In many systems, why is the capillary tube soldered to the suction line?

The capillary tube is typically soldered to the suction line primarily for heat exchange purposes. The suction line carries refrigerant vapor that is warm after absorbing heat from the conditioned space. When the capillary tube is soldered to the suction line, it allows for some heat transfer between the warm suction line and the refrigerant in the capillary tube, which can help in ensuring the refrigerant is in a proper state for optimal performance as it enters the evaporator.

This heat exchange can help maintain a more efficient system operation, as it may assist in raising the temperature of the refrigerant in the capillary tube, reducing the risk of liquid refrigerant reaching the compressor, which can cause damage. By enabling this heat transfer, the system's overall efficiency and reliability can be improved.

While the other reasons listed may have elements of truth regarding their needs in system design, the primary reason for soldering the capillary tube to the suction line is indeed for heat exchange purposes, making this the correct understanding of the scenario.