Subcooling

[chillyblue]

Subcooling
i know how it happens how to read & measure it, but i'm just not convienced that i can or do experience it.

question 1
i have always been told that to drain a condensor properly you need to locate the liquid receiver inlet lower than the condenser outlet.
if the above is not the case you will reduce the condensing surface area, increasing you head pressure and also increase subcooling.

Question 2
Most condensing units have vertical condensers with the liquid receiver inlet valves half way up the condensing surface, with the pipework running down to the base of the unit and then back up to the receiver inlet valve, Nice liquid trap!!!!!
does this mean that the condenser would have to be halve full before liquid starts to enter the liquid receiver??? if so thats a 50% reduction of the condensing surface.

question 3
To obtain subcooling (in a standard condenser not one with a subcooling circuit) liquid must sit in the condenser and be 100% liquid, ( if there is any vapour present the vapour needs to condense before the liquid can subcool). for liquid to sit in the condenser there must be no flow or the liquid receiver is full.

Perhaps i'm looking too deep!!!!

your advice and help would be appriciated

CB

[Pooh]

Subcooling
i know how it happens how to read & measure it, but i'm just not convienced that i can or do experience it.

question 1
i have always been told that to drain a condensor properly you need to locate the liquid receiver inlet lower than the condenser outlet.
if the above is not the case you will reduce the condensing surface area, increasing you head pressure and also increase subcooling.

Question 2
Most condensing units have vertical condensers with the liquid receiver inlet valves half way up the condensing surface, with the pipework running down to the base of the unit and then back up to the receiver inlet valve, Nice liquid trap!!!!!
does this mean that the condenser would have to be halve full before liquid starts to enter the liquid receiver??? if so thats a 50% reduction of the condensing surface.

question 3
To obtain subcooling (in a standard condenser not one with a subcooling circuit) liquid must sit in the condenser and be 100% liquid, ( if there is any vapour present the vapour needs to condense before the liquid can subcool). for liquid to sit in the condenser there must be no flow or the liquid receiver is full.

Perhaps i'm looking too deep!!!!

your advice and help would be appriciated

CB

Chillyblue

Question One.
It is the pressure difference across the metering device that causes the liquid to flow from the condenser not gravity.
Question Two.
See the answer to question one.
Question Three.
As the liquid is cooled below its saturation temperature by the cooling medium it has got to become subcooled, if there is a sight glass fitted and it is clear you have subcooled liquid.

I think you are getting confused by thinking the liquid moves by gravity rather than due to the pressure difference across the metering device.

Ian

[NoNickName]

1) It's a good practice to have the receiver in at a lower level than the condenser out.
2) As liquid progresses its path to TXV, temperature and pressure decrease. The liquid level in the condenser is lower than in the liquid riser. But you are conceptually right. That's a liquid trap.
3) No. That explains with a mass flow balance between vapour condensing and liquid leaving the receiver.

[NH3LVR]

I will add something to the answers on question one. It does not apply to smaller systems.
Larger system use a equalizing line from the top of the receiver to the top of the condensor. The drain line from the condensor is sloped back to the receiver.
Otherwise the condensor will fill with liquid and subcool. In fact excessive subcooling is one indication troubles have developed.

[Peter_1]

1) It's a good practice to have the receiver in at a lower level than the condenser out.
2) As liquid progresses its path to TXV, temperature and pressure decrease. The liquid level in the condenser is lower than in the liquid riser. But you are conceptually right. That's a liquid trap.
3) No. That explains with a mass flow balance between vapour condensing and liquid leaving the receiver.

NoNickName, is this the new company you're working for?
We buy these machines via ECR Belgium.

[NoNickName]

NoNickName, is this the new company you're working for?
We buy these machines via ECR Belgium.

Yep. ECR is a good customer.

[Gary]

I will add something to the answers on question one. It does not apply to smaller systems.
Larger system use a equalizing line from the top of the receiver to the top of the condensor. The drain line from the condensor is sloped back to the receiver.
Otherwise the condensor will fill with liquid and subcool. In fact excessive subcooling is one indication troubles have developed.

This only applies to systems where the condenser is outdoors and the receiver is indoors.

In cold weather, the condenser may be at a lower pressure/temperature than the receiver, because the receiver is in a warm machine room.

This can stop the flow of liquid from the lower pressure condenser to the higher pressure receiver. This is a common problem in large rack systems, and is usually resolved by equalizing the condenser and receiver pressures, thus allowing gravity to move the liquid.

In all other systems where the condenser and receiver are subjected to the same ambient conditions, any liquid gathering in the bottom of the condenser is pushed through to the receiver, regardless of which is higher.

[Gary]

question 3
To obtain subcooling (in a standard condenser not one with a subcooling circuit) liquid must sit in the condenser and be 100% liquid, ( if there is any vapour present the vapour needs to condense before the liquid can subcool).

This just simply isn't true. Subcooling can and does occur where there is still vapor present.

For example, take a jug of refrigerant, place it on a block of ice and put a heating pad on top. You now have subcooled liquid on the bottom, superheated vapor on the top and saturation at the vapor/liquid interface... all in the presence of each other.

In a slightly undercharged system, you can measure subcooling on the liquid line even though there are vapor bubbles in the sight glass.

You can measure a small amount of superheat at the compressor inlet and have that compressor flooding with liquid droplets.

[mohamed khamis]

Hi there,

In a simple term, if u observe in some cars the exhaust gas has a black colour and this indicates to some amount of fuel has not been fully burnt well in the engine although the rest is fully burnt. Similarly, if the refrigerant mass flow rate which is metered by TXV is larger slightly than is required for cooling load the refrigerant leaves the evaporator in superheat phase (smaller than designed) but with some droplets of liquid. The same for the condenser and subcooling.

Gary u gave an example any brain can accept it but the refrigerant jug is subjected to two different modes of heat exchange (cooling and heating) and definitely the two phases (superheat and subcooling) can be occured in this example.

I think chillyblue he objects how the extisting of the two phases with one different mode of heat exchange which is the cooling in the condenser.

I think all the receiver should be down the condenser or at least at the same level

[Gary]

Question 2
Most condensing units have vertical condensers with the liquid receiver inlet valves half way up the condensing surface, with the pipework running down to the base of the unit and then back up to the receiver inlet valve, Nice liquid trap!!!!!
does this mean that the condenser would have to be halve full before liquid starts to enter the liquid receiver??? if so thats a 50% reduction of the condensing surface.

In this, you assume that liquid in two connected containers must seek an equal level. This is only true if both containers have the same vapor pressure.

If the vapor pressure in one of those containers is increased, the liquid is pushed to the other container.

The condenser has a compressor discharge connected to its inlet, giving it a higher vapor pressure than the receiver. This pressure difference pushes the liquid from the condenser to the receiver.

[chillyblue]

The condenser has a compressor discharge connected to its inlet, giving it a higher vapor pressure than the receiver. This pressure difference pushes the liquid from the condenser to the receiver.

Hi Gary thanks for your help
are you saying that the pressure in the reciever is lower than that of the condenser, i thought they would both be the same??

as far as the liquid vapour mix is concerned i was always taught that it can be neither superheated or subcooled if there was a mix, the following was taken from the sporlan site which explains superheat/ subcooling and says that you can only have subcooling in the liquid line from the receiver not to it or in it!!! (perhaps i have mis read or interpreted it.

When a system employs the use of a liquid receiver, there can be no subcooling at the surface of the liquid in the receiver. The reason is that when liquid refrigerant and vapor exist together, they must obey the P-T relationship or the refrigerant must be saturated. In our example the measured
pressure in the receiver is 146 psig; the refrigerant in
the receiver must therefore be at 110°F.
Once a solid column of liquid is formed, subcooling of the refrigerant can take place by lowering its temperature with the use of liquid-suction heat exchangers, subcoolers, or from lower ambient temperatures surrounding the line. Subcooling is a lowering of a temperature below the saturation

point or boiling point. In our illustration in Figure 2.

Many thanks for the help

CB

[Gary]

Hi Gary thanks for your help
are you saying that the pressure in the reciever is lower than that of the condenser, i thought they would both be the same??

The compressor generates the pressure and through friction alone there must be a slight difference... and if a liquid "plug" forms in the line between, the difference would be greater until that liquid is pushed through.

as far as the liquid vapour mix is concerned i was always taught that it can be neither superheated or subcooled if there was a mix, the following was taken from the sporlan site which explains superheat/ subcooling and says that you can only have subcooling in the liquid line from the receiver not to it or in it!!! (perhaps i have mis read or interpreted it.

When a system employs the use of a liquid receiver, there can be no subcooling at the surface of the liquid in the receiver. The reason is that when liquid refrigerant and vapor exist together, they must obey the P-T relationship or the refrigerant must be saturated. In our example the measured
pressure in the receiver is 146 psig; the refrigerant in
the receiver must therefore be at 110°F.
Once a solid column of liquid is formed, subcooling of the refrigerant can take place by lowering its temperature with the use of liquid-suction heat exchangers, subcoolers, or from lower ambient temperatures surrounding the line. Subcooling is a lowering of a temperature below the saturation

point or boiling point. In our illustration in Figure 2.

This is a little misleading. In the example I gave earlier of the jug on ice with a heating pad on top, saturation exists at the vapor/liquid interface.

If a receiver is half full, the bottom half can be subcooled, because the liquid below the center is not touching the vapor. Saturation exists only where the vapor meets the liquid. In reality, unless the receiver is very large in relation to the system, there is a churning, changing dynamic condition with the overall effect moving progressively towards subcooling.

A vapor bubble can be surrounded by subcooled liquid, with saturation only at the surface of the bubble, the liquid further from the bubble surface being subcooled. This is why you need more than one degree of subcooling at the TXV to provide solid liquid.

A liquid droplet can be surrounded by superheated vapor, with saturation only at the surface of the droplet, the vapor further from the droplet surface being superheated. This is why you need more than one degree of superheat at the compressor inlet to provide solid vapor.

[Billy Ray]

Chilly Blue,

It sounds quite plausible that Gary is right, although i was tendering to side with your opinion.

All that i've been told in twenty years is vapour/liquid in a receiver therfore = saturation.

This said, a few years ago whilst discussing this same phemonimen with a Research Scientist from Copeland, he had 'mapped' a receiver with a multitude of sensors & described the same effect as Gary.

Sounds like back to the drawing board for us old (mid age) fridgies.

Billy Ray

Advid refrigeration enthuiast from Norfolkshire, England.

[US Iceman]

Subcooling is a lowering of a temperature below the saturation point....

That is the accepted definition of subcooling.

However, you should recognize the same effect can be created by increasing the pressure of the liquid above the equivalent pressure for any specific saturation temperature.

The best example of this is liquid sitting in a vertical pipe. The pressure at the bottom of the pipe is higher than the pressure at the top. The pressure increase is due to static head of the liquid sitting in the pipe.

The pressure increase at the bottom of the pipe generates an "equivalent effect" of subcooling, although the actual liquid temperature does not change.

You also see a similar phenomena when trying to calculate non-condensable gases.

[Billy Ray]

Your right Mr Iceman,

The harder you try to understand, the more difficult it becomes.

Billy Ray

[Gary]

Sounds like back to the drawing board for us old (mid age) fridgies.

Hey, I'm mid-age... you guys are kids... LOL

[Gary]

Your right Mr Iceman,

The harder you try to understand, the more difficult it becomes.

Billy Ray

I suspect the primary reason for teaching it the wrong way is that the wrong way is easier to teach.

[US Iceman]

One of the most difficult things to realize is it is hard to throw out something you learned because it keeps getting in the way.

[chillyblue]

Thanks guys thats a great help,

The concern reguarding the condenser was in an artical i read in the service enginers section of the institute of refrigeration, they described condensers that were not higher than the reciever would not drain properly and would cause high head pressures due to "locking liquid in the bottom of the condenser reducing its surface area.

As with the subcooling the things you are told you take at face value, especially from reliable sources like the IOR, and sometimes these facts only confuse things more or put another spanner in the works which was'nt actually there in the first place???

CB

[wambat]

What I find interesting is that the author contradicts himself by saying” When a system employs the use of a liquid receiver, there can be no sub cooling at the surface of the liquid in the receiver" which is correct but then he says” In our example the measured pressure in the receiver is 146 psig; the refrigerant in the receiver must therefore be at 110°F”, but John Tomczyk says it right when he says “Once the sub cooled liquid exits the condenser, the receiver receives and stores the liquid. The liquid level in the receiver will vary if the metering device is throttling open or closed. Usually receivers are on systems when thermostatic expansion valves are used as metering devices. The sub cooled liquid in the receiver may lose or gain sub cooling depending on the temperature of the area surrounding the receiver. If the sub cooled liquid is warmer than the receiver’s surroundings, then the liquid will reject heat to the surroundings and sub cool even more.

[US Iceman]

The sub cooled liquid in the receiver may lose or gain sub cooling depending on the temperature of the area surrounding the receiver. If the sub cooled liquid is warmer than the receiver’s surroundings, then the liquid will reject heat to the surroundings and sub cool even more.

Absolutely right. This is a subtle point all too often forgotten.

[chillyblue]

I would go with that as well.

[chillyblue]

I have just charged a system which is brand new and have concentrated just on the subcooling aspect, these were my readings.

Plant discription,
refrigerant R404A
unit ;- Tecumseh condensing unit with liquid reciever
air on 13 deg C
air off 17 deg C
operating temp -20 deg C
expansion device TEV
suction pressure

The discharge pressure was taken from the receiver outlet (as there is no port on the compressor discharge)

shortly after startup 135psi pipe temp at condenser outlet 16 deg C sight glass bubbling (2deg subcooling)

next reading 135psi pipe 17 deg C sight glass bubbling (1deg subcooling)

i added gas too clear the sight glass and get more subcooling

Next reading 145psi pipe 18deg C sight glass clear (2.5deg subcooling)

next reading 145psi pipe 16deg C sight glass clear (4.5deg subcooling)

next reading 150psi pipe 17deg C sight glass clear (4.5deg C subcooling)

More gas added

next reading 155psi pipe20deg C sight glass clear (2.5deg C subcooling)

next reading 160psi pipe 21deg C sight glass clear (3 deg C subcooling)

My questions

1. I was on the understanding that if you over charged a system you would get high subcooling (mine seemed to drop, get lower)
2. surely if you keep adding gas it will just end up filling the reciever, so how do you gain more subcooling as it's not in the condenser any longer than it was before
3. subcooling will be affected by the flow of refrigerant through the system, if the TEV is wide open the flow through the condenser will be faster (less subcooling) than if the valve is throttled down the flow through the condenser would be slower and give more time to cool i.e. more subcooling.
4. Do i still need to add more refrigerant?? is the system still under charged??

In my experience i would be satisfied that the system is properely charged and that if i added more refrigerant i would just be filling the receiver unnecessarily, and then leaving no space for pump down.

Totally confused.

CB

[The MG Pony]

I've develuped a habit of starting with a 1 pound charge for small systems and let it get the box within opperating range of temp, then if I get good sub cooling and clear sight glas I take away gas just till it starts to buble then start to add gas till it stops, then 1 extra ounce after that for good messure.

Mind you this is R-134a and R-22 systems.

With systems with receivers I've found this gives a verry good charge. Think I'll have to assemble a modle system and plaster the thing with probes when I get back to my home town!

[Gary]

The discharge pressure was taken from the receiver outlet (as there is no port on the compressor discharge)

shortly after startup 135psi pipe temp at condenser outlet 16 deg C sight glass bubbling (2deg subcooling)

next reading 135psi pipe 17 deg C sight glass bubbling (1deg subcooling)

i added gas too clear the sight glass and get more subcooling

Next reading 145psi pipe 18deg C sight glass clear (2.5deg subcooling)

next reading 145psi pipe 16deg C sight glass clear (4.5deg subcooling)

next reading 150psi pipe 17deg C sight glass clear (4.5deg C subcooling)

More gas added

next reading 155psi pipe20deg C sight glass clear (2.5deg C subcooling)

next reading 160psi pipe 21deg C sight glass clear (3 deg C subcooling)

Temperatures should be taken on the liquid line at the receiver outlet, not the drip line at the condenser outlet. You are measuring temperature on the wrong line.

[nike123]

Hi, chillyblue!
What is your waiting period after you adding gas? You should let at least fifteen minutes after toping up that system stabilize and then read subcooling.

[chillyblue]

Hi Gary

Sorry thats a mistake the temperatures i toke were on the receiver outlet line.


Hi Nike123
The temperatures were taken at times straight after topup and then about 10-15 minute intervals.

CB

[nike123]

Hi Nike123
The temperatures were taken at times straight after topup and then about 10-15 minute intervals.

CB

From what you said and your readings I suppose that after toping up you have 2,5K subcooling and with time subcooling rise.to level of 4.5K
Then you topped up again and you have 2,5K of subcooling and that value rise with time to 3K.
I thing that you didn't waited long enough to system stabilize when you took last reading, or the system did not stabilized fast enough to took that reading.
I would also check relation of TEV functioning with subcooling drop and rise.
If TEV hunting, than also subcooling oscillates.

[Gary]

air on 13 deg C

You are trying to charge a system in cold weather without head pressure control, which is a little more complicated.

Block off part of the condenser until the SCT is 90-100F/32-38C. This simulates warmer weather. You will need to keep adjusting the blockage as you fill the system in order to maintain within this range.

When the evaporator outlet superheat is down to normal range (with the evaporator air on temp also in range), there is enough to do the job (minimum charge). When the sight glass clears, there is enough to ensure solid liquid at the TXV (normal charge).

Charge until the sight glass is clear or the subcooling is 15F/8.5K (maximum charge), whichever comes first.

[chillyblue]

i have noticed with several systems i have charged the following.
I have a twin fan air cooled condenser with one of the fans operating as soon as the unit is operating and the other fan operates via head pressure control.
Why is it that the sight glass can be totally clear, no bubbles until the second condenser fan cuts in, then it starts bubbling, then the second fan cuts out and the sight glass clears again.

Cheers

C.B

[SteinarN]

Lets assume the system is running stable at a pressure slightly less than the cut in pressure for fan no2. Then the ambient increase ever so slightly, enough to cause the cut in of no2. The head pressure decreases imidiately due to increased condencer capacity. The refrigerant in the receiver has a higher temperature (and pressure) than the new lower condencing temperature/pressure and starts to boil off refrigerant gas. The outflow of refrigerant from the condenser is greatly reduced or stopped completely due to the rapid reduction in pressure in the condenser and the slower reduction in presure in the receiver. The result of this is the refrigerant level in the receiver drops to the level of the outlet tube, or even under the level of the outlet tube in some cases. And the sight glas start to bubble, foam, or if the refrigerant level drops enough, the sight glas get clear with only gas in it for a short while.

At the same time the condenser fills up with the refrigerant now missing from the receiver. The last part of the condenser (increasingly larger last part) is now filled with increasingly subcooled liquid. As the liquid refrigerant in the receiver has boiled off enough gas to lower the temperature and pressure of the remaining refrigerant, the flow from the condenser is resumed. At this point the condenser pressure is still higher than what it eventually will be at stable running conditions. This is beceause the size of the effective condenser is at this point reduced due to a large part of the condenser filled with subcooled liquid. As this subcooled liquid start to enter the receiver it cools the refrigerant still in the receiver as well as the receiver itself, it condenses some of the gas in the receiver further lowering the pressure. The pressure in the receiver falls more. The receiver starts to fill up with liquid and the sight glas clears. As the amount of subcooled liquid in the condenser decreases as it flows rapidly out of the condenser and into the receiver, the degree of subcooling decreases also. The condensing pressure decreases slightly more as the effective condenser size increases as the subcooled liquid exit the condenser. In the end balance is restored with a new lower condensing pressure than was the starting point.

[chillyblue]

Great explanation thanks.

so would you say that the system is undercharged?

C.B.

[nike123]

Nice explanation Steinar. I learned something new here.

[SteinarN]

Great explanation thanks.

so would you say that the system is undercharged?

C.B.

It is hard to tell whether it is enough refrigerant in a system when one fan switch on and off causing periodic bubbles in the sight glass as you observed. The exception is when there is a level sight glass in the receiver. Then I charge acording to the actual refrigerant level in the receiver and not paying attention to the liquid line sight glass.

Try to get the system running stable with only one or with both fans. Adjust the fan head pressure control if necessary to do this. Charge refrigerant as normal.

[US Iceman]

Lets assume the system is running stable at a pressure slightly less than the cut in pressure for fan no2. Then the ambient increase ever so slightly, enough to cause the cut in of no2. The head pressure decreases imidiately due to increased condencer capacity. The refrigerant in the receiver has a higher temperature (and pressure) than the new lower condencing temperature/pressure and starts to boil off refrigerant gas. The outflow of refrigerant from the condenser is greatly reduced or stopped completely due to the rapid reduction in pressure in the condenser and the slower reduction in presure in the receiver. The result of this is the refrigerant level in the receiver drops to the level of the outlet tube, or even under the level of the outlet tube in some cases. And the sight glas start to bubble, foam, or if the refrigerant level drops enough, the sight glas get clear with only gas in it for a short while.

At the same time the condenser fills up with the refrigerant now missing from the receiver. The last part of the condenser (increasingly larger last part) is now filled with increasingly subcooled liquid. As the liquid refrigerant in the receiver has boiled off enough gas to lower the temperature and pressure of the remaining refrigerant, the flow from the condenser is resumed. At this point the condenser pressure is still higher than what it eventually will be at stable running conditions. This is beceause the size of the effective condenser is at this point reduced due to a large part of the condenser filled with subcooled liquid. As this subcooled liquid start to enter the receiver it cools the refrigerant still in the receiver as well as the receiver itself, it condenses some of the gas in the receiver further lowering the pressure. The pressure in the receiver falls more. The receiver starts to fill up with liquid and the sight glas clears. As the amount of subcooled liquid in the condenser decreases as it flows rapidly out of the condenser and into the receiver, the degree of subcooling decreases also. The condensing pressure decreases slightly more as the effective condenser size increases as the subcooled liquid exit the condenser. In the end balance is restored with a new lower condensing pressure than was the starting point.

Now that is one of the best explanations I have seen about this!

It also presents some very good information on why you should not just charge a system using the site glass!

Two thumbs up SteinarN.

[Billy Ray]

Steinar,

thats well described.. i have seen this effect many times on larger receivers.

The best way so see this effect in 'real -life' is on a receiver wth two sight glasses with a slight system under charge. Typically a good size horizontal Bitzer receiver has two sight glasses, one near the bottom, one near the top.

Put a torch (flash light) to the bottom sight glass & look through the top sight glass.

The effect is quite stunning. You can see the liquid pulsating into the receiver. The flow literally stops & restarts. The vapour in the receiver clears & changes to fog-like condition.

This all happens in a pattern as the condenser fans cycle. The particular plant that i have seem this effect to this extent had a large condenser with only two fans, as you described Chiily Blue.

Billy Ray

[SteinarN]

Steinar,

thats well described.. i have seen this effect many times on larger receivers.

The best way so see this effect in 'real -life' is on a receiver wth two sight glasses with a slight system under charge. Typically a good size horizontal Bitzer receiver has two sight glasses, one near the bottom, one near the top.

Put a torch (flash light) to the bottom sight glass & look through the top sight glass.

The effect is quite stunning. You can see the liquid pulsating into the receiver. The flow literally stops & restarts. The vapour in the receiver clears & changes to fog-like condition.

This all happens in a pattern as the condenser fans cycle. The particular plant that i have seem this effect to this extent had a large condenser with only two fans, as you described Chiily Blue.

Billy Ray

That's why I dont like to control one fan with a on/off pressure control on a two or three fan condenser. I feel it to be a to simple solution. Not energy efficient either. I usually install a small VFD and control the fans in parallel. You get an absolutely stable condensing pressure then. And a reduction in the condenser fan power consumtion as well.

[Kh1971]

Hi, can you explain again the foaming issue on the sight glass

[nike123]

Hi, can you explain again the foaming issue on the sight glass

Why again? Does the explanation gone somewhere, or missing?

[chillyblue]

Back to subcooling

I've been experimenting again, as follows. just to add the freezer is working fine, i'm just experimenting.

Copeland condensing unit operating a freezer

refrigerant R22
room temperature minus 17deg C
air on temp 16deg C
air off temp 21deg C
discharge pressure 135psi / 24deg C
suction pressure 16psi / minus 24deg C
temperature reading taken at condenser outlet or reciever outlet (did not seem to change between the two) around 24deg C with a clear sight glass.
One thing i did notice which is very strange, the outlet temperature of the condenser/receiver went higher than the condensing pressure/temperature relationship (is this possible).

Why can i never achieve any subcooling, i feel that i am being robbed of the privilege.

Thanks in advance for your assistance.

CB

[SteinarN]

Back to subcooling

the outlet temperature of the condenser/receiver went higher than the condensing pressure/temperature relationship (is this possible).

Why can i never achieve any subcooling, i feel that i am being robbed of the privilege.

Thanks in advance for your assistance.

CB

In real life you can not have a higher outlet tempereature than is the condensing temperature. It's probably measuring error, either your thermistor, your gauge or both.

Regarding the amount of subcooling it is several threads here where that subject has been discussed lively. Some argues you always have (measurable) subcooling and often a large subcooling. My opinion is that you never have any (measurable) amount of condenser outlet subcooling in a simple, stable running system with receiver as you describe.
This is assuming no non-condensables in the system, not any measurable pressure loss between condenser outlet and receiver, and the receiver not mounted significantly higher than the condenser.

[chillyblue]

Regarding the amount of subcooling it is several threads here where that subject has been discussed lively. Some argues you always have (measurable) subcooling and often a large subcooling. My opinion is that you never have any (measurable) amount of condenser outlet subcooling in a simple, stable running system with receiver as you describe.
This is assuming no non-condensables in the system, not any measurable pressure loss between condenser outlet and receiver, and the receiver not mounted significantly higher than the condenser.

Sorry i don't quite understand what you are trying to say.

CB

[SteinarN]

I ment to say I have trouble to understand how it can be possible with substantial subcooling on most systems. Thus I find your measurments where you dont have any subcooling absolutely normal.

[chillyblue]

I ment to say I have trouble to understand how it can be possible with substantial subcooling on most systems. Thus I find your measurments where you dont have any subcooling absolutely normal.

Thanks, i'm glad i'm not the only one i have never experienced large amounts of subcooling unless it is produced artifically via a vessel pre cooling the liquid or condensers with a dedicated subcooling circuit.
Most books refer to 4 to 7 degC of subcooling or the system is undercharged.
I've charged many systems that run perfect(in my opion), with full sight glasses and no subcooling. i would guess that if you continued to charge the system with refrigerant perhaps the subcooling will come??? but at a cost of reducing the condensing surface area and causing excessively high discharge pressures, and electrical current.
Is it that the manufacturers reduce condenser and reicever sizes???

Most condensing units i work with the receiver sits infront of the condenser fan so is subject to the air off temperature, would that make a difference??

Cheers CB

[US Iceman]

Most condensing units i work with the receiver sits infront of the condenser fan so is subject to the air off temperature, would that make a difference??

Sure. You have to remember the refrigerant can absorb heat or reject heat to or from the ambient. If the liquid is absorbing heat it can start to flash (or decrease any subcooling it may contain).

[Gary]

One thing i did notice which is very strange, the outlet temperature of the condenser/receiver went higher than the condensing pressure/temperature relationship (is this possible).

No, it isn't possible. You need to calibrate your instruments.

[US Iceman]

Another possibility for warmer liquid lines is if the liquid seal is lost somewhere and warm gas flows with the liquid.

[chillyblue]

No, it isn't possible. You need to calibrate your instruments.

I assume when you say that it is not possible, you mean because there is liquid present.
Am i right in saying that when a liquid is present the temperature cannot rise above the
temperature/pressure, and when there is no liquid present the temperature cannot drop below temeperature/pressure relationship

Hope that makes sense
Cheers CB

[SteinarN]

I assume when you say that it is not possible, you mean because there is liquid present.
Am i right in saying that when a liquid is present the temperature cannot rise above the
temperature/pressure, and when there is no liquid present the temperature cannot drop below temeperature/pressure relationship

Hope that makes sense
Cheers CB

That's a good understanding of the temp/pressure relation limits.

[chillyblue]

Also in theory then you can only subcool liquid to a temperature to which its surrounding area is. (unless you insulate everything).

In my example then, i will not be able to subcool the liquid any lower than the air off temperature of the fans that is hitting the receiver(+21degC), because the liquid in the reciever will evaporate and instanly decrease any additional subcooling that had been achieved, altering pressures and temperatures to balance the high side. (discharge pressure 135psi / 24deg C)so in effect subcooling of 3 deg.C would be about the maximum i could achieve without the use of artificial subcooling, or without insulating everything.

CB