Do split-unit aircons benefit from having their liquid and gaseous pipes in contact?

[kaon]

Question is pretty much given away in the title...
I noticed that the liquid and gaseous lines are often separately insulated with foam, and yet from what I understand about refrigeration, I'd expect to have efficiency gains if the 2 lines are in good thermal contact with each other.

More context:
I am speaking of common home-use air-conditioners, in tropical climates (30C (86F), 85%RH) where there is a indoor evaporator-fan-coil-unit linked with an outdoor condenser-compressor-fan-unit, via 2 copper pipes.

I expect the temperature of the liquid line to be slightly warmer than outdoor-air, and the gaseous line to be colder than indoor-air, so if we let the 2 pipes be in thermal contact, (for starters, don't insulate them separately, do put them in the same foam tube), wouldn't we get more efficiency for free?

What am I missing? Why are those lines normally insulated separately? (Yes, I'm a lay person...)

TIA

[taz24]

I noticed that the liquid and gaseous lines are often separately insulated with foam, and yet from what I understand about refrigeration, I'd expect to have efficiency gains if the 2 lines are in good thermal contact with each other.

Definatley not.

What am I missing? Why are those lines normally insulated separately? (Yes, I'm a lay person...)

TIA

They are insulated seperatley because they are two seperate sides of the system.
(Unlike a normal fridge system were liquid subcooling and suction superheat is improved by the pipes being in contact) Air conditioning systems expand the liquid in the outside unit. This means that the smaller supply pipe (normaly be the liquid line) is actualy carring saturated liquid and is in effect part of the evap.
If this pipe was not insulated then all the duty would be wasted by transfering straight to the suction.

Cheers taz.

[Brian_UK]

Well done Taz, said it in one

[Thermatech]

One reason why manufactures have part expanded liquid at the outdoor unit is for reduced refrigerant noise at the indoor unit. As pure liquid passes through an expansion device & starts to 'flash off' it tends to generate some noise. If this noise is at the outdoor unit then the manufacturer can make big noise about very quiet indoor unit.
But this makes good quality insulation of the liquid line very important for efficiency because as Taz explained it is in effect part of the evaporator coil.

[star882]

Air conditioning systems expand the liquid in the outside unit. This means that the smaller supply pipe (normaly be the liquid line) is actualy carring saturated liquid and is in effect part of the evap.

Not always. In some A/Cs, the liquid line is the liquid line.

[mohamed khamis]

Hi Kaon
Welcome to this forum
It does not matter the liquid line is expanded outside or inside... I know what u meant... u want to make simple suction-line-heat exchanger by soldering the liquid line exit from the reciever with the suction line before enerting the compressor and insult the both lines togther. This is exactly like in the domestic refrigerator.

By exploiting the heat from the liquid line to be more subcooled and on the same time increase the degree of superheat....
This subject was hashed in many threads and it is one of them, in a short, this heat exchanger is benficail in terms of enhancemnt in system COP but in AC application it just increase the subcooling on expense of an increase in energy consumption for the compressor so it is frequently omitted from the AC application
Try this link
http://www.refrigeration-engineer.co...heat+exchanger

[paul_h]

Hi Kaon
Welcome to this forum
It does not matter the liquid line is expanded outside or inside... I know what u meant... u want to make simple suction-line-heat exchanger by soldering the liquid line exit from the reciever with the suction line before enerting the compressor and insult the both lines togther. This is exactly like in the domestic refrigerator.

By exploiting the heat from the liquid line to be more subcooled and on the same time increase the degree of superheat....
This subject was hashed in many threads and it is one of them, in a short, this heat exchanger is benficail in terms of enhancemnt in system COP but in AC application it just increase the subcooling on expense of an increase in energy consumption for the compressor so it is frequently omitted from the AC application
Try this link
http://www.refrigeration-engineer.co...heat+exchanger

No, it's different with domestic a/cs because the liquid line isn't warm, the 'liquid line' is actually part of the evaporator if the flow control is in the outdoor unit. There's no gains in this setup. And if the unit is reverse cycle, you'll actually be warming up the liquid line (in the traditional sense of liquid line) with the discharge line when on heating mode.

[paul_h]

Not always. In some A/Cs, the liquid line is the liquid line.

In larger units that is often the case, but in small domestic splits there is no room for a flow control in the evaporator. When someone says split system, I think of wall, cassette or ceiling/floor console systems, ie. visible indoor units.
Ducted systems are split systems by definition, but I normally don't call them "split systems", but call them ducted systems, or ducted splits.
So when someone says split system, they're normally the one's with the expansion in the outdoor unit. While ducted can be either way, inside or outside.
I'm not saying you're wrong and should think of it like me, but non ducted splits nearly always have expansion in the outdoor, and most ducted splits made in the last decade have expansion in the outdoor. And the one's that don't are a real pain. It sucks having to weld in new valve while in a cramped, hot ceiling space. So maybe for that reason my mind blocks them out

[taz24]

Not always. In some A/Cs, the liquid line is the liquid line.

you are right.
I was being general because the guy was asking a basic question

I should of put in The majority of domestic and small comercial.

Cheers taz.

[mohamed khamis]

No, it's different with domestic a/cs because the liquid line isn't warm, the 'liquid line' is actually part of the evaporator if the flow control is in the outdoor unit. There's no gains in this setup.

Oh this is first time of me to know that otherwise I define Liquid line wrongly, as my knowledge the liquid line is defined as the line of high pressure liquid exit from condenser (which is in outdoor unit)until reaches the TXV, all of this run is named liquid line. And also according to common information this line is almost hot not warm its average temperature equals to the condensing temperature minus the subcooling degree (normally 5 to 11 K). And the condensing temperature is normally from 45 C to 60C and therefore the liquid line temperature is 40 C to 50C and how is not warm in either domestic or not domestic AC.

There's no gains in this setup

How?. The suction line is defined as the line from the evaporator to the compressor which is in the outdoor unit and its temperature equals to 7.4 + 11 = 18C and inevitably there is heat transfer between both lines and it can be exploited in the heat exchanger and heat gain.

. And if the unit is reverse cycle, you'll actually be warming up the liquid line (in the traditional sense of liquid line) with the discharge line when on heating mode.

[/QUOTE]

Thisis another issue, the cycle of the refrigeration is completely changed from cooling mode to heating mode. If u have reverse cycle u should have reverse (four positions) with two check valves and if add suction line heat exchanger u will complicate the design but it can be installed easily with the cooling mode circuit and it will be isolated automatically in the heating mode.


cheers

[paul_h]

Oh this is first time of me to know that otherwise I define Liquid line wrongly, as my knowledge the liquid line is defined as the line of high pressure liquid exit from condenser (which is in outdoor unit)until reaches the TXV, all of this run is named liquid line. And also according to common information this line is almost hot not warm its average temperature equals to the condensing temperature minus the subcooling degree (normally 5 to 11 K). And the condensing temperature is normally from 45 C to 60C and therefore the liquid line temperature is 40 C to 50C and how is not warm in either domestic or not domestic AC.


How?. The suction line is defined as the line from the evaporator to the compressor which is in the outdoor unit and its temperature equals to 7.4 + 11 = 18C and inevitably there is heat transfer between both lines and it can be exploited in the heat exchanger and heat gain.

I don't understand what you are saying, but the 'liquid line' from a split with the flow control in the outdoor unit is not at high pressure. The HP in the condenser might be 2000kpa, but the 'liquid line' coming out of it is only about 800 kpa and not warm, as it is after the capillary or TX valve. It's still refered to as the liquid line because it is full of liquid refrigerant that hasn't expanded, but it isn't at high side pressure.

[mohamed khamis]

I don't understand what you are saying, but the 'liquid line' from a split with the flow control in the outdoor unit is not at high pressure. The HP in the condenser might be 2000kpa, but the 'liquid line' coming out of it is only about 800 kpa and not warm, as it is after the capillary or TX valve. It's still refered to as the liquid line because it is full of liquid refrigerant that hasn't expanded, but it isn't at high side pressure.

After this clarification i don not find any word to comment except thank u so much for this clarification.

Have a nice day

[kaon]

I don't understand what you are saying, but the 'liquid line' from a split with the flow control in the outdoor unit is not at high pressure. The HP in the condenser might be 2000kpa, but the 'liquid line' coming out of it is only about 800 kpa and not warm, as it is after the capillary or TX valve. It's still refered to as the liquid line because it is full of liquid refrigerant that hasn't expanded, but it isn't at high side pressure.

Hi all,

Thank you all for the explanations, this is a great place.
The main points I've learned are (and pls clarify if these are still not exactly right)

1. the "liquid line" of a typical domestic split unit is actually part of the evaporator and that the flow control (aka "expansion valve"?) is somewhere in the outdoor condenser housing. I suspected this, since I noticed that both lines usually feel cold. (the "liquid line" is even colder than the "gas line" in domestic splits, right?)

2. The advantages of putting flow control in the outdoor unit: less noisy, more compact indoor unit, and "sucks having to weld in new valve while in a cramped, hot ceiling space." (care to elaborate? is there an unreliable valve in the flow control system?)


Further questions:

Q1:
If I did not care about noise, compactness, and ease of servicing, and I simply wanted better efficiency, then would it be advantageous to have the flow control at the indoor unit, and let the 2 lines be in thermal contact?

Q2:
Anyone know of a good webpage that lists the various layouts of split-units? One that shows pressures, temperatures of each part, where the expansion valve is?

Q3:
How do "expansion valves" work really? Does that term refer to the same things as "the term flow control"?

Q4:
For a typical domestic split, does the heat-exchange which I intended in my original post take place in the outdoor unit?
i.e. insulated gas line reaches the outdoor unit, then comes into thermal contact with the section of warm-pipe that is in between the end of the condensing coils and the expansion valve?

Sorry, I know this would be a lot easier to communicate with a sketch.

Q5:
Taz mentioned "saturated liquid" being in the liquid line of domestic splits as opposed to warm freshly compressed and condensed liquid. I'm not comfortable with the distinction between these 2 states of "liquid"... care to elaborate?
My intuitive feel is: you allow the freshly condensed liquid to pass into a lower pressure space, it sucks in heat in wanting to expand, but the pipe-insulation limits this heat absorption, so the liquid line gets cold to a steady state where heat that leaks thru the insulation lets only a small portion of the liquid vaporize, and this is called the "saturated liquid".


BTW, "subcool", "superheat", and "saturated liquid" aren't in the RE glossary!

PS: I haven't considered reverse cycle, my climate is hot and humid all year round.

[The MG Pony]

Personally? I'd put the expansion valve on the evap, and keep the liquid line separate and un-insulated and heavily insulate the suction line to keep the SST low as possible, dense gas means good capacity! More so then a slightly cooled liquid line!

[taz24]

Hi all,

Thank you all for the explanations, this is a great place.
The main points I've learned are (and pls clarify if these are still not exactly right)

1. the "liquid line" of a typical domestic split unit is actually part of the evaporator and that the flow control (aka "expansion valve"?) is somewhere in the outdoor condenser housing. I suspected this, since I noticed that both lines usually feel cold. (the "liquid line" is even colder than the "gas line" in domestic splits, right?)

2. The advantages of putting flow control in the outdoor unit: less noisy, more compact indoor unit, and "sucks having to weld in new valve while in a cramped, hot ceiling space." (care to elaborate? is there an unreliable valve in the flow control system?)


Further questions:

Q1:
If I did not care about noise, compactness, and ease of servicing, and I simply wanted better efficiency, then would it be advantageous to have the flow control at the indoor unit, and let the 2 lines be in thermal contact?

Yes

Q2:
Anyone know of a good webpage that lists the various layouts of split-units? One that shows pressures, temperatures of each part, where the expansion valve is?

Try http://www.sporlan.com/main.shtm

Q3:
How do "expansion valves" work really? Does that term refer to the same things as "the term flow control"?

Expansion valves are only glorified holes.
A hole reduces the flow of liquid refigerant and a presure drop is caused.
When you lower the pressure of a refrigerant you lower the tempreature of the refrigerant.
TEV's, automatic, manual, capilary tubes, and high pressure, low pressure float valves are some methods of throtling the refrigerants.

Q4:
For a typical domestic split, does the heat-exchange which I intended in my original post take place in the outdoor unit?
i.e. insulated gas line reaches the outdoor unit, then comes into thermal contact with the section of warm-pipe that is in between the end of the condensing coils and the expansion valve?

Sorry, I know this would be a lot easier to communicate with a sketch.

Just think of your normal fridge system

Comp, Cond, Metering device and evap.

Were the components are in the circuit does not mater as long as they are in the right order. You can put them in any place but the effiecency of the system needs to be maintained at all times.

Q5:
Taz mentioned "saturated liquid" being in the liquid line of domestic splits as opposed to warm freshly compressed and condensed liquid. I'm not comfortable with the distinction between these 2 states of "liquid"... care to elaborate?

There are three states or phases of the quality of the refrigerant.
Pure gas or vapour, Pure liquid and then a mixture of both.
Liquid can be below the boiling piont or at boiling point.
Thinki of a glass of water.
Water in a glass on the table at any temp up to 99.9degC is subcooled.
Water boiling at 100degC is now saturated.
Steam at 100degC is saturated.
Steam at 101degC = is now superheated.

Refrigerant is the same just the temp changes.

My intuitive feel is: you allow the freshly condensed liquid to pass into a lower pressure space, it sucks in heat in wanting to expand, but the pipe-insulation limits this heat absorption, so the liquid line gets cold to a steady state where heat that leaks thru the insulation lets only a small portion of the liquid vaporize, and this is called the "saturated liquid".

Yep that is right.

BTW, "subcool", "superheat", and "saturated liquid" aren't in the RE glossary!

PS: I haven't considered reverse cycle, my climate is hot and humid all year round.

Dont wory about it yet.


Cheers taz.

[The MG Pony]

http://www.refrigerationbasics.com/1024x768/rb1.htm < You really should start here.

If impatient and willing to sacrifice learning a bit of in depth info skip here: http://www.refrigerationbasics.com/1024x768/rb2.htm

Good basic info on charging: http://www.refrigtech.com/charge.html

Good basic tutorial on TXVs: http://hvacwebtech.com/marksM2.htm