In theory, you could make a refrigeration system using air as the working fluid:
Compress air. Air temp increases due to adiabatic heating.
Cool air back down to room temperature.
Expand air. Air temp decreases to below room temp due to adiabatic cooling.
Use cool air for refrigeration.
Real-world systems typically use some kind of CFC/HCFC as the working fluid. The one advantage about using these compounds that I can readily see is that evaporation/condensation allows for the transfer of much greater amounts of heat energy - which means more cooling capacity without resorting to outlandish pressures/temperatures/flow rates on the hot side of the system.
Are there other properties of CFCs/HCFCs that make them well-suited for use as working fluids in refrigeration systems?
That’s basically it: those compounds simply work better than anything else we’ve tried at that specific task. It’s a bit like asking why do we put rubber-like compounds on wheels, is it because those work better than all-steel tyres? Why, yes, yes it is.
Actually, the best working fluid for evaporation cycle heat pumps is Ammonia.
It transfers even more heat with a given mass flow. It is inexpensive and easy to work with. It is still used in large scale industrial heat pumps.
It is also very dangerous when it leaks out.
A big reason for the popularity of the CFCs / HCFCs etc is that they are very safe, while also being quite good in all other properties as a refrigerant (environmentally, not so much).
Thermal transfer is also an issue. Gases can’t contain as much heat per volume as liquids. So you need many times more volume of gas than liquid in the system, esp. key areas.
If the medium in the “evaporator” coils inside the fridge was still a gas, it would have to be pumped thru much faster (higher pressure) with more coils/fins/etc. Similarly, but not nearly as much, the “condenser” coils outside the fridge would have to be larger, etc.
So, more energy to move the amount of “cold” around plus more metal. Lose, lose.
you basically summed it up in your OP. they work the best at transferring heat using practicable operating pressures. CO2 can be used as a refrigerant, but compared to CFCs, HFCs, or HFO-1234, CO2 requires insanely high working pressures.
hydrocarbons like propane or isobutane work well as refrigerants too, but have that whole “flammability” problem.
There are fewer lumps of coal burned at your power plant to drop your house temp from 77 to 72 on a 95 degree day when you use one of those refrigerants. CO2 takes more coal than NH3. Water or air would take much much more. Also time.
It already takes quite a lot of power (and thus $$$) to run the refrigeration systems we have now, and they tend to be kind of bulky too. We COULD use compressed air, or peltier/thermoelectric coolers, but then we’d be trying to do what’s already quite energy-intensive at significantly worse efficiencies. I think peltier coolers are, at best, 1/4 as efficient as typical refrigerant-based systems, and they’re more expensive to build as well. Compressed air systems are even worse because air is a terrible heat conductor (think about how a single half-inch water pipe can deliver enough heat to a radiator for an entire room), plus the noise! We don’t use these systems for the same reason that you don’t replace your Chevy Suburban with an 18-wheeler if you’re already hard-up for gas money.
They are better because the working fluid goes through the liquid to gas phase as it expands into the evaporator. This provides a considerable increase in cooling compared to just expanding a gas. They would need a huge heat exchanger (no longer called an evaporator) and larger compressors to use a non condensing gas.
As mentioned, ammonia systems are simple. I had an ammonia refrigerator in my motor home and all it needed to work was either a small flame when on the road, or a heater when plugged into a 120 outlet. No compressor at all.
I worked on the solar dynamic system for the Space Station which needed a very high temperature phase transition fluid to store the solar energy for use in the dark portions of each orbit. They developed a LiF-CaF2 wax which melted at 1040 K. Very interesting project but it was dropped in favor of the more established solar cells.
Air refrigeration takes more power and there is the problem of frost.
CO2 is extremely high pressure and not as efficient.
Water has the highest latent heat of evaporation, but requires extremely low pressures and with low pressure differiential not very efficient.
NH3 is better, but people are afraid of it. And when used in a small AC unit or house hold appliance and begins to leak it stinks.
The best properties of CFCs and HCFC, non toxic. good latent heat of evaporation, safe to work with, and systems can be designed with very high efficiencies. The bad side if exposed to high heat they break down to dangerous gasses. Being a heavy gas when a leak in a closed room happens they can displace the oxygen in the room. And then there is the claim about Ozone depleation.
When using ammonia you have to be a little more careful with the metals you use, and in keeping the ammonia free of contaminants. The risk of stress corrosion cracking can be overcome by picking the right metals, but that tends to drive up costs. That cost increase may be offset by the efficiency or utility value of using an ammonia coolant system, but probably not in consumer refrigerators.
Propane seems to be a nearly ideal substitute for the increasingly proprietary and expensive non-CFC refrigerants. I think it has similar thermal efficiency to R-22 and the only real drawback is its flammability. But is that REALLY such a big deal? I can understand the apprehension for use in automobile a/c systems, especially since the condenser coil is front and center in any collision, but in refrigerators and building HVAC systems, is it really any more dangerous than the gas furnaces and similar appliances we already use?
The pentane marking could be for the blowing agent used in the spray foam insulation. It needs to have the foam removed and “degassed” before the rest of the fridge is scrapped.
But see, what’s the risk? You can have a high rise with gas kitchen appliances and furnaces and that doesn’t seem to be a problem. So why are hermetically sealed air conditioners a problem?
The issue is that no firefighter wants to go into a location which has a fire and which now has two or three new explosive devices (fridge, AC, and deep freezer) that are being heated rapidly.
At work, the local fire house has made a map of our building/labs which contain pressurized gas cylinders (just argon, nitrogen, and helium) and denoted that these are areas where:
if there isn’t fire, make sure to keep it away (same with the organic solvents, etc.)
if there is fire, put it out ASAP
but
if the fire is raging in these areas, CLEAR the neighborhood and use the truck and ladder nozzles.
Pressurized vessels are no joke and in a fire the pressure can easily go up 10x and if you add flammability, you escalate the problem.
yep, this. there was a kerfluffle with HFO-1234yf before automotive started using it; Daimler found that in an (unintentional) crash test, 1234yf exposed to flame (caused by oil igniting on the hot engine) decomposed into HF (hydrogen fluoride) gas, which is- to put it mildly- pretty nasty. On the other hand, when exposed to flame the current R-134a decomposes into the same compounds, and the old R-12/R-22 would break down into phosgene. which ain’t all that fun to be around either.