The reservoir of refrigerant between condenser & metering device

[sevensages]

TL;DR Summary

Why would more of the condenser's heat exchange capacity being used to subcool already condensed liquid (as opposed to changing vapor to liquid) cause each particle of refrigerant to be subcooled for a longer duration in the condenser? Is it because the already subcooled refrigerant is further cooling each particle of refrigerant through conduction?

When I was looking at the list of related threads to my thread "How does metering device in an A/C cause flash gas", I noticed an interesting thread titled "How does low charge on an A/C cause the subcooling to be low". PeterDonis and fourthindiana had an interesting conversation on that thread. PeterDonis really broke down and explicitly spelled out how low charge on an A/C causes the subcooling to be low. That thread has been closed, probably due to time constraints (to prevent necroposting). The thread is years old.

Here is a link to that thread:

https://www.physicsforums.com/threa...an-a-c-cause-the-subcooling-to-be-low.957119/

In the first reply to that thread, PeterDonis brought up the significance of the reservoir of liquid refrigerant between the condenser and the metering device. fourthindiana seemed to completely fail to understand the significance of the reservoir of liquid refrigerant between the condenser and the metering device. I think that this is worthy of discussion.

PeterDonis wrote:

"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."

In post 4, PeterDonis wrote:
"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."

fourthindiana responded by asking "If you have more subcooled liquid refrigerant in the condenser, does that somehow increase the subcooling? If so, how?"

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

Further in the thread, PeterDonis writes that if there is more refrigerant in the system, that causes each particle of refrigerant to be subcooled for a longer duration in the condenser and in the reservoir of liquid refrigerant going to the metering device.
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I don't think that fourthindiana ever quite grasped the gist of this. fourthindiana has not posted in a year and a half. There is a line through his or her screen name, which probably indicates that fourthindiana disabled his or her account. So I think that fourthindiana is unlikely to come back and clarify his or her opinions on this. I'm hoping PeterDonis and others here can clarify.

Why would more of the condenser's heat exchange capacity being used to subcool already condensed liquid (as opposed to changing vapor to liquid) cause each particle of refrigerant to be subcooled for a longer duration in the condenser? Is it because the already subcooled refrigerant is further cooling each particle of refrigerant through conduction? I'm asking if it is because the reservoir of refrigerant is cooling each particle of refrigerant (as opposed to only the refrigerant only losing heat into the fins of the condenser through conduction, which is powered by the outdoor fan removing heat from the fins by convection).

It seems to me that there are two ways that an air-conditioner cools off refrigerant before the refrigerant reaches the metering device: 1# by conduction through the fins of the condenser, and this conduction is increased by the outdoor fan removing heat by convection and 2# by the reservoir of liquid refrigerant removing heat (by conduction) from each new particle of refrigerant to go through the condenser and enter the liquid line of the lineset. Is my understanding correct?

Edited to add summary

 

[PeterDonis]

Why would more of the condenser's heat exchange capacity being used to subcool already condensed liquid (as opposed to changing vapor to liquid) cause each particle of refrigerant to be subcooled for a longer duration in the condenser?

Because the condenser can't pick and choose what it's transferring heat from. At any given location on the condenser, heat is being transferred from whatever is flowing through the condenser at that location, to the air blowing past that location. If what is flowing through the condenser at a given location is a mixture of saturated liquid and vapor, then heat will be transferred from that to the air, which will contribute to condensing the vapor to liquid. If what is flowing through the condenser at a given location is subcooled liquid, then heat will be transferred from that to the air, which will further subcool the liquid.

So how much of the condenser's total heat exchange capacity gets used to subcool liquid, as opposed to changing vapor to liquid, depends on how much of the condenser has subcooled liquid flowing through it, as opposed to a liquid/vapor mixture. In a properly charged A/C system in the range of operating conditions for which it is designed, that will usually be a significant portion of the condenser, because it is much better to have some extra subcooling than to risk not having a complete phase change inside the condenser. So the system is designed to have a significant amount of subcooling to provide a buffer.

From the standpoint of an individual parcel of refrigerant flowing through the condenser, what determines how much it gets subcooled is what fraction of the time it takes to flow through the condenser is spent in phase change vs. subcooling. Again, as above, the system will be designed to ensure that there is a substantial time spent subcooling under the design operating condition to ensure that there is always a complete phase change, even if actual operating conditions vary. But there is no way to pick and choose--you can't tell the liquid to stop transferring heat once it has condensed completely, i.e., to stop subcooling; as long as there is a temperature difference, it will transfer heat.

In the original thread you refer to, because I was responding to a somewhat different issue, I wasn't completely clear about where the subcooling occurs; subcooling occurs inside the condenser, not between the condenser and the expansion device. See below.

1# by conduction through the fins of the condenser

As above, this is where the heat transfer from refrigerant to outside air occurs, yes. Note that, as you imply in your next clause here, while the heat transfer through the condenser fins is by conduction, the actual driver of the rate of heat transfer is convection of the outside air past the fins; in other words, the best way to get more heat transfer out of the condenser is to increase the airflow past it.

2# by the reservoir of liquid refrigerant removing heat (by conduction) from each new particle of refrigerant to go through the condenser and enter the liquid line of the lineset.

Here your understanding is incorrect. There is no static "reservoir" of liquid refrigerant. All of the refrigerant in the system is continuously flowing through it. Conduction from one parcel of refrigerant to another is negligible.

 

[sevensages]

Because the condenser can't pick and choose what it's transferring heat from. At any given location on the condenser, heat is being transferred from whatever is flowing through the condenser at that location, to the air blowing past that location. If what is flowing through the condenser at a given location is a mixture of saturated liquid and vapor, then heat will be transferred from that to the air, which will contribute to condensing the vapor to liquid. If what is flowing through the condenser at a given location is subcooled liquid, then heat will be transferred from that to the air, which will further subcool the liquid.

So how much of the condenser's total heat exchange capacity gets used to subcool liquid, as opposed to changing vapor to liquid, depends on how much of the condenser has subcooled liquid flowing through it, as opposed to a liquid/vapor mixture. In a properly charged A/C system in the range of operating conditions for which it is designed, that will usually be a significant portion of the condenser, because it is much better to have some extra subcooling than to risk not having a complete phase change inside the condenser. So the system is designed to have a significant amount of subcooling to provide a buffer.

From the standpoint of an individual parcel of refrigerant flowing through the condenser, what determines how much it gets subcooled is what fraction of the time it takes to flow through the condenser is spent in phase change vs. subcooling. Again, as above, the system will be designed to ensure that there is a substantial time spent subcooling under the design operating condition to ensure that there is always a complete phase change, even if actual operating conditions vary. But there is no way to pick and choose--you can't tell the liquid to stop transferring heat once it has condensed completely, i.e., to stop subcooling; as long as there is a temperature difference, it will transfer heat.

In the original thread you refer to, because I was responding to a somewhat different issue, I wasn't completely clear about where the subcooling occurs; subcooling occurs inside the condenser, not between the condenser and the expansion device. See below.As above, this is where the heat transfer from refrigerant to outside air occurs, yes. Note that, as you imply in your next clause here, while the heat transfer through the condenser fins is by conduction, the actual driver of the rate of heat transfer is convection of the outside air past the fins; in other words, the best way to get more heat transfer out of the condenser is to increase the airflow past it.Here your understanding is incorrect. There is no static "reservoir" of liquid refrigerant. All of the refrigerant in the system is continuously flowing through it. Conduction from one parcel of refrigerant to another is negligible.

I am trying to reconcile what you wrote with my intuition of what would determine how much an individual parcel of refrigerant is subcooled.

When I read the thread "How does low charge cause low subcooling", I was trying to figure out what determines how much each individual parcel of refrigerant gets subcooled. I thought that if an individual parcel of refrigerant is only subcooled due to heat transfer from the refrigerant to the condenser fins (and the metal refrigerant linse that the refrigerant goes through), then the total amount of refrigerant in the unit would not affect how much each individual parcel of refrigerant gets subcooled. The outdoor fan motor does not blow any faster if there is more (or less) refrigerant in the air-conditioner. So, at first, this was a big mystery to me. The question on my mind was the following: How does adding more refrigerant to an air-conditioner make each individual parcel of refrigerant get subcooled more? Last night, I thought that the answer to the question was that if you add more refrigerant to the unit, that makes each individual parcel of refrigerant get subcooled more because the reservoir of liquid refrigerant in the liquid line and at the bottom of the condenser further subcools each individual parcel of refrigerant. I was so confident that i had solved this mystery that I thought it was a bit foolish of me to create this thread and ask you about it. I thought I was asking a question in which I had obviously found the solution. Boy was I wrong! You're telling me that my solution is incorrect!

My intuition tells me that if an individual parcel of refrigerant is ONLY subcooled due to heat transfer from the refrigerant to the condenser fins (and the metal refrigerant lines that the refrigerant goes through), then the amount of the refrigerant in the system should not affect how much each individual parcel of refrigerant gets subcooled. Why does my intuition tell me that? Because the outdoor fan motor spins at the same speed regardless of how much or how little refrigerant is in the air-conditioner, and I don't see any reason for heat to go from each individual parcel of refrigerant to the condenser fins and then to the outdoor air any faster if there is more refrigerant in the air-conditioner.

I suppose you probably learned about physics in depth and learned this stuff in college. If you learned about how air-conditioners worked by reading books on your own BEFORE you went to college, would your intuition also tell you that if an individual parcel of refrigerant is ONLY subcooled due to heat transfer from the refrigerant to the condenser fins (and the metal refrigerant lines that the refrigerant goes through) , then the amount of refrigerant in the system should not affect how much each individual parcel of refrigerant gets subcooled?

I don't know how to reconcile your statement that the reservoir of liquid refrigerant does not subcool each individual parcel of refrigerant in any significant amount with what my intuition tells me. If your intuition would tell you the same thing that my intuition tells me, how do you reconcile this?

 

[sevensages]

Because the condenser can't pick and choose what it's transferring heat from. At any given location on the condenser, heat is being transferred from whatever is flowing through the condenser at that location, to the air blowing past that location. If what is flowing through the condenser at a given location is a mixture of saturated liquid and vapor, then heat will be transferred from that to the air, which will contribute to condensing the vapor to liquid. If what is flowing through the condenser at a given location is subcooled liquid, then heat will be transferred from that to the air, which will further subcool the liquid.

So how much of the condenser's total heat exchange capacity gets used to subcool liquid, as opposed to changing vapor to liquid, depends on how much of the condenser has subcooled liquid flowing through it, as opposed to a liquid/vapor mixture. In a properly charged A/C system in the range of operating conditions for which it is designed, that will usually be a significant portion of the condenser, because it is much better to have some extra subcooling than to risk not having a complete phase change inside the condenser. So the system is designed to have a significant amount of subcooling to provide a buffer.

From the standpoint of an individual parcel of refrigerant flowing through the condenser, what determines how much it gets subcooled is what fraction of the time it takes to flow through the condenser is spent in phase change vs. subcooling. Again, as above, the system will be designed to ensure that there is a substantial time spent subcooling under the design operating condition to ensure that there is always a complete phase change, even if actual operating conditions vary. But there is no way to pick and choose--you can't tell the liquid to stop transferring heat once it has condensed completely, i.e., to stop subcooling; as long as there is a temperature difference, it will transfer heat.

In the original thread you refer to, because I was responding to a somewhat different issue, I wasn't completely clear about where the subcooling occurs; subcooling occurs inside the condenser, not between the condenser and the expansion device. See below.As above, this is where the heat transfer from refrigerant to outside air occurs, yes. Note that, as you imply in your next clause here, while the heat transfer through the condenser fins is by conduction, the actual driver of the rate of heat transfer is convection of the outside air past the fins; in other words, the best way to get more heat transfer out of the condenser is to increase the airflow past it.Here your understanding is incorrect. There is no static "reservoir" of liquid refrigerant. All of the refrigerant in the system is continuously flowing through it. Conduction from one parcel of refrigerant to another is negligible.

I would like for other PF members to comment on what PeterDonis wrote in post 2. I would like a second opinion and third opinion, etc. If you are a mechanical engineer who specializes in HVAC, I would especially like to see your opinion. Does everyone else here agree with what PeterDonis wrote in post 2?

Specifically, do you agree with PeterDonis that the reservoir of liquid refrigerant at the bottom of the condenser does not further subcool each individual parcel of refrigerant by conduction (in any more than a negligible way)?

 

[PeterDonis]

@sevensages please don't quote an entire post when responding. If you're responding to a particular statement from a post, just quote that. Otherwise you can just reference the post by number (e.g., post #2).

 

[PeterDonis]

my intuition

...looks to me like it needs retraining, since it is not considering significant variables. See below.

the total amount of refrigerant in the unit would not affect how much each individual parcel of refrigerant gets subcooled

Yes, it does, because it affects both the pressure of refrigerant in the condenser, which affects its saturation temperature (and therefore affects the rate of heat transfer to the air through the fins), and the mass flow rate of refrigerant through the system, which affects the heat transfer rate everywhere in the system that heat transfer occurs. It is true that the airflow rate across the condenser is not changing, but that is not the only relevant variable.

A good exercise (which, IIRC, was proposed by "old jim" in the other thread) is to start a system at very low charge and gradually increase the charge, holding everything else constant (ambient air temperature, airflow rate across the condenser, compressor RPM), and observe what happens to the key operating parameters. As was noted in the old thread, with very low charge, the high side pressure will be so low that the saturation temperature of the refrigerant in the condenser will be lower than ambient, so the refrigerant will never even condense. As you add charge, the high side pressure increases, which increases the saturation temperature, until finally that temperature goes above ambient and refrigerant starts to condense inside the condenser. Most people's untrained intuition will not even be considering high side pressure and its effect on the saturation temperature of the refrigerant in the condenser. (IIRC that was the case for fourthindiana's intuition in the old thread.)

You're telling me that my solution is incorrect!

Yes.

I suppose you probably learned about physics in depth and learned this stuff in college.

I studied physics in college, but not HVAC. All that I learned about HVAC systems, I learned during the time I spent as an automotive HVAC engineer. Automotive HVAC is actually more interesting because there are more variables that can change, as compared to a home HVAC system. A home system's compressor RPM and condenser airflow are constant; that's not true for an automotive system since both engine RPM and vehicle speed affect those variables, and those things are changing.

If your intuition would tell you the same thing that my intuition tells me

What my intuitions were before I spent time actually learning how HVAC systems worked is irrelevant, as, unlike you, I never tried to solve HVAC problems using my untrained intuition. I knew enough to know that my untrained intuition was of no value, and that the best way to learn how HVAC systems actually worked was to, well, learn how they actually worked, by actually learning what the relevant variables were and how changes in them affect the system. The best way to train your intuition to do that is to watch an instrumented system in operation and see how changes in variables affect it. As I said above, I spent time as an automotive HVAC engineer and got to do that.

 

[russ_watters]

For what it's worth, I'm an HVAC engineer but I work in applications (cleanroom and lab design), not designing the equipment itself. That means my understanding/use of refrigerant systems is pretty "standard" -- I look at the design conditions and don't dig into the details of what can go wrong. So I've not really dug into this issue much myself.

However, maybe I can explain what's going on here in my own words, using a couple of extreme scenarios:

Let's say your refrigerant charge is way, way too high. You essentially have an all-liquid system at very high pressure, with no evaporating or condensing happening. Liquid is denser than gas, so this liquid travels much slower through the condenser than a gas would.

A very low pressure system would be all gas, with the mirror image problem.

A properly filled system is going to be somewhere in between. Indeed, the fill is going to be calibrated specifically to partially fill the condensing unit (and evaporator). Not enough refrigerant means it has to stay gas for longer because for a fixed total volume, lower mass of refrigerant means more is gas. Too much refrigerant means it all has to be condensed sooner, for the opposite reason. The difference then is more or less time/space in the condenser for the all liquid refrigerant.

 

[PeterDonis]

Let's say your refrigerant charge is way, way too high. You essentially have an all-liquid system

Not really, because your compressor will fail since it's not designed to pump liquid, only gas. (The key term is "liquid slugging".) Even if we imagined a compressor that could somehow magically pump liquid, it wouldn't have nearly the same compression ratio, so the high side pressure would drop sharply and the refrigerant would end up being gas again anyway. It's simply not possible for a standard A/C system to be pumping only liquid everywhere for any length of time.