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

## Main Question or Discussion Point

I know that having both high superheat and low subcooling on an Air-Conditioner (A/C) is a strong indication that an air-conditioner does not have enough refrigerant.

When the charge on an A/C is low, there is less refrigerant that enters the evaporator. When there is a lower amount of refrigerant entering the evaporator, it takes less heat to boil the refrigerant in the evaporator. When it takes less heat to boil the refrigerant in the evaporator, the refrigerant in the evaporator boils quicker. When the refrigerant in the evaporator boils quicker, this causes the refrigerant to both absorb heat quicker and to absorb more total heat. This causes the superheat to increase.

However, I don't really understand why low charge on an A/C causes the subcooling to be low. When the charge on an A/C is low, less refrigerant enters the condenser. When less refrigerant enters the condenser, it seems to me that the refrigerant would condense quicker since less heat has to be given off to condense a small amount of liquid than a large amount of liquid. If a small amount of refrigerant would condense quicker than a large amount of refrigerant, I would expect that low charge would cause the refrigerant to shed its heat quickly, which would cause subcooling to be high. However, I have always read and heard that a low charge causes the subcooling to be low.

How does low charge on an A/C cause the subcooling on an air-conditioner to be low?

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PeterDonis
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How does low charge on an A/C cause the subcooling on an air-conditioner to be low?
You are not taking into account the effect of the reservoir of liquid refrigerant between the condenser and the expansion device.

In a properly charged A/C system, there is a substantial reserve of liquid refrigerant between the condenser and the expansion device. This liquid is subcooled. By how much is it subcooled? Well, that depends on how much cooling it receives once it has fully condensed to liquid in the condenser. (Seems obvious, right?) And that in turn depends on how much reserve refrigerant there is in the system: heuristically, the more reserve refrigerant there is, the more time the liquid can spend as liquid being subcooled, before it reaches the expansion device and enters the evaporation phase of the cycle. (Many systems have a liquid reservoir between the condenser and the expansion device for this very purpose.)

Now, what happens if you decrease the charge? It seems obvious: you will decrease the amount of reserve refrigerant in the system, which decreases the amount of liquid in the reservoir between the condenser and the expansion device, which means the liquid will spend less time in that portion of the system, which means it will subcool less. That is why low charge causes low subcooling.

You might want to also reconsider your analysis of why low charge causes superheat at the evaporator outlet to be high.

You are not taking into account the effect of the reservoir of liquid refrigerant between the condenser and the expansion device.
Before I read your reply here, I did not know that the reservoir of liquid refrigerant between the condenser and the expansion device was a factor in how much subcooling there is in a system. Are you talking about the accumulator?

In a properly charged A/C system, there is a substantial reserve of liquid refrigerant between the condenser and the expansion device. This liquid is subcooled. By how much is it subcooled? Well, that depends on how much cooling it receives once it has fully condensed to liquid in the condenser. (Seems obvious, right?) And that in turn depends on how much reserve refrigerant there is in the system: heuristically, the more reserve refrigerant there is, the more time the liquid can spend as liquid being subcooled, before it reaches the expansion device and enters the evaporation phase of the cycle. (Many systems have a liquid reservoir between the condenser and the expansion device for this very purpose.)

Now, what happens if you decrease the charge? It seems obvious: you will decrease the amount of reserve refrigerant in the system, which decreases the amount of liquid in the reservoir between the condenser and the expansion device, which means the liquid will spend less time in that portion of the system, which means it will subcool less. That is why low charge causes low subcooling.
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?

Another question has popped up in my mind: Why is it the case that the more reserve refrigerant there is, the more time the liquid can spend as liquid being subcooled?

Is that the case because if you have a lot of refrigerant, that excess of refrigerant in the reservoir physically impedes the flow of refrigerant from the condenser to the expansion device, causing the liquid refrigerant to spend more time in the condenser?

You might want to also reconsider your analysis of why low charge causes superheat at the evaporator outlet to be high.
What did I get wrong in my original analysis of why low charge causes superheat at the evaporator outlet to be high?

PeterDonis
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I did not know that the reservoir of liquid refrigerant between the condenser and the expansion device was a factor in how much subcooling there is in a system.
The term "reservoir" might be misleading. It does not mean there is necessarily an actual extra device in the system to hold extra liquid refrigerant. (In automotive systems with TXV's, there usually is such a device, called a receiver/drier. In orifice tube systems, there is usually no such device, but the lower part of the condenser serves a similar function.) It just means that there is some amount of liquid refrigerant in the system, and that amount decreases if the refrigerant charge decreases.

Are you talking about the accumulator?
No. The accumulator is between the evaporator and the compressor, and accumulates gaseous refrigerant. I'm talking about liquid refrigerant between the condenser and the expansion device.

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?
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 saturation point. As charge is reduced, that amount decreases.

Why is it the case that the more reserve refrigerant there is, the more time the liquid can spend as liquid being subcooled?

Is that the case because if you have a lot of refrigerant, that excess of refrigerant in the reservoir physically impedes the flow of refrigerant from the condenser to the expansion device, causing the liquid refrigerant to spend more time in the condenser?
No. At least, I've never seen any indication that back pressure of this sort is significant in A/C systems.

What did I get wrong in my original analysis of why low charge causes superheat at the evaporator outlet to be high?
You failed to consider the effect of decreased subcooling on what happens in the evaporator. If the liquid refrigerant at the expansion device is subcooled less, it will evaporate more quickly inside the evaporator and will be superheated more when it exits the evaporator. That is why superheat goes up when charge is reduced. All of the stuff you talk about in your analysis is irrelevant.

More generally, you keep on thinking in terms of "less refrigerant" inside a particular device (evaporator, condenser, whatever). But there is no such thing as "the amount of refrigerant" inside any part of the system. Refrigerant is flowing continuously throughout the system; it doesn't sit in one device until something is finished and then move on. The only unit of analysis in which you can usefully think of an amount of refrigerant is the whole system. For a correct analysis of an individual device, you need to look at the flow rate through the device, not the "amount of refrigerant" inside it.

The term "reservoir" might be misleading. It does not mean there is necessarily an actual extra device in the system to hold extra liquid refrigerant. (In automotive systems with TXV's, there usually is such a device, called a receiver/drier. In orifice tube systems, there is usually no such device, but the lower part of the condenser serves a similar function.) It just means that there is some amount of liquid refrigerant in the system, and that amount decreases if the refrigerant charge decreases.
Thanks for the clarification.

No. The accumulator is between the evaporator and the compressor, and accumulates gaseous refrigerant. I'm talking about liquid refrigerant between the condenser and the expansion device.
The reservoir device I was thinking of is the receiver (just as you mentioned in the first paragraph of your reply). I know you were talking about liquid refrigerant between the condenser and the expansion device.

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 saturation point. As charge is reduced, that amount decreases.
Why is the critical variable how much of the condenser is occupied by subcooled liquid refrigerant (as opposed to a liquid/vapor mixture at the saturation point)? If you have more subcooled liquid refrigerant in the condenser, does that somehow increase the subcooling? If so, how?

I mean, if you have four ounces of liquid refrigerant in the condenser, how would that cause you to have more subcooling than you would have had if you had 2 ounces of liquid refrigerant in the condenser?

No. At least, I've never seen any indication that back pressure of this sort is significant in A/C systems.
Okay. That rules out that back pressure of that sort is the critical factor.

You failed to consider the effect of decreased subcooling on what happens in the evaporator. If the liquid refrigerant at the expansion device is subcooled less, it will evaporate more quickly inside the evaporator and will be superheated more when it exits the evaporator. That is why superheat goes up when charge is reduced. All of the stuff you talk about in your analysis is irrelevant.
Good point.

More generally, you keep on thinking in terms of "less refrigerant" inside a particular device (evaporator, condenser, whatever). But there is no such thing as "the amount of refrigerant" inside any part of the system. Refrigerant is flowing continuously throughout the system; it doesn't sit in one device until something is finished and then move on. The only unit of analysis in which you can usefully think of an amount of refrigerant is the whole system. For a correct analysis of an individual device, you need to look at the flow rate through the device, not the "amount of refrigerant" inside it.
Interesting.

PeterDonis
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If you have more subcooled liquid refrigerant in the condenser, does that somehow increase the subcooling? If so, how?
Because more of the condenser's heat exchange capacity is being used to subcool already condensed liquid, as opposed to changing vapor to liquid.

Because more of the condenser's heat exchange capacity is being used to subcool already condensed liquid, as opposed to changing vapor to liquid.
Got it! Wow! You did it again (just like on my "superheated vapors and Gay-Lussac's Law" thread).

I believe I understand this completely now. Good explanation.

PeterDonis
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Thank you for teaching me this information.

PeterDonis
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Thank you for teaching me this information.
You're welcome!

PeterDonis, I was reviewing this four month old thread today, and it occurred to me that I don't understand something that you said.

The critical variable is how much of the condenser is occupied by subcooled liquid refrigerant, as opposed to a liquid/vapor mixture at saturation point. As charge is reduced, that amount decreases.
You never told me how a reduction in charge causes a lower percentage of the condenser to be occupied by subcooled liquid and a higher percentage of the condenser to be occupied by a liquid/vapor mixture at saturation point.

My house has an air-conditioner that is supposed to have 6 pounds of refrigerant in it. Let's say that when my air-conditioner is properly charged with 6 pounds of refrigerant, 40% of my condenser is occupied by a subcooled liquid refrigerant. Therefore, let's say that the other 60% of my condenser is occupied by a liquid/vapor mixture at saturation point. According to what you are saying, if my system develops a leak and the total amount of refrigerant in my air-conditioner drops from 6 pounds to 4 pounds, there would be two effects on my condenser: #1 Less of my condenser would be occupied by subcooled liquid refrigerant and ,thus, #2 more of my condenser would be occupied by a liquid/vapor mixture at saturation point. Why would a reduction of the total charge in my system cause less of my condenser to be occupied by subcooled liquid refrigerant and more of my condenser to be occupied by a liquid/vapor mixture?

PeterDonis
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You never told me how a reduction in charge causes a lower percentage of the condenser to be occupied by subcooled liquid and a higher percentage of the condenser to be occupied by a liquid/vapor mixture at saturation point.
Vapor expands, liquid doesn't. As the absolute quantity of liquid in the condenser decreases, the percentage of the condenser that the liquid occupies will as well.

Vapor expands, liquid doesn't. As the absolute quantity of liquid in the condenser decreases, the percentage of the condenser that the liquid occupies will as well.
Okay. Good point. You've cleared it up well. Thank you.

PeterDonis, I know that your explanation of why low charge in an A/C causes low subcooling is correct. I am making this post in an effort to fully understand your explanation because I don't currently full understand it. In other words, I am not arguing with you in this post. I'm just trying making this post and asking these questions to understand your explanation.

More generally, you keep on thinking in terms of "less refrigerant" inside a particular device (evaporator, condenser, whatever). But there is no such thing as "the amount of refrigerant" inside any part of the system. Refrigerant is flowing continuously throughout the system; it doesn't sit in one device until something is finished and then move on. The only unit of analysis in which you can usefully think of an amount of refrigerant is the whole system. For a correct analysis of an individual device, you need to look at the flow rate through the device, not the "amount of refrigerant" inside it.
After I read your post #4 on this thread back in October, I started thinking of the effect of low charge level on subcooling terms of amount of refrigerant in the whole system instead of thinking in terms of "less refrigerant" in the condenser, and your explanation made sense to me, and your explanation still makes sense to me when I think of this issue in terms of the amount of refrigerant in the whole system as opposed to "less refrigerant" in the condenser. Back in October, your explanation made me think that I fully understood this. But it occurred to me yesterday that I don't fully understand this. I really don't fully understand why I cannot think of this issue in terms of "less refrigerant in the condenser". You did cover this briefly in post #4. You said "Refrigerant is flowing continuously throughout the system; it [refrigerant] doesn't sit in one device until something is finished and then move on." I agree with you 100% that refrigerant is flowing continuously throughout the system. I agree with you 100% that refrigerant does not sit in one device until something is finished and then move on.

When I run my air-conditioner at my house, the refrigerant does flow continuously throughout the system. Refrigerant does not sit in one device until something is finished and then move on. But at any one instant my compressor is pumping refrigerant, there is an EXACT amount of refrigerant in my system. There might be 2.00 pounds of refrigerant in my condenser at 12:05:00 p.m. Then there might be 2.03 pounds of my refrigerant in my condenser thirty seconds later at 12:05:30 p.m.

Just because the weight fluctuates and there will be an infinitesimally smaller or infinitesimally larger amount of refrigerant in my condenser from second to second, there is still an exact amount of refrigerant in my condenser at any instant in time.

Do you agree with me that it is a fact that there is an exact, definite amount of refrigerant in my condenser at any instant in time?

If you agree with me that it is a fact that there is an exact, definite amount of refrigerant in my condenser at any instant in time, why does it lead me down the wrong path when I try to understand how low charge causes low subcooling when I think of this issue in terms of "Less refrigerant in the condenser?"

PeterDonis
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Do you agree with me that it is a fact that there is an exact, definite amount of refrigerant in my condenser at any instant in time?
Yes. But it doesn't mean what you think it means. Consider one second, and then the next second. At each second, there is some exact, definite amount of refrigerant in the condenser. But not only can that amount change from one second to the next, which particular molecules of refrigerant are included in the amount will change from second to second, because the refrigerant is flowing. Your thinking of it in terms of "less refrigerant inside the condenser" implicitly assumes that that refrigerant is sitting still inside the condenser for some period of time. It isn't. Similarly, your thinking of it in terms of "less refrigerant entering the evaporator will boil quicker" assumes that some parcel of refrigerant, whose size depends on the amount of charge in the system, enters the evaporator, gets boiled, and then exits, and then the next parcel enters. That's not correct; refrigerant is continously flowing throughout the system. So there is no "amount of refrigerant inside the condenser" or "amount of refrigerant being boiled inside the evaporator" at any time; these are not batch processes happening to a fixed parcel of refrigerant at a time, they are continuous processes happening in a continuously flowing system.

jim hardy
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I often say "Use everyday sensory experiences to sharpen our grasp of the fundamentals".

The last airconditioner i charged, an ancient one ton "windowshaker" in my garage, gave me a good demonstration.
I could put my hands directly on the evaporator because i was working right in front of it.
As you know, you can feel a small difference in temperature by using two hands.

As i began adding Freon i felt both the entry tube at the bottom and the exit tube at the top . The difference was startling
Then i noticed by feeling the front of the evaporator i could tell how far up the evaporator was the transition from liquid mix to dry gas - it warms quickly above that level.
As i added refrigerant i felt two effects
1. Air coming out got colder
2. The transition point moved progressively farther up the evaporator.

I stopped when the unit was using roughly the lower 2/3 of the evaporator for phase change and upper 1/3 for superheat, .as detected by feel .
I figured that left me "headroom" against overfill
and is about the only way to estimate state of charge when you don't have a high side pressure tap.

The unit works great
but far more important to me is @PeterDonis 's points are now intuitive.

Try it yourself. There's nothing like a visceral demonstration of physics to make sense of it.

old jim

late edit
if you try that method you'll be better able to estimate your transition point by feeling the tubes at edges of evaporator, if they're accessible. Mine was an ancient Sears with copper tubes that i could reach.

Last edited:
PeterDonis, even when I think of this in terms of the amount of refrigerant in the entire system instead of the amount of refrigerant in one device, I still don't understand this 100%. I will only think of this in the terms that you say are the right way to think about it in this post.

PeterDonis, back in October, I initially said "If you have more subcooled liquid refrigerant in the condenser, does that somehow increase the subcooling? If so, how?"

You responded with the following pithy statement:
Because more of the condenser's heat exchange capacity is being used to subcool already condensed liquid, as opposed to changing vapor to liquid.
I'm assuming that if you have a large pot of 50 degree liquid water on a 10 pound block of ice, and if you have a small pot of 50 degree liquid water on a different 10 pound block of ice, the small pot of 50 degree liquid water will cool faster.

How do you reconcile this with the fact that when you have more subcooled liquid refrigerant in the condenser, there is more thermal inertia to overcome to further subcool the liquid refrigerant?

In other words, when you have more subcooled liquid refrigerant in the condenser, yes, more of the condenser's heat exchange capacity is being used to subcool already condensed liquid, as opposed to changing vapor to liquid, but there is also proportionally just as much thermal inertia to overcome. When you have more liquid refrigerant in the condenser, why does the extra thermal inertia to overcome not cancel out the fact that more of the condenser's heat exchange capacity is being used to subcool already condensed liquid?

PeterDonis
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I'm assuming that if you have a large pot of 50 degree liquid water on a 10 pound block of ice, and if you have a small pot of 50 degree liquid water on a different 10 pound block of ice, the small pot of 50 degree liquid water will cool faster.

How do you reconcile this with the fact that when you have more subcooled liquid refrigerant in the condenser, there is more thermal inertia to overcome to further subcool the liquid refrigerant?
Don't you see how you keep thinking of things in terms of batches instead of flows? There is no "block of refrigerant" inside the condenser, or anywhere else. Refrigerant is continuously flowing through the condenser, and the rest of the system. It doesn't just sit there and wait to be condensed or subcooled.

Perhaps it might 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. Does that help?

Have you read post #16? It describes an actual dynamic process of adding charge to a system and seeing the effects. @jim hardy is describing the effects on the evaporator, but you could do something similar with the condenser: as charge was added, the level in the condenser where the phase change ends and subcooling begins would move, and the reasons are similar in both cases.

jim hardy
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When you have more liquid refrigerant in the condenser, why does the extra thermal inertia to overcome not cancel out the fact that more of the condenser's heat exchange capacity is being used to subcool already condensed liquid?
Compare the heat of vaporization to specific heat of liquid. It's about 338X greater.

It's trivial to subcool .
The thermal inertia to achieve 10 degrees subcooling is small,
you have to remove just 3% of the heat you removed to condense. ($\frac{2.967}{100.5}$ rounds off to 3%)
So it's nowhere near a 1::1 cancellation.

old jim

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Don't you see how you keep thinking of things in terms of batches instead of flows? There is no "block of refrigerant" inside the condenser, or anywhere else. Refrigerant is continuously flowing through the condenser, and the rest of the system. It doesn't just sit there and wait to be condensed or subcooled.
Yes. When I wrote post #17, I tried to avoid that mistake of thinking of things in terms of batches instead of flows, but I fell into the trap again.

Perhaps it might 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. Does that help?
It does help me understand this a little bit.

Have you read post #16? It describes an actual dynamic process of adding charge to a system and seeing the effects. @jim hardy is describing the effects on the evaporator, but you could do something similar with the condenser: as charge was added, the level in the condenser where the phase change ends and subcooling begins would move, and the reasons are similar in both cases.
Yes. But I have added charge to a system and saw the effects on subcooling myself several times at the trade school I attend.

In my opinion , some of the most pithy points you have made about this are in post #4 when you told me "If you decrease the charge, there will be less liquid refrigerant in the system" and when you told me about how "Vapor expands, liquid doesn't" in post #12.

When I wrote post #17 this morning, I thought I had found a hole in your reasoning, but now you have convinced me that there is no hole in your reasoning. What happened was that I fell into the mistake of thinking of this in terms of batches instead of flows.

I think I understand this now.

Compare the heat of vaporization to specific heat of liquid. It's about 338X greater.

View attachment 239298

It's trivial to subcool .
The thermal inertia to achieve 10 degrees subcooling is small,
you have to remove just 3% of the heat you removed to condense. ($\frac{2.967}{100.5}$ rounds off to 3%)
So it's nowhere near a 1::1 cancellation.

old jim
That's a good point also.

PeterDonis, Jim Hardy, and other PF members, I found an excerpt in the book Refrigeration and Air Conditioning Technology by John Tomczyk, Eugene Silverstein, Bill Whitman, and Bill Johnson that gives an explanation of why low charge causes low subcooling that does not seem to mention anything that PeterDonis mentions in his explanation of why low charge causes low subcooling. I'm hoping that you people could read the authors' explanation and tell me whether or not you agree with it and tell me how this works. The authors' explanation relies on at least two of uncashed checks, IMO.

The excerpt that I am going to include in this post comes from chapter 29, and chapter 29 is titled Troubleshooting and Typical Operating Conditions for Commercial Refrigeration. The excerpt is in a section of Chapter 29 called "Low Refrigerant Charge." Here is the excerpt from the book that explains how low charge causes low subcooling: "An undercharge of refrigerant will produce low compressor amperage because the higher superheat coming back to the compressor inlet will cause the inlet vapors to expand, which decreases their density. Low-density vapors entering the compressor will mean low refrigerant flow through the compressor. This will cause low amperage (amp) draw, because the compressor does not have to work as hard compressing the low-density vapors. This low refrigerant flow may also cause refrigerant-cooled compressors to overheat. Due to the low refrigerant flow rate through the compressor and system that occurs with an undercharge of refrigerant, the 100% saturated-vapor point in the condenser will be very low, causing low condenser subcooling. The condenser will not receive enough refrigerant vapors to condense them to a liquid and feed the receiver (if the system has one). Condenser subcooling is a good indication of how much refrigerant charge is in the system. Low condenser subcooling may mean a low charge, whereas high condenser subcooling may mean an overcharge" (page 780).

I'm especially interested in the sentence that I put in boldfaced green font. I don't see how the authors' explanation of how low charge causes low subcooling does not contradict PeterDonis' explanation, but PeterDonis or others here might be able to show me how the authors' explanation does not contradict PeterDonis' explanation.

To me, the green sentence has two uncashed checks. In other words, the green sentence has two premises which the author just assumed without proving.

Does having a low amount of refrigerant in the system cause the refrigerant to flow through the compressor and the system slowly? If so, how does having a low amount of refrigerant in the system cause the refrigerant to flow through the compressor and the system more slowly?

I'm assuming that "the 100% saturated-vapor point in the condenser will be very low" means that the saturation temperature in the condenser will be very low. If you think it means something else, please let me know and tell me what you think it means and why.

How does the refrigerant having a slow flow rate through the compressor and the rest of the system cause the 100% saturated-vapor point in the condenser to be very low?

PeterDonis
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I don't see how the authors' explanation of how low charge causes low subcooling does not contradict PeterDonis' explanation, but PeterDonis or others here might be able to show me how the authors' explanation does not contradict PeterDonis' explanation.
There's no contradiction; the book is just describing what's happening from a different viewpoint, with an emphasis on different things.

The gist of what they are saying is that, with a lower saturation temperature in the condenser, more of the heat transfer in the condenser ends up going to the phase change instead of to subcooling. Which is just another way of saying that there is less subcooled liquid in the system, which is what I was saying.

But there do seem to me to be things the book is leaving out. See below.

how does having a low amount of refrigerant in the system cause the refrigerant to flow through the compressor and the system more slowly?
By "flow rate" I assume the book means mass flow rate. To see why mass flow rate is reduced with low charge, perhaps it's worth taking a step back and asking what parameters of the system do not change when the amount of charge changes (this will come in handy later on in the post as well).

If we assume that external conditions are held constant, then the following parameters should be fixed:

- The compressor's compression ratio;

- The compressor's RPM;

- The temperature of air at the upstream side of the condenser;

- The mass flow rate of air across the condenser;

- The temperature of air at the upstream side of the evaporator;

- The mass flow rate of air across the evaporator.

The first two of those imply that the volume flow rate at the outlet of the compressor is constant (i.e., independent of the charge). But if the charge is lowered, the density of the refrigerant will be lower (because, as the source explains, superheat at the compressor inlet will be higher so the vapor will be less dense). So the mass flow rate will decrease if the charge is lowered. And since the mass flow rate will be the same throughout the system (assuming the system is sealed and there are no leaks), lower charge leads to lower mass flow rate throughout the system.

I'm assuming that "the 100% saturated-vapor point in the condenser will be very low" means that the saturation temperature in the condenser will be very low.
I think so. See below.

How does the refrigerant having a slow flow rate through the compressor and the rest of the system cause the 100% saturated-vapor point in the condenser to be very low?
I think they left out a step here. The slower flow rate in itself won't change the saturation temperature. What makes the saturation temperature lower is that the pressure inside the condenser is lower. In fact, pressures throughout the system will be lower if the charge is lowered. The book you reference hints at this when it talks about compressor amperage being lower: what that means is that the compressor is drawing less power, because it's doing less work on the refrigerant, which means the compressor outlet pressure will be lower; and that pressure drives the pressures everywhere else in the system. So we have lower pressure and lower mass flow rate of refrigerant, which makes sense: it should take less pressure to push less mass flow.

Next, the heat transfer will be lower at the condenser if the charge is lowered. Why? We said above that the air temperature at the upstream side of the condenser and the mass flow rate of air across the condenser are fixed. And we know that the saturation temperature of the refrigerant inside the condenser is lower--which means the temperature of the refrigerant in general will be lower (because most of the refrigerant inside the condenser is at the saturation temperature). That means the difference in temperature between the refrigerant and the air is smaller; and that makes the heat transfer at the condenser lower.

All of this gives us another way to think about why there is lower subcooling. The heat transfer process in the condenser has three stages: first, cooling superheated vapor down to saturation temperature; second, the phase change from vapor to liquid; and third, subcooling the liquid. As the total heat transfer is lowered with a lower charge, where does the reduction get taken from? It can't get taken from the phase change: the heat transfer required for that is fixed. It can't be taken from the superheat to saturation cooling phase: that heat transfer is fixed by the compressor outlet superheat (which, if anything, is likely to increase with a lower charge). So the only place the reduction can come from is subcooling: hence, lower charge means lower subcooling.

I emphasize once again that these are not different, contradictory explanations for the lower subcooling. They are just different ways of describing the process that is taking place, with emphasis on different aspects of it. The only issue I see with the book excerpt you gave is that it left out the effect of pressure on the saturation temperature inside the condenser.

jim hardy
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The only issue I see with the book excerpt you gave is that it left out the effect of pressure on the saturation temperature inside the condenser.
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.
----------------------------------------------------------------------------
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" .

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

Assume a unit sitting outside on your workbench on 90 degree day.
We remove every bit of refrigerant (and properly reclaim it of course).
The we start the compressor (neglect there's no cooling for it - this is just a thought experiment).
Of course pressure everywhere is zero(30" vacuum) so there's no back pressure meaning the compressor is unloaded. So current is low.

Now add Freon 22 until low side pressure at the compressor comes up to one atmosphere, 14.7 psia (0psig).
That'll be superheated because Freon's saturation temperature at 0psig is between -41 and -42, and we're working outdoors on a 90 degree day.
Here's a snip from the Freon 22 Properties table at https://wcec.ucdavis.edu/wp-content/uploads/2012/08/DuPont-R22-thermo_prop.pdf

note also there's an entropy column that we'll use in a minute...
so let's find 0psig and 90°F in the superheat table

Entropy at 90°F and 0psig is 0.28034
Entropy has a physical meaning but you can think of it as just GPS coordinates to help you navigate the thermo table.
It'll let us find the compressor's exit temperature.
Let's just assume a compression ratio of 5 for this experiment.
so the high side pressure will be 5 X 14,7psia = 73.5 psia (can i just call it 74 ?)
Now it's a thermodynamics basic that reversible compression (no heat added) doesn't change the entropy
so let's find 0.28034 in the entropy column (S) for 74 psia.

Hmm. That entropy lies between 230 and 240 °F.

So the Freon leaves the compressor around 230 to 240 °F and cools quickly down to 90F in the condenser , and 90°F is still superheated.
It cools so quickly because there's no phase change meaning not very many BTU's are involved yet.
That's why when you first start charging you feel the discharge line get hot but not the condenser. The gas cools quickly to ambient.
Looking at that superheat table's enthalpy in column H, the BTU change from 235-ish to 90 °F is around 143 - 117, just 26 BTU per pound of Freon for the 135 degree cooldown..

So next the Freon goes through the expansion valve where it gets throttled back down to 14.7psia
It's another Thermo fact that throttling is a constant enthalpy process (H-column)
so we can go back to the 14.7 psia (0psig) table and look for where is 117.337 enthalpy in the H column...
here it is
between 70 and 80F

the Freon enters the evaporator still superheated at 75-ish and warms quickly to 90 , again no phase change.
and that's why you feel the low side line just barely cool when you first start charging.

We continue adding refrigerant until discharge pressure becomes high enough to make liquid at 90F..
which is from the saturation table , 168.4 psig (183...psia)

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 HLiquid 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

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

and we move more heat because the heat exchangers are now moving that energy rich "latent heat of vaporization" instead of paltry superheat and subcooling - look at the enhalpy change in Latent Enthalpy H column compared to a few degrees in either liquid or vapor H column.

So - the purpose of this tome is to get your mind to envision the significance of phase change to refrigeration
and make it intuitive that BTU-wise,
Subcooling and Superheating are pennies
Phase change is dollars
---------------
so -----
you want a goodly portion of both heat exchangers to be working with dollars not pennies
and the only reason for having superheat at all is to keep from feeding liquid into the compressor..

Here's the diagram Thermodynamicists use to describe a refrigeration cycle
It shows zero subcooling in condenser and zero superheat in evaporator
a real cycle would overshoot the saturation line a little bit , compromise is part of any design.

But now you've seen how to navigate the R22 thermo table and can find the temp & pressure at all five corners of that curve.
and you will find it useful to imagine yourself inside the sealed system flowing along with the refrigerant.
I suggest you practice finding the points in the thermo table for temperatures and pressures that you measure in your lab

When we begin to add numbers to our mental image we are deepening our understanding

again i apologize for the scrambledness of this presentation

just that the math never "clicked" for me until i felt that temperature profile on evaporator and condenser change as i added charge.
Our sense of touch goes through different routes in our brain to the cerebrum than does reading a refrigeration manual or thermo book. Maybe we use more of our brain that way, i really dont know...
That's why i say "Use your everyday sensory experience to refine your everyday science"
and that you ask these questions says you are one who ponders things until they make sense. You will excel .

I hope the above helps.

Print yourself a thermo table for whatever refrigerant you use in school lab and practice navigating it with real measured temperatures and pressures. Teacher will love it.

Good luck in your career -

old jim

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