What indicates low suction pressure in a capillary tube system?

In a capillary tube system, low suction pressure is primarily indicated by low head pressure. The suction pressure in the low side of the system is directly related to the amount of refrigerant vaporizing in the evaporator. When the suction pressure is low, it typically indicates that there is an insufficient flow of refrigerant from the evaporator back to the compressor, leading to a corresponding drop in pressure.

This low suction pressure can cause several issues, including inadequate cooling and potential damage to the compressor if it continues to operate under these conditions. A low head pressure, which reflects reduced system efficiency, correlates with this scenario because the refrigerant must absorb heat to vaporize properly. If it's not absorbing heat effectively due to low suction pressure, the head pressure will also fall as a direct consequence.

Other signs you might encounter, such as frost on the evaporator or bubbles in the sight glass, could indicate other refrigerant flow issues but are not direct indicators of low suction pressure in this context. Frost, for instance, might suggest that the evaporator is not receiving enough refrigerant, but this does not specifically measure the suction pressure as head pressure does.