Refrigerant Capillary Tube Calculator

A refrigerant capillary tube is a small-bore, fixed restriction used to meter liquid refrigerant between the high-pressure condenser and low-pressure evaporator. Its narrow passage creates the pressure reduction needed for evaporation without moving parts. Domestic refrigerators, small air conditioners, dehumidifiers, and compact heat pumps often use this simple device because it is inexpensive, quiet, and reliable. The tube diameter and length must nevertheless suit the refrigerant, compressor, cooling load, and expected operating pressures. This calculator combines tube geometry with an entered average velocity to estimate volumetric flow. The circular flow area is pi times diameter squared divided by four, and volume flow is area multiplied by velocity. Refrigerant density converts that value to mass flow, while representative latent heat converts mass flow to an approximate cooling capacity. The displayed pressure drop is the difference between inlet and outlet pressure. Reynolds number is density times velocity times diameter divided by dynamic viscosity and indicates whether the assumed flow is broadly laminar, transitional, or turbulent. Real capillary selection is more complicated than a single steady-state model. Refrigerant flashes as pressure falls, so density, viscosity, quality, and velocity change along the tube. Entrance losses, bends, roughness, subcooling, oil circulation, manufacturing tolerance, ambient heat transfer, and charge level also influence performance. The result should therefore be treated as an engineering estimate, not a replacement for manufacturer selection charts, validated simulation software, or prototype testing. Diameter has an especially strong effect on restriction. A small change in bore can cause a large change in flow, while added length generally increases resistance. A tube that passes too much refrigerant may flood the evaporator and raise suction pressure; one that passes too little may starve the evaporator and reduce capacity. Designers commonly begin with published compressor and refrigerant data, then trim tube length during controlled testing. Use consistent measurements and realistic operating values. Inlet pressure should exceed outlet pressure, diameter and length must be positive, and velocity should represent flow inside the tube rather than line velocity elsewhere in the system. Temperature is included to document the operating point, although this compact estimate uses representative liquid properties for each refrigerant. Always follow refrigerant safety rules, pressure-vessel practices, and applicable environmental regulations when building or servicing a system.