A length of thin tubing connected to the high-pressure side (liquid line) from its inlet side and to the low-pressure side of the system (the evaporator) from its outlet side. Liquid refrigerant is forced to flow through the capillary tube by these two forces. Because of the small inside diameter of the capillary tube, a constriction in the flow of refrigerant is created in the sealed system. This constriction maintains the pressure difference between the high and low side. Without continually maintaining this pressure difference, the vaporization and liquification of the refrigerant would not be possible. When refrigerant reaches the larger space of the evaporator (by the suction power of the compressor), the low pressures in this environment immediately cause the refrigerant to vaporize and absorb the heat from the evaporator. Commonly used in commercial and household refrigerators, freezers, and window or rooftop-type air conditioners, capillary tubes are installed between the filter-drier on the liquid line and the evaporator. If a capillary tube must be replaced for any reason (such as a restriction that cannot be cleared with a tube cleaner), it is most important to replace it with one of exactly the same length and inside diameter. The diameter is measured by a capillary tube sizing kit. Capillary tubes are used in different sizes and lengths according to the capacity of the unit (see fig. 45b, 45c, 45d, 45e).