The capillary tube is the simplest type of refrigerant flow control device and may be used in place of an expansion valve. The capillary tubes are small-diameter tubes through which the refrigerant flows into the evaporator. These devices, which are widely used in small hermetic-type refrigeration systems (up to 30 kW capacity), reduce the condensing pressure to the evaporating pressure in a copper tube of small internal diameter (0.4—3 mm diameter and 1.5-5 m long), maintaining a constant evaporating pressure independently of the refrigeration load change. These tubes are used to transmit pressure from the sensing bulb of some temperature control device to the operating element. A capillary tube may also be constructed as a part of a heat exchanger, particularly in household refrigerators.
With capillary tubes, the length of the tube is adjusted to match the compressor capacity. Other considerations in determining capillary tube size include condenser efficiency and evaporator size. Capillary tubes are most effective when used in small capacity systems.
Figure 3.23 shows an excellent diagram of a practical vapor compression refrigeration system with all control devices.