Too much subcool, is this possible?

Hi all

I'm trying to work something out here with a system that uses a TEV.

I have quite a low head pressure and the liquid coming out of the condenser is subcooled by more than 10K.

Now, correct me if I'm wrong, but if I take the load off the evaporator, then if the TEV works properly and the system is charged correctly, it should fill the evaporator with liquid refrigerant and close down completely, bringing the back pressure to 0 psi.

I have tried this, but although the superheat appears to be right and there is subcooled liquid coming out of the condesnser, the TEV won't shut down. Could it be the head pressure is too low due to "over condensing"?

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Originally Posted by carlfoster

Now, correct me if I'm wrong, but if I take the load off the evaporator, then if the TEV works properly and the system is charged correctly, it should fill the evaporator with liquid refrigerant and close down completely, bringing the back pressure to 0 psi.

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Hi carlfoster why are you expecting the TEV to fully close down.

Lrac

Hi LRAC

I expect the TEV to close down because, as I understand it, if the evaporator fan is switched off, the load is reduced to such an extent that the valve will not need to feed liquid into it, and so it closes to maintain the superheat level.

This is a technique I was instructed to use when I was apprenticed, but to be honest I never fully understood it:confused:

A TEV always has a minimum flow, a zero flow isn't possible.

You are talking about a TEV, with just a capillary tube to a bulb, no cables, sensors or control module?

If the answer is yes, then it will not fully shut.

The TEV needs to be adjusted to maintain the superheat.
If the discharge pressure is too low then you need to fit a head pressure control that slows down the condensor fan.

Yes, there is such a thing as too much (condenser outlet) subcooling. I like to draw the line at 9K, but up to 11K is acceptable. More than this tells us that liquid is backing up in the condenser, taking up room and thus making the condenser less efficient.

But to answer the real question, I agree with everyone else that TEV's have a minimum flow.

I have never seen a TEV which would shut tightly enough to prevent a overfeed situation if a coil iced up, or the fans stoped.
Perhaps if they were brand new?
I have never heard of a "Minimum Flow" for a TEV.
But I am not an expert on TEV'S.

10K of subcooling isn't too bad, if there's an extra subcooling circuit one could expect even up to 4K more.

What type of TEV do you have and what's your suctionpressure (and type of ref.)?

[QUOTE=Gary;60981].... I like to draw the line at 9K, but up to 11K is acceptable. More than this tells us that liquid is backing up in the condenser, taking up room and thus making the condenser less efficient....
QUOTE]

Gary, not always, for example in the case when you're using an additional subcool coil for this or a mechanical subcooling.
Profroid makes condensors where the last tubs on the lower part are going upwards again.

In a capillary fed system without receiver, I agree.

Thanks all for your explanations. I have mostly worked with fridge and freezer systems using TEVs, and due to the low temps, run at low pressures. On these systems, you could get a very low return, down to zero psi even.

I think that what you are saying about it though probably applies to air conditioning which is what I'm having trouble understanding now.

I have 2 identical a/c systems, both running on R407C, but one with TEV and the other with a cap tube.

For some reason, the one with the cap tube is pushing out a lower temperature, about 1 or 2 degrees. I have subcooled liquid going to the TEV, about 6K subcool, and a head of 12 Bar, which I've created using a variable condenser fan. The low pressure is steady at about 4.5 Bar.

The other unit runs at 14 Bar, and has a return of 5 Bar. The condenser fan is running faster. I think it has more refrigerant in, but only slightly.

So what do you think the reason for the difference could be?

P.S. The one with a cap tube has a cooler liquid line too, even though the HP is higher, and the compressor is running cooler.

I had to read your post a number of time but still not 100pc sure which is which?

TEV Unit: 12Bar, 4.5Bar & 6K Subcool

CAP Unit: 14Bar, 5Bar & Cooler liquid line

The CAPtube unit compressor will be circulating more gas because it is more dense. The higher head will reduce the compressor capacity but it will still circulate more gas because of the much higher low pressure. The gas will also pick up more heat each time it circulates because it hits the CAPtube cooler.

The air flow and wet bulb in and out is another thing.

Reason for the difference may lay in difference in airflow over the evaporator and temperature diffence between the air entering the two units.

The condensor size and even a little dirt in the condensor will shift everything!

Comparing a tev-system with a cap system will get you confused and futhermore, are the compressors the same size and if they're inverters, are they running the same speed! Get my drift?

If both units are working fine and delivering outlet temperatures needed why bother!

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Originally Posted by Tomking

I had to read your post a number of time but still not 100pc sure which is which?

TEV Unit: 12Bar, 4.5Bar & 6K Subcool

CAP Unit: 14Bar, 5Bar & Cooler liquid line

The CAPtube unit compressor will be circulating more gas because it is more dense. The higher head will reduce the compressor capacity but it will still circulate more gas because of the much higher low pressure. The gas will also pick up more heat each time it circulates because it hits the CAPtube cooler.

The air flow and wet bulb in and out is another thing.

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Thanks for the reply and for clarifying my post :o

I don't understand why you say the gas in the cap tube unit is more dense? Is it because it's running cooler?

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Reason for the difference may lay in difference in airflow over the evaporator and temperature diffence between the air entering the two units.

The condensor size and even a little dirt in the condensor will shift everything!

Comparing a tev-system with a cap system will get you confused and futhermore, are the compressors the same size and if they're inverters, are they running the same speed! Get my drift?

If both units are working fine and delivering outlet temperatures needed why bother!

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Sanderh, I have to compare the 2 because, as an air conditioning manufacturer, we need to provide the most efficient solution, and this is a testing phase for us. Both units are brand new, have the same compressors, evaporators, condesnsers etc. The only difference is one has a cap tube and the other a TEV. I just need to determine which is going to be the bst and easiest route to follow.

I have compared the inlet temperatures and they are essentially the same.

Oh, and there is no inverter.

So, it has become apparent that you can have too much subcool. Does anyone know why this is so?

Sorry, you're right, I didn't read everything!

So what's the purpos of the ac, what's it going to be cooling?

Is the load going to be constant or changing all the time?

And yes, you can have too much subcool, but don't want to since it will, as been talked about before "destroy" the capacity of your system! and will lower the capacity of your condensor.

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Originally Posted by carlfoster

I have 2 identical a/c systems, both running on R407C, but one with TEV and the other with a cap tube.

For some reason, the one with the cap tube is pushing out a lower temperature, about 1 or 2 degrees. I have subcooled liquid going to the TEV, about 6K subcool, and a head of 12 Bar, which I've created using a variable condenser fan. The low pressure is steady at about 4.5 Bar.

The other unit runs at 14 Bar, and has a return of 5 Bar. The condenser fan is running faster. I think it has more refrigerant in, but only slightly.

So what do you think the reason for the difference could be?

P.S. The one with a cap tube has a cooler liquid line too, even though the HP is higher, and the compressor is running cooler.

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Even if your systems were properly charged, which they are not, and had equal condenser airflow, which they do not, you would still be comparing apples and oranges, because the systems are not identical.

No two systems are truly identical, whatsmore one of these is a TEV system and the other is a cap tube system. An apple and an orange.

I don't see the point in your experiment.

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Originally Posted by Sanderh

Sorry, you're right, I didn't read everything!

So what's the purpos of the ac, what's it going to be cooling?

Is the load going to be constant or changing all the time?

And yes, you can have too much subcool, but don't want to since it will, as been talked about before "destroy" the capacity of your system! and will lower the capacity of your condensor.

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The a/c will be used for applications requiring closed control, usually computer rooms etc.

The load is generally constant, and the temperature normally a small range of 18 - 21 degrees.

I see your point about the condenser capacity being lowered, which I understand if the condenser was sized properly, but unfortunately, I believe the condenser is probably a bit large for this application, hence has the capacity for more than the normal liquid capacity.

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Even if your systems were properly charged, which they are not, and had equal condenser airflow, which they do not, you would still be comparing apples and oranges, because the systems are not identical.

No two systems are truly identical, whatsmore one of these is a TEV system and the other is a cap tube system. An apple and an orange.

I don't see the point in your experiment.

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Thanks for your points, I do understand that no 2 systems are exactly the same, but surely they shouldn't differ that much?

Also, why do you belive they are not properly charged?

The point of this is to determine the most cost effective method of manufacturing the units, while trying to maintain system efficiency. It might cost more in parts to make a TEV controlled system, but it might be a quicker turn around and easier to determine correct flow control than using a capilliary tube.

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Originally Posted by carlfoster

Thanks for your points, I do understand that no 2 systems are exactly the same, but surely they shouldn't differ that much?

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In order to compare them, you need to control the variables: Given equal airflow at equal wetbulb temperatures through both coils and equal subcooling and equal superheat, the two systems will operate very similarly. But they will only operate similarly under these specific conditions.

If you then equally change the heat loads, the other variables will no longer be equal (especially the superheat) and the TEV system will perform better because it has superior ability to adjust to changing heat loads.

If the loads are relatively constant, and the variables are adjusted for those constant conditions, then either system will perform about the same.

Ok gary, thanks.

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Originally Posted by Gary

Yes, there is such a thing as too much (condenser outlet) subcooling. I like to draw the line at 9K, but up to 11K is acceptable. More than this tells us that liquid is backing up in the condenser, taking up room and thus making the condenser less efficient.

But to answer the real question, I agree with everyone else that TEV's have a minimum flow.

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Hi Gary,
Why would you want to limit sub-cooling?

In theory, I don't see any problem with liquid sub-cooling. All it does is increase your refrigeration effect by reducing the evaporator inlet quality.

You say that the liquid backs up in the condenser and makes it less efficient.

If this were true, condsider the following.

The liquid sub-cooling is a measure of the liquid refrigerant in the system. Too much charge and the sub-cooling increases. After all, where else can the liquid go?

This reduces the effective condensing area (not the efficiency) of the condenser and the head pressure goes up.

But what if I added another condenser. I know its not practical but I'm using it to illustrate the point. This 2nd condenser would actually be a sub-cooler that I can fill with liquid and get the benefit of the extra sub-cooling.

In this case, would you still say that high sub-cooling is a problem?

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Originally Posted by nh3simman

But what if I added another condenser. I know its not practical but I'm using it to illustrate the point. This 2nd condenser would actually be a sub-cooler that I can fill with liquid and get the benefit of the extra sub-cooling.

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You're right, it's indeed very practical, we do it from time to time and even more, you can buy these condensors like you describe those but they're then integrated in the standard coil (bends going up again instead of all going/flowing down)

I would say now that it's not really a matter of too much subcool, but rather a matter of the head pressure being too low, resulting in a low suction and therefore icing up (in a high temp application that is).

So, I think a lot of subcool is fine as long as the head pressure is ok.

Let's say the outdoor temp is 75F.

The refrigerant is condensing at 95F.

The (condenser outlet) liquid line temp is 80F.

Subcooling is the difference between condensing temp and liquid line temp. In this case 15F.

The liquid line temp can go no lower than the ambient temp (75F), so the subcooling isn't going to get any higher than 20F (95F-75F=20F) without raising the condensing temp.

If we keep adding refrigerant, this is exactly what will happen, because the condenser is filling up with liquid, driving up the head pressure.

With an extra condenser, we can add a lot more liquid before this happens. But when it does happen, we know that it is because the condensing temp is going up, not because the liquid line temp is going down. The liquid line temp can go no lower than the ambient temp.

BTW, with an extra condenser, we might expect even lower condensing temp, i.e. less subcooling, not more. Else that extra condenser is not a condenser at all, it is a receiver.

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Originally Posted by Gary

The liquid line temp can go no lower than the ambient temp.

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If you used a liquid to suction gas heat exchanger, the sub-cooling could be greater.

In principle I just see no problem with high sub-cooling.

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Originally Posted by nh3simman

If you used a liquid to suction gas heat exchanger, the sub-cooling could be greater.

In principle I just see no problem with high sub-cooling.

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Agreed. But then it would not be condenser outlet subcooling. We are discussing condenser outlet subcooling. High TEV inlet subcooling is a very good thing. High condenser outlet subcooling is a very bad thing.

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Originally Posted by Gary

....you would still be comparing apples and oranges, because the systems are not identical.

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Is this a typical English expression? We say comparing apples with pears...

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For some reason, the one with the cap tube is pushing out a lower temperature, about 1 or 2 degrees. I have subcooled liquid going to the TEV, about 6K subcool, and a head of 12 Bar, which I've created using a variable condenser fan. The low pressure is steady at about 4.5 Bar.

The other unit runs at 14 Bar, and has a return of 5 Bar. The condenser fan is running faster. I think it has more refrigerant in, but only slightly.

So what do you think the reason for the difference could be?

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You have somewhat overcharged your cap tube system as compared to your TEV system. Thus, you are stacking liquid in the condenser providing it more opportunity to become subcooled. This also explains the higher discharge pressure.

The TEV system likely has less refrigerant in it, but you are maintaining a somewhat controlled discharge pressure by diminishing the air over the coil with your fan control. You are not backing liquid up into the condenser, thus you have less subcooling.

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For some reason, the one with the cap tube is pushing out a lower temperature, about 1 or 2 degrees.

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Are you saying that the cap tube system is providing a cooler air temperature? It is entirely possible. You have a lower suction pressure with the TEV system, so I would expect that you have a higher superheat, all other things considered equal.

The one measurement you have failed to share is the superheats between the two systems. I suspect you have room to decrease the superheat with the TEV by making a minor adjustment to it... trusting that you have a 6 K subcooling.

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Originally Posted by Dan

You have somewhat overcharged your cap tube system as compared to your TEV system. Thus, you are stacking liquid in the condenser providing it more opportunity to become subcooled. This also explains the higher discharge pressure.

The TEV system likely has less refrigerant in it, but you are maintaining a somewhat controlled discharge pressure by diminishing the air over the coil with your fan control. You are not backing liquid up into the condenser, thus you have less subcooling.


Are you saying that the cap tube system is providing a cooler air temperature? It is entirely possible. You have a lower suction pressure with the TEV system, so I would expect that you have a higher superheat, all other things considered equal.

The one measurement you have failed to share is the superheats between the two systems. I suspect you have room to decrease the superheat with the TEV by making a minor adjustment to it... trusting that you have a 6 K subcooling.

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I can't quite remember all the details as I worked on these systems a while ago now and have sorted the problem out. The superheat on the cappilliary system was very low, so possible liquid coming back along the suction.

What I figured out was that the system with the capilliary tube wasn't behaving properly, and I believe the reason for this was that the cappiliary tube was too short and too small, only 600mm long. When the system shut off it took way too long for the pressures to equalise and the compressor had problems starting again.

I therefore decided to remove the cappilliary tube and fitted an expansion valve instead. The system worked better after that.

A question I do have with these systems is with regard to the evaporator design. It is a fairly small one with 3 rows of coils, but it also has a distributor. So, I've found it hard to calculate the right cap tube size as the distributors also reduce the pressure and you wouldn't need the standard length or cappiliary. This makes it difficult because shorter cap tubes affect the flow drastically with any small change in size.

So, is it normal to fit a cappiliary tube on to an evaporator with distributors, or should expansion valves be fitted.

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So, is it normal to fit a cappiliary tube on to an evaporator with distributors, or should expansion valves be fitted.

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I would say no, but if you bypass the distributor itself, and feed each distributor circuit with a cap tube, I don't see any reason why it wouldn't perform once you guessed the cap tube correctly.

Anyway, nice to see that you have your problem sorted out. I often jump into discussions too late. :)

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Originally Posted by carlfoster

What I figured out was that the system with the capilliary tube wasn't behaving properly, and I believe the reason for this was that the cappiliary tube was too short and too small, only 600mm long. When the system shut off it took way too long for the pressures to equalise and the compressor had problems starting again.

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Under design conditions (indoor, outdoor temperature/humidity), given the ideal cap tube size, the condenser outlet subcooling will be not too high nor too low AND the evaporator outlet superheat will be not too high nor too low.

If we charge the system until the subcooling is borderline low, and the superheat is low, then the cap tube is not restrictive enough.

If we charge the system until the subcooling is borderline high, and the superheat is high, then the cap tube is too restrictive.

you have to use a pressure control device for condenser or you have to control condenser fans