Why must refrigerant in the suction line be slightly warmer than the saturation temperature?

The reason the refrigerant in the suction line needs to be slightly warmer than the saturation temperature is primarily to prevent liquid slugging. In an air conditioning system, the refrigerant should be in a gaseous state as it enters the compressor. If the temperature of the refrigerant in the suction line is at or below the saturation temperature, it can lead to the formation of liquid refrigerant. When this liquid refrigerant enters the compressor, it can cause damage or even catastrophic failure, referred to as liquid slugging.

By ensuring that the refrigerant is warmer than the saturation temperature, it helps to guarantee that the refrigerant is fully vaporized, allowing for smoother operation of the compressor and maintaining the integrity of the system. This is a crucial aspect of maintaining both the performance and longevity of air conditioning systems.