Most people already know that capillary tubes are what helps medical technicians in a lab draw blood for testing. However, not many realize that the same technology is also used in their kitchen refrigerator. A capillary tube is made of ultra-thin, rigid material (plastic or glass) which enables liquid to flow up the tube against gravity through a phenomenon called capillary action. The diameter of the tube governs how high liquid rises - the smaller the inside of the tube, the further up liquid can climb .
Capillary action occurs due to intermolecular forces between liquid and a solid surface. If the tube is small enough, the natural cohesion within water causes surface tension. When this tension is combined with the adhesive forces between liquid and a hard surface, water is propelled forward . For a visual example, place a glass tube in a cup of water and watch it rise, or watch this clip:
Water molecules tend to stick together (cohesion) and adhere to the surface they touch (adhesion). Whenever adhesion overcomes cohesion, capillary action occurs. The shorter the distance, the stronger the pull from the water molecules sticking to the capillary tube surface .
In major appliances like refrigerators and air conditioning units, capillary tubes made of copper act as throttling devices. As refrigerant moves from the condenser into the capillary tube, its pressure rapidly drops due to the tube's small diameter. Normally, the refrigeration system experiences a pressure drop across the capillary. When the system stops, pressure equalizes on both sides of the tube. Upon compressor restart, the load is reduced .
Unlike an automatic throttling valve, the non-adjustable capillary tube cannot control refrigerant flow. This means the flow can vary based on changing conditions. For example, higher atmospheric pressure increases condenser pressure, while a lighter refrigeration load decreases evaporator pressure, altering liquid flow through the capillary. Although designed to work best under specific conditions, a properly selected capillary tube still functions well in various situations, according to our research.
When installing capillary tubes, it's important to remember the following:
Since capillary pressure equalizes when the refrigeration unit stops, be careful not to overcharge the refrigerant. Overcharged systems raise compressor discharge pressure and cause overloads. Knowing the exact refrigerant amount is critical . Brazing a capillary tube to a condenser coil is a job for experienced professionals, as the small diameter risks blockage during the brazing process.
Accumulators, cylindrical vessels made by pouring copper into a small hollow space, sit between the evaporator and compressor. They serve the important function of separating any liquid particles that may exit the evaporator with the refrigerant. The collected liquid refrigerant then slowly vaporizes and reaches the compressor. In cases of drastic evaporator load drops, accumulators also prevent refrigerant flooding to the compressor .
The small inside diameter of capillary tubes makes them susceptible to blockage by large particles. Blockages starve the evaporator, reduce suction power, decrease refrigerator cooling, and force the compressor to work harder. This increases head pressure and risks overheating. While many blockages occur during installation or assembly, they can also develop during normal operation .
Blockage can be calculated by comparing the nitrogen flow rate through a new capillary tube to the flow rate through the blocked tube, using identical measurements and the ASHRAE test method .
Inadequate boundary lubrication leads to severe metal wear and can result from design issues, poor construction, processing chemicals, using non-recommended lubricants, or metal slag infiltration. Using the wrong lubricant or processing chemicals incompatible with the refrigeration lube can form residue that completely blocks a capillary tube .
Historically, non-polar CFCs and mineral oils were used in most refrigeration systems. Dirty parts after manufacturing weren't an issue, as non-polar processing oil is soluble in non-polar refrigeration oil. However, the introduction of polyol esters and HFCs led to chemical compatibility problems due to polarity differences between these and mineral oils .
New polar lubricants needed to replace non-polar oils to match HFC refrigerants. Extreme temperatures also affect chemical solubility, with the evaporator being the coldest area where incompatible chemicals precipitate. These precipitates can restrict flow if they accumulate at the capillary tube entrance. Thorough rinsing before adding refrigeration oil can remove them and prevent issues .
In summary, capillary tubes play a crucial role in both medical and refrigeration applications. Understanding how they work, proper installation techniques, and potential issues can help ensure optimal performance in refrigeration systems.