Carbon dioxide as coolant

We've got pressurised sulphur, cooling an industrial / mining region in the middle of the Maxwell Mountains. SO HOW LOW CAN WE GO

Carbon-dioxide has its triple-point well beneath our temperature and pressure: 216.55 K, 0.517 MPa. Its critical-point is 304.13 K, but this much at 7.38 MPa which isn't Ishtar Terra's problem. Whilst we're chilling the air, we may as well chilly down that bit further. And we won't even change the pressure!

The aim here is 250 K (-4° F) but maintaining 5-6 MPa pressure.

In the not-quite-human-and-computer-habitable tier, nitrogen (Venus has a lot of it, it's just swamped by CO₂) would get sucked in with new CO₂, as the old CO₂ condenses. Fairly soon this chamber hosts 5 MPa wintry nitrogen over a pool of CO₂. Those interested in the chemistry may read Goos, Riedel, Zhao, & Blum, "Phase diagrams of CO₂ and CO₂–N₂ gas mixtures and their application in compression processes", Energy Procedia 4 (2011), 3778-85.

Already this has given to the colony its server-room and ice-box. Neutral atmo and freezing cold... what's not to love?

If some adjacent room's oxygen level is boosted to 0.4% we should be able to breathe in it. Although I'd wear a mask to warm this air before it got into my lungs. And goggles to keep the freezing and pressurised air out of my eyes and ears. Then there's the issue that nuclear submarines in Earth's seas don't operate beyond 4.5 MPa and... Yeah the miners will mostly be robots.

The liquid 250 K CO₂ is the refrigerant in Venus' deep interior. More: the miners under Venus should just use that instead of water down-pit. Water is used - to widen cracks in the rock, as it freezes and expands.

Getting rid of CO₂ ponds on the bottom is hardly a problem: just shut off the A/C, open the airlocks, and get the miners to safety. After the surrounding rock boils the ponds and raises their gas - close the locks again and restart the A/C.