Non condensables??

[ChillTechnican]

Hi all, ive been having trouble with a chiller unit going out on high pressure. The unit is on R404a and the high pressure is set to 350 psi, the unit did have an automatic reset high pressure switch but this was hammering everything when it was cutting out and coming back in. The unit is placed between the chiller and the wall of another building but the prevailing wind blows up the units date and can almost stop the wind flow. So we are considering moving the condenser on to top of chiller , but when i was there i decided to take a look at the sub cooling.. is it possible to have a higher sub cooling temp than the sct ?? does the unit have air or a mix of refrigerants in it?? or have i stuffed up taking the measurements. High was 235 psi or 36 deg C and the temp of pipe was 47 deg C, i was using a laser thermometer to test liquid line at reciever outlet.
cheers for any advice.

[nike123]

When using laser (IC) thermometer you must adjust emissivity according to measuring surface if your instrument have that function, or you need to wrap 1 layer of black electrical isolating tape around pipe of witch you are measuring temperature.
You must be sure in your measured data before acting upon it.

[US Iceman]

The unit is placed between the chiller and the wall of another building...

Check the airflow on the air-cooled condenser to make sure it is not recirculating. An adjacent building wall and discharge air off of a condenser can do funny things to air flow.

Measure the temperatures of the air flow entering the condenser (all around the condenser inlet). If the air temperature measured is higher than the ambient dry bulb temperature away from the condenser you more than likely have air recirculation problems.

The warmer entering air reduces the condenser capacity and there is not much you can do to fix this than move the condenser to a location where it receives free air flow on all four sides.

[Gary]

I keep a laser around for identifying hot spots (loose connections, burnt contacts, etc.) in electrical panels. Aside from that, I consider them unreliable and therefore pretty much useless.

[Springbok]

Hey ChillTechnician...I would rather use a dial thermometer to be strpped onto the pipe,rather than a lazer,i find it more accurate readings are given.The temperature you have is pretty warm for the liquid line.Best to check the air flow in that surrounding area before thinking of air or moisture ( non condensables ) in the system.Is the sight glass fully charged?Good luck buddy...

[Gary]

High was 235 psi or 36 deg C...

What was the ambient temp? And perhaps more relevant, what was the condenser entering air temp?

[ChillTechnican]

Hey guys thanks for all replys, the system was fully charged but i was wondering if overcharged, ambient/air on was 22 C at time but no wind blowing.The feedback i get about laser is to only use it for pointing at a/c grilles to get an idea if they are working or similar indication . I will invest in a press on type today. Thanks all!!

[Gary]

Hey guys thanks for all replys, the system was fully charged but i was wondering if overcharged, ambient/air on was 22 C at time but no wind blowing.The feedback i get about laser is to only use it for pointing at a/c grilles to get an idea if they are working or similar indication . I will invest in a press on type today. Thanks all!!

This gives you a TD of 36C - 22C = 14K/25F which is well within normal range. You can forget about noncondensables and probably overcharge, too.

The problem, as you have correctly identified, is almost certainly location in relation to the prevailing wind and the solution would be relocation and/or baffles to redirect the wind/prevent recirculation.

[Dave Sewell]

Gary appears to have covered most avenues but somehow dicounts non-condensibles.
Disconnect the electrical supply to the compressor bot not the condenser fans.
Allow them to run idle for at least 30mins til all components are at ambient temperature then from the High side port measure the pressure and cros refrence to the saturated pressure relevant to ambient temp.
Well above would indicate air although your thermometer indicates an overcharge.
Also bear in mind refrigerant of this type should be charged in the liquid phase.
Have we a partial imbalance in the refrigerant composition?

DAVE

[Gary]

Gary appears to have covered most avenues but somehow dicounts non-condensibles.

If the system had any appreciable amount of non-condensables, the TD would be much higher... but then it never hurts to check.

[Gary]

Disconnect the electrical supply to the compressor bot not the condenser fans.
Allow them to run idle for at least 30mins til all components are at ambient temperature then from the High side port measure the pressure and cros refrence to the saturated pressure relevant to ambient temp.

Hmmm... you may want to close the king valve and pump down the system first.

[SteinarN]

The correct way to determine any existence of non condensables is to measure the liquid subcooling at condenser outlet or receiver outlet. Assuming only minor hight differences and not any measurable pressure differences between condenser/receiver and not overfilled system there should be no measurable subcooling at condenser/receiver outlet. Any subcooling here is a certain symtom of non condensables in the system.

When there is sufficient refrigerant in the system, any non condensables will accumulate in the receiver bc only liquid is drawn out of the receiver and the non condensables (gasses) cant find the way to the bottom of the receiver where the outlet pipe is.

An example:
Lets say the liquid temperature (condenser outlet and receiver outlet) is 30 degres C, but you measure a condensing/receiver pressure of 15,1 bar. At 30 condensing/liquid outlet, the pressure should have been 13,1 bar! In this instance you have 2 bar additional pressure (15,1-13,1) in the receiver caused by non condensables. Lets say the gas phase volume of the receiver is 5 litres, then you have 10 litres of noncondensables at atmospheric pressure in the system. (10 litres at 1 atmosphere equals 5 litres at those 2 atmospheres additional pressure.) This is assuming that the temperature of the liquid surface in the receiver is the same temperature as the condenser outlet/receiver outlet. This assumtion should be valid in a system with short pipes between condenser and receiver, not any excess pressure loss between condenser outlet and receiver outlet and not considerably temperature difference between the condensing temperature and the surrounding air of the receiver.

When the system is stopped, the non condensables have a possibility to escape from the receiver and expand/dilute into a much larger volume of the system. That 10 litres of non condensables, when expanded/diluted in, lets say, 20 litres of the system, will cause an additional pressure above the saturated pressure of only 0,5 bar. (10 litres at 1 bar equals 20 litres at 0,5 bar) Thus it is far more accurate to measure the receiver outlet pressure and temperature when the system is running than measure those values at stand still in order to determine the presence of any non condensables.

[SteinarN]

We can do some more reasoning in the above example.

If we add 2,5 litres (roughly 2,5 kg) refrigerant to the system, then the gas volume of the receiver will be reduced to 2,5 litres. Those 10 litres of non condensables would result in a 4 bar (10 litres gas compressed to 2,5 litres give a pressure of 4 bar) additional pressure above the saturated liquid pressure at receiver outlet. Thus adding additional refrigerant, above when there is sufficient refrigerant in the system to get a clear sight glass, will lead to increased condensing pressure and increased liquid subcooling in a system with noncondensables in it.

If there was no non condensables in the system, then there would not be any increase in the condensing pressure before the receiver got completely filled up.

[Gary]

The correct way to determine any existence of non condensables is to measure the liquid subcooling at condenser outlet or receiver outlet. Assuming only minor hight differences and not any measurable pressure differences between condenser/receiver and not overfilled system there should be no measurable subcooling at condenser/receiver outlet. Any subcooling here is a certain symtom of non condensables in the system.

Sorry, but it doesn't work that way. On any system, if there is sufficient refrigerant to clear the sightglass, or even with an occasional bubble, there will be substantial subcooling at the receiver outlet.

[SteinarN]

Sorry, but it doesn't work that way. On any system, if there is sufficient refrigerant to clear the sightglass, or even with an occasional bubble, there will be substantial subcooling at the receiver outlet.

Thats not my experience. At all my systems there is a very close relationship between the receiver pressure and the outlet temperature. I have had some transport refrigeration units with large subcooling, lets say 10 degres C. I have vacumed and filled new refrigerant and the subcooling has disapeared AND the condensing pressure got substantially reduced.

How du you explain that a bubble of warm and pure refrigerant gas can exist in the liquid pipe sorrounded by substantially subcooled liqiud . If there was such a bubble it would very quicly lose its latent heat to the sorrounding cool liquid and dissapear completely. If there is any measurable substantly subcooling at the receiver outlet, then it is either non condensables in the receiver or there is an other refrigerant mix in the system than it should be.

[Gary]

How du you explain that a bubble of warm and pure refrigerant gas can exist in the liquid pipe sorrounded by substantially subcooled liqiud .

The same way I would explain droplets of liquid in a superheated suction line. The refrigerant is moving and there is carryover. If this were not true, we would only need 1 degree of superheat at the compressor inlet to ensure a solid flow of vapor.

[SteinarN]

The same way I would explain droplets of liquid in a superheated suction line. The refrigerant is moving and there is carryover. If this were not true, we would only need 1 degree of superheat at the compressor inlet to ensure a solid flow of vapor.

I didnt realy see an answer to my question.

If there is sufficient liquid in the receiver to completely cover the inlet of the outlet tube, so that none of the gas in the receiver can enter the outlet tube, where does the ocasional bubbles in the sightglas come from?

Do you agree that any gaseous refrigerant, sorrounded by liquid subcooled refrigerant, must have a higher temperature than the sorrunding subcooled liquid?

How can any of the liquid subcooled refrigerant, drawn from the receiver, produce any gaseous refrigerant, visible in the sightglas, without any measurable pressure drop or any heat supplied?

This is the essence of the whole refrigeration process, that the temperature and the pressure of the refrigerant determines the possible states that same refrigerant can be in.

[Gary]

The liquid in the receiver is subcooled. As it enters the tube, it is subcooled. If the liquid level barely covers the outlet tube, such that a bubble gets through occasionally, it is still subcooled.

Obviously, we are not going to agree on this. You will need to prove it to yourself by taking some measurements.

[techie]

hey well sounds like it could be your filter drier could be partially blocked check the temp on either side of filter drier then check your expansion valve and orifice then try bring the discharge pressure to 250 psi and make sure the sight glass is full and stays full while doing it.check the amps pulling on your compressor to could be restriction or overcharged

[SteinarN]

Obviously, we are not going to agree on this.

Seems that this is the only thing we agree on. We have a different understanding of the physics in a refrigeration system. I accept you have your understanding based on your experience and measurment you have done, but i have another understanding of the basics in a refrigeration system based on my knowlegde of physics and my experience and measurments i have done.

[SteinarN]

hey well sounds like it could be your filter drier could be partially blocked check the temp on either side of filter drier then check your expansion valve and orifice then try bring the discharge pressure to 250 psi and make sure the sight glass is full and stays full while doing it.check the amps pulling on your compressor to could be restriction or overcharged

Our little "discussion" is about the fundamentals of a refrigeration system, not any spesific troubleshooting.

[techie]

id say move the unit to where it can get proper ventilation and check your pressures then it could be that it not getting proper air circulation pressure must be on the high side between 220 and 290 psi comfortably jst because its set at 350 doesnt mean it must work at 350 350 is just a cut out however if its not cutting out at 350 then the pressure switch is stuffed

[Gary]

Seems that this is the only thing we agree on. We have a different understanding of the physics in a refrigeration system. I accept you have your understanding based on your experience and measurment you have done, but i have another understanding of the basics in a refrigeration system based on my knowlegde of physics and my experience and measurments i have done.

This is not something I have pulled out of my hat. I have invested an enormous amount of research on a wide variety of systems, regarding this very point.

What I have found is that the sight glass clears at 10-15F subcooling, depending upon the velocity of the refrigerant flow. If your experience tells you otherwise, then it is what it is, and we can only agree to disagree.

[Gary]

Our little "discussion" is about the fundamentals of a refrigeration system, not any spesific troubleshooting.

Uhhh... Actually, we are having a side discussion about fundamentals. The main thread is in fact about troubleshooting a specific system.

[SteinarN]

Uhhh... Actually, we are having a side discussion about fundamentals. The main thread is in fact about troubleshooting a specific system.

Well, i have to give you right about that. Guess i got a bit engaged

But that is the only thing i give you right about

[Gary]

Here is something to ponder:

Static gas laws tell us that you can't have superheat in the presence of liquid, nor subcooling in the presence of vapor.

Place a half full container of refrigerant on a block of ice, with a heating pad on top. We now have superheated vapor at the top, subcooled liquid on the bottom and saturation at the vapor/liquid interface... all in the presence of each other.

How would you explain this?

[The Viking]

Steinar/Gary,

I do find the discussion interesting.
From previous post I know Gary's definition of sub-cooling.
But Steinar, can you just confirm that we are talking about saturated temperature of gas at pressure measured by gauge, less true measured temperature of gas?
If we agree on this then, unfortunately, I have to agree with Gary.....

[SteinarN]

The superheated gas in the top of the bottle isnt in contact with the saturated liquid in the liquid/vapor interface. The more or less superheated gas in the middle of the bottle insulates the more superheated gas in the top.

The same applies to the liquid in the bottom which isnt in contact with the saturated liquid/vapor interface.

However there is a flow of heat from the warm gas down to the cool liquid. As soon as the heat/cool source is removed the temperatures will start to equalice and we end up with a saturated liquid/vapor mix in the end.

To take this reasoning a step further, we can substitute the bottle with an evaporator. We can se one cirque as a very narrow and long bottle. At the inlet of one cirque we have a saturated mix of liqiud and vapor coresponding to the liquid/gas interface in the bottle. At the outlet of the cirque we have (normaly) superheated gas, coresponding to the top of the bottle. The same applies in a condenser.

I cant se any problem with containing superheated gas and subcooled liquid in the same chamber as long as we supply and/or remove heat as we do in an evaporator and a condenser. If no heat is added or removed, then we will end up with saturated gas/liquid.

[SteinarN]

Steinar/Gary,

I do find the discussion interesting.
From previous post I know Gary's definition of sub-cooling.
But Steinar, can you just confirm that we are talking about saturated temperature of gas at pressure measured by gauge, less true measured temperature of gas?
If we agree on this then, unfortunately, I have to agree with Gary.....

I didnt catch the meaning of your question
English isnt my native language.

Saturated temperature of gas at pressure measured by gauge, less true measured temperature of gas gives the subcooling of the gas

However:
Saturated temperature of liquid at pressure measured by gauge, less true measured temperature of liquid give the subcooling of the liquid.

And:
True temperature of gas less saturated temperature of gas measured by gauge gives the superheat of the gas.

[nike123]

SteinarN, you (and any one interested) should go here
http://hvac-talk.com/vbb/showthread.php?t=56495
and read whole thread. I just reading it and it seems that could give us some answers.

[SteinarN]

SteinarN, you (and any one interested) should go here
http://hvac-talk.com/vbb/showthread.php?t=56495
and read whole thread. I just reading it and it seems that could give us some answers.

17 pages

It take days to read it.

On page 3 now.

[SteinarN]

From that link i see one instance where we can have substantly subcooled liquid from the receiver. That is in the case of a hotgas by-pass valve delivering hotgass directly into the top of the receiver in order to maintain condensing pressure in cold ambient. That hot gass will maintain pressure in the receiver and displace a portion of the refrigerant into the condenser in a similar facion as non condensables will do. Thereby a large portion of the end of the condenser will contain liquid refrigerant, gradually acheive more subcooling towards the condenser outlet. The liquid will loose some of its subcooling when it enters the receiver and receives some heat from the warm gas there but still maintain substantly subcooling at the receiver outlet. But this requires a constant supply of hot gas into the receiver.

[Gary]

SteinarN, you (and any one interested) should go here
http://hvac-talk.com/vbb/showthread.php?t=56495
and read whole thread. I just reading it and it seems that could give us some answers.

An interesting discussion for sure. There are few people in this industry I would consider to be true authorities and Andy Schoen is one of them.

[frank]

There are few people in this industry I would consider to be true authorities and Andy Schoen is one of them.

AKA - Prof Sporlan ?

[Gary]

AKA - Prof Sporlan ?

Yep... haven't seen him posting lately...

[Gary]

17 pages

It take days to read it.

On page 3 now.

On page 16, near the bottom, Andy states that subcooling is not needed on systems with large receivers. Perhaps this explains the differences in our experiences.

In my books, I tell people to charge a TXV system to a clear sight glass or 15F/8.5K subcooling, whichever comes first. I'm thinking that covers it.

[nike123]

In my books, I tell people to charge a TXV system to a clear sight glass or 15F/8.5K subcooling, whichever comes first. I'm thinking that covers it.

Is that subcooling same with low ambient and low load condition as with high ambient and high load condition.
Or, do we need to charge when one of these extremes is satisfied.
__________________

[US Iceman]

On page 16, near the bottom, Andy states that subcooling is not needed on systems with large receivers.

To be honest I have not read the whole thread on the other site, but I question a blanket statement like this. If you have liquid rising up to the TXV's you will need subcooling no matter what size the receiver is.

On the other hand, if you have head pressure controls and flooded condensers in the winter time you have more than enough subcooling for almost anything you want to do.

Personally, I do not like blanket statements at all without defining the parameters on which they apply. It is the same danger as using rules of thumb for everything when they only apply to limited circumstances.

The subject of subcooling keeps coming up in different threads and I think we need to keep in mind the two ways subcooling is created.

One is the traditional way in which the liquid refrigerant is cooled below it's saturation temperature. In this case you have an actually lowered the refrigerant temperature but not it's pressure.

The other is when the liquid pressure is increased above the pressure equal to the saturation point the liquid exists at before the additional pressure is applied. In this case you have increased the pressure of the liquid but not changed it's temperature.

The entire discussion is based on one of the above and the specific conditions encountered in the system.

[Gary]

Oops... I misquoted Andy. What he said was that condenser subcooling was not needed on systems with large receivers. I would take that to mean subcooling in the drop leg between condenser and receiver.
.

[Gary]

The subject of subcooling keeps coming up in different threads and I think we need to keep in mind the two ways subcooling is created.

One is the traditional way in which the liquid refrigerant is cooled below it's saturation temperature. In this case you have an actually lowered the refrigerant temperature but not it's pressure.

The other is when the liquid pressure is increased above the pressure equal to the saturation point the liquid exists at before the additional pressure is applied. In this case you have increased the pressure of the liquid but not changed it's temperature.

Yes, lowering liquid temperature is good subcooling, raising saturated condensing temperature is bad subcooling, keeping in mind that subcooling is the difference between these two.

In any case, to get back to the original point, subcooling does not necessarily indicate non-condensables even if it is excessive subcooling. Excessive subcooling is more likely an indication of overcharge, although it might (less likely) indicate non-condensables.

[Gary]

To get back to the subject of this thread, even though we don't know what the actual subcooling temperature is, we know that the TD is 14K/25F, which is not even close to being excessive and therefore the subcooling is highly unlikely to be excessive, whether from overcharge or non-condensables.

[US Iceman]

...subcooling does not necessarily indicate non-condensables even if it is excessive subcooling.

Almost, but... doing the calculations with the temperature and pressure readings can cause the two (subcooling or non-condensables) to be interpreted as or appear to be the same thing.

This is a difficult thing for people to accept because the only way to prove it is have a connection on the condenser drain.

If you open the connection and liquid refrigerant comes out, you have subcooling.

If you open the connection and vapor bleeds out, you have non-condensables.

...raising saturated condensing temperature is bad subcooling...

I quite agree. Dan mentioned this once before as one possible effect.

But you have to remain aware that pressure can be increased by other means also. A refrigerant pump is one way. Another is static head in liquid!

PH charts are the way to follow this, or vapor pressure curves for the refrigerant in question. You can do this is with pressure/temperature tables also but the lack of reference lines make it less apparent.

[Refrigerologist]

The correct way to determine any existence of non condensables is to measure the liquid subcooling at condenser outlet or receiver outlet. Assuming only minor hight differences and not any measurable pressure differences between condenser/receiver and not overfilled system there should be no measurable subcooling at condenser/receiver outlet. Any subcooling here is a certain symtom of non condensables in the system.

When there is sufficient refrigerant in the system, any non condensables will accumulate in the receiver bc only liquid is drawn out of the receiver and the non condensables (gasses) cant find the way to the bottom of the receiver where the outlet pipe is.

An example:
Lets say the liquid temperature (condenser outlet and receiver outlet) is 30 degres C, but you measure a condensing/receiver pressure of 15,1 bar. At 30 condensing/liquid outlet, the pressure should have been 13,1 bar! In this instance you have 2 bar additional pressure (15,1-13,1) in the receiver caused by non condensables. Lets say the gas phase volume of the receiver is 5 litres, then you have 10 litres of noncondensables at atmospheric pressure in the system. (10 litres at 1 atmosphere equals 5 litres at those 2 atmospheres additional pressure.) This is assuming that the temperature of the liquid surface in the receiver is the same temperature as the condenser outlet/receiver outlet. This assumtion should be valid in a system with short pipes between condenser and receiver, not any excess pressure loss between condenser outlet and receiver outlet and not considerably temperature difference between the condensing temperature and the surrounding air of the receiver.

When the system is stopped, the non condensables have a possibility to escape from the receiver and expand/dilute into a much larger volume of the system. That 10 litres of non condensables, when expanded/diluted in, lets say, 20 litres of the system, will cause an additional pressure above the saturated pressure of only 0,5 bar. (10 litres at 1 bar equals 20 litres at 0,5 bar) Thus it is far more accurate to measure the receiver outlet pressure and temperature when the system is running than measure those values at stand still in order to determine the presence of any non condensables.

I cannot agree with your statement. Non condensables accumulate at the top of the condenser. These non condensibles take up room that is normally used in cooling the superheated refrigerant. Therefore the condenser is effectively smaller and there will be little or no subcooling or worse still the refrigerant will remain superheated.
Tol check for non condensibles you must fit your gauges, switch the system off and allow the equipment to reach the average ambient. Read the pressure and compare for a saturated pressure given for that ambient. If it is 14.7lbs above that expected then air is present.

[Refrigerologist]

To make things clear an overcharge of refrigerant will increase the amount of sub-cooling of the liquid at the liquid line. Non-condenables will reduce sub-cooling at the liquid line for reasons that are set out in my last post. These are facts and not assumptions!

[Refrigerologist]

To make things even clearer: Think about a dirty condenser. The condenser efficiency is lowered. It is made for all intensts and purposes too small for the job. Non-condenables do the same thing as they take up room in the condenser and CANNOT BE CONDENSED, THINK ABOUT IT! Therfore the non-condensables effectively make the condenser smaller by occupying space that the refrigerant should be using to be cooled. The air is NOT driven out of the condenser, it remains where it is trapped at the top!

[Refrigerologist]

Another point I should make is that a system is not necessarily fully charged because it has a full sight glass. The liquid line temperature should be checked to ascertain the actual amount of sub-cooling. Many times it is necessary to add further refrigerant to a system, even though the sight glass is clear. Try it some time phone a chiller manufacturer and find out their recommended sub-cooling figures. Then if you are having to recharge or commission a system you may find that by charging only to a full sight glass the amount of sub-cooling is higher that which is recommended.

[Refrigerologist]

On page 16, near the bottom, Andy states that subcooling is not needed on systems with large receivers. Perhaps this explains the differences in our experiences.

In my books, I tell people to charge a TXV system to a clear sight glass or 15F/8.5K subcooling, whichever comes first. I'm thinking that covers it.

Gary I never read this post, but yes you have hit it in a nutshell. Systems should be charged to the sub-cooling value of refrigerant at the liquid line. Sight glasses are only a guide o what is happening.

As for some of these other posts, I cannot believe what I am reading!

[US Iceman]

As for some of these other posts, I cannot believe what I am reading!

I guess it depends on which ones you are reading.

You can't make broad statements and expect them to hold true for every case.

Non condensables accumulate at the top of the condenser.

Not all the time. I have modified my answer in this edit.

SteinarN is getting to the heart of the problem. Where the non-condensables appear also depends on how the condensers are piped. If you have a liquid seal (P-trap) in the condenser drain line the non-condensables get trapped on top of the liquid seal. If the liquid seal disappears or the drain line is not trapped at all, the non-condensbales do flow down into the receiver. That makes a lot more sense!

[SteinarN]

To make things clear an overcharge of refrigerant will increase the amount of sub-cooling of the liquid at the liquid line. Non-condenables will reduce sub-cooling at the liquid line for reasons that are set out in my last post. These are facts and not assumptions!

In a normal fin and tube condenser, the non condensibles will acumulate in the receiver. The refrigerant is flowing through the tubes in the condenser at maybe 1-3 m/s. How can the non condensables molecules know they shall apply emergency brakes in order to not be sweept along with the refrigerant gas and liquid towards the outlet of the condencer?

The non condensibles will acumulate in the receiver, adding additional part pressure in the receiver, thereby restrict the exit of refrigerant out of the condenser. The result is that the end of the condenser will be full of liquid refrigerant gradually acheiving substantly subcooling towards the outlet of the condenser. Condensing pressure will rise as well.

[SteinarN]

Another point I should make is that a system is not necessarily fully charged because it has a full sight glass. The liquid line temperature should be checked to ascertain the actual amount of sub-cooling. Many times it is necessary to add further refrigerant to a system, even though the sight glass is clear. Try it some time phone a chiller manufacturer and find out their recommended sub-cooling figures. Then if you are having to recharge or commission a system you may find that by charging only to a full sight glass the amount of sub-cooling is higher that which is recommended.

I always install receivers with sight glass at the bottom and if available at the top also. Then i have an exelent supervision of the actual refrigerant level in the system. When i charge the system, i charge until the ball in the lower sight glass floats. At that point i have a clear liquid line sight glass, or maybe with an ocasional bubble. If i should charge until i got a subcooling of lets say 5C at condenser or receiver outlet, i would have to charge the receiver completely full. I dont care of the subcooling as long as i have liquid some way up from the bottom in the receiver.

Some says the proper amount of subcool is, lets say 8C. If i design a system with a dt condenser at 8K, how can it be possible to acheive a subcooling of 8C?

I have several systems with floating head running with condenser dt as low as 5K in low load condition. What is your recomended subcooling in such a case?