One of the hardest things for me to grasp was that superheat and subcooling act differently when the system is overcharged (or undercharged for that matter). In a fixed metering system, somehow it made sense that with too much refrigerant that the superheat decreases. It decreases because there is too much refrigerant in the evaporator coil. Remember, the evaporator coil is designed to boil off all of the refrigerant in the first 80-90% of the coil to ensure superheat is added and only vapor gets back to the compressor.
So, if we force too much refrigerant into the evaporator coil, it floods the evaporator coil and it is 100% full of liquid refrigerant and no superheat is added, because all of the heat is still changing the state of the refrigerant.
But... why does the subcooling increase? What happens is that the metering device restricts the flow of refrigerant. When the system is overcharged the liquid refrigerant begins to back up into the condensing coil. Since it is backed up it spends more time in the condensing coil where it picks up more subcooling.
In a fixed metering device an:
Here's a short video on how to measure subcooling (TXV)
So, if we force too much refrigerant into the evaporator coil, it floods the evaporator coil and it is 100% full of liquid refrigerant and no superheat is added, because all of the heat is still changing the state of the refrigerant.
But... why does the subcooling increase? What happens is that the metering device restricts the flow of refrigerant. When the system is overcharged the liquid refrigerant begins to back up into the condensing coil. Since it is backed up it spends more time in the condensing coil where it picks up more subcooling.
In a fixed metering device an:
- Overcharged system - Has low or no superheat and high subcooling
- Undercharged system - Has high superheat and no or low subcooling
Here's a short video on how to measure subcooling (TXV)