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DaBit
19-09-2002, 09:48 AM
I am reading different stories about the system performance gain when using suction gas <-> liquid line heat exchangers. Most people agree that with HC refrigerants (R600a is widely used here in Europe) there is a gain in both performance and COP when using suction gas <-> liquid line heat exchange. But for HFC refrigerants like R134a the opinions differ. Some people say that there is an increase in both performance and COP, some say that COP increases but performance does not, some say that both performance and COP decreases.

I understand that SG <-> LL heat exchange increases liquid subcool and thus decreases/eliminates flashing in the expansion device. This fact alone might increase performance.

I also understand that SG <-> LL heat exchange increases suction gas superheat, and therefore discharge line temperatures. This might be a problem when operating at low evaporating temperatures/pressures.

But say that we have an R134a system, and enough subcool on the liquid to prevent flashing at the expansion device. Does adding a SG <-> LL heat exchanger increase performance and/or COP?

My theory is that it does at least increase performance. My theory is als follows: increased subcool on the liquid increases the amount of energy taken from the evaporator per kg. of refrigerant (less energy needed to cool the boiling liquid refrigerant down to evaporating temperatures). The extra subcool appears as extra suction gas superheat at the compressor (due to the heat exchange). This does not decrease performance since with most (at least simple piston type) compressors the volume displaced per second is the same due to the fixed motor speed. The extra superheat appears as an increase in discharge heat, which is removed by the condenser. However, amp draw of the compressor probably increases due to the higher load on the compressor. Therefore, I would say that the the COP stays approximately the same.

Can anyone point me to the flaws in this theory and elaborate on the subject? I would like to understand why things happen instead of just accepting them.

superheat
19-09-2002, 01:43 PM
You are trading heat. By subcooling the liquid you reduce flash gas. That is not a gain. Gas that flashes in the evap is not available to cool air. Suction gas heated by the LL heat xchanger is not available to cool air either.
You are right about increased discharge superheat.
By increasing suction superheat, you reduce volumetric efficiency. The evap depends on the MASS flow rate, lbs/min boiling in the evap. The compressor has a fixed volume. The ratio of volume to mass is density. As the temp of a gas goes up, the density goes down. This means every stroke of the piston move less ***** mass. This will decrease amp draw a little, but decreases capacity more.

I can not address HC refrigerants, I do not know about them.
To me LL heat exchanger is an engineering detail. The compressor motor will be cooled less by the hotter gas, so it need a higher class motor. In the old days with more semihermetics, That was not a factor. (Air cooled motors) In the near future engineers might tell us not to use as many heat exchangers on medium temp systems.

Many small systems lack the proper superheat at the compressor. the heat exchanger will help there. This also is a bigger detail with air cooled semihermetics. A tin can has enough heat to boil small amopunts of liquid in the suction line.

These are my reflections on heat exchangers.

DaBit
19-09-2002, 02:46 PM
By increasing suction superheat, you reduce volumetric efficiency. The evap depends on the MASS flow rate, lbs/min boiling in the evap. The compressor has a fixed volume. The ratio of volume to mass is density. As the temp of a gas goes up, the density goes down. This means every stroke of the piston move less ***** mass. This will decrease amp draw a little, but decreases capacity more.

Oops, you are completely right about this. I did not think about this effect.


Many small systems lack the proper superheat at the compressor. the heat exchanger will help there.

So, what is proper superheat? I understood that you need just enough superheat to prevent liquid refrigerant returning to the compressor. Excess superheat seems undesirable to me. Is there a good reason to use more (or less) superheat at the compressor inlet?

When reading between the lines, I conclude that in your opinion a SG<->LL heat exchanger does not give a performance or COP increase. Am I correct?

Gary
19-09-2002, 03:32 PM
Increased subcooling makes the evaporator more efficient by reducing flash gas. The resultant increase in evaporator heat transfer increases suction pressure, thereby increasing density.

Compressor inlet superheat is controlled by adjusting the refrigerant charge on fixed orifice systems, or by adjusting the spring pressure on TXV systems and insulating the suction line on both.

The bottom line for a suction/liquid heat exchanger (with proper superheat control) is increased mass flow, increased performance, and increased COP.

In a closed loop system, mass flow is equal at all points (conservation of mass flow).

We would all agree that installing a suction/liquid heat exchanger increases compressor inlet superheat. We then take steps to decrease that superheat. In other words, we increase the flow through the metering device.

Again, in a closed loop system, mass flow is equal at all points. If we increase the flow through the metering device, we increase the flow throughout the system.

Compressor inlet superheat should be 15-20F.

superheat
19-09-2002, 03:56 PM
Heat exchanger increases subcooling at the expense of superheat. It increases the performance of the evap, but might not increase evap pressure. A large evap on a refrig system has plenty of time to expand the *****. I did a study with AC systems with the ability to bypass the heat exchanger. The pressure did not increase. With a low temp system a couple of degrees hotter evap is a larger percentage increase. It will make a bigger difference. Increasing the superheat decreases the density of the return gas. Which effect is larger? A heat exchanger makes less sense in an AC system. It makes more sense with a semihermetic. I need to pull out my engineering homework to analyse the system performance. I am not fully convinced that a heat exchanger will increase system performance. There have been alot of changes in HVAC in the last few years. Heat exchangers need to be studied more. They are inexpensive. They have no parts to break. Heat exchangers would never be a bad thing, but I would not say they always increase system performance.

Dan
19-09-2002, 11:12 PM
They have no parts to break. Heat exchangers would never be a bad thing, but I would not say they always increase system performance.

Actually, my experience is that heat exchangers do have parts to break, expecially with gas defrost. In a small container, we take a liquid line from warm to cold and a suction line from cold to warm. I am not a fan of heat exchangers for this reason.

Prof Sporlan
20-09-2002, 04:22 AM
Specifically, increased subcooling increases the net refrigerating effect of the refrigerant, which will reduce refrigerant velocity within the evaporator. As a far as improving its performance, it depends upon the effect of the reduced refrigerant velocity has on the evaporator. Highly subcooled liquid will, in fact, invariably reduce the efficiency of the evaporator unless the evaporator is specifically designed for it. Velocities can be reduced to the point where you will have "sewer flow" of the refrigerant, and make it unable to properly wet the inside surfaces of the evaporator tubes.

If the suction/liquid line heat exchanger is resolving a flas gas problem ahead of the TEV, by all means use it. Flash gas in the liquid line in itself can and will cause problems with system performance.

The suction/liquid line heat exchanger will increase superheat entering the compressor unless some means is used to reduce it. Increased superheat will, of course, reduce density of the refrigerant vapor entering the compressor, and thus flow rate.

Normally, the trade off is increased subcooling and net refrigerating effect versus reduced flow rate from the compressor and little or no effect in evaporator performance. The thermodynamics, however, can be a bit involved here. A definitive answer as to the benefit of a suction/liquid line heat exchanger, assuming flash gas in the liquid line is not a factor, requires testing or some sophisticated modeling.

In the Prof's humble opinion, use a suction/liquid heat exchanger only if you are concerned about flash gas in the liquid line. :D

zolar1
20-09-2002, 06:16 AM
I see that there isn't any mention of refrigerant charge when using Heat Exchangers.

I assume that you are using a pre-determined amount of refrigerant (i.e. 5 pounds without a H.E. and 5 pounds with a H.E.)

Suppose you increased the refrigerant change proportionally (or adjusted the TEV) to match the capacity of the Heat Exchanger to the point that you get SOME liquid in the suction side before the heat exchanger. Perhaps a flooded evaporator scenerio?

How about adding a very small cap tube 'bleed-off' from the high side to just before the heat exchanger?

Has anyone tried this with a cap tube system (other than me)?

It would seem to me that if you kept your superheat adjusted properly, a heat exchanger might be a very useful tool.

Domestic refrigerators are plenty cheap enough for experimentation.

Zolar

Roger Goetsch
26-09-2002, 06:52 AM
I thought we hashed this one out a long time ago! The following may sound technical, but if you actually do it, it's way easy.

Get yourself a pressure-enthalpy diagram for any refrigerant, chart a hypothetical system, then extend the condensing liquid line 10 extra degrees or so into the subcooled liquid region and match it by extending the evaporating liquid line by an equal ten degrees into the superheat region.

Even if you follow the isentropic compression lines (ideal compression) you can see at a glance how any increase in superheat pushes you into a less favorable isentropic curve.

Now in the real world, other things happen. If you put the HX close to the compressor, returning heat to the compressor that might have been dissipated from the liquid line on its way to the evaporator, you have messed up (technical term).


Originally posted by Prof Sporlan
Specifically, increased subcooling increases the net refrigerating effect of the refrigerant, which will reduce refrigerant velocity within the evaporator. As a far as improving its performance, it depends upon the effect of the reduced refrigerant velocity has on the evaporator. Highly subcooled liquid will, in fact, invariably reduce the efficiency of the evaporator unless the evaporator is specifically designed for it. Velocities can be reduced to the point where you will have "sewer flow" of the refrigerant, and make it unable to properly wet the inside surfaces of the evaporator tubes.

Now there's something that never occurred to me! (Thanks, Prof!)(Who IS that masked man?) Probably unusual, but knowing how cost control requires optimizing for design conditions and little else, worth considering.

lin
26-09-2002, 04:00 PM
Assume that with HX, the gas out of evap still have same superheat as without HX, so it absorb more heat in there.

But after HX, it is superheated more, in the p-h diagram, u will see compressor need more power to compress it to high pressure.

So if the More heat divide by more power large than original COP, the Cop increase, otherwise decrease.

Some refrigerant is more sensitive in superheat, like NH3, beacause they have higher isentropic exponent, the COP will decline.

baker
27-09-2002, 04:20 AM
Some laboratory data on how increased superheat can improve COP can be found at:
http://ctan.unsw.edu.au/pub/archive/HC/papers/iir96p1r.pdf

DaBit
10-10-2002, 12:32 PM
I did a little testing with my small system using R134a, a small TEV to feed the coaxial evaporator and evaporating at approximately -25 &deg;C. The performance gain from using a simple SG<->LL heat exchange (lines routed together. Liquid R134a going to the TEV is cooled from 28 &deg;C to 17 &deg;C) is quite minimal, but there is a performance gain of about 5-7%.

Gary
11-10-2002, 09:33 AM
The test is meaningless without full system data. Everything is connected to everything else, and everything effects everything else. It's like pushing on a balloon. It bulges elsewhere.

Low side:

Evap air/water in temp
Evap air/water out temp
SST or low side pressure
Suction line temp near compressor

High side:

Cond air/water in temp
Cond air/water out temp
SCT or high side pressure
Liquid line temp near condenser

These are the minimum readings needed.

DaBit
11-10-2002, 11:32 AM
Well Gary, I will provide you the data soon.
I only did a quick check, and thermally coupling the lines improved the amount of heat I can feed the system while maintaining temp. This was a quick test I needed to run before I could start insulating. Insulating was necessary to prevent icing and keeping the false load due to heat leaks minimal.

Nico
06-11-2002, 10:08 AM
I think Lin has the right approach here. I remember in the early day's my teacher told me that for R12 you have some gain, but for R22 it makes no sense. I am lazy enough to wait and see who brings the clear sientific answer.

At the same time, I have seen so many installations screwed up by heat exchangers where the TX bulb was put right after the heat exchanger. In this case the control hunt is amplyfied due to the change in (liquid) flow. For cappilairy and HP / LP float systems there is nothing to gain in evaporator performance. Together with the experiance that heat exchangers tend to leak, I would agree with prof sportlan: use them as little as possible.

DaBit
06-11-2002, 11:13 AM
Routing the liquid and suction lines together gives me a little performance boost. I did not check the COP, but I am less interested in COP than in capacity.

Various studies I have read on the internet showed an 5-15% increase in both COP and capacity for HFC and HC (R134a, R404a, R600a, R290) refrigerants, and little to no effect with CFC and HCFC (R12/R22/R502) refrigerants.

superheat
06-11-2002, 02:52 PM
Some HCFC retrofits beg for some type of heat exchange. I have seen them with no subcool and no superheat. The only way to run them is with heat exchange.

Is this board in any agreement as to application of heat exchangers. I put them in all low temp walk-ins. Would most people say that is the only place to put them? Would many people say don't even put them in any low temp boxes?

gas_n_go
24-06-2007, 08:09 PM
I will say this about heat exchangers, as far as parts not breaking, I had a sys with the exchanger mounted right at the coil. It busted inside and was leaking straight liquid in the suction line. I was getting flood back at my compressors. shut my txv all the way down. pulled apart the valve to check my cartridge size (Q body), finally changed the valve. Problem still persisted. Lightbulb lit in my head, realized I had a bad exchanger, but not till after i unecessarily replaced the txv. Egg on my face but was a good learning experience.

Now I have a question a little bit off track but following the same theme. I remember this like it was yesterday. I was in my mothers womb and thought to myself, why dont we attach the liquid line to the suction line and a calculated distance from our evap out. Theoretically what Im looking to achieve is a transference of sensible heat that detours around my evap. I want my suction line to absorb btu's from my liquid line and provide superheated gas going back to my compressor and increase my txv by supplying it with a subcooled liquid. I lived with this mental quandry for many years and I finally began my apprenticeship and I made this inquery to one of my mentors. He told me it was a mathimatical wash in terms of energy and was pointless. Could someone please explain to me if thats true or not, and why????

lana
25-06-2007, 05:23 AM
Hi there,

I think your theory is correct. Heat Exchanger does the same thing.
I have seen systems in which liquid line was passed directly inside suction pipe, i.e. pipe inside pipe. I am not saying this is correct but it does the same heat transfer. The problem I see is the practical difficulties involved and future leakage that might occur. When using heat exchanger then everything is much easier.

This is my opinion anyway.

Cheers

mohamed khamis
25-06-2007, 11:23 AM
Some HCFC retrofits beg for some type of heat exchange. I have seen them with no subcool and no superheat. The only way to run them is with heat exchange.

Is this board in any agreement as to application of heat exchangers. I put them in all low temp walk-ins. Would most people say that is the only place to put them? Would many people say don't even put them in any low temp boxes?

Hi everybody

Don not oppress the SG<>LL heat exchanger, it sometimes gives an improvement in the system efficiency and sometimes impairs it according to the application field in which it will be installed. In a short, it is favorably installed in the application of low evaporating temperature under -1°C but if it is installed in other applications that for a safety considerations in terms of preventing compressor from flood backing and to prevent the liquid line from gas flushing and likely to excess vibration in liquid line. So the question may be raised how this heat exchanger help the system efficiency or ruin it this comes from the following: Firstly it should be establish a good foundation for some relationships:

System efficiency (COP) = Qe / Wcomp

Qe = refrigerant mass flow * (evaporator outlet enthalpy - evaporator inlet enthalpy)

Wcomp = refrigerant mass flow * (compressor outlet enthalpy - compressor inlet enthalpy)

Mass flow through the TXV = cd*Ao*Sqrt (2*pressure difference* inlet refrigerant density)

Mass flow thru the compressor = Cylinder size * RPM/60*volumetric efficiency

The mass flow rate at all point in the cycle must be the same otherwise the system is unbalance.


Firstly, as it is known the SG<>LL heat exchanger job is to increase the subcooling and superheating degree.

The increase in subcooling degree (lower inlet refrigerant temperature at TXV) results in an increase in refrigerant density at TXV inlet, therefore the mass flow rate inevitably is increased thru the TXV for the same other parameters. On the other hand, as the inlet enthalpy to the evaporator must be increased without any doubts. The net product is the existence of SG<>LL heat exchanger will certainly enhance the evaporator cooling capacity as it is done in the mechanical subcooling system case. Now to ensure the action will be occurred to the system COP we have to look at the compressor power.

The increase in superheating increment on P-h diagram does not tell what will be happened to the compressor enthalpy difference “compressor work”. I expected without any modifications in the inlet and outlet pressure the compressor enthalpy difference will be maintained the same if u draw on h-s diagram and I have verified that by using CATT program and I found the increment in compressor enthalpy difference is very slight 2%. However, this is superficial looking, the existence of heat exchanger leads to increase the pressure drop (unless it was soldered LL with SG) the excessive of pressure drop can lead to a significant increase in compressor work not power. Alternatively, the increase in suction gas temperature will lead to a decrease in its density and in turn the volumetric efficiency will indeed be dropped as a result of two sides (drop in refrigerant density and suction pressure drop). Therefore, the mass flow rate thru compressor has tendency to drop but there is a rule of mass conservation which tells the mass flow rate must be constant along with the cycle. Thus, the compressor must mince itself to offset the excess of TXV feeding by raising the drawing electrical current to handle the excess in TXV feeding. The net outcome is the compressor power in also increased. Therefore, two competing effects at a work. The increment in cooling capacity attempts to enhance COP but the increment in compressor power plays against that. Therefore, the system COP can be improved or hurt in conjunction with the installation of SG<>LL heat exchanger.
However, the judgment sentence can be given in case of the low evaporating temperature applications except in Ammonia or R-22 application. The reason of benefits in low Tev is as long as Tev is low the latent heat to convert liquid to vapor is large and u have to assist the evaporator by adding mechanical superheating such as in accumulator or using SG<>LL heat exchanger without harming the compressor power because at this case the density of the refrigerant is in fact very low and compressor refrigerant capacity is very high so the superheating has no significant effect at this moment. However, in ammonia and R-22 it is not preferred due to the increase in suction gas temperature means increase in discharge temperature and therefore the compressor oil viscosity will be negatively effect due to the high discharge temperature especially in NH3 and R-22, the polytropic index is quite big and any rise in suction temperature will has severe influence on the discharge temperature. Sorry for long post

Cheers

mohamed khamis
26-06-2007, 03:13 AM
Hi everybody

Don not oppress the SG<>LL heat exchanger, it sometimes gives an improvement in the system efficiency and sometimes impairs it according to the application field in which it will be installed. In a short, it is favorably installed in the application of low evaporating temperature under -1°C but if it is installed in other applications that for a safety considerations in terms of preventing compressor from flood backing and to prevent the liquid line from gas flushing and likely to excess vibration in liquid line. So the question may be raised how this heat exchanger help the system efficiency or ruin it this comes from the following: Firstly it should be establish a good foundation for some relationships:

System efficiency (COP) = Qe / Wcomp

Qe = refrigerant mass flow * (evaporator outlet enthalpy - evaporator inlet enthalpy)

Wcomp = refrigerant mass flow * (compressor outlet enthalpy - compressor inlet enthalpy)

Mass flow through the TXV = cd*Ao*Sqrt (2*pressure difference* inlet refrigerant density)

Mass flow thru the compressor = Cylinder size * RPM/60*volumetric efficiency

The mass flow rate at all point in the cycle must be the same otherwise the system is unbalance.


Firstly, as it is known the SG<>LL heat exchanger job is to increase the subcooling and superheating degree.

The increase in subcooling degree (lower inlet refrigerant temperature at TXV) results in an increase in refrigerant density at TXV inlet, therefore the mass flow rate inevitably is increased thru the TXV for the same other parameters. On the other hand, as the inlet enthalpy to the evaporator must be increased without any doubts. The net product is the existence of SG<>LL heat exchanger will certainly enhance the evaporator cooling capacity as it is done in the mechanical subcooling system case. Now to ensure the action will be occurred to the system COP we have to look at the compressor power.

The increase in superheating increment on P-h diagram does not tell what will be happened to the compressor enthalpy difference “compressor work”. I expected without any modifications in the inlet and outlet pressure the compressor enthalpy difference will be maintained the same if u draw on h-s diagram and I have verified that by using CATT program and I found the increment in compressor enthalpy difference is very slight 2%. However, this is superficial looking, the existence of heat exchanger leads to increase the pressure drop (unless it was soldered LL with SG) the excessive of pressure drop can lead to a significant increase in compressor work not power. Alternatively, the increase in suction gas temperature will lead to a decrease in its density and in turn the volumetric efficiency will indeed be dropped as a result of two sides (drop in refrigerant density and suction pressure drop). Therefore, the mass flow rate thru compressor has tendency to drop but there is a rule of mass conservation which tells the mass flow rate must be constant along with the cycle. Thus, the compressor must mince itself to offset the excess of TXV feeding by raising the drawing electrical current to handle the excess in TXV feeding. The net outcome is the compressor power in also increased. Therefore, two competing effects at a work. The increment in cooling capacity attempts to enhance COP but the increment in compressor power plays against that. Therefore, the system COP can be improved or hurt in conjunction with the installation of SG<>LL heat exchanger.
However, the judgment sentence can be given in case of the low evaporating temperature applications except in Ammonia or R-22 application. The reason of benefits in low Tev is as long as Tev is low the latent heat to convert liquid to vapor is large and u have to assist the evaporator by adding mechanical superheating such as in accumulator or using SG<>LL heat exchanger without harming the compressor power because at this case the density of the refrigerant is in fact very low and compressor refrigerant capacity is very high so the superheating has no significant effect at this moment. However, in ammonia and R-22 it is not preferred due to the increase in suction gas temperature means increase in discharge temperature and therefore the compressor oil viscosity will be negatively effect due to the high discharge temperature especially in NH3 and R-22, the polytropic index is quite big and any rise in suction temperature will has severe influence on the discharge temperature. Sorry for long post

Cheers
Hi there

There are some notes on the previous post which are:

Mass flow thru the compressor = Cylinder size * RPM/60*volumetric efficiency* refrigerant density at compressor inlet


... because at this case the density of the refrigerant is in fact very low and compressor refrigerant capacity is very high so the superheating has no significant effect at this moment.

Cheers

there is a correction which is

...because at this case the density of the refrigerant is in fact very high (as a result of decreasing in TEV)and compressor refrigerant capacity is high so the superheating has no significant effect at this moment.

Cheers

Chef
26-06-2007, 12:30 PM
Interesting analysis and conclusions but we had always thought that idea of a SG-LL xfer is to allow the superheat to be set much lower and improve the evapoator performance. Try putting half the superheat value though a p-h diagram and you see about 7-10% in cooling capicity (safely) and shorter compressor cycle times. The down side is actual power absorbed is equivelently higher but if the system is designed to have the exchanger then a larger compressor volumetric flow rate can be installed. Quicker pull down but more power so there is no free lunch.

If pull down time is the key fit one but if efficiency is paramount leave it out, cheaper.

Chef

mohamed khamis
27-06-2007, 06:24 AM
Interesting analysis and conclusions but we had always thought that idea of a SG-LL xfer is to allow the superheat to be set much lower and improve the evapoator performance. Try putting half the superheat value though a p-h diagram and you see about 7-10% in cooling capicity (safely) and shorter compressor cycle times. The down side is actual power absorbed is equivelently higher but if the system is designed to have the exchanger then a larger compressor volumetric flow rate can be installed. Quicker pull down but more power so there is no free lunch.

If pull down time is the key fit one but if efficiency is paramount leave it out, cheaper.

Chef
HI Chef

Thanks for courtesy. there are two issues should be discussed. Firstly, The lowering in evaporator superheat does not enhance the cooling capacity for the same evaporating pressure. The more superheated gas temperature exits from the evaporator indicates to more cooling load imposed on the evaporator and thus the TXV has to treat that by opening wider and accordingly the more evaporator capacity, this on the operation mode scale. on the design mode scale, if two evaporators (A&B) has the same operational parameters but the exit enthalpy of refrigerant (i.e. superheat degree) for A is higher for that for B. Definitely the design of A is more effective or in another term the cooling capacity of A is the bigger. if u put half the superheat value though a p-h diagram you will see the exit enthalpy is reduced about 7-10% and thus the cooling capacity will pursue the reduction.

cooling capacity = mr*(hexit-hinlet) and it is not (hinlet-hexit).

Secondly, yes the pull down relies on the system capacity and instantaneous cooling load as well the thermal storage of the enclosure in which the HVAC system is installed. In effect, the more cooling capacity the shorter pull down time and this is occurred with the system installed with SG-LL heat exchanger due to the increase in cooling capacity as a result of increase the subcooling degree and TXV feeding capacity.
I wish it is clear

Cheers:)

mohamed khamis
27-06-2007, 07:09 AM
Try putting half the superheat value though a p-h diagram and you see about 7-10% in cooling capicity (safely).

Chef

I attached to u a file to show if the point A (outlet exit from evaporator) is shifted to right side the enthalpy of this point is increased (not decreased) in turn the cooling capacity is increased

rbsguzman
29-09-2008, 12:26 AM
Oops, you are completely right about this. I did not think about this effect.



So, what is proper superheat? I understood that you need just enough superheat to prevent liquid refrigerant returning to the compressor. Excess superheat seems undesirable to me. Is there a good reason to use more (or less) superheat at the compressor inlet?

When reading between the lines, I conclude that in your opinion a SG<->LL heat exchanger does not give a performance or COP increase. Am I correct?
Low superheat should not affect compressor at all, as long as you have good superheated vapor at compressor suction inlet, the use of a suction heat exchanger will depend of what kind of refrigerant you have, in some cases you cannot use a heat exchanger on low temp R-22 systems, but having low temp gas vapor is very useful due to it lowers heat of
compression (high disch temperatures)THR , increases VE, density of gas and therefore increasing RE (refrigeration effect) throughout the evaporator and mass flow rate and reduces the HP requirement so some items need to be taken into account before saying that low superheat affects the compressor mechanically. I know for fact that the best way to know what's going to happen in a refrigeration cycle is to analize entalpies, constant entropies, compressor performance inf. In conclusion there are many methods of preventing flash gas such as, mechanical sub-coolers, liquid pumps, natural sub-cooling, well selected TXV's etc. I have read in these forums that 15-20F compressor superheat is good and that's not true.
Thanks, Roberto Guzman
PD. The use of a suction heat exchanger is mostly used to prevent flooding backs not to eliminate flash gas. You might need something more effective than that.

RefrigNoob
02-04-2009, 02:00 PM
Here's my take on LL<>Suction Hx. Please bear in mind that I've only been in refrigiration for about 3 months now.

I ran couple of sims and tests and read a good paper on the subject.

Yes, a LL<>Suction Hx does increase refrigiration capacity. But at a cost. The cost is an increase in the temperature difference between air-on and suction gas temp.

There are also so many differnt types of refrigiration systems out there, so let me also state that the following is based only on the field in which I have my grand total of 3 months experience. Transportation refrigiration units, specifically trailer units. Dry evap, dry condensor, recipricating compressor, 404A. Something like a TK Slx400.

Here are some figures. I'm not going to go into great detail on RH% in these figures, but it remains unchanged in the simulation.
Air-on Evap = 0°C
Evaporation temp = -15.3°C
Air-on Cond = 30°C
Condensation temp = 51.2°C
Qe=21.1kW
Qc=30.6kW
This is without a Liquid<>Suction Hx.

Now we add a Hx and we have 10°C heat exchange between the LL and SG.
Air-on Evap = 0°C
Evaporation temp = -16.9°C
Air-on Cond = 30°C
Condensation temp = 52.6°C
Qe=23kW
Qc=32.2kW

The cooling capacity did go up. But, we know that you never get something for nothing. Law of energy conservation.

Note how after the system has re-stabilised that Evaporation temp actually decreased. Eventhough the air-on Evap temp is still 0°C.

All this means, is that though you score some kW Qe at mid range, you won't be able to go down as cold as you could.

I know that our compressors (Bock FKX-40 560 TK) can't really pump down further than an Evaporation temp of -40°C. And for my purposes (cooling a sealed cargo box where Evap air-on = Box temp) there is a direct relationship between my box temp and evaporation temp. In the first set of figures evaporation temp is 15°C under box temp. In the second set of figures evaporation temp is 17°C below box temp (evap air-on). Which means, if I'm aiming for a box temp of around -25°C, and my compressor can only pump down to -40°C (Evaporation temp), I can't use a LL<>SG Hx. Though the relationship is probably not as linear as I'm describing, one can easily see what I'm trying to illustrate.

A LL<>SG HX will give more capacity at medium temps, at the expense of low down capability. Use of them should be considered according to application, but as a rule of thumb, they should only be used to eliminate flashing at the TXV. So around 1-3°C temp transfer. If you still have flashing, consider revising your LL.

As I said in the beginning, I'm still very new to refrigiration, so I'm open to comments regards my oberservations and conclusions.

RefrigNoob
02-04-2009, 03:14 PM
Now I thought about it some more...
If you really want a LL<>SG Hx to really work for you you should setup your system as follows.
Let's assume you have a LL<>SG Hx that will transfer 7°C of heat between the LL and SG. Then, sithout the HX installed you can set your TX valve to 0°C superheat, and check your charge for 0°C sub-cooling. Then install your Hx. Now your compressor is safe with 7°C superheat, and your TXV gets clean liquid that's been sub-cooled by 7°C.

And you score condensing and evaporation capacity, because those coils don't have to do the superheating and sub-cooling anymore. That job has now been delegated to the Hx.

SteinarN
02-04-2009, 06:10 PM
Have a look at this http://www.refrigeration-engineer.com/forums/showthread.php?t=11946 tread.

EMS AE
18-05-2010, 02:47 AM
A newbie to this forum.

<meta http-equiv="Content-Type" content="text/html; charset=utf-8"><meta name="ProgId" content="Word.Document"><meta name="Generator" content="Microsoft Word 9"><meta name="Originator" content="Microsoft Word 9"><link rel="File-List" href="file:///C:/Users/Dad/AppData/Local/Temp/msoclip1/01/clip_filelist.xml"><!--[if gte mso 9]><xml> <w:WordDocument> <w:View>Normal</w:View> <w:Zoom>0</w:Zoom> <w:DoNotOptimizeForBrowser/> </w:WordDocument> </xml><![endif]--><style> <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0in; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:8.5in 11.0in; margin:1.0in 1.25in 1.0in 1.25in; mso-header-margin:.5in; mso-footer-margin:.5in; mso-paper-source:0;} div.Section1 {page:Section1;} --> </style> The intricacies of refrigerants are not within the realm of my particular forte, which is LVS-BAS/EMS. However, the issues that Suction to LL heat exchangers (SLHE) under some conditions increase pump requirements while at other time increases capacity performance... In my realm I see resolution top be rather simple… lighten the load. For instance, change out R-22 to a quality R-22a. Otherwise, I can only advise one review the % of actual refrigerants in the products they use as the amounts can vary within a product class.
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Another configuration where the SLHE may be beneficial: Instead of co-mingling LL/SL heat, the LL ports as usual but the SL ports are used to circulate cool water, if economically available.
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Inversely, another configuration for the SHLE: The SL ports remain as usual however, the LL ports are used to circulate economically available hot water (such as solar). SHLE upsizing may be necessary depending on one’s application.
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Careful consideration should be given as to alternative energy temperature control and SLHE mounting location. For split HP systems the two alternative SHLE uses offer reasonable benefits if water potential is already available.

D.D.KORANNE
18-05-2010, 11:07 AM
Refrigerant specific heat also is point one will have to take into account to calculate the rise in temp of the refrigerant returning from evaporator . Higher the specific heat higher the rise in gas temp in ll suction gas heat exchanger . Higher the temp gain in suction gas , lower the liquid sub-cooling gain in capacity . It needs to balanced out.

norseman
18-05-2010, 01:52 PM
Hi to all.

I am not an expert on the issue, but I have worked on several types of blastfreezers to rapid freeze blood bags. One type used a 3 cylinder Copeland 7.5hp I think. (many Years ago) One cylinder was used to feed a heat exchanger. The other two cylinders made the discharge trough the hx so the discharge line out of the unit was below zero C. First time I saw a discharge line who was insulated due to the very low temperature on the line to the chest cabinet of 360 ltr. Inside this cabinet the 2/3 of the volume at the lower part was a huge evaporator and some wild running fans. Without load the R502 system dropped the temperature in the cabinet may be 10c a minute. I am sure that it was a good reason for the very sub cooled refrigerant on this unit. Later days I am into a contact freezers for the same duty using a Copeland scroll. The same is done here, the discharge is subcooled not by the suction line, but by a small HX who use the some of the liquid to precool the discharge to around 0C before entering the TX.
Both unit types have to be as fast as possible to pull down and probably also to give the most capacity as possible. The last one on 404A pull to a suction of about -60c so it is seriously low suction pressures on them. Anyway, a cold discharge is the way they prefer and I am sure that it does it's mission on them.
:eek: