Hello everyone. This thread is more or less a followup on this thread.

Since my last post in that thread some things have happened. I had a long chat with an airconditioning installation/repairman who was just curious about my adventures. After a few hours of chatting about the difficulties of refrigerating a PC, he gave me a lot of interesting stuff, including a refrigeration manifold with 4 valves, a few solenoids, sight glasses, schraeder valves, expansion valve for R404a/R507, high side pressostate, pipe and 1kg of R507.

I did not have to pay a single dime for it. The only thing he wants in return is that I keep him updated.

Of course, I am very thankful for his help, especially the refrigeration manifold, since now I will finally be able to provide Gary with the required measurements.

Since my current system has it's problems, I will start all over. It's not that much work to redo the piping etc. since I am getting pretty quick in it. The target is: -35 °C / -31F secondary coolant temperature @ 150W load.

I definitely want your comments on the design, and possible improvements.

Here is the first shot at the new chiller design with R507:


Since I will use R507 in a compressor designed for R134a, I have to adjust the operating conditions to fall within the compressor's limits. This means:
- Suction pressure 0.8 .. 2 bar absolute. (R507: -51 °C .. -31 °C / -60F .. -24F)
- Condensing pressure < 16 bar absolute. (R507: 33°C / 91F)

When I keep pressures within these limits, the amount of work to be done for the compressor should be approximately the same as the amount of work to be done for R134a with -10 °C .. -30 °C evaporating and 55 °C .. 60°C condensing.

Since I am shooting for -35 °C / -31F secondary coolant temp, I have to evaporate at approximately -40 °C / -40F. This is within the suction pressure limit.

To keep the condensing pressure within the given limit, I will use a liquid cooled condenser with 4-8L (1-2 gallons) of water and an air-cooled radiator (a heater core from a European car called Opel Ascona). The large amount of water functions as a buffer, and it is sufficient to keep condensing temperature low during startup, when extra heat is dumped into the condenser. When the system is at operation conditions, the amount of generated heat is low enough for the air cooled radiator to get rid of with very little rise in temperature.

I cannot find a minimal suction gas temperature in the Danfoss datasheet, but I can increase suction gas temperature by performing suction gas<->liquid line heat exchange. This seems to be very good for the COP in R507 systems anyway; even more than R134a benefits from it. Can anyone confirm or deny this? I know this 'medal' has two sides, and we discussed the benefits of a SG<->LL HX earlier.

To keep everything safe, I will mount a pressostate (is that the correct word), which will cut out the compressor when condensing pressure reaches 18 bars.

To use a low amount of refrigerant, I will use a receiver just large enough to hold the entire amount of refrigerant in case a pipe becomes clogged. Or course the receiver should not run empty when suction pressure is high.

The expansion valve will be a Danfoss TUAE stainless steel R404a/R507 valve, designed for -60 .. -20 °C (-76F .. -4F) operation, -20 °C MOP, external pressure equalisation. This will protect the compressor a bit during startup.

The refrigerant distributor. I plan to use a few pieces of 0.8mm / .031" I.D. captube to distribute refrigerant over multiple 1/4" evaporator coils. I still need to find out how long I should make those tubes, but a rough guess would be 30cm / 1ft. I just need a bit of pressure drop to obtain a decent distribution of refrigerant, but how much pressure drop I need: I don't know.

In the evaporator, I will try to cram as much 1/4" pipe into the (copper) shell as possible, divided into 3-4 circuits. 12 meters / 36ft must be possible, maybe even more. My current evaporator uses 3 meters of 3/8" pipe, and Gary told me to use at least 4 times that amount to minimize the TD between refrigerant and secondary coolant. Translated to 1/4" pipe this is even a longer piece of pipe to obtain the same amount of surface area.

Then, there is the secondary coolant circuit. This circuit uses a water/methanol mixture and a small pump to minimize the pump motor heat dumped into the secondary coolant.

Please note that theoretically the circulation direction of the secondary coolant is wrong (not counterflow), but in practice I need a bit of head above the pump's inlet to keep it running smoothly at such low temps. And I need the refrigerant to flow from top to bottom.

Comments, anyone? I would really appreciate tips on the evaporator and refrigerant distributor, and any general tips.