Subcooling

[Thana]

Having been in the refrigeration trade for some years I was wondering wether someone could tell me what is the correct way to measure subcooling? I can tell you what it is etc.. but to me there seems to be some confusion on where to measure and take temperature/pressure readings.
My point is
1. Do you take the Discharge pressure and liquid temperature or..
2. Do you take the liquid pressure and liquid temperature?

So can anybody clear this up for me?

Thanks in advance..

[nike123]

Having been in the refrigeration trade for some years I was wondering wether someone could tell me what is the correct way to measure subcooling? I can tell you what it is etc.. but to me there seems to be some confusion on where to measure and take temperature/pressure readings.
My point is
1. Do you take the Discharge pressure and liquid temperature or..
2. Do you take the liquid pressure and liquid temperature?

So can anybody clear this up for me?

Thanks in advance..

You take pressure close to condenser where you have access valve. If you don't have access valve near the condenser, than you take it at compressor outlet and take in consideration pressure drop from compressor to condenser. Or if you have after condenser till expansion valve any access point, you could use it, taking in consideration pressure drop.
Best place is immediately (10in/20cm) after condenser outlet.
There, you take also temperature reading of pipe which represents temperature of subcooled liquid refrigerant.

When you take temperature readings of pipes (in general) stay away at least 10in/20cm from parts of large masses (rotalock valves, 4way valve, liquid filter etc...)

[Thana]

Thanks nike123, so I take from what you have said that the discharge before the condenser is the most comman place? and not the liquid line after the condenser.

[nike123]

That depend on where you have access valve for pressure check. On some systems, only place is near compressor, then on other systems you have plenty service valves. Ideally, it should be at same place where you take temperature reading. Further you are from this point measurement is less accurate. Ad to that pressure gauge and temperature probe and thermistor inaccuracy, and you end up with big error. And that error could lead you to wrong decisions. Always make sure that your measuring is accurate enough.

For example, if you are working on air-to water chiller, their pipes are short and pressure drops are small, then you could take pressure reading anywhere and it is still accurate enough.
On the other hand, when you work on split-system you have only one access point for pressure reading and you could have 15 m (or more) of pipe. That reading should consider pressure drop in pipe.

[powell]

The optimim place to measure sub-cooling is at the inlet of the TEV. But I realize that most systems do not have an access fitting there. If you have a problem job, install an access fitting.

The liquid line is the only place to measure subcooling. The outlet of receiver is ok but you must account for pressure drop if there is long run to the inlet of the TEV.

Measure the LLP and LLT at the inlet of the TEV. On closed coupled systems this may be measured at the outlet of the receiver.

Use a P/T chart and convert the pressure to temperature and subtract the measured temperature from the P/T chart temperature.


•

[Tesla]

Hi Thana
Powell is spot on with the correct answer here. I found a tool (superheat calculation kit) from yellow jacket to measure the refrigerant temperature directly not the pipe temp. After using the tool I found a large error/differance between pipe and liquid temp, hence I only use the tool to measure SH or SC.

[SteinarN]

Hi Thana
Powell is spot on with the correct answer here. I found a tool (superheat calculation kit) from yellow jacket to measure the refrigerant temperature directly not the pipe temp. After using the tool I found a large error/differance between pipe and liquid temp, hence I only use the tool to measure SH or SC.

I have the same kit. (actually two of them) It did a big ahh to me. I did use clamp on probes earlier, but didnt get reasonable readings with them. Cold suctionlines seemed always to be warmer than they could be, showing several degrees of superheat even when flooding the evaporator. Liquidlines seemed to have several degrees subcooling even when they couldnt contain subcoold liquid.

That changed when i started to use that kit. I guess faulty temp readings is the reason so many argue they get subcooled liquid at the condenser outlet in units with receiver. When you have a receiver with 50cm pibe between it and the condenser outlet, it is absolutely 100% impossible to have any measurable degree of subcooling if the receiver isnt full of liquid and you dont have uncondensables in the system.

[nike123]

The optimim place to measure sub-cooling is at the inlet of the TEV. But I realize that most systems do not have an access fitting there. If you have a problem job, install an access fitting.

The liquid line is the only place to measure subcooling. The outlet of receiver is ok but you must account for pressure drop if there is long run to the inlet of the TEV.

Measure the LLP and LLT at the inlet of the TEV. On closed coupled systems this may be measured at the outlet of the receiver.

Use a P/T chart and convert the pressure to temperature and subtract the measured temperature from the P/T chart temperature.


•

That is place where you measure total subcooling. Condenser subcooling is measured at condenser outlet.
Place depends on what subcooling are you measuring.
Still, I don't know why I am fixated at condenser subcooling in this post.

[nike123]

Hi Thana
Powell is spot on with the correct answer here. I found a tool (superheat calculation kit) from yellow jacket to measure the refrigerant temperature directly not the pipe temp. After using the tool I found a large error/differance between pipe and liquid temp, hence I only use the tool to measure SH or SC.

Did you mean this?


How much is it cost? It looks something worth to buy.
Then again, how accurate is that thermometer?

[Gary]

Where you measure subcooling depends on why you are measuring subcooling. For my purposes, I need to know the receiver outlet subcooling. On systems that do not have a receiver, I want to know the condenser outlet subcooling.

[nike123]

I have the same kit. (actually two of them) It did a big ahh to me. I did use clamp on probes earlier, but didnt get reasonable readings with them. Cold suctionlines seemed always to be warmer than they could be, showing several degrees of superheat even when flooding the evaporator. Liquidlines seemed to have several degrees subcooling even when they couldnt contain subcoold liquid.

That changed when i started to use that kit. I guess faulty temp readings is the reason so many argue they get subcooled liquid at the condenser outlet in units with receiver. When you have a receiver with 50cm pibe between it and the condenser outlet, it is absolutely 100% impossible to have any measurable degree of subcooling if the receiver isnt full of liquid and you dont have uncondensables in the system.

From what I know, if liquid exist it is subcooled!
When mater change state it is either subcooled or superheated. It cannot be in between.
Or maybe I am wrong.

Saturation:
Saturation is simply the term used to describe the point where a change of state in a substance is taking place. For water at sea level, the boiling temperature is 212 degrees F. Therefore, we say the saturation (boiling temperature) is 212 degrees. As soon as the temperature of the steam is heated above it’s “saturation” temperature, it has been superheated. Refrigerant that has boiled (turned into a vapor) at 40 degrees has a saturation temperature of 40 degrees. If the refrigerant vapor is heated to 41 degrees it is no longer saturated, it is then superheated by 1 degree. Remember, only a gas or vapor can be superheated. Superheat is any temperature of a gas or vapor above it’s saturation temperature.

[Thana]

Thanks guys very intresting...I like the sound of that little gismo there, where can you buy one and ruffly how much?

[thermo prince]

An old ICI Arcton' handbook once used a nice phrase for Saturated Condensing Temperature (SCT) : they called it the
" formation temperature " of refrigerant liquid ( with of course a corresponding formation pressure or saturated condensing pressure for each given temp).
Any further sensible cooling ( sensible being that temperature gain / loss measurable on a thermometer) of this high pressure formed liquid at the condenser outlet / receiver tank outlet or up to the tx valve is subcooling. As Gary and Nike say, it depends on what subcooling (SC) you are interested in - condenser only SC or total SC.

Thana , I dont mean this rather simple explanation is for you or the other seasoned HVACR guys here but I like the term 'formation temperature' and use it with rookie transport trainees and feel it can be helpful to all apprentices/improvers online here in getting their heads around what's really happening at certain heat exchange devices.

P.S Steinar and Nike123 - it would be interesting if you ran some side by side comparison of this Yellow Jacket 'fluid' measurement tool readings in conjunction with the typical Fluke/Testo/Fieldpiece pipe measuring probes. See if there's any offset or correction factor that can hold true in given ranges - i.e the variance of pipe clamp to the fluid probe temp readings.

regards
T-P

[nike123]

P.S Steinar and Nike123 - it would be interesting if you ran some side by side comparison of this Yellow Jacket 'fluid' measurement tool readings in conjunction with the typical Fluke/Testo/Fieldpiece pipe measuring probes. See if there's any offset or correction factor that can hold true in given ranges - i.e the variance of pipe clamp to the fluid probe temp readings.

regards
T-P

If I got him I will make some comparison with my no name IR thermometer. Other thermometer then IR I currently don't poses.
Still I am pretty much satisfied with results from IR thermometer. I do regular checks of him at known surfaces and it is accurate enough (within 1°C):

[SteinarN]

An old ICI Arcton' handbook once used a nice phrase for Saturated Condensing Temperature (SCT) : they called it the
" formation temperature " of refrigerant liquid ( with of course a corresponding formation pressure or saturated condensing pressure for each given temp).
Any further sensible cooling ( sensible being that temperature gain / loss measurable on a thermometer) of this high pressure formed liquid at the condenser outlet / receiver tank outlet or up to the tx valve is subcooling. As Gary and Nike say, it depends on what subcooling (SC) you are interested in - condenser only SC or total SC.

Thana , I dont mean this rather simple explanation is for you or the other seasoned HVACR guys here but I like the term 'formation temperature' and use it with rookie transport trainees and feel it can be helpful to all apprentices/improvers online here in getting their heads around what's really happening at certain heat exchange devices.

P.S Steinar and Nike123 - it would be interesting if you ran some side by side comparison of this Yellow Jacket 'fluid' measurement tool readings in conjunction with the typical Fluke/Testo/Fieldpiece pipe measuring probes. See if there's any offset or correction factor that can hold true in given ranges - i.e the variance of pipe clamp to the fluid probe temp readings.

regards
T-P

Actually, i have. Carrier Vector Freezer -35 evaporating. Couldnt measure less than -31 with a clamp on probe even with fully open TXV regardless of how i pressed or adjusted the clamp. A small adjustment of the clamp could change the reading with 5 degres instantly. Tried several types of clamps.

With that nice kit i got instantly a reading of -34/-35. When i adjusted the valve i observed a hunting in the temperature between -25/-31 every 30 sec. I couldnt see any hunting at all with that clamp on. Couldnt get it stop hunting regardless of how i adjusted the valve. At least in measuring superheat on freezers this kit is essential. Its probably easier to measure more normal temperature subcooling or superheat on coolers with clamps.

Another thing. Liquid refrigerant hasnt to be subcooled. Saturated liquid is by definition not subcooled. If you dont have saturated liquid, then you have subcooled liquid, as you can have at condenser outlet on a unit without receiver.

[Pyebesi]

Dear All,
Grateful if you can find me a pc board and compressor for a Daikin Air conditioner RSXY10HJW1.

Thanks

[The Viking]

Dear All,
Grateful if you can find me a pc board and compressor for a Daikin Air conditioner RSXY10HJW1.

Thanks

>>>>Moderator, please move to it's own thread<<<<

Pyebesi,
Your best bet would be to contact your nearest Daikin dealer.

Closest Daikin branch is probably in SA, click HERE for details.

[patrickj]

supreheating and subcooling are two very important factors in a refrigertion system.In the case of a compressor the discharge is always superheated.That means the discharge temperature of the refrigerant gas is more than the saturation temperature corresponding to the dischrge pressure.
Similerly after condensation if the temperature of the refrigerat liquid is lower than that of the saturation temperatures corresponding tothe condensing pressure, then we call the liquid is subcooled.

This property can be very useful in trouble shooting


PATRICKJ

[marc5180]

Did you mean this?


How much is it cost? It looks something worth to buy.
Then again, how accurate is that thermometer?

Where can i buy one of those? I really need an acurate piece of kit for superheat purposes?

[SteinarN]

Where can i buy one of those? I really need an acurate piece of kit for superheat purposes?

I think its produced by Yellow Jacket. Here in Norway i paid somthing like USD200 for it.

[marc5180]

I've looked online but i can't find anywhere?

[nike123]

I've looked online but i can't find anywhere?

Price in USA is 98 USD.
I sent query about nearest representative at this page:
http://www.yellowjacket.com/Contact.asp
I suggest you, to do the same.

[marc5180]

Sent them an email this morning, so hopefully should hear back from them soon. Thanks

[Refrigerologist]

I have the same kit. (actually two of them) It did a big ahh to me. I did use clamp on probes earlier, but didnt get reasonable readings with them. Cold suctionlines seemed always to be warmer than they could be, showing several degrees of superheat even when flooding the evaporator. Liquidlines seemed to have several degrees subcooling even when they couldnt contain subcoold liquid.

That changed when i started to use that kit. I guess faulty temp readings is the reason so many argue they get subcooled liquid at the condenser outlet in units with receiver. When you have a receiver with 50cm pibe between it and the condenser outlet, it is absolutely 100% impossible to have any measurable degree of subcooling if the receiver isnt full of liquid and you dont have uncondensables in the system.

To say you cannot have sub-cooled liquid at the outlet of the condenser must be incorrect even when a reciever is installed. Sub-cooling takes place in approximately the last third of the condesnser surface area. Therefore it stands to reason that the liquid leaving at the outlet is still sub-cooled. I think you are being misled by the notion that because liquid and vapour are present in the receiver then this renders sub-cooling impossible in the final bit of condenser to receiver pipe work. By that theory it would be impossible to have any subcooled liquid in a short liquid line, and we would be wholly reliant on sub-cooling taking place in the liquid line. By your own logic, This would have to take place a significant distance from the reciever. Remember it is a solid, (if that is the correct word), column of liquid entering the receiver, just as it should be a solid column of liquid leaving the reciever. The liquid only comes in contact with the vapour inside the reciever vessel.

This is the tricky area, the concept of a sub-cooled liquid in contact with its' vapour. But, I believe somebody explained, (in another thread), that the lower part of the receiver vessel that contains liquid is at a lower temperature than the vapour at the top. I believe somebody else explained that the continuos evaporation from the liquid is in equilibrium with the returning condensing vapour, thus causing a slight reduction in temperature of the liquid in comparison to the vapour. And this explained why the liquid at the withdrawl tube remained sub-cooled.

[US Iceman]

I do hope this discussion of subcooling does not keep jumping to every thread that has the word subcooling in it.

[SteinarN]

I do hope this discussion of subcooling does not keep jumping to every thread that has the word subcooling in it.

I'm backing out of this little discussion for a while. Have to take some measurments and a couple photos one day tho, and post in the proper thread.

[Refrigerologist]

I do hope this discussion of subcooling does not keep jumping to every thread that has the word subcooling in it.

Iceman, or indeed Gary, you have good reps. What are your thoughts on the subject? The original post was where should you take a measurement for sub-cooling? There are some conflicting arguments, (inculding my own), as to where in the system the refrigerant is sub-cooled, and I feel we need some clarification. I am not saying I am right, more that this is what I have learned from my Tech college course, by experience and from the engineers who trained me. I am open to the idea that my understanding of refrigeration fundamnetals may be incorrect! After all maybe I misunderstood what was being taught and have perpetuated the 'myth' But, I would need a plausible explanation if that is the case.

For the record, I would normally take 2 temperature readings one at the condenser outlet, the other a short way down the liquid line from the receiver, if installed. As for guage position, this is wholly dependent on what service ports are available. If a liquid service port is available this is where I prefer to fit the pressure guage. If not then it must be the compressor discharge service port.

[US Iceman]

When I have some time to spend on this I will start a new thread in the hopes we carry on a interesting discussion.

[750 Valve]

What are your thoughts on the subject? The original post was where should you take a measurement for sub-cooling?

Like has been mentioned, take the temp reading and the pressure reading at the same place. This place depends on what you want to read, cond subcooling or even total subcooling (measure at inlet to txv in this case). Subcooled liquid can (and usually does) occur in the condenser, after being fully condensed and can and also usually occurs in the reciever and all the way down the liquid line to the metering device.