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If it weren't for immigrants also needing the NHS, there'd be less requirement for NHS staff of any ethnicity.

In 2-tier UK she'd get locked up for saying that

Not even pedantic to point out that you meant absolute temperature, not pressure :giggle:

I had occurred to me that in principle the cycle could work with a permanent gas, but I assumed there must be big advantages in using a phase-change refrigerant. I wasn't aware that air was actually used in a practical system so thanks for that.

That's right, for any "permanent gas" ie one that cannot be liquefied by pressure alone at room temperature, there are 2 options - store it as a compressed gas at high pressure, or cool it, compress it (at moderate pressure), and keep it as liquid in a Dewar vessel (aka vacuum flask). Which to...

If you mean oxygen, hydrogen, nitrogen, air - only when cooled to below critical temperature, -140°C for air. Propane, eg has critical temperature 96°C so can be liquefied by pressure alone. In a freezer, the refrigerant must be liquid at the P and T in the condenser (heat dump grid), and gas at...

Are those curves for the UPS3? Perhaps I'm reading them wrong, but from the points with the square blobs, 3.5m head, 1.8 m^3/h and ~ 0.04kW, I calculate efficiency 44%, not 69.2%. The latter figure would be impressive overall efficiency for a small pump. It's a while since I looked at it, but I...

OK

OK, would have been better to have added it earlier. But I still don't see what's different about this system, incorporates usual materials. It might still be best to add inhibitor and deal with the leaks. Depends where the leaks are. If it's a rad it probably needs replacing. I doubt steel pipe...

It's much easier to add some than replace all the pipes!

Am I missing something here? Heating systems typically have several different materials - steel rads, copper pipework, aluminium, cast iron or stainless steel boiler, plastic. With inhibitor kept up to scratch it works OK. What's the problem with some steel pipework?

As I said in #61, I think there is some liquid refrigerant when the whole thing is at ambient temperature. When the compressor has just stopped so the freezer is at -18° more of it will be liquid.. I know how propane tanks work! It's a liquid at storage pressure and the pressure stays constant...

I believe the UPS3 has high efficiency, perhaps at the expense of lower reliability. I would prefer a cheaper pump and to spend a few ££ more a year on electric, as most of the extra electric power ends up in the water anyway.

Don't know whether you found these sites Design a Refrigeration System - The Engineering Mindset and Refrigerant R-134a Not sure I have the inclination (or likely the ability) to follow all that, but at least the vapour pressure of R134a at various temperatures agrees with my calcs!

I was thinking of what the pressure is when the compressor stops but the freezer is still at -18°. In your scenario my guess is the refrigerant is partly liquid. The instructions say position the thing and leave it 24 hours before switching on, presumably for the liquid to gravitate down. If it...

Good points. I don't remember seeing that thread at the time or I'd have stuck my oar in! Amazing the installers didn't try turning the pump speed down. My rule is use lowest speed at which the system works. Confirms my suspicion of the USP3.

I suppose that's possible, but my guess is it would still work, but the freezer going above design temperature in line with ambient above design. This is interesting! If any of the refrigerant is to remain as liquid (in the heat dump grid) when the compressor is stopped, the pressure must be...

It certainly seems reasonable that the refrigerant for a freezer taking in heat at -19° and rejecting it at ~ 35° would be different from an AC unit taking in warm air at 20° and returning it at say 15°. Whether that's the case in practice I don't know. In any case, the pressure after the...

I was wondering about that, as I don't have any figures. I checked vapour pressure of refrigerants using the Antoine equation and R410a is 22bar (320 psi) at 35°C, so 400 psi sounds about right (35° could be too low). Is that the compressor differential? Any idea what the pressure settles down...

I don't see why the process would just stop. At moderate ambient temperature the compressor runs On/Off. As ambient rises the On times get longer, till at ambient design limit it runs continuously. Above that the freezer temperature rises.