The capillary tube — the simplest of refrigerant controls — is the most popular method for metering refrigerant flow from the condenser to the evaporator in compression-type refrigeration systems. Capillary tubes are used instead of thermal expansion valves.
The length and diameter of the capillary tube is chosen to match the flow capacity of the tube to the pumping capacity of the compressor. These seamless tubes are made of copper and act as a throttling valve to meter the amount of refrigerant entering the evaporator. There is only one set of conditions at which a capillary tube will operate at maximum efficiency.