- #36
PeterDonis
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fourthindiana said:I'm assuming that if the temperature is 90 F, and the refrigerant is at atmospheric pressure (which is zero psig), the refrigerant will be a vapor, but I'm only assuming that because I read your post #34 before I wrote this reply.
You don't have to assume anything. @jim hardy posted the tables for the refrigerant, which clearly show, as he posted, that it is in fact a vapor at 90 F and atmospheric pressure. Those tables are derived from actual measurements.
fourthindiana said:Before I read your post #34, I would have probably guessed that the refrigerant would be a liquid because I would associate low pressures with coldness, and I would associate coldness with liquid rather than vapor.
This is why you shouldn't guess. You should look at the actual measured properties of an actual refrigerant. These general heuristics you are using will lead you astray.
fourthindiana said:Based on the fact that your post #34 says that if you keep the temperature constant, and if you raise the pressure, eventually the refrigerant will condense to a liquid, I suppose that the R-22 would be in liquid form at 90F and 184 psia.
Again, you don't have to suppose, and you don't have to base it on what I said. You can base it on the actual facts: the table your textbook has, which, again, is based on actual measurements of that refrigerant. Similar tables exist for any refrigerant that is in common use, not to mention many other fluids. For example, this Google search gives links to similar tables for water (which is used as a working fluid in many commercial power plants):
https://www.google.com/search?clien...rmodynamic+tables+for+water&ie=utf-8&oe=utf-8
fourthindiana said:Did you mean to say that reducing pressure doesn't have to reduce the temperature?
Oops, yes, I did. Sorry for the typo.
fourthindiana said:I didn't know that reducing the pressure reduces the saturation temperature.
You weren't aware that, for example, water boils at a lower temperature high up in the mountains as compared with sea level? This comes into play when people cook in, for example, Denver:
https://en.wikipedia.org/wiki/High-altitude_cooking
fourthindiana said:the only reason that how much of the condenser is occupied by liquid refrigerant being subcooled determines the amount of subcooling is that how much of the condenser is occupied by liquid refrigerant being subcooled determines how much time each parcel of refrigerant spends being subcooled.
This is the major reason, yes. I think there are also small effects due to the change in saturation temperature with pressure, but I would have to look at the detailed numbers.
In other words, the causal logic is:
More of condenser occupied by liquid refrigerant -> More time spent being subcooled by each parcel -> More subcooling
But saying that time spent being subcooled is the critical variable (which is post #4 said was not correct) would imply that the causal logic is:
More time spent being subcooled by each parcel -> More of condenser occupied by liquid refrigerant -> More subcooling
Which is backwards.
fourthindiana said:that sounds like more evidence that my understanding is correct.
It means that #1 is the only viable option of the two you gave, yes. But that in itself doesn't tell you what the causal logic is. See above.