is usually below 5 ℃.


2. The increase in condensation pressure hinders the flow of refrigerant, causing a large amount of accumulation in the condenser, leading to an increase in condensation pressure. The condensation pressure of household air conditioners is generally between 1.4 and 1.6 MPa, but in case of ice blockage failure, it may reach over 2.1 MPa. Meanwhile, due to the decrease in superheat of the return air, the suction pressure of the compressor will also increase.


3. Capillary frost and ice blockage cause a sudden drop in local temperature of the capillary, and the wall of the tube will frost extensively, even covered with ice edges. Indoor units may also experience dripping or frosting.


4. The system vacuum and vibration ice blockage result in insufficient liquid supply to the evaporator, increased gas ratio, and increased noise. The compressor may experience suction and vibration due to excessive gas intake.


5. Frequent start stop or jump stop of the compressor caused by ice blockage can lead to low evaporation temperature and frequent triggering of low-pressure protection, resulting in repeated start stop of the compressor. In severe cases, the compressor may trip and shut down directly. In addition to the above phenomena, capillary ice blockage may also cause a decrease in refrigeration capacity and an increase in energy consumption. But these phenomena are not unique to ice blockage and require further examination and diagnosis.


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Methods for eliminating capillary ice blockage


Once it is determined that the fault is caused by capillary ice blockage, timely measures need to be taken to eliminate it.


Common handling methods include:


1. The first step in refrigerant recovery is to use a recovery machine to extract all the refrigerant from the system, in order to avoid direct discharge that may cause environmental pollution and resource waste. The extracted refrigerant must be purified with a dry filter before it can be reused.


2. Replacing capillaries is usually necessary for capillaries with severe ice blockage. Pay attention to recycling residual refrigerant during disassembly to avoid emptying. When selecting a new capillary, it is necessary to strictly follow the specifications and models indicated on the equipment nameplate, and the size cannot be changed arbitrarily. Ensure smooth direction, secure fixation, and no compression deformation during installation.


3. Clean the system and replace the filter element by repeatedly flushing the system with nitrogen or R11 refrigerant to remove impurities, carbon deposits, and moisture from the pipelines and components. If necessary, the compressor should also be disassembled to clean the crankcase. Replace the capillary inlet filter and dry filter element, and refill with qualified refrigerant.


4. Check whether the thermal expansion valve and solenoid valve can operate normally and whether the valve core is flexible. If a malfunction is found, it needs to be repaired or replaced.


5. Adjust the condensation and evaporation temperature according to environmental conditions and equipment conditions, adjust the condensation temperature to 40-50 ℃, and the evaporation temperature is generally not lower than -5 ℃. If necessary, the condensing air volume can be appropriately reduced to increase the condensing pressure.


6. In addition to maintenance, it is also important to prevent capillary ice blockage in the system.


The following measures can be taken:


7. Control the refrigerant water content and regularly check the refrigerant water content. If it exceeds the standard, it should be replaced or purified in a timely manner. When adding refrigerant, it is essential to use specialized charging equipment to prevent air and moisture from entering.


8. Install a drying filter on the liquid tube near the capillary inlet, and install a suitable size drying filter to intercept moisture and solid impurities. Regularly inspect and replace the filter element, usually every 1-2 years.


9. The use of anti icing capillary tubes with smooth inner walls and gradually shrinking cross-sections can effectively prevent ice crystals from gathering and blocking at the inlet.


10. Control the circulation of compressor lubricating oil and select lubricating oil with appropriate viscosity to control the oil content of the system. If necessary, install an oil separator in the return oil pipeline to reduce the amount of oil entering the capillary tube.


11. To avoid low load and intermittent operation, the refrigeration system should maintain continuous operation under rated conditions as much as possible, avoid frequent start stop and long-term low load. Under these operating conditions, the system has poor oil circulation, insufficient liquid subcooling, and is more prone to ice blockage.


12. Ensure the quality of installation and construction. When installing refrigeration equipment, it is necessary to ensure the cleanliness of the pipelines and the welding of the connection parts is firm. The compressor should be placed steadily, with tightly fixed pipelines to avoid vibration and wear. In summary, capillary ice blockage is a common malfunction in refrigeration systems, which can seriously affect refrigeration efficiency and equipment reliability. To eliminate ice blockage, it is necessary to identify the cause and take measures such as cleaning and replacement. At the same time, attention should also be paid to the prevention of ice blockage, controlling moisture, oil pollution, and impurities from the source, optimizing system operating conditions, and improving design and installation quality. Only through comprehensive management can the frequency of capillary ice blockage be fundamentally reduced, ensuring the stable and efficient operation of refrigeration equipment.


Comprehensive governance


1. Reasonable design of capillary length and inner diameter


The length and inner diameter of capillaries need to be accurately calculated based on parameters such as the cooling capacity, refrigerant type, and evaporation temperature of the refrigeration system. Generally, formulas or table lookup methods can be used to determine. hair


Fine tube inner diameter (mm)=0.128 x (flow coefficient K/pressure drop coefficient Δ p) ^ 0.25


The capillary length (m)=pressure drop coefficient Δ p/(Z × D ^ 4.75) In the equation, the flow coefficient K is related to the evaporation temperature and refrigerant type, the pressure drop coefficient Δ p is taken as 0.5-0.8, and the resistance coefficient Z is taken as 3.5-3.8, D is the inner diameter of the capillary tube (mm).


Choosing the appropriate capillary size can match the throttling pressure drop with the required flow rate, avoiding ice blockage caused by excessive or insufficient throttling flow.


2. Strictly inspect the quality of refrigerants. When purchasing refrigerants, it is necessary to choose reputable and qualified suppliers, request product quality inspection reports, and ensure that purity and moisture content meet the standards. For recycled refrigerants, they must undergo professional purification treatment and pass the test before they can be used. Poor quality refrigerants not only exacerbate icing, but also corrode pipelines, producing toxic substances and posing safety hazards.


3. Timely refrigerant replacement is necessary when the refrigeration system is operating for a long time and has severe pollution. When replacing, it is necessary to first recover the old refrigerant and rinse it repeatedly with nitrogen or R11 until the discharged liquid is clean and transparent. Then refill with new refrigerant and record the filling amount for future inspection.


4. Improving the detection and diagnosis technology for ice blockage faults in conventional refrigeration system maintenance mainly relies on empirical judgment and simple measurement. In order to improve maintenance efficiency and accuracy, it is urgent to develop intelligent ice blockage detection and diagnosis equipment. Real time monitoring of capillary wall temperature distribution using infrared thermal imaging can be used to determine the location and degree of ice blockage; Using an online moisture meter to continuously detect changes in refrigerant moisture content; Develop a capillary vibration and noise spectrum analysis system to detect anomalies caused by ice crystal aggregation and detachment early on.


5. Strengthen the training and management of practitioners to address common faults such as capillary ice blockage. Systematically explain the principles and knowledge, develop operational standards, and standardize operating procedures to improve the skill level of frontline employees. At the same time, strengthen management and assessment, and strictly supervise behavior.


6. Pay attention to the construction of standards and regulations in the refrigeration industry, accelerate the formulation of national and industry standards for the design, construction, operation and maintenance, maintenance, and disposal of refrigeration systems, and standardize specific requirements for capillary tube selection, installation, and fault diagnosis.


7. Conduct basic research on the mechanism of capillary ice blockage, investigate the micro processes of ice blockage nucleation, growth, and detachment, reveal the interaction laws between water, oil pollution, solid particles, and ice crystals, and construct a mathematical and physical model for the formation of ice blockage. Advanced experimental methods such as high-speed photography and particle imaging velocimetry can be used to obtain the flow and heat transfer laws of the ice blockage process. Through mechanism research, it is expected to identify the key factors affecting ice blockage, providing theoretical guidance for suppressing ice blockage and optimizing capillary design.


8. On the basis of revealing the mechanism of ice blockage, the development of new anti ice blockage capillary structures can be carried out. For example, coating hydrophobic and anti icing coatings on the inner wall of capillaries to reduce the adhesion of ice crystals; Processing spiral grooves to guide the continuous flow of condensate and suppress ice crystal aggregation; Design a variable cross-section sleeve composite structure to form a vortex in the inlet section, separating solid particles from the pipe wall.


9. Explore the application of alternative refrigerants for refrigeration, such as traditional fluorocarbons such as R12 R22 and others have good thermal performance, but high cost ODP and GWP values are high, facing pressure to eliminate them. And new alternative working fluids, such as R134a R410A, R600a, etc. have low toxicity and minimal ozone depletion, but their thermal properties differ significantly from Freon. The flow boiling characteristics in capillaries are not yet clear, and the risk of ice blockage needs to be evaluated.


10. Conducting energy-saving optimization research on refrigeration systems. The energy efficiency of refrigeration systems not only depends on the efficiency of compressors, but also on the throttling performance of capillaries. While preventing ice blockage, it is also necessary to optimize the throttling and expansion process of the capillary tube to reduce irreversible losses. Two phase flow numerical simulation and other methods can be used to optimize the pipe type, pipe length, and ring pipe layout, so as to achieve the best match between the refrigerant temperature and pressure after throttling expansion. At the system level, collaborative optimization of the design and operating parameters of the capillary tube, evaporator, and condenser should be carried out to maximize the system COP and achieve overall energy conservation and efficiency enhancement of the refrigeration system while meeting the refrigeration capacity.




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