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

[fourthindiana]

Guys

i apologize up front for the seeming scrambledness of this post
but i think to teach refrigeration properly needs a combination of textbook theory and practical hands-on

and this is the result of a few hours' trying to put myself in @fourthindiana 's shoes.
Here goes. If this only confuses things please advise and i'll delete it.

jim hardy, I did read your entire post when you posted this on Tuesday. On post #28 on Tuesday, I wrote the following in response to your post: "Thank you for your insightful post, jim hardy. Your 'reducto ad adsurdum' method helps me understand this." However, frankly, I did not really understand your post on Tuesday. I mostly just said that your post helps me understand this because I felt indebted to you and didn't want you to think you went to the trouble for nothing since you obviously spent so much time and energy on this. I work at a job in addition to attending a trade school as a full time student. I was on a big sleep deficit when I read your post on Tuesday night. Your post is just too complicated for me to understand when I am tired. However, I would really like to be able to understand your post. I think you and others here could help me understand your post.

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Sometimes it's a useful analytical tool to make a thought experiment that takes things to the extreme. I think it's called "Reducto ad Absurdum".

I don't think I have ever explained something to someone else using a thought experiment that takes things to extreme. But I think I have used a thought experiment that takes things to extreme myself before to help me understand scientific principles.

What if you reduce the charge so low that the compressor can't raise pressure enough to make any liquid anyplace in the system ?

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.

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?

It'd be in superheat everywhere.

I understand that if the refrigerant gas never condenses to liquid, the refrigerant would be superheated everywhere.

I'm having a lot of connection issues on my computer. Sometimes I lose my connection and I lose all the content I've wrote in my posts. I will respond to your post in this post and another post.

 

[fourthindiana]

at that point we have zero subcooling. So if there's any liquid in the condenser it's not much.We are just beginning to involve phase change.
Look at the H_Liquid column - it's down to 36.158 BTU/lb
Suction pressure should be $\frac {discharge ~pressure} {compression~ ratio}$ = 183/5 = 36.6psia
which is the pressure in the evaporator
so go back to the saturation table for that pressure
that liquid is going to evaporate at about -3°F, and absorb 94 BTU/lb as it does
but there's not much liquid so it evaporates quickly and gets ever more superheated as it continues toward compressor suction
View attachment 239385

As we add more refrigerant we can make more than just a tiny droplet of liquid in the condenser
so the liquid level in there will rise
and the greater amount of liquid that's entering the evaporator will fully evaporate further up the evaporator before beginning its superheat

I think I got the gist of what you were saying until you got to the chart you included right above the sentence "As we add more refrigerant we can make more than just a tiny droplet of liquid in the condenser." Here is my interpretation of what you're saying here: "The liquid refrigerant absorbs 94 BTU/lb as the liquid refrigerant evaporates. 94 BTU/lb is a lot more than the 9.608 BTU/lb the liquid refrigerant absorbs before phase change. Therefore, phase change is dollars and subcooling and superheating is pennies."

But what about the fact that your chart says that the vapor enthalpy is 104 BTU/lb? 104 BTU/lb>94 BTU/lb ---Based on this fact, couldn't someone say that superheating is more dollars than phase change?

I don't have much of a scientific background. My only scientific background that is relevant to this subject is Chemistry class in my junior year of high school, and physics class is my senior year of high school. I graduated from high school in the year 2000, so I am even rusty as far as the limited amount of what I learned in these classes in high school.

When I see your chart saying BTU/lb of Enthalpy for liquid, latent, and vapor, I know that it is about heat somehow, but I don't know exactly what it means.

As I recollect, entropy somehow expresses the "chaos/orderliness" of the heat of a system, and that is 100% of what I know about entropy. In other words, I know extremely little about what entropy means.

Please help me understand this.

 

[fourthindiana]

Because more of the condenser's heat exchange capacity is being used to subcool already condensed liquid, as opposed to changing vapor to liquid.

Back in October, I said I posted that I understood how low charge causes low subcooling immediately after I read this post of yours. I do understand this to a degree. Probably 99.9% of HVAC students would say that they fully understand this when they understand it on the level that I currently understand it. I'm in the 0.1% of other HVAC students.

In post #18, you told me that it would help to rephrase things in terms of time: "when you have more subcooled refrigerant inside the condenser, that means each tiny little parcel of refrigerant, as it flows through the condenser, spends more time being subcooled." That makes sense to me 100%. If it weren't for what you wrote in post #4, I would leave it at that.

In post #3, I wrote "In a nutshell, my understanding of what you're saying is that if you decrease the charge in a system, the liquid refrigerant will spend less time in the condenser, which means it will subcool less. Is that correct?"

In post #4, you replied "No. If you decrease the charge, there will be less liquid refrigerant in the system. It's not a matter of time spent in the condenser; at least, that's not the critical variable. The critical variable is how much of the condenser is occupied by subcooled liquid refrigerant, as opposed to a liquid/vapor mixture at the saturation point. As charge is reduced, that amount decreases."

There are only two ways that I can think of that having more of the condenser's heat exchange capacity being used to subcool already condensed liquid can cause increased subcooling: 1# each parcel of refrigerant spends more time being subcooled or 2# some parcels of refrigerant never get subcooled at all if less of the condenser is occupied by subcooled liquid refrigerant.

I don't think that I am thinking of this in terms of batches in this post because I am using the terms that you used.

How does having more of the condenser's heat exchange capacity being used to subcool already condensed liquid (as opposed to changing vapor to liquid) increase the subcooling if time is NOT the critical factor?

 

[PeterDonis]

I did not know that the compressor has to raise pressure to condense gas to liquid.

If the temperature is 90 F, and the refrigerant is at atmospheric pressure, will it be a gas or a liquid? How about 90 F and 184 psi (absolute)?

I thought that gas changes to liquid strictly based on the temperature.

We usually think of phase changes as being triggered by changes in temperature at constant pressure; but phase changes can also be triggered by increasing pressure at constant temperature. That's what @jim hardy was describing: you start out with a gas, raise the pressure, and eventually it will condense to a liquid.

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

Not necessarily. First, reducing pressure doesn't have to reduce the pressure (it depends on how the volume is allowed to change). Second, reducing pressure does reduce the saturation temperature; so even if the actual temperature is reduced, it might not be reduced as much as the saturation temperature is reduced. So reducing pressure might make it less likely that a gas would condense to a liquid, if the saturation temperature decreases more than the actual temperature does.

The advice @jim hardy gave to look at the actual data tables for an actual refrigerant is a good one. You really can't think of this stuff purely in the abstract. You need to know the actual properties of an actual fluid, and pick some actual concrete numbers and investigate their consequences. That's what @jim hardy did in his post, and that post is an excellent example to follow.

the vapor enthalpy is 104 BTU/lb?

Don't confuse enthalpy with the change in enthalpy. The changes are what are important.

I know that it is about heat somehow, but I don't know exactly what it means.

You actually don't need to know exactly what it means; just knowing that it's "about heat somehow" is enough for this discussion. You can dig into a thermodynamics textbook if you want the gory details, but I don't think they're really essential here. "Heat content" is enough.

entropy somehow expresses the "chaos/orderliness" of the heat of a system

Yes, but again, you don't need to know the gory details for this discussion. The only function entropy plays in this analysis is that it remains constant during compression, so you can figure out the properties of the compressor out state by knowing the entropy of the compressor in state and the compression ratio (which gives you the outlet pressure from the inlet pressure).

This is another reason why knowing the properties of an actual refrigerant, by looking at charts like the ones @jim hardy used, is helpful: it let's you figure out values for the key thermodynamic properties (enthalpy, entropy, pressure, temperature) without having to understand all the gory details about what they mean. HVAC technicians have to do that all the time: they usually don't know the gory details either, but they still have to be able to figure out what's wrong with a system and fix it.

If it weren't for what you wrote in post #4, I would leave it at that.

Post #4 did not say that a parcel does not spend more time being subcooled if there is more charge. It said the time is not "the critical variable". The critical variable is how much of the condenser is occupied by liquid refrigerant being subcooled, vs. saturated refrigerant undergoing phase change. The fact that more of the condenser is occupied by liquid refrigerant if there is more charge is the reason why a parcel spends more time being subcooled.

1# each parcel of refrigerant spends more time being subcooled or 2# some parcels of refrigerant never get subcooled at all if less of the condenser is occupied by subcooled liquid refrigerant.

#2 can't be right, because the thermodynamic properties of the refrigerant at a fixed location inside the system, in a steady state, should not change from one parcel to the next. So each parcel passing the exit of the condenser, for example, should have the same subcooling if everything is in a steady state. Otherwise it wouldn't be a steady state.

 

[fourthindiana]

If the temperature is 90 F, and the refrigerant is at atmospheric pressure, will it be a gas or a liquid?

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.

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.

How about 90 F and 184 psi (absolute)?

I looked at a chart in my textbook about temperature/pressure relationships of R-22. The chart in my textbook says that the corresponding pressure for R-22 at 90 F is 168.4 psig. 168.4 psig is 183.1 psia. 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.

We usually think of phase changes as being triggered by changes in temperature at constant pressure; but phase changes can also be triggered by increasing pressure at constant temperature. That's what @jim hardy was describing: you start out with a gas, raise the pressure, and eventually it will condense to a liquid.

Not necessarily. First, reducing pressure doesn't have to reduce the pressure (it depends on how the volume is allowed to change).

Based on the context, I think that you meant to say that reducing pressure doesn't have to reduce the temperature. Did you mean to say that reducing pressure doesn't have to reduce the temperature?

Second, reducing pressure does reduce the saturation temperature; so even if the actual temperature is reduced, it might not be reduced as much as the saturation temperature is reduced.

I didn't know that reducing the pressure reduces the saturation temperature.

So reducing pressure might make it less likely that a gas would condense to a liquid, if the saturation temperature decreases more than the actual temperature does.

Interesting.

The advice @jim hardy gave to look at the actual data tables for an actual refrigerant is a good one. You really can't think of this stuff purely in the abstract. You need to know the actual properties of an actual fluid, and pick some actual concrete numbers and investigate their consequences. That's what @jim hardy did in his post, and that post is an excellent example to follow.

Interesting.

Don't confuse enthalpy with the change in enthalpy. The changes are what are important.

Okay, but some of jim hardy's post still went over my head.

You actually don't need to know exactly what it means; just knowing that it's "about heat somehow" is enough for this discussion.

I think I need to know more about what it means because I don't fully understand jim hardy's post.

Post #4 did not say that a parcel does not spend more time being subcooled if there is more charge. It said the time is not "the critical variable". The critical variable is how much of the condenser is occupied by liquid refrigerant being subcooled, vs. saturated refrigerant undergoing phase change. The fact that more of the condenser is occupied by liquid refrigerant if there is more charge is the reason why a parcel spends more time being subcooled.

It's my understanding that 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. Is my understanding correct?

#2 can't be right, because the thermodynamic properties of the refrigerant at a fixed location inside the system, in a steady state, should not change from one parcel to the next. So each parcel passing the exit of the condenser, for example, should have the same subcooling if everything is in a steady state. Otherwise it wouldn't be a steady state.

To me, that sounds like more evidence that my understanding is correct.

 

[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.