R507 in an R134a compressor: any danger?

[Gary]

Why? There is not much pressure drop over suction line piping between the outlet of the evaporator and the compressor inlet. Mass flow is low, the 3/8" piping is wide enough to reduce gas speed in the suction line and therefore friction losses, and there is only 45cm / 1.5ft of piping between the evaporator outlet and compressor inlet.

The measurement of the gas temperature from the evaporator can be considered accurate since the small themocouple is mounted inside the receiver in a piece of 1/4" copper, where the suction gas passes over directly.

SST is not gas temperature. It is the temperature at which the liquid is evaporating inside the evaporator. This can be determined by reading the pressure and consulting a pressure/temperature chart.

I am planning on mounting a pressure gauge on the compressor's service port. I have more space over there to mount such a gauge.

This would be very helpful.

During pulldown everything worked fine, and the TEV showed it's best side by providing a much quicker pulldown than the capillary tube. However, as soon as the temperature and load reached their lowest points, the TEV became instable and started oscillating by injecting not enough refrigerant , an increasing amount of refrigerant up to the point where liquid was returned to the compressor, and then reducing the amount of refrigerant again until the evaporator starves.

Caused by lack of heat load due to insufficient evaporator surface area. This is confirmed by the very wide difference between SST and water temp (high evap TD). To be sure, there is a point at which the TEV can become unstable, but that point would be at a much lower temperature, given ample heat transfer. This is one area where (within reasonable limits) more is better. That limit will be reached when the TD drops below about 3K. And there are ways to take it even lower. You need a lot more surface area.