Subcooling & Refrigerants

[US Iceman]

After all of the posts from the "how to size receiver" thread I thought I would post some information in a separate thread.

I think most of us agreed on the effects of subcooling for mass flow and volume flow. I wanted to take some time to put this into some charts so that it is easier to see the effect of subcooling on a refrigerant.

The charts listed below are based on using ammonia with an evaporating temperature of 20°F (-6.6°C) and a condensing temperature of 95°F (35°C).

Note: The mass and volume flow charts are all based on 1 Ton of Refrigeration. It's Friday afternoon, OK?

A similar set of charts can be made for any refrigerant and evaporating temperature.

The x axis is listed as ammonia liquid temperature. This is based on the condensing temperature minus the amount of subcooling equals the liquid temperature to the TXV, in this case.

So we can state the subcooling equals 0 degrees at 95°F (35°C). If we have five degrees of subcooling the liquid temperature will be 90°F (30°C).

As the subcooling is increased the liquid refrigerant temperature decreases. At the same time, the required mass and volume flow also decrease.

When selecting the TXV, the valve capacity is based on the liquid temperature feeding the TXV. The "TXV correction sheet.pdf" from Sporlan shows different correction factors for various liquid temperatures. A correction factor of 1.0 is found at 86°F. That just happens to be the operating point Sporlan selected.

If the liquid temperature is 70°F (21.1°C), we would have 25°F of subcooling if our condensing temperature was 95F (35°C). The capacity correction factor increases from 1.0 to 1.05. Just 5%.

This is one of the correction factors used for selecting the TXV to match the evaporator capacity and the compressor capacity. The 5% only corrects the TXV capacity for the additional subcooling. That's all.

One thing to be aware of is how are you getting the subcooling? It is not free! Something has to provide this; another refrigeration system, pre-cooling, etc.

However, these charts do show the positive effect of subcooling. You can either increase the existing system capacity, or, select smaller compressors to do the same job.

The evaporator and condenser are still selected for the required cooling loads and heat rejection.

Does everyone agree with this?

[Lc_shi]

It's agood summary. Subcooling is useful to capacity improvement.
Basic concept is very important.We're puzzled by phenomenon many times.If we can come down to the basics,we can deal with them in the proper way.
Subcooling is a good example Hope Iceman can lead more looking easy but significant topics later. It 's beneficial for every one.

regards
LC

[Peter_1]

I agree, just a small rectification...subcooling can be free, well at least...without adding additional energy.
You know what I mean.

It shoud perhaps be a good idea if we cut and paste that 'Liquid receiver thread here'

As soon as I have the answers of some coil manufacturers, I will post them.
I wrote the same mail to Goedhart, Helpman, Heatcraft, Guntner, Kuba, ECO, Luve, Searle.

[Peter_1]

LC posted in another thread also a question for a simulation package where you can calculate new working points if one parameter shifts.

Same could be used here: if we subcool the liquid, liquid mass flow decreases, volume coming out the evaporator decreases (is it decreasing anyway b) but compressor still pumps same volume. So evaporator temperature will drop and so the LMTD will increase whereby capacity of the evaporator increases.
Is the volume coming out the evaporator is decreasing because LMTD is increased, so more heat was added to the colder striating liquid.
It's sometimes confusing. Better learned for something else

Think that I need to re-read the the very basic thermodynamics because all these can be set of course in formulas but some things can't be predicted like the behavior in an evaporator which was designed differently.
This will be something when I retire I think.

[Andy]

Hi
water cooled condensers lend themselves to some very economic subcoolers.
Suction liquid heatexchangers subcool liquid and improve performance, but only by improving liquid quality and with addtional energy input at the compressor.

We use heat recovery in most new installations, this offers us the possibility for economic subcooling.
We elevate the condensing temperature to 45 deg c, condense the refrigerant in the water cooled heat recovery condenser, pass the condensate thru the pressure regulation valve into the condenser, which is controlled by the pack controller at a condensing pressure of 30 to 34 deg c. this further subcools the liquid to usually about 20 deg c (dependant on ambient, lets face it that is the ambient in summer in Ireland).
This addtional subcooling is acheived with very little fan input power and with the condenser cooled by a very small water pump the power input is actually lower than a conventional system.

Typical C.O.P for a supermarket installation is raised from 2.74 to 3.18
even with considering the floating haed in the winter this still saves energy compared to a conventional system, and the there is still all that free heat to be considered, typically meeting all but the start up heating loads of a supermarket and all the hot water needs for washing.

Kind Regards. Andy

[US Iceman]

Hi guys,

It seems my comment about "no free subcooling" started the ball rolling. I did this intentionally for a reason. I'll explain why.

Subcooling can be created by many different methods;

1) Liquid pressure is increased with the temperature held constant (a pump or static pressure in a liquid column for example)

2) Cooling the liquid refrigerant with some other cold fluid (constant pressure but a decrease temperature)

The purpose of subcooling as I see it is to increase the NRE (Net Refrigerating Effect, or enthalpy difference) of the refrigerant to reduce the required mass flow and/or to provide 100% liquid to the TXV's.

In example 1 above the subcooling is provided by the increase in pressure by the refrigerant pump. Some energy is required for the pump motor. Static pressure from a liquid column is free, if the liquid is running downhill.

In example 2, whatever produces the cold temperature to subcool the refrigerant may have some energy cost. Say a separate refrigeration was used to subcool the main liquid line. The small refrigeration system used for subcooling requires energy.

The example Andy offered is a good one. The heat removed from the liquid refrigerant is used to heat a water supply. Here you get subcooling and heating (very much like a heat pump, where you are reusing the heat energy).

subcooling can be free, well at least...without adding additional energy.
You know what I mean.

I do indeed. My comment on the "no free subcooling" was intended to get people to think of how they use subcooling in a refrigeration system.

If a system is designed without subcooling and then does not work due to flash gas in the liquid line, a subcooler can be added to provide 100% for the TXV. This will result in effects very similar to what Peter describes. The system tries to re-balance in some way to meet the operating conditions. This is based on the installed components of the system.

On the other hand, if the system were designed with the additional subcooling a smaller compressor can be used since the required mass flow (& volume flow) is now reduced. With this, you can balance the components before the system is installed.

One last question: If liquid refrigerant is cold, does this mean the refrigerant is subcooled?

[frank]

One last question: If liquid refrigerant is cold, does this mean the refrigerant is subcooled?

Only if the liquid temperature is below the saturation temperature

[Peter_1]

...In example 2, whatever produces the cold temperature to subcool the refrigerant may have some energy cost. Say a separate refrigeration was used to subcool the main liquid line. The small refrigeration system used for subcooling requires energy.

And the subcooler in a condensor? This is for free.

Took this out of an odler thread where I posted

We even installed once (Carrefour) a pack (+/- 300 kW) with a condenser +/- 70 ft high on the roof. The pack was in the cellar at -2 level, so +/- 90 ft high discharge lines and liquid lines.
The store was at ground level.
My intention was to install the receiver right under the condenser and use the liquid column down to subcool it enormous.
Conclusion of the engineering office: it will rust there and this will not give you such a great benefits.
I even argued that underneath the condenser it's mostly dry, warm, even proposed a stainless steel tank...
It's installed now in the very warm cellar besides the pack and liquid lines goes up again two levels to the cabinets. Can you imagine this
Sometimes, the basics ars so difficult for some who know it all.

It's not always easy to predict exactly what will happen if system parameters change.

You can therefore perhaps calculate very precise at what condtions a system will run but as soon humidity, temperature pressure changes, system will find a new balance point.

Well, you can only speak about subcooling if you know also the corresponding pressure.

But as subcooling increase, that's what I think, this is always an advantage for the whole system.

Another question, if the service valve is covered under ice, will this mean that we have liquid slugging.

[Andy]

And the subcooler in a condensor? This is for free.

No and Yes, no if fans have to be run to cool yes if the subcooling is by low ambient
Another question, if the service valve is covered under ice, will this mean that we have liquid slugging.

Depends on the evaporation, if it was 0deg c yes if the evaporation was -30 deg c I would say no.

Kind Regards. Andy

[US Iceman]

Only if the liquid temperature is below the saturation temperature

Right to the point Frank.

I see a fair amount of people who seem to think cold liquid is subcooled. I posted that question to keep the discussion rolling for other members who may not be totally familiar with the refrigerant behavior under various conditions.

The flip side of this is if the pressure is increased above the saturation temperature you also have subcooling, which is what Peter describes below:

Well, you can only speak about subcooling if you know also the corresponding pressure.

Peter was right on when he stated:

My intention was to install the receiver right under the condenser and use the liquid column down to subcool it enormous.

And followed by:

But as subcooling increase, that's what I think, this is always an advantage for the whole system.

All of these seem like basic statements, but to a young person starting out this may prove to be beneficial.

Another question, if the service valve is covered under ice, will this mean that we have liquid slugging?

A very good question Peter. This is usually another question that will result in different answers.

My short answer is No. The valve temperature in the question is below freezing 0C (32F). Frost and ice will form below this temperature. On a low temperature system the suction valve temperature will probably be lower than 0C if it is working correctly.

The tech should check the suction pressure and temperature. You can still have suction superheat (sometimes a lot of it) and still be below freezing. It is the suction superheat that is the important factor to reduce slugging, not if ice is present.

I think the main point of this thread is to check the pressure and temperature relationships. Always measure temperature and pressure and compare it to the refrigerant charts.

[Peter_1]

Well the question better was: do you then have automatically liquid slugging.

The answers were of course correct. I hadn't expect something else from those who posted.

I'm sure of those who posted that they understand their basics.

Why I asked this question: I know several technicians - one has +/- 25 years experience -and when he sees a frosted service valve on a compressor, he automatically conclude " Liquid slugging' and he turns in the TEV some turns.
Even if the evaporator is frosted, turning that TEV!!

Or another one I told already on RE: he needs for a repair a Danfoss TEV with orifice 3 but he has only a n°4.
No problem...fit a 4 and turn in that valve some turns and a 4 becomes a 3.

What can we all make it so difficult and what can refrigeration be so easy

Saw that advert on the right bteween Dez's adverts?------>
Looking for ana expereicned NH3RE
Are these realy normal wages for a NH3 tech in the UK?
Geeeeeeez.
What am I doing here.

[US Iceman]

...several technicians - one has +/- 25 years experience -and when he sees a frosted service valve on a compressor, he automatically conclude " Liquid slugging' and he turns in the TEV some turns.
Even if the evaporator is frosted, turning that TEV!!

I see this a lot.

You are right, why do we have to make this so hard? The pressure/temperature tables tell you almost everything you need. Gauges, thermometers, your eyes, hands and ears can tell you almost everything else.

I did see the advert for the NH3RE. Is that a good wage over there?

[Andy]

Saw that advert on the right bteween Dez's adverts?------>
Looking for ana expereicned NH3RE
Are these realy normal wages for a NH3 tech in the UK?
Geeeeeeez.
What am I doing here.

Good engineers are hard, no impossible to find. We are going to place an advert in RE this week for three engineers.
Ireland has some good engineers, but the good ones are well looked after and not for moving, the pay is not the deciding factor in Ireland.

I was earning very close on that 10 years ago, but doing some serious hours

The going rate for a good industrial engineer is just about what is advertised, commercial may be a little less, but dependant on area.

What about the US, Iceman.

Kind Regards. Andy

I was offered $100k Canadian 5 years ago

[US Iceman]

HI Andy,

Some of the service people I worked with years ago made more than anyone else at the manufacturing firm I worked at.

Except for the salesman, and those select few who had golden doors on their offices. You know... the corner office guys!

The overtime will really add up fast.

You are right about the lack of good people with experience. They are very small in numbers. The good ones usually are well looked after and don't move around.

[Dan]

I see a fair amount of people who seem to think cold liquid is subcooled. I posted that question to keep the discussion rolling for other members who may not be totally familiar with the refrigerant behavior under various conditions.

This is actually a pet peeve of mine. We too often use the word "subcooling" when all we mean is "cooling." Actually, you want minimal subcooling. Any more subcooling than necessary to provide high quality liquid to the TEV is actually wasteful because you should be lowering the pressure at that point to save on energy.

We use the word "subcooling" too loosely. There is nothing wrong with using the word "cooling" in discussions about liquid temperature and valve capacity, because we are only considering enthalpy which has little to do with pressure. Better yet, just state the liquid temperature and you have said mostly what is necessary. At that point you can discuss pressure, which is the other entirely unrelated dynamic in TEV sizing.

The flip side of this is if the pressure is increased above the saturation temperature you also have subcooling, which is what Peter describes below:

Again subcooling is undesirable. What is one of the key indications of noncondensibles in a system? Inordinate subcooling. Condensers have this uncanny ability to deliver liquid temperature at or near the design TD (10, 15, or 30 deg F above design ambient 95% of the time. Even with noncondensibles that cause the discharge pressure to escalate to pressures 50 or more degrees F above design, thus providing tremendous subcooling at tremendous cost.

I think a strong case can be made that any subcooling above 20 deg F is a waste of energy.

[US Iceman]

Dan brings up some good points.

What is one of the key indications of noncondensibles in a system? Inordinate subcooling.

Noncondensables do appear as though you were looking at a lot of subcooling. In fact this is rather easy to verify on an ammonia system since we usually have purge valves on the condenser coil outlets.

If the purge valve is cracked open a little bit and you get gas, then you have noncondensables in the condenser coil.

If the purge valve is cracked open a little bit and you get liquid, then you have the condenser coil with too much liquid in it. (sometimes you can have both liquid and noncondensables as Dan mentioned)

This can be caused by several things: head pressure controls (flooding the condenser coil), or, too small of an equalizing line between the receiver and evaporative condenser, or a combination of too small of an equalizing line and liquid traps on the condenser coil which are not deep (tall) enough.

Any more subcooling than necessary to provide high quality liquid to the TEV...

I can go along with this. However, in the example Peter posted earlier using the static head to generate subcooling is useful. If the receiver was placed at the low point in the system and then the liquid had to be piped vertically again, he would have needed some subcooling to provide a high quality liquid at the higher TXV's.

In his example, placing the receiver on the roof with the liquid line running down would have provided the desired effect for a high quality liquid to the TXV's at a lower level. This would be an example of free subcooling. No fans, pre-coolers, or pumps required.

Better yet, just state the liquid temperature and you have said mostly what is necessary. At that point you can discuss pressure, which is the other entirely unrelated dynamic in TEV sizing.

I fully agree Dan. I normally use the term "liquid feed temperature". Find the liquid temperature you need to provide 100% liquid to the TXV, and then worry about the minimum inlet pressure for the TXV.

I would recommend the subcooling value be checked during the design stage to ensure the actual liquid temperature is lower than the saturation temperature of the minimum liquid feed pressure to ensure 100% liquid at all conditions.

[Andy]

I think a strong case can be made that any subcooling above 20 deg F is a waste of energy.

I am going to stick my neck out and say not necessarily.

When recovering heat from the discharge gas we use a water cooled condenser, the existing condenser is used to subcool the liquid down to 20 deg from a condensing pressure of 32 deg c. that is close to 20 deg f of subcooling. Further more we use a valve to hold the discharge and the water cooled heat recovery condenser at 45 deg c condensing. dropping the pressure from 45 to 32 deg thru it and flashing of gas to cool the liquid down to 32 deg c, there may be only a small quanity of flash gas at this point as the water cooled condenser has a 2 -3 deg c subcooling as well.


I would put a limit of 20 deg c on the conventional cycle. Use of 10 deg bore hole water, will allow this to be obtained, (this will be subcooling not cooling if a receiver pressure regulator is used).

One system I have recently designed has 45 deg c of subcooling, but this is mechanical subcooling using an economiser on a screw compressor.

Kind Regards, Andy

[Peter_1]

First, BESC5240, I think you can bring some light in the darkness regarding the fact what will happen when we feed a TEV with liquid at a temperature equal or lower then evaporating temperature.

Dan, what is a pet peeve?

Any more subcooling than necessary to provide high quality liquid to the TEV is actually wasteful because you should be lowering the pressure at that point to save on energy

This is in my opinion only true if you subcool by the conventional ways.
Placing the receiver on the a roof is one example (think BESC5240 remembers this example, it was in Antwerp)

A Little bit subcooling is indeed enough for a proper functioning of the TEV but it improves a lot the NRE and the COP of the compressor.

Especially if you can do it with 'free-cold' like Andy mentioned with water or ground water.
We made an offer last week where we want to subcool the liquid with the radiant heating of the underfloor of a freezer. This energy is for free and we have to do it anyway.


Andy we do it the same, I posted some pictures some months ago in another thread.

But the air condensor is also used not only to subcool but if water reaches very hot temperatures near discharge temperatures, you need the air condenser also to codeines further the discharge gasses.
To hold sufficient pressure, we regulate the water pump.

We proposed also 20 K subcooling as Andy mentioned.
But what happen in the evaporators with 45 K subcooling? Didn't you go below evaporating pressure?
This sounds interesting, especially this was mentioned - If I remember well -in the thread 'Liquid Receiver'

[Andy]

it anyway.

But the air condensor is also used not only to subcool but if water reaches very hot temperatures near discharge temperatures, you need the air condenser also to codeines further the discharge gasses.
To hold sufficient pressure, we regulate the water pump.

We proposed also 20 K subcooling as Andy mentioned.
But what happen in the evaporators with 45 K subcooling? Didn't you go below evaporating pressure?
This sounds interesting, especially this was mentioned - If I remember well -in the thread 'Liquid Receiver'

Peter,
if you have the opertunity try the condensing pressure regulation valve between the heat recovery condenser and the aircooled condenser, and move the head control connection to the aircooled condenser, not the pack discharge. The COP increase is very interesting, and near enough free.
This would ofcourse only work if the heat recovery is sized to capture all the THR

On the 45 deg subcooling the plant is a blast freezer, evaporative condenser at 34 deg c, Evaporation at the rooms -43 deg c and -40 deg c at the compressor, with the intermediate at say -14 to -16 deg c producing liquid off at -11 deg c.

We are putting heat recovery on this, but it is only 96kW, a de superheater, so I will see if I can create some subcooling.
The plant is layed out with the condenser above the receiver, which is above the rooms (plant is actually on steelwork above the rooms).

Kind Regards. Andy

[Peter_1]

Can you make a little schematic of it Andy please?

[Andy]

Peter
I will try, but no promises, I would have do it on cad and convert.
If you PM me I will email you the full cad preliminary drawing, hot gas defrost and all

Kind Regards. Andy

[Andy]

Hi Peter
please see attached drawing, for heat recovery, subcooler condenser and mechanical subcooler.

Kind Regards. Andy

[NoNickName]

Hi Peter
please see attached drawing, for heat recovery, subcooler condenser and mechanical subcooler.

Kind Regards. Andy

I think few compressor manufacturers call this circuit "economizer". We have a few installed. Working a charm.

[Dan]

Further more we use a valve to hold the discharge and the water cooled heat recovery condenser at 45 deg c condensing.

Andy, you are artificially holding up the discharge pressure to create this subcooling in order to provide a high quality of heat. This throws a new element into the COP equation. How much does this cost in energy compared to other available means of heating, for example. No longer "conventional" refrigeration as Peter might say.

Believe it or not, there is a trend in supermarkets to take advantage of lower discharge pressures whenever they are available. Using, instead, electric defrost and supplemental heat instead of heat reclaim and gas defrost. The argument is persuasive in the example of artificially maintaining higher discharge pressures to accomplish a gas defrost compared to letting the discharge pressure float down and using electric defrost. DelHaize appears to be going in this direction.

Your example, Andy, of mechanical subcooling could just as well be phrased as mechanical liquid cooling, because the pressure of the liquid is unrelated to the temperature you are trying to achieve. During hot days it is only coincidental that you are subcooling the liquid.

If you are artificially maintaining a higher discharge temperature than needed during winter, for example, you are most likely doing it for a reason unrelated to the simple compressor COP.

I share your belief that we should direct available heat to desired applications such as hot water and space heating. But we should also not create any heat unnecessarily where we could otherwise save energy. Newer controllers, stepping valves, etc are opening avenues that will take further advantage of lower discharge pressures available in temperate climates.

Dan, what is a pet peeve?

"an opportunity for complaint that is seldom missed; "grammatical mistakes are his pet peeve"
complaint - an expression of grievance or resentment"

I guess I used it correctly, although I have a much lighter attitude about it than that particular definition connotes.

I started in this trade as a technical writer. I found a single Installation Manual referring to the same valve with 7 different names. It was a "holdback" valve, a "discharge pressure regulating" valve, a "flooding valve", a "winter control" valve, an "A7" valve, "condenser pressure regulating" valve, and I cannot remember the 7th one but I think it was the brand name "Flo-Con."

How impossible it was for me to understand all these valves. So I am a stickler for words and what they mean. On occasion, I adopt one or two as a pet. You have met the one I named "subcooling."

Lowering liquid temperature improves COP and NRE. Not Subcooling. If we must use the word, let's call it "cooling." Subcooling defines a temperature pressure relationship and implies that you have an opportunity to lower discharge or liquid pressure. When you have non-condensibles in your system it doesn't "appear" that you have subcooling, you actually have subcooling.

Thank you all for your courtesy in your arguments. I feel much better now and promise not to take my pet for a walk for another year.

[US Iceman]

Subcooling occurs when the actual liquid temperature is cooled below it's saturation temperature. And I agree it should be understood the use of the term subcooling is indeed based on the principles of the temperature/pressure relationship of the respective refrigerant.

This is the traditional definition of subcooling and agrees with Dan's use of the term cooling for lower liquid temperatures.

A literal interpretation of subcooling is it's temperature dependency at a constant pressure.

By cooling the liquid refrigerant below the saturated condensing temperature (at a constant pressure) the NRE and COP does improve, assuming the system has been properly designed to utilize the "cold" liquid refrigerant.

A secondary and equally important reason for utilizing "colder" liquid is to ensure 100% quality liquid to the expansion device.

To this extent the use of the term subcooling is normally applied.

However, by applying the basic principles of the pressure/temperature relationship that define the term "subcooling" we can also determine other types of similar behavior.

One of these other types is the use of static pressure (head) to increase the liquid pressure at a constant temperature. In this example, the temperature is not cooled/lowered, but the liquid pressure is increased.

This again meets the definition of subcooling, since the saturation temperature of the liquid at the higher pressure (due to the static head) is greater than the actual liquid temperature. In this case, "cold" is a relative term implied by definition and calculation.

An additional example of the principles of subcooling is in the determination of non-condensables in the refrigeration system.

The presence of non-condensables is determined by the use of the temperature/pressure relationship. If the condensing pressure is elevated due to non-condensables, the liquid refrigerant will exist at a lower actual temperature.

However, the liquid temperature is not cooled below the design condensing temperature, rather it exists at a temperature lower than the equivalent saturation temperature of the higher condensing pressure due to the non-condensable gases.

While these meets the basic definition of the principles of subcooling, in this case no benefit is derived for the NRE or COP. The liquid is not colder than the design condensing temperature so the enthalpy difference is not increased for the NRE. In this example, the discharge pressure is simply higher than desired and detracts from the system performance and efficiency.

In other words, subcooling "appears" to be present by calculation of the recorded data. In this situation the "subcooling" is non-useful. No benefit is derived by the refrigeration system.

In reality this is similar to the use of the term superheat. There are instances where superheat is useful and contributes to the NRE, and others where an increase in superheat can contribute to a loss of system capacity. The latter case is one where the superheat is non-useful.

I believe the key to understanding the terms of subcooling and superheat in the relevance to refrigeration systems is based on the terms "useful" or "non-useful".

If no benefit is derived to increase the system efficiency or performance it can be called non-useful.

For this reason I believe it is necessary to have a context for the use of the terms subcooling and superheat.

That context is if the term (subcooling or superheat) provides a useful benefit that adds to the system performance and results in any increase to the overall efficiency.

[US Iceman]

Dan also brings up a very valid point about the "cost" of subcooling. In effect, we have to be aware of the requirement to provide a high quality liquid to the expansion device. Any subcooling required to deliver the liquid to this expansion device is a mandatory requirement in the first place.

Secondly, it is equally valid to say colder liquid will increase the NRE and COP of the refrigeration system.

At the same time, the compressor is using primary energy (electricity) to generate the discharge pressure. If the discharge pressure is artificially maintained at a higher pressure to generate heat recovery, the subcooling of the liquid and heat recovery comes at some "cost".

If liquid subcooling is being done to cool the liquid from a higher than normal discharge pressure, you may have a lot of subcooling present, but the system will be using more energy to do so.

I believe the statement made by Dan was to limit the subcooling to 20 degrees F. If the discharge pressure is limited to the minimum it takes to condense the refrigerant, less energy is used.

In other words, keep the discharge pressure as low as you can during the majority of the time. And then subcool the liquid to some temperature to ensure adequate delivery of the liquid to the expansion device.

The amount of subcooling that would be required would be minimal as Dan suggested. Conversely, if a side port on a screw compressor is used to provide additional subcooling by an economizer the additional subcooling can be used to reduce the design mass flow also. This provides a further improvement at very little cost.

Heat recovery can still be accomplished at the low discharge pressure as a lot of heat energy is still available by condensing. The one thing that may not be present is the temperature though.

Heat recovery is based on two criteria: heat and temperature.

The effectiveness of this is in how these are generated. Condensing provides a lot of heat energy, but the temperature may not be high enough to support the requirements.

The overall efficiency of the heat recovery is based on the total energy used to provide it. It may be more cost effective to use the condenser to recover the condensing energy at low discharge pressure and use another means to provide the final desired temperature.

[BESC5240]

First, BESC5240, I think you can bring some light in the darkness regarding the fact what will happen when we feed a TEV with liquid at a temperature equal or lower then evaporating temperature.

If you would want to feed liquid with a temperature lower then the evaporation temperature 'To' , you would need an external 'system' with a temperature lower then To.

If you could achieve this, you would be expanding liquid. At the outlet of the TEX there would be 100% liquid (or even 'subcooled liquid'). No standard DX coil is designed to be fed with 100% liquid. The limit to the subcooling you should apply is not the liquid temperature before the expansion valve (or subcooling of the liquid) but the gas/liquid rate of the mixture comming out of the TEX.This is the x-value in the logp-h diagram: it should always be higher then 20% in DX systems. If not you would get a bad distribution and a bad heat exchange.

[US Iceman]

The limit to the subcooling you should apply is not the liquid temperature before the expansion valve (or subcooling of the liquid) but the gas/liquid rate of the mixture coming out of the TEX.

This is the x-value in the log p-h diagram: it should always be higher then 20% in DX systems. If not you would get a bad distribution and a bad heat exchange.

In other words, if the liquid is subcooled too low the quality discharging from the the TEX could be less than 20%. Below this value (x=20%), insufficient flash gas will form to provide the liquid distribution into the evaporator coil. If you have uneven liquid distribution into the coil, the result will be poor heat transfer performance.

I believe BESC5240 went straight to the heart of the problem. You need flash gas to distribute the liquid into a DX coil (although this does not apply to a liquid overfeed system).

If the resulting expansion process with very cold liquid cannot produce sufficient flash gas you have evaporator performance problems.

If the liquid could be cooled below the evaporating temperature, no flash gas will form until the liquid is warmed up to the bubble point line on the P-H diagram where x=0 (all saturated liquid).

All of this may seem like an introduction to thermodynamics and engineering but I don't think it really is. These are the basic details of how refrigerants work and can help you with system analysis in the field.

[Peter_1]

Here I am again, USIceman and BESCie, this need some further explanation...why you need flash gas in a distributor?

Perhaps a stupid question but can't we see the distributor as a 'liquid-splitter' to the different coils?

[US Iceman]

Hi Peter,

A very good explanation of this is provided in the Sporlan Refrigerant Distributor book (pages 3 to 5).

http://www.sporlan.com/20-10.pdf

This will explain it better than I can.

Perhaps a stupid question but can't we see the distributor as a 'liquid-splitter' to the different coils?

This is certainly not a stupid question Peter. In fact it is a very good one.

The flash gas is required to help promote equal distribution to each evaporator coil circuit. This is also the main reason why the distributor tubes have to be the same length. Poor distribution of liquid and flash gas can cause the TEV to hunt.

[NoNickName]

Poor distribution of liquid and flash gas can cause the TEV to hunt.

I second Peter, isn't it true that the sensing bulb should be installed after the coil? In that case the bulb should only sense vapour, more or less superheated, but definitely vapour. If not, then the coil is overfed and TEV should reduce the feeding.

On the other hand, my experience is that when the TEV has flash gas and liquid before it, then it hunts.

Am I missing something?

[US Iceman]

I second Peter, isn't it true that the sensing bulb should be installed after the coil? In that case the bulb should only sense vapour, more or less superheated, but definitely vapour. If not, then the coil is overfed and TEV should reduce the feeding.

On the other hand, my experience is that when the TEV has flash gas and liquid before it, then it hunts.

Am I missing something?

The bulb is placed at the evaporator outlet as per the instructions.

The distributor tries to provide a uniform mixture of the flash gas and liquid that exits the TEV into each coil circuit. If the coil circuits are fed with an uneven amount of flash gas and liquid each circuit load is different, so each coil circuit can have different pressure losses and heat absorbing capacity.

If one circuit has more liquid than one of the other circuits, the circuit with more liquid can have liquid exiting the circuit and reach the TEV bulb. This causes the TEV to close (low superheat has been sensed).

Now the other circuits are starved of liquid. You have unequal coil loading and lack of capacity. When superheated vapor hits the bulb, the TEV opens. The valve will open and close (hunt) to try to maintain the evaporator superheat.

If the evaporator circuits and distributor are not properly selected none of it works well.

Flash gas upstream of the TEV will cause a similar situation.

You can also experience the same thing if the coil circuits are not equal length. The coil manufacturer does this during the design.

I have seen this also occur when someone tries to make a home made distributor. A real mess!

[Andy]

Andy, you are artificially holding up the discharge pressure to create this subcooling in order to provide a high quality of heat. This throws a new element into the COP equation. How much does this cost in energy compared to other available means of heating, for example. No longer "conventional" refrigeration as Peter might say.

discharge pressure float down and using electric defrost. DelHaize appears to be going in this direction.

Your example, Andy, of mechanical subcooling could just as well be phrased as mechanical liquid cooling, because the pressure of the liquid is unrelated to the temperature you are trying to achieve. During hot days it is only coincidental that you are subcooling the liquid.

If you are artificially maintaining a higher discharge temperature than needed during winter, for example, you are most likely doing it for a reason unrelated to the simple compressor COP.

I share your belief that we should direct available heat to desired applications such as hot water and space heating. But we should also not create any heat unnecessarily where we could otherwise save energy. Newer controllers, stepping valves, etc are opening avenues that will take further advantage of lower discharge pressures available in temperate climates.


.

Dan
as far as I can calculate there is no energy penlty for thr system we use. We do add extra compressor energy to elevate the head pressure, but what we loose in one hand we gain in the other, by subcooling in the now un-needed aircooled condenser. If we had no heat recovery we could float the head in the winter and improve our COP to 3.38 from 3.18, but for us this is only 6 months of the year, the summer COP for a convensional system is 2.74. In the summer (all but say 1 month) we still need some heating, due to the cabinet cooling of the shops.
Our COP is around 3.18 the year round with reduced condenser fan input power.
A conventional system with floating head in the winter
would have an average COP of 2.96 (average of 3.38 and 2.74)

From what I see it pays to recover the heat, free haet and enhanced efficiency in the refrigeration system.

But I am willing to be proved wrong

Kind Regards. Andy

[Dan]

I never thought of unequal distribution with liquid temperatures below the design evaporator temperature. So you could actually lose refrigeration effect while implementing an accidental liquid recirculation system. Interesting to think about. What's an expansion valve to do?

By the way, Iceman, I think you explained some things I was attempting to say regarding subcooling better than I did. You introduced the concept of useful and non-useful.

[US Iceman]

Hi Dan,

What's an expansion valve to do?

All it can do is try to work.

It has been my experience that by using the concepts of useful and non-useful in the context of subcooling and superheat discussions you can get the point across.

If either one does not provide some benefit to system performance or efficiency it is a wasted effort.

Most of the people who posted to this were moderators. Some were regular RE members, but all came with very good thoughts and ideas.

I feel it is these types of discussions that help the younger people a lot. This is the sort of thing you don't learn in school, but can provide some good background information for service tech's and system designers.

Peter had a very good question too about the ice on the suction valve. It all comes down to using the refrigerant tables and the basics.

After all of the discussion on subcooling pros & cons. I have another question related to this subject. I will warn you that this is a term I developed and use, but it points out another area that can be addressed here.

What is positive subcooling and negative subcooling?

I"ll provide a hint: It has to do with two stage systems and booster compressors.

[Peter_1]

I never thought of unequal distribution with liquid temperatures below the design evaporator temperature. So you could actually lose refrigeration effect while implementing an accidental liquid recirculation system. Interesting to think about. What's an expansion valve to do?

Dan, that was exactly my way of thinking, it's perhaps a hypothetical question, but it surprises me too.

Positive and negative subcooling, will think of it today while working but I think we will soon need to make yellow arrows we install inside the lines to point the directions where the gass needs to flow.
The gass molecules will become to confused by all this

[Renato RR]

My intention was to install the receiver right under the condenser and use the liquid column down to subcool it enormous.
.

Thanks,Peter.I was blind but now I see.

Renato

[Lc_shi]

find a article about"subcool" ,attached below.
maybe of some help

rgds
LC

[The MG Pony]

Hey all, I'm just geting into this industry (Still a student just starting) and fell in love with TXVs and this article and the one about the receivers and such has been a majour help Thanks for all this great info and help

[US Iceman]

HI The MG Pony,

That is an unusual screen name. How was that developed?

I'm glad you enjoyed the threads. With all of the experience of the various members here we can have some very revealing discussions.

Please feel free to jump in on any topic.

[The MG Pony]

I play a machine gunner dod and I have an affinity to ponies so it made sense, and I use a dual Xeon rig

So my full screen name is "Xeon the MG Pony" on DoD so if you play log on to HBMS Server.

FYI: DoD = Day of defeat,
Das MG 42 schÜtze