How does a low charge on an A/C cause the subcooling to be low

[PeterDonis]

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.

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.

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

Did you mean to say that reducing pressure doesn't have to reduce the temperature?

Oops, yes, I did. Sorry for the typo.

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

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.

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.

 

[jim hardy]

Sometimes I lose my connection and I lose all the content I've wrote in my posts.

Try the "reload" button on the page. Usually i can find one of the drafts that PF saves every few minutes.I'll try to address your questions.
I think you just haven't thought through them .. that happens when we're entering a new field.
Much of learning is after all just discovering what we already know.
Our everyday experience teaches us a lot yet we don't trust ourselves to use it in our new field. That's natural because we're so accustomed to making mistakes (at least i am)

I don't understand the premises of this question. I did not know that the compressor has to raise pressure to condense gas to liquid. I thought that gas changes to liquid strictly based on the temperature. If anything, I thought that reducing pressure would have the opposite effect on gas. If anything, I thought that reducing pressure would make it more likely that a gas would condense to liquid since reducing pressure reduces the temperature and low temperatures cause gas to condense, not high temperatures.

You already know that a gas condenses to liquid depending on what are its temperature AND pressure.
High school chemistry taught us that 'sticky' molecules have a high boiling point.
Air is a mix of not sticky oxygen and nitrogen molecules . Nitrogen liquifies (or boils) at -320F and oxygen at -297F. (at one atmosphere of pressure)

Water which has quite sticky molecules boils/liquifies at +212F (at the same pressure)
You know from your automobile temperature gage that 220 is in the normal range... not boiling because of the pressure cap on the radiator.

Freon is no different, just its molecules are somewhat less sticky so it changes phase at lower temperatures than water.
Heat being molecular motion, it'll help you to envision boiling as the molecules shaking so violently they escape their stickiness and separate into a gas .
Pressure pushes them together , so as pressure goes up it takes more heat to shake them apart.
That's why both pressure AND temperature affect boiling point.

Low pressure makes it easier for the molecules to separate and stay apart. So lowering pressure encourages evaporation not condensation.

Why does the compressor have to raise pressure to make liquid in the condenser?
Are you just saying that the compressor has to raise pressure to make liquid in the condenser only because the compressor has to have pressure to move the refrigerant from the compressor to the condenser?

No,
i'm saying that Freon in the condenser where it's a balmy 90F has to be at its saturation pressure to condense.
And that pressure is, from the thermo table, 168.4 psia.
If the compressor can't push pressure that high it won't condense.gotta go - supper's on

old jim

 

[jim hardy]

wow i see you guys are making progress so i'll stand by a while ..

Carry On, Gentlemen!

 

[PeterDonis]

You know from your automobile temperature gage that 220 is in the normal range... not boiling because of the pressure cap on the radiator.

AFAIK no car nowadays uses pure water as engine coolant, but either some kind of synthetic coolant or that mixed with water, so the actual boiling point of the stuff in the cooling system will be higher than shown in your chart. For example, when I was an automotive powertrain cooling engineer, the standard coolant we used, 50/50 water/glycol, boiled at 262 F at 15 psig. I know that not just from reading the charts for the standard coolant mix but from personal experience during several hot weather trailer towing tests.

 

[fourthindiana]

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.

The table I referenced from my textbook gives the pressure that R-22 will have at a given temperature. I don't think that the table I referenced from my textbook is giving the saturation pressure of R-22 at a given temperature though.

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:

I was aware of the fact that water boils at a lower temperature high up in the mountains as compared with sea level. However, I never thought to apply that same scientific principle over to refrigerants. As jim hardy says, much of learning consists of discovering what we already know.

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.

So if a higher percentage of the condenser is occupied by liquid that is subcooling, the pressure of the refrigerant changes since it is a liquid instead of a gas, and this causes the refrigerant to have a different saturation temperature?

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

Okay. I get it.

 

[PeterDonis]

The table I referenced from my textbook gives the pressure that R-22 will have at a given temperature.

If all you know is pressure, you can't tell what the temperature will be. You need to know either the volume or the density. And that's only for the gas phase, for fluids for which the ideal gas law is a good approximation. And the temperature for a given pressure will change as the volume or density changes, so if actual temperature is being given, there would have to be multiple tables to cover a range of volumes or densities, with each one specifying the volume or density it applies to.

I don't think that the table I referenced from my textbook is giving the saturation pressure of R-22 at a given temperature though.

If all the table is specifying is pressure, the only thing that makes sense is that it's telling you saturation temperature. See above.

I would have to see the textbook to be able to figure out what it's telling you.

 

[PeterDonis]

So if a higher percentage of the condenser is occupied by liquid that is subcooling, the pressure of the refrigerant changes since it is a liquid instead of a gas, and this causes the refrigerant to have a different saturation temperature?

No. The pressure in the condenser is determined by the pressure at the outlet of the compressor. It is the same everywhere in the condenser, and it doesn't change due to the phase change. The reason the pressure is higher if there is more charge is that the compressor outlet pressure is higher; and the reason for that is that with more charge in the system, you are putting more fluid into a constant volume so pressures everywhere in the system will be higher.

 

[fourthindiana]

You already know that a gas condenses to liquid depending on what are its temperature AND pressure.

I did learn that gas condenses to liquid depending on what are its temperature and pressure in my high school chemistry class twenty years ago. However, I had forgotten about that fact when I read your post #24 this morning.

High school chemistry taught us that 'sticky' molecules have a high boiling point.
Air is a mix of not sticky oxygen and nitrogen molecules . Nitrogen liquifies (or boils) at -320F and oxygen at -297F. (at one atmosphere of pressure)

I would be agreeing just for the sake of being agreeable if I agreed that high school chemistry taught me that "sticky" molecules have a high boiling point. Perhaps I did learn in high school chemistry that sticky molecules have a high boiling point. However, I cannot remember whether or not I learned that in high school chemistry class. I dunno.

Freon is no different, just its molecules are somewhat less sticky so it changes phase at lower temperatures than water.
Heat being molecular motion, it'll help you to envision boiling as the molecules shaking so violently they escape their stickiness and separate into a gas .

That makes sense.

Pressure pushes them together , so as pressure goes up it takes more heat to shake them apart.
That's why both pressure AND temperature affect boiling point.

I definitely learned this in high school chemistry class.

Low pressure makes it easier for the molecules to separate and stay apart. So lowering pressure encourages evaporation not condensation.

I learned this in high school chemistry class also.

You said it best: much of learning consists of discovering what we already know.

No,
i'm saying that Freon in the condenser where it's a balmy 90F has to be at its saturation pressure to condense.
And that pressure is, from the thermo table, 168.4 psia.
If the compressor can't push pressure that high it won't condense.

I have a table in my textbook that gives the pressure of R-22 at different temperatures. I didn't know that the pressures of R-22 at each different temperature are the saturation pressures of R-22.

 

[fourthindiana]

If all the table is specifying is pressure, the only thing that makes sense is that it's telling you saturation temperature. See above.

Good point. You're probably right.

I would have to see the textbook to be able to figure out what it's telling you.

Here is a photograph of the table I am referencing from my textbook:

 

[PeterDonis]

Here is a photograph of the table I am referencing from my textbook:

By itself that doesn't show enough context for me to see what it's a table of. But the 90 F entry in the 22 column says 168.4, which matches the table that @jim hardy posted in his entry, so this looks like a table of saturation pressures vs. temperature.

 

[fourthindiana]

No. The pressure in the condenser is determined by the pressure at the outlet of the compressor. It is the same everywhere in the condenser, and it doesn't change due to the phase change. The reason the pressure is higher if there is more charge is that the compressor outlet pressure is higher; and the reason for that is that with more charge in the system, you are putting more fluid into a constant volume so pressures everywhere in the system will be higher.

To me, this sounds like you are reversing the causal logic.

In post #35, I asked if the only reason that a higher percentage of the condenser being occupied by subcooling liquid increases the subcooling is that a higher percentage of the condenser being occupied by subcooling liquid makes each parcel of refrigerant keep subcooling for a longer amount of time. You responded that there are small effects due to the change in saturation temperature with pressure. I don't see how a higher percentage of the condenser being occupied by subcooling liquid would cause small effects on the saturation temperature with pressure.

If you are saying that a higher percentage of the condenser being occupied by subcooling liquid would cause more subcooling by small effects on the saturation temperature with pressure, it seems to me like your causal logic is wrong or I am not understanding something.

Are you just saying that part of the reason that increasing the amount of refrigerant in the system will increase the subcooling is that increasing the charge will cause a small effect on the saturation temperature with the pressure but you are not saying that the higher percentage of the condenser being occupied by subcooling liquid is directly causing the small effect on the saturation temperature?

 

[PeterDonis]

I don't see how a higher percentage of the condenser being occupied by subcooling liquid would cause small effects on the saturation temperature with pressure.

That's not what I said. The small effects on saturation temperature with pressure has nothing whatever to do with how much of the condenser is occupied by subcooled liquid. I was simply pointing out that, as you change the charge, the pressure changes, therefore the saturation temperature of the refrigerant changes.

 

[fourthindiana]

By itself that doesn't show enough context for me to see what it's a table of. But the 90 F entry in the 22 column says 168.4, which matches the table that @jim hardy posted in his entry, so this looks like a table of saturation pressures vs. temperature.

Here is the entire text with the table: "Figure 3.15 This chart shows the temperature/pressure relationship in in. Hg vacuum, or psig. Pressures for R-404A and R-410A are average liquid and vapor pressures."

 

[fourthindiana]

That's not what I said. The small effects on saturation temperature with pressure has nothing whatever to do with how much of the condenser is occupied by subcooled liquid. I was simply pointing out that, as you change the charge, the pressure changes, therefore the saturation temperature of the refrigerant changes.

Good. I was hoping that you would say that the small effects on saturation temperature with pressure has nothing whatever to do with how much of the condenser is occupied by subcooled liquid. I like it when everything makes sense to me.

In your post #36, you were just telling me that there is another reason that there are two reasons that increasing the charge increases subcooling: 1# increasing the charge increases the percentage of the condenser being occupied by subcooling liquid and 2# increasing the charge increases the saturation temperature of the refrigerant.

I finally feel like I understand this 100%. Well done. You would have been a good physics/engineering professor.

 

[PeterDonis]

In your post #36, you were just telling me that there is another reason that there are two reasons that increasing the charge increases subcooling: 1# increasing the charge increases the percentage of the condenser being occupied by subcooling liquid and 2# increasing the charge increases the saturation temperature of the refrigerant.

Yes. More precisely, we now have two causal chains. The first is the one I gave in post #36:

More of condenser occupied by liquid refrigerant -> More time spent being subcooled by each parcel -> More subcooling

The second is the effect I was thinking of when I mentioned that higher pressure increases the saturation temperature:

Higher saturation temperature -> Higher difference between the saturation temperature and the outside air temperature -> More heat transfer per unit time from refrigerant to air -> More subcooling

To make the distinction clearer, I'll rewrite those two causal chains but now adding an extra notation (in italics) to each one:

More of condenser occupied by liquid refrigerant -> More time spent being subcooled by each parcel for the same heat transfer per unit time -> More subcooling

Higher saturation temperature -> Higher difference between the saturation temperature and the outside air temperature -> More heat transfer per unit time from refrigerant to air for the same time spent being subcooled -> More subcooling

 

[PeterDonis]

Well done. You would have been a good physics/engineering professor.

Thanks!

 

[fourthindiana]

Thanks!

You're welcome.

 

[fourthindiana]

PeterDonis, in the OP (almost a year ago), I asked how low charge on an A/C causes the subcooling to be low. I gave my explanation for why low charge on an A/C causing low subcooling is counterintuitive. You told me that I am not taking into account the effect of the reservoir of refrigerant between the condenser and the expansion device. You were highly skilled at explaining the shortcomings of my own thought process on this. You answered various questions about this based on my point of view.

I am curious as to how you would explain how low charge on an A/C causes low subcooling when you are just explaining this from scratch, not rebutting someone else's explanation of it. I suppose you could give a highly informative explanation of how low charge causes low subcooling in, say, 200 words or less.

How does low charge on an A/C cause low subcooling?

 

[PeterDonis]

I am curious as to how you would explain how low charge on an A/C causes low subcooling when you are just explaining this from scratch, not rebutting someone else's explanation of it.

Look at post #24 from @jim hardy as an example of such an explanation. He simply describes the process of adding charge to the system and seeing what happens.

More generally, you seemed all through this thread, and still seem, to be looking for "the" explanation of how low charge "causes" low subcooling. But that very way of framing the question misleads you. There is not a single explanation and there is not a single cause. There is a whole system which is continuously flowing, and as you change the charge level, various things happen; and there are various ways of looking at the whole process. Focusing on one particular thing will not help you understand the whole process; rather, you need to understand the whole process first, and then you can simply "read off" particular things from the whole process.