The capillary tube is a key component in mini refrigeration system. Its function is to throttle and regulate the flow of refrigerant. However, during actual operation, the capillary tube is prone to “oil blockage” failures, leading to insufficient refrigerant flow and decreased system performance. This article provides a detailed analysis of the causes, symptoms, diagnosis, and treatment of oil blockage in capillary tubes. It also proposes a series of measures to prevent and resolve oil blockage issues, offering valuable reference for the maintenance and repair of refrigeration equipment.
I. Causes of Oil Blockage in Capillary of Mini Refrigeration System
(A) Excessive Oil Content in Compressor Exhaust
During operation, a small amount of lubricating oil enters the small refrigeration system along with the compressor exhaust. If the oil content in the exhaust is too high, oil can accumulate in the capillary tubes, causing oil blockage. Common reasons for excessive oil discharge include excessive oil filling, improper compressor model selection, and low oil viscosity.
(B) Failure of Filter-Drier
The filter-drier in the mini cooling system can adsorb moisture, impurities, and some oil from the refrigerant. Once the filter-drier becomes saturated or fails, a large amount of oil will enter the capillary tubes, increasing the risk of oil blockage.
(C) Poor Heat Dissipation of the Condenser
The condenser is a key component in the mini refrigeration system for dissipating heat. Its heat dissipation capacity directly affects the condensing pressure and temperature. Poor heat dissipation of the condenser can lead to more oil entering the capillary tubes along with high-temperature and high-pressure liquid, easily causing blockage. Factors causing poor heat dissipation include improper equipment layout, dirty or clogged fins, and condenser fan malfunctions.
(D) Insufficient Refrigerant
Leaks in the small refrigeration system can lead to insufficient refrigerant, causing a decrease in system suction pressure and evaporation temperature. Under low-temperature conditions, the refrigerant’s ability to dissolve oil is poor, and the oil in the liquid pipe can easily precipitate and accumulate in the capillary tubes, causing blockage.
(E) Long-term Low-load Operation
When the compact refrigeration system operates at low loads or experiences frequent start-stops, the flow rate in the capillary tubes is low, and the oil carrying capacity is poor, making it easy for oil to deposit in the tubes. This situation is common in improperly used household air conditioners and refrigerators.
II. Symptoms and Diagnosis of Capillary Oil Blockage in Mini Refrigeration System
(1) Reduced Frost Formation on the Evaporator and Poor Cooling Performance
Oil blockage in the capillary tube restricts the flow of refrigerant, resulting in insufficient refrigerant entering the evaporator. Consequently, there is less frost formation on the evaporator surface, and the frost layer is thin and uneven, leading to poor cooling performance and slow room temperature reduction.
(2) Abnormally Low Evaporation Temperature
Due to the oil blockage in the capillary tube, the refrigerant inside the evaporator is insufficient, causing the evaporation pressure to be excessively low. For an R22 system, the normal evaporation temperature ranges from -5°C to 5°C, but in the case of an oil blockage, it can drop below -20°C. A pressure gauge or thermometer can be used to measure the evaporation pressure and temperature to determine if there is an oil blockage.
(3) High Compressor Suction Superheat
Capillary tube blockage reduces the liquid saturation level in the evaporator, increasing the compressor suction superheat. In an R22 system, the normal superheat is 5-10K, but it can rise to 20-30K when there is an oil blockage. The compressor suction temperature and pressure can be measured to calculate the superheat and diagnose the oil blockage condition.
(4) Increased Condensation Pressure and Current
A blocked capillary tube causes an increase in condensation pressure, leading to higher power consumption by the compressor and a corresponding increase in current. Condensation pressure can be measured with a pressure gauge, and current can be detected with a clamp meter. Both measurements can serve as auxiliary criteria for diagnosing oil blockage.
(5) Small Temperature Difference Between Capillary Tube Inlet and Outlet
Under normal conditions, the temperature at the outlet of the capillary tube is 15-20°C lower than at the inlet. However, with an oil blockage, this temperature difference is significantly reduced. A thermometer or infrared temperature sensor can be used to measure the inlet and outlet temperatures, and a difference of less than 5°C can indicate an oil blockage.
III. Solutions for Oil Blockage in Capillary of Mini Refrigeration System
(A) Proper Installation and Usage
Design and install the micro refrigeration system reasonably, selecting capillary tube diameters and lengths that match the unit. Avoid frequent start-stops or long-term low-load operation. Regularly clean the condenser to ensure its heat dissipation performance.
(B) Timely Replacement of Filter-Drier
Select an appropriate filter-drier based on the refrigerant type and system capacity, and regularly inspect and replace it. Signs such as discoloration of the desiccant or increased pressure drop indicate the need for replacement. Install two-stage filters in series if necessary to further reduce oil content.
(C) Inspection and Adjustment of Refrigerant Quantity
Strictly control the refrigerant charge in the compact system using a charging machine or electronic scale. Regularly check for leaks and promptly replenish refrigerant if insufficient to ensure the system operates at a reasonable charge level.
(D) Cleaning or Replacement of Capillary Tubes
For capillary tubes with severe oil blockage, chemical or mechanical cleaning methods can be used to clear the blockage. Chemical cleaning involves injecting a solvent into the tubes to dissolve and flush out oil stains, while mechanical cleaning uses steel wires or cables to scrape off oil deposits. Replace damaged or severely deformed capillary tubes with new ones.
(E) Optimization of Compressor Lubrication and Oil Return
Select refrigeration oil with appropriate viscosity and control the oil volume within a reasonable range. Improve the compressor exhaust filter mesh to enhance oil filtering effect. If necessary, install an oil separator on the exhaust pipe to further reduce oil content.
Conclusion
Oil blockage in capillary tubes is a common fault in small refrigeration equipment, which can seriously affect the refrigeration performance and energy efficiency. To eliminate oil blockage, comprehensive considerations should be made from the aspects of design, installation, usage, and maintenance, and targeted measures such as cleaning and replacement should be taken. In daily operation, it is necessary to strengthen the monitoring and diagnosis of capillary tubes to timely detect and eliminate potential oil blockage hazards, thus ensuring the safe and economic operation of refrigeration equipment.
About MOIR
MOIR is a company focusing on mini refrigeration systems, including Small Liquid Chiller Units, DC Condensing Units, and DC Air Conditioning Units. All systems are hermetically sealed, charged, and tested in nominal conditions prior to shipment, with low maintenance. With complete traceability comes standard here and along with a full ISO 9001 International Quality Management System, you always can rely on our many years of experience in the small portable cooling sector and benefit from our high process and quality level.
