Heat Pump TXV with 80 Degree Evaporator?

[jeff5may]

If you stick a TXV of higher tonnage than your compressor will supply, the system will hunt constantly trying to reach a balance point. No matter how big an orifice you stick in, the pump can only move so much gas. In a flooded evaporator system, you have a violently boiling evaporator. This violent boiling gets worse as the temperature/capacity goes up, so sizing the heat exchange surfaces gets to be a trial and error design situation. Also, a flooded evaporator develops no superheat. The superheat is gained only after the liquid boils off.

Flooded setups always have some kind of header, separator or tank above the evaporator to catch the froth and keep it out of the compressor. This is where the superheat begins to develop, if any. The low side float valve, when employed, is in this tank, and acts like a toilet float valve to maintain a constant liquid level in the burp/flash/separator tank. Again, think huge scale here: these systems are usually in the hundreds or thousands of tons of capacity range. When you have a 10 gallon surge tank, it's fairly easy to mainain it at 3 gallons full.
More advanced systems also employ a suction line / liquid line heat exchanger which adds superheat to the suction line close to the compressor inlet. The heat is robbed from the warm liquid line just prior to the metering device.