PDA

View Full Version : COP and Superheat - does is it help?



Chef
04-09-2012, 12:21 AM
There any many references that say COP improves with superheat in some gases and not in others. R12 was classically described as an improver with SH whilst R22 was said to have worse COP values as SH increased.

So far I have not been able to show this increase in COP and so ask if anyone knows of a good example/article that descibes it.

All I have seen is that SH pushes the compression cycle into flatter entropy zones and hence greater compression work plus it reduces the compressor efficiencies and both are bad for COP.

Also as SH increases so the quality increases which reduces the refrigeration effect. Combine all 3 effects and COP falls of very quickly with SH.

Thanks

Chef

Teuria
04-09-2012, 04:00 AM
Hi

I think this is effective to all refrigerants, :


All I have seen is that SH pushes the compression cycle into flatter entropy zones and hence greater compression work plus it reduces the compressor efficiencies and both are bad for COP.

But if you found such refrigerant wich does not act like others, let me know :)

shooter
04-09-2012, 11:13 AM
look for the program coolpack it is from denmark, here you can make calcs with sh and cop.

Chef
05-09-2012, 06:51 AM
look for the program coolpack it is from denmark, here you can make calcs with sh and cop.

Interesting as coolpack shows an increase in COP with increasing SH.

Lets take some extreme values and have condensing at 35C and the evaporator at -10C so now we add 50C superheat and get an improvement in the COP.

This means the evaporator is at 40C (thats +40C) which makes little sense. Not sure how they calculate the various states but in reality that cant possibly be true so just for fun I tried 100C superheat and my COP went up yet again.

So coolpack does an energy balance and does not consider what the physical conditions in the freezer are, it is up to the user to change the evaporator temperature to allow realistic SH realisation.

Bottom line is it has not solved the dilema.

Chef

mad fridgie
05-09-2012, 10:18 AM
Have you played with useful and non useful super heat, and pressure drop which also shows as increase non useful super heat.

Chef
06-09-2012, 05:50 AM
Back to my original question - there are several (many) references that show an increase in COP with SH but I am unable to find this.

Coolpack gives unrealistic results as having 50C SH in a fridge system is not possible but it allows the calculations to continue. Hmmm. Concerning.

If one assumes a box temperature and then adjusts evaporator temperature to include SH then the results are more reasonable and the COP does decrease. I wonder if everyone else uses the software that way?

SURESH YADAV
06-09-2012, 12:19 PM
Dear
Effect on COP by increase in SH depends on the thermodynamic characteristics of refrigerant. If refrigerant have higher gamma value decrease the COP. Because increase in power consumption more as compare to improve in refrigeration effect. Super heat is mostly used for compressor safety not for improving the COP.

shooter
12-09-2012, 08:04 PM
Yes with rising SH the COP goes up, but do not forget the return temp must be also very high, so yes it is correct what coolpack does.

desA
18-09-2012, 08:02 AM
Hi Chef,

The answer to your question would surely be answered by running incremental (evap) SH trials on a complete system?

Programs like Coolpack assume that as SH is varied, that all Te,sat & Tc,sat remain constant. Is this the case, in reality? What results does your own simulator predict?

Chef
21-09-2012, 04:36 AM
Yes with rising SH the COP goes up, but do not forget the return temp must be also very high, so yes it is correct what coolpack does.

So what you are saying is that 50C SH is feasable from a system where the ambient is maybe 30C. So you say more SH the more COP and Coolpack is right!!!

You must explain how we build this thing.

Maybe you could put the numbers into Coolpack yourself and then post a suitable design for the fridge you have.

Chef
21-09-2012, 04:53 AM
Hi Chef,

The answer to your question would surely be answered by running incremental (evap) SH trials on a complete system?

Programs like Coolpack assume that as SH is varied, that all Te,sat & Tc,sat remain constant. Is this the case, in reality? What results does your own simulator predict?

To answer my question which was "Does anyone knows of a good example/article that describes COP increasing with SH" would be great.

I really only want to know if you have any good referance material so doing an incremental SH trial on a complete system is a bit of an overkill.

Programs like Coolpack assume that as SH is varied, that all Te,sat & Tc,sat remain constant. Is this the case, in reality?

Well that is my point. If the predictions are not real how are we to apply them in everyday life.

What results does your own simulator predict?

Not what Coolpack does. As you say "is this the case in reality" and I am trying to get to the reality.

mad fridgie
21-09-2012, 07:20 AM
Come on chef, spill the beans, whats your giving that coolpack does not.

No system really is even in steady state, there are very few exceptions.

desA
21-09-2012, 08:30 AM
[at]Chef : Chill-beans sent post-haste via an urgent ocean drop overhead. Be on the look-out.

I stand by my view that there are always a few ways to arrive at an answer:
1. Experimentation on real system.
2. Controlled, accurate simulation, where all variable free to float to steady-state as SH varied.

It would be interesting to see how your internal simulator stacks up in a controlled test situation. Precisely how SH is raised, will contribute (I'd imagine) as much to the system change, as anything. E.g. cutting back refrigerant mass-flow, or adjusting evap fan speed etc.

Anyway, this thread promises to be an insightful one, if all parties can pull together on the same side of the investigation. :)

Chef
21-09-2012, 11:59 PM
Come on chef, spill the beans, whats your giving that coolpack does not.

No system really is even in steady state, there are very few exceptions.

Well as I said if you put 50C SH into Coolpack it just plods along and gives very unrealistic answers. It is not possible to continue adding SH and increase the COP ad infinitum so there is something wrong me says.

What I see is a change in the evap temp so the box temp stays the same and this is real world so the SH is accounted for. My COP drops of quickly at 50C SH

Chef
22-09-2012, 12:20 AM
[at]Chef : Chill-beans sent post-haste via an urgent ocean drop overhead. Be on the look-out.
Please use Yellow Box Ticker code 47590023 for GPS location.

I stand by my view that there are always a few ways to arrive at an answer:
1. Experimentation on real system.
Full system experimentation is massive overkill for my original question - does SH improve COP and is there any good articles about it. Even well founded opinions would be just as good.

2. Controlled, accurate simulation, where all variable free to float to steady-state as SH varied.
Thats exactly what I am doing but disagreements with other resources like Coolpack need to be addressed, understood and resolved. If my memory serves me well i believe you are a believer in SH being a COP riser so your input as to what conditions it will help is what we need.

It would be interesting to see how your internal simulator stacks up in a controlled test situation. Precisely how SH is raised, will contribute (I'd imagine) as much to the system change, as anything. E.g. cutting back refrigerant mass-flow, or adjusting evap fan speed etc.
What sort of controlled test situation, test against what, already tested it against Coolpack and it disagrees.


Anyway, this thread promises to be an insightful one, if all parties can pull together on the same side of the investigation.
If we are all on the same side then we will all agree and we will get nowhere, maybe you can explain a little more your comment here.
:)

My reply in blue of course.

desA
22-09-2012, 06:27 AM
I doubt that you are going to find the literature you may be searching for, on RE. There may be bits & pieces scattered throughout academic literature, but perhaps not quite in the form you are looking for.

A few thoughts;
1. If someone has ready access on a test rig (heat-pump, or refrig circuit), we can run up a set of parallel trials - for hard comparison purposes. (Thinking here of MF, as my test lab is still packed away in SE Asia. Will only have access to that in a few months again).
2. You (Chef) then run trials using your simulator.
3. Coolpack 'snapshots' are taken along the test trajectory.
4. Suggest we also use Emerson 'Select 7.6', or 'Duprex 3.2' as additional comparators.
5. We bring all findings to the table, & compare notes along the exploration trajectory.

We could all learn a great deal from this exercise & it would be fun, to boot.

mad fridgie
22-09-2012, 12:35 PM
Ok, I am struggling with the question and the answers.
If we have refrigerant that has fully boiled, then we add further heat, and if the heat is useful, then at this point only the system must be more efficient.
So we have 2 different points we have same pressure on with some addition energy and one with a lot of additional energy.
Is the question or the implied answers about what happens in the compressor or what is happening to the system.???????? ???????? ???????

xxargs
22-09-2012, 03:58 PM
if super heating from liquid line make better performance or not is depend of two thing

1. If high or low specific heat to increase refrigerant up/down one degree in gas or liquid state (not phase change) .

2. how much latent heat in phase change between liquid and vapor

For example ammonia have very high latent heat on phase change ie. low mass flow comparing to cooling power and low specific heat so even small amount of joule transfer in SH-exchanger with liquid line, punish system badly on COP and make very heat high side output gas temperature after compression.

Other side, take example of R125 or HFO1234yf is have very low latent heat (need high mass flow to make some cooling power and have high specific heat) you _need_ SH-exchanger with liquid line to make any acceptable COP (or you take with you so much heat in warm liquid line in to evaporator so most of cooling power goes to cool liquid itself down to evaporator temperature and you have low quality of refrigerant - ie. very little of mass refrigerant stay in liquid state i evaporator after expand and most already vaporize without any use for cooling room) and also to make secure with SH at refrigerant are dry after compression step so you not have refrigerant steam in exhaust side and wash away oil from inside cylinders, cylinder head, valves etc. inside compressor and wear out this to fast.

Propane (R290) and isobutan (R600a) have lite of this problem with steam in exhaust if not have some degree SH in suction line with compressor with low heating of suction gas before entering compression step (to many mistake done about not respect this fact and make short life equipment depend of bad lubrication in cylinder head in the end) and winning some COP if carefully balanced between high pressure side temperature and make liquid side cooling so close as possibly to evaporator temperature before expansion - in this way you can in theory make same or slightly better performer with propane compare to R134a without SH-exchanger with liquid line.

- but if you use pure R125 and HFO1234yf this problem is much worse, risk to working with wet steam inside compressors and COP going down badly if not using heavy grade transferring SH-exchanger with liquid line - all depend of low latent heat compare to high specific heat

For refrigerant as R12 and R134a have very small improve COP with SH-exchanger with liquid line and for bigger equipment not invest it in most case - exception white wares as fridges and freezer for for kitchen and houses ie. mostly put in capillaries inside suction line or welded/pressed/soldered parallel some distance (but often in little to late step for best performance ie. liquid already starting to vaporize in capillary line before put together with suction line).

R22 have slightly punish on COP and rise hot gas temperature very fast if try SH-exchanger with liquid line

Chef
23-09-2012, 02:59 AM
I doubt that you are going to find the literature you may be searching for, on RE. There may be bits & pieces scattered throughout academic literature, but perhaps not quite in the form you are looking for.
I am looking for it in any form so happy to get any reference out there.

A few thoughts;
1. If someone has ready access on a test rig (heat-pump, or refrig circuit), we can run up a set of parallel trials - for hard comparison purposes. (Thinking here of MF, as my test lab is still packed away in SE Asia. Will only have access to that in a few months again).
The prospect of a test rig showing COP increases for a few degrees of SH is unrealistic, unless it is at JPL or similar. My rig is set up for cap tubes not for SH and COP trials so that is out as well.
2. You (Chef) then run trials using your simulator.
Already done that and the results dont agrree
3. Coolpack 'snapshots' are taken along the test trajectory.
That is where I got one comparison from as already posted.
4. Suggest we also use Emerson 'Select 7.6', or 'Duprex 3.2' as additional comparators.
OK you can do that bit and see if you find COP going up or down in those programs
5. We bring all findings to the table, & compare notes along the exploration trajectory.
Table the results from 4 above so we can see if thay all predict the same thing
We could all learn a great deal from this exercise & it would be fun, to boot.

Answers in blue

Chef
23-09-2012, 03:08 AM
Is the question or the implied answers about what happens in the compressor or what is happening to the system.???????? ???????? ???????

It is about the system I suppose as it concerns the COP - what happens in the compressor is only part of it.

Differant programs and differant answers is a concern. If I fiddle Coolpack with the evaporator temperature it gets closer

Chef
23-09-2012, 03:29 AM
xxargs - a very detailed reply and thankyou for the input.

You are bringing in a SH-exchanger but initial tests were not on a system with a SH exchanger and that will come later.

For refrigerant as R12 and R134a have very small improve COP with SH-exchanger with liquid line and for bigger equipment not invest it in most case - exception white wares as fridges and freezer for for kitchen and houses ie. mostly put in capillaries inside suction line or welded/pressed/soldered parallel some distance (but often in little to late step for best performance ie. liquid already starting to vaporize in capillary line before put together with suction line).

Even if the liquid is vapourising in the capillary the addition of SC will still be felt just the same if was applied before the capillary. Any cooling of the fluids in the tube will have an effect no matter where they occur.

The main question in this case is how effective is the capillary tube in a suction line acting as an effecient heat exchanger as the area of the tube is so small it cant really have an effective heat transfer between the 2 fluids. It may help a little but this cant be called a SH-exchanger.

I realise various gases have differant properties when doing these calculations and at the moment I am looking at R134a, R12 and R22. Although R12 is defunct it is still the standard for all comparisons.

mad fridgie
23-09-2012, 05:24 AM
I think we need to look at an extreme example.

We have fluid that needs to be cooled single pass 50C to 5C. So we need a evaporation temp below 5C, say 0C

We could have then up to 50K of useful superheat available. Lets say 24K superheat as this practically in range of most compressors.

Will this give a better COP than say having a conventional 6K Sh, and does vary due to the refrigerant?

And if it is better, what effect does this have on the design of the evap heat exchanger.

desA
23-09-2012, 08:11 AM
Right then, Chef. Please set the example/s & lets get cracking.

Peter_1
23-09-2012, 07:38 PM
Right then, Chef. Please set the example/s & lets get cracking.

Hello friends, Chef, MadF and surely DesA (greetings to Pam)...this seems again to be a very intersting thread.
Just read very fast this interesting thread..asmost threads started by you guys. :p
Chef, I did the same excercise a long time ago with the Bitzer software (and I think we discussed this once with DesA)
Result of this is now that in fact, we just build a DX-GSHP for my daughter's new house where we're using this theory (as far as I'm following correctly your explanations) We assembled yesterday this heatpump and we took several pictures of it.
We will add additional and controlled energy to the suction gasses before they enter the compressor to achieve - we hope- much higher discharge temperatures at a low HP and an increased COP due to the much higher and controlled SH. We therefore will 'sacrifice' low temperature water to gain high much higher temperature water. Perhaps the additional costs will be lost if we see once running that the theory was wrong but anyhow, we tried it at least.

xxargs
23-09-2012, 11:04 PM
I try some writting her (Im not native English - so excuse for misspellings word and grammar)

If we take AC for cars in desert area with 2 degree C of evaporator and 70 degree C in condenser temperature no SH or SC on pipes before condenser/evaporator

compressor assume 1 in isentropic effectivity for more easy calculation (ie. ideal situation...)

(i know - is ugly way to put numbers)





R134a
Temperature
C


Pressure Mpa
Enthalpy kJ/kg
Entrophy kJ/kg
Density kg/m^3








without SH
exchanger



















2
0,31
251,62
0,93
15,46




Cold gas from evaporator




77,04
2,12
291,04
0,93
106,5
-39,42


work to the compressor




70
2,12
156,14
0,54
996,25
134,9


heat from condenser




2
0,31
156,14
0,58
29,66




After ex-valve
quality 0,51551




2
0,31
251,62
0,93
15,46
-95,48


Cooling power from evaporator














3,42


Heating-COP














-2,42


Cooling-COP














266,86


kg/hour refrigerant for 10 kW heat














17,26


m^3 i suction volume per hour for heat














95,87


cc efficiency volume on compressor on 50 Hz (3000 rpm) for 10 kW heating














267,87


liter liquid refrigerant before pressure loss in TXV for 10 kW heat








377,04

kg/hour refrigerant for 10 kW cooling








378,46

liter refrigerant/hour for 10 kW cooling power








24,38

m^3 i suction volume per Hour for 10 kW cooling








135,46

cc efficiency volume on compressor on 50 Hz (3000 rpm) for 10 kW cooling




and for HFO1234yf (new refrigerant for Car to replace R134a)



HFO1234yf
Temperature

C


Pressure Mpa
Enthalpy kJ/kg
Entrophy kJ/kg
Density kg/m^3








without SH
exchanger



















2
0,34
201,99
0,74
18,83




Cold gas from evaporator




70
2,04
232,02
0,74
137,88
-30,03


work to the compressor
Q = 0,95 => steam in vapor out from compressor




70
2,04
141,07
0,53
882,47
90,95


heat from condenser




2
0,34
141,07
0,57
29,86




After ex-valve
quality = 0,62384




2
0,34
201,99
0,79
18,83
-60,92


Cooling power from evaporator














3,03


Heating-COP














-2,03


cooling-COP














395,82


kg/hour refrigerant for 10 kW heat














21,02


m^3 i suction volume per hour for heat














116,76


cc efficiency volume on compressor on 50 Hz (3000 rpm) for 10 kW heating














448,54


liter liquid refrigerant before pressure loss in TXV for 10 kW heat








590,94

kg/hour refrigerant for 10 kW cooling








669,64

liter refrigerant/hour for 10 kW cooling power








31,38

m^3 i suction volume per Hour for cooling 10 kW









174,31

cc efficiency volume on compressor on 50 Hz for 10 kW cooling





You lose COP going from R134a to HFO1234yf and volume flow on liquid line almost doubling compare to R134a for same cooling power and system need bigger pipe with more volume inside or accept higher flow rate and compressor have almost 30% bigger pumping suction volume for same cooling power.

If done HFO1234yf system witch suction SH with liquid line, is possibly make better COP and lower mass and volume flow (but how this work in car in hot temperature and maximum fan to cooling down compartment short after start and suction gas already hot from evaporator before SH-exchanger, this is different story)





HFO1234yf
Temperature
C


Pressure Mpa
Enthalpy kJ/kg
Entrophy kJ/kg
Density kg/m^3








with SH-
exchanger



















2
0,34
201,99
0,74
18,83




Cold gas from evaporator



2(g) -> 53 (g)
0,34
252,37
0,91
14,96
50,36

heat from SH- exchanger




53 (g) -> 110 (g)
2,04
293,37
0,91
92,37
-41


work to the compressor





110(g) -> 70 (liq)
2,04
141,07
0,47
882,47
152,3


heat from condenser



70(liq) -> 39(liq)
2,04
91,33
0.32
1048
-50,36

heat to SH- exchanger




39(liq) -> 2 (liq/g)
0,34
141,07
0,57
29,86




After ex-valve
quality = 0,317




2
0,34
201,99
0,79
18,83
-110,66


Cooling power from evaporator














3,71


Heating-COP














-2,7


cooling-COP














237


kg/hour refrigerant for 10 kW heat














12,6


m^3 i suction volume per hour for heat














70,03


cc efficiency volume on compressor on 50 Hz for 10 kW heating














226,48


liter liquid refrigerant before pressure loss in TXV for 10 kW heat








325,32

kg/hour refrigerant for 10 kW cooling








310,4

liter refrigerant/hour for 10 kW cooling power








17,3

m^3 i suction volume per Hour for cooling 10 kW









95,97

cc efficiency volume on compressor on 50 Hz for 10 kW cooling



With good heating transfer SH-exchanger seem HFO1234yf working very good - better than R134a without SH-exchanger and almost all this is depend to make cooler liquid line before TXV and quality of evaporated mass going from 0.62 of liquid mass for self-cooling to only 0.317 of liquid mass and have almost double of usable liquid mass in evaporator to absorb heat.

But i real world you have heating from compressor easily exceed maximum 110 degree C of hot gas temperature from compressor, SH-exchanger not work perfectly and we have also starting process before we have any cool suction gas from evaporator to feed SH-exchanger...


Interesting notice how this look in using DiMetylEther (short DME) in car-using situation - this single refrigerant is very similar to R12 an R134a in pressure at same temperature



DME
Temperature C

Pressure Mpa
Enthalpy kJ/kg
Entrophy kJ/kg
Density kg/m^3








with out SH -
exchanger



















2
0,29
522,82
1,93
6,26




Cold gas from evaporator




84,87
1,81
613,14
1,93
35,6
-90,32


work to the compressor




70
1,81
264,72
0,91
578,47
348,42


heat from condenser




2
0,29
264,72
0,99
15,56




After ex-valve
quality = 0,49706




2
0,29
522,82
1,93
6,26
-258,1


Cooling power from evaporator














3,86


Heat-COP














-2,86


Cool-COP














103,32


kg/hour refrigerant for 10 kW heat














16,5


m^3 i suction volume per hour for heat














91,65


cc efficiency volume on compressor on 50 Hz for 10 kW heating














178,62


liter liquid refrigerant before pressure loss in TXV for 10 kW heat








139,48

kg/hour refrigerant for 10 kW cooling








241,12

liter refrigerant/hour for 10 kW cooling power








22,27

m^3 i suction volume per Hour for cooling 10 kW








123,73

cc efficiency volume on compressor on 50 Hz for 10 kW cooling




DME have high latent heat compare to most other refrigerant except ammonia, so not need so much liquid refrigeration mass to give lot of cooling power

(i hope is not made to much mistake on all calculation and numbers above and make wrong conclusion depend of them in later text here.)

Some compare:





for 10 kW cooling at 2 degree C for evaporator and 70 degree C on condenser
R134a
HFO1234yf
HFO1234yf with SH exchanger and 110 degree C hot gas from (ideal) compressor
DME
(dimetyl ether)


mass flow kg/hour
377
591
325
139


volume flow liq liter/hour
379
669
310
241


cc on compressor at 50 Hz (3000 rpm)
135
174
96
123,7


suction volume m^3/hour
24,38
31,4
17,5
22,3


cooling-COP
2,42
2,03
2,7
2.86




DME not winning to have SH exchanger

DME have almost same burning heat as methanol/etanol in air - around half of energy per kilo compare burning propane/butane - Need also higher lever of leak gas blend in air before lowest explosion level compare to HC (around 3.5 percent in air, HC have 2 percent limit) and if make design right on for example car-AC with smaller pipes (going from 8 mm to 6 mm on liquid line ex.) etc. it needs only 150 - 180 gram DME refrigerant compare to standard small/medium car with 470 gram R134a load for same cooling power..

ie. load is close to allowed maximum HC-refrigerate mass inside fridge and freezer for indoor using (max 150 gram HC) with free placing and no demand of extra ventilation.


With HFO1234yf you need going up some size on pipes and bigger load of refrigerant to take more mass-flow and if using SH-exhanger with some usable area inside, take also lot of liquid volume. I expect around 30% more refrigerant with HFO1234yf compare to R134a for same cool-power.

But.. last time i look in new car with new HFO1234yf filled - newer see any trace of SH-exhanger in cooling loop, evaporator seems not enforced/rugged. TXV still inside compartment area with O-rings-sealing as standard in many car, and build pretty same as ordinary R134a-system, but now we have flammable refrigerant with poison fumes if burning...

So they either take down cooling capacity a bit or work with high mass flow and lower COP in system, so after pretty tales about carbon oxide foot-print and so on, seems in the end cheating and only using cheapest solution provided and payed from refrigerant makers in time had possibility to make serious move to non synthetic and nature and more effective existed refrigerant...

---

Is very lively discussion over year if people slightest thinking to using HC as propane and isobutane-blend in car-AC - but now is flammable refrigerant is provided from 'right side' - seem now OK even if refrigerant burning make poisonous hydrofluoric gas in some percent in smoke (0.1 percent hydrofluoric gas in breathing air is deadly even in very short time as couple of seconds).

As consumer as me want have some choice between refrigerant in car in case of burning and for me is very easy to select between car with 700 grams of HFO1234yf and giving poison smoke if burning compare 150 gram of DME for same cooling power and with burning have same smoke as over Trangia stove burning ethanol...

Chef
24-09-2012, 05:07 AM
OK here is the standard set of conditions to start us of.

It is a simple freezer with just 4 components, the evaporator, the compressor, the condenser and the expansion device. There is no HX between suction/liquid at this time.

Freezer box temperature -20C +/-0.1C
Condensing temperature 30C and no SC
SH out of the evaporator is 3C
Compressor is 50% efficient at all points in the cycle (for the time being)
All lines that need to be are insulated.
You may assume good heat transfer coefficients in both the condenser and evaporator of 1C delta T.

We will use R134a for this stage.

So we need to show the COP and any increase in COP when we add a further 5C to the SH giving a total of 8C SH out of the evaporator.

At great risk of being controversial no one is allowed to use an exolube or endocube or whatever it is called to improve efficiency.

Chef
24-09-2012, 05:27 AM
xxargs - that is a lot of data you posted there but I have to ask how is it really relevant to the question in hand. We are looking for the truth of whether SH increases the COP and the I suppose a comparison of gases is critical eventually I was hoping to do that later - ie after we see whether the COP goes up or down with one or 2 gases and then see if others act differently.

I also understand the relevent inherrant safety about differant gases and at a real life level it is important but at a conceptual level we could even choose a gas as UrFl to see what the numbers predict. I mean Uranium Flouride is not your everyday fridge techy gas and has some enviromental issues.

If you have any data that shows how the COP improves with the SH rise it would be great. IChoose any gas you like and we can adapt the base line rules to suite.

mad fridgie
24-09-2012, 05:48 AM
Hi

Pete, yes done this trick many times, very highly suited to one pass water heating systems with very low flow rates, big water temp splits and long thermal length on the heat exchanger.

Not sure if this what Chef is discussing, as the benefits the heat side for sure.

Chef, just a little confusion. you have cond and evap at delta T of 1C, it is not possible to have useful superheat higher than 1C, without the use of an liquid suction heat exchanger. Or is this just a typo and should read 10C delta T

xxargs, great English spelling and grammar (I wish mine was as good, and good info as well. Keep up the work!

Chef
24-09-2012, 06:52 AM
Mad - The whole idea to set the dT to 1C is make sure excessive SH cannot be added as seems to be the case in Coolpack.

I do not know what the other programs predict but as DesA is in charge of the comparisons we shall see.

Your idea of a 50C liquid to be cooled in a counterflow exchanger is a bit outside the realm of the discussion at the moment but may be relavanrt later. The same rules will apply when this is analysed I suppose but is it realistic.

Just want to look at stuff you can but of the shelf.

mad fridgie
24-09-2012, 08:03 AM
Mad - The whole idea to set the dT to 1C is make sure excessive SH cannot be added as seems to be the case in Coolpack.

I do not know what the other programs predict but as DesA is in charge of the comparisons we shall see.

Your idea of a 50C liquid to be cooled in a counterflow exchanger is a bit outside the realm of the discussion at the moment but may be relavanrt later. The same rules will apply when this is analysed I suppose but is it realistic.

Just want to look at stuff you can but of the shelf.

But in coolpack, you should split the superheat between useful and non useful. So in your example you can only have 1K useful and 2K non useful. (increasing to 7K non useful)

So increase the system superheat, then you must say it is the non useful superheat that has to increase. (exclude the S/L heat exchanger)

Peter_1
24-09-2012, 06:55 PM
If I put some values in Solkane, then COP only increases with a usefull superheat (in the evaporator which is of course normal) If you add Sh along the suction line, then COP decreases fast. As long as the energy to superheat doesn't come from the room or the cooled product itself, then the COP will allways decrease. That's what my elbow is saying me.

Chef
25-09-2012, 12:34 AM
So taking what MAD says and no SH (usefull) is allowed outside of the evaporater I get these numbers.

3C SH gives COP 1.776
8C SH gives COP 1.558

This is for a box temperature of -20C in both cases and dT is 1C.

This was done by putting the TXV bulb at the outlet of the evaporater and setiing my dT to 1C, then as I vary the SH control I get the COP values as above so I am still seeing a reduction in COP as I increase my SH.

Peter - you get an increase in COP using Solkane (?) so we are working at odds here or inputting different values or something is wrong?

mad fridgie
25-09-2012, 12:49 AM
So taking what MAD says and no SH (usefull) is allowed outside of the evaporater I get these numbers.

3C SH gives COP 1.776
8C SH gives COP 1.558

This is for a box temperature of -20C in both cases and dT is 1C.

This was done by putting the TXV bulb at the outlet of the evaporater and setiing my dT to 1C, then as I vary the SH control I get the COP values as above so I am still seeing a reduction in COP as I increase my SH.

Peter - you get an increase in COP using Solkane (?) so we are working at odds here or inputting different values or something is wrong?

Without do the numbers your results look correct!

Peter,s COP only increases if the superheat is useful, in other words energy is removed from the product or product cooled area.

Chef
25-09-2012, 04:57 AM
Without do the numbers your results look correct!

Peter,s COP only increases if the superheat is useful, in other words energy is removed from the product or product cooled area.

My SH is useful as it comes from the evaporator but just because it useful in terms of taking heat away from the box/product does mean it is useful in a cycle as the COP falls.

The more you increase the SH inthe the TXV the further down the evaporator pressure gets depressed and so COP falls off.

My initial comment still stands - I have not seen a COP increase with SH.

Peter_1
25-09-2012, 05:59 AM
Peter,s COP only increases if the superheat is useful, in other words energy is removed from the product or product cooled area.
Isn't that the same what I said MF but in other words? COP will decreaseas long as SH doesn't comes from the room or product

Peter_1
25-09-2012, 06:04 AM
Solcane
with 50% compressor effficiency
Troom = -20°C, Te = -21°C SH suction line is 3K -> COP 1.95
Troom = -20°C, Te = -21°C SH suction line is 8K -> COP 1.91
This seems normal although numbesr are a little different .

Peter_1
25-09-2012, 06:17 AM
....
The more you increase the SH inthe the TXV the further down the evaporator pressure gets depressed and so COP falls off.
..
Chef, the SH isn't created in the TXV but at the end of the evaporator and then further not usefully outside the cold room. If it comes from the room, even from the suction line inside the room, then energy that needed to be removed from the room was removed, so this energy was useful.
So with the energy needed to evaporate the refrigerant, we add some additional energy also taken from the room to become a total energy retracted versus needed energy. If we add some more energy,then TE will also slightly increase, TC as well but influence of Te is much bigger resulting in a COP increase.

In fact, I guess this explanation wasn't usefull :-) for you at all and it's me not following for 100% due to a 'language-misunderstanding-barrier' from my side?

Chef
25-09-2012, 06:40 AM
Solcane
with 50% compressor effficiency
Troom = -20°C, Te = -21°C SH suction line is 3K -> COP 1.95
Troom = -20°C, Te = -21°C SH suction line is 8K -> COP 1.91
This seems normal although numbesr are a little different .

The SH must be taken from the Troom and at your conditions this does not seem feasible.

So you have Troom at -20C and Te at -21C but how does Troom add 8C to the SH - it is not possible unless Troom was at -11C. My guess is that both Coolpack and Solcane (do not know this program) both allow you to vary the SH outside of practical limits.

If you want an 8C SH from your room at Troom=-20C then Te has to be -29C. That will indeed reduce the COP.

Your results show a drop in COP but not nearly enough for the extra 5C.

Also I get 1.776 and you get 1.95 so a difference here as well.

Chef
25-09-2012, 06:51 AM
Chef, the SH isn't created in the TXV but at the end of the evaporator ?

I got this and the bulb is positioned at the end of the evaporator and is set to 8C so the TXV and the bulb act together to control the SH. That is how it was input to the program. It seemed to understand.

So if my temperature is set to -20C then the system keeps on running till the Te is -29C and the SH is 8C.

Peter_1
25-09-2012, 07:34 AM
So you have Troom at -20C and Te at -21C but how does Troom add 8C to the SH - it is not possible unless Troom was at -11C. My guess is that both Coolpack and Solcane (do not know this program) both allow you to vary the SH outside of practical limits.

If you want an 8C SH from your room at Troom=-20C then Te has to be -29C. That will indeed reduce the COP.

Your results show a drop in COP but not nearly enough for the extra 5C.

Also I get 1.776 and you get 1.95 so a difference here as well.
Of course Chef, in a room at -20°C, you can't add 8SH in a normal, practical way. But you can do it of course with an in-line heat exchanger p.e. But I thought you guys were more discussing theoretical values to prove that Coolpack calculates something wrong


I also noticed the different values for the COP both program gives you. Solkane is from Solvay and can be downloaded for free at their website. It has a nice graphical interface, better than Coolpack but less features than Coolpack.
Honeywell also made something similar and I tested it some time (in SI units) but found very strange results when calculating cycles. Seems that the authors of this program doesn't care about there mistakes although it was nice made.

mad fridgie
25-09-2012, 08:31 AM
Peter sorry for stepping on your toes! Not my intention!

Coolpack and the like in my opinion is to used by those who understand the refrigeration process. So the issue of what the superheat is, is determined by the knowledge on the person imputing the data.

I agree you can almost insert what ever data you want and it will calculate it.

The areas in Coolpack that I use, do not reference the load itself, (ie room temp), so that being the case, they level of superheat useful or not, can be what ever you want, coolpack also does not give warnings about very discharge temps (even though is does calculate the temp), which is also a practical issue

Peter_1
25-09-2012, 12:57 PM
MF, no you didn't step on my toes, not at all, I only have sometimes doubts if I understand correct what I read.
Ever tried Solkane?

Peter_1
25-09-2012, 06:32 PM
Honeywell link http://www.honeywell-refrigerants.com/refrigerant-modeling-software-and-apps/?goback=%2Egde_2107555_member_108269853 very similar to Coolpack but more graphics

Chef
26-09-2012, 07:03 AM
Peter - I downloaded a copy of Solkane and it is very nice stuff, lots of respect for the authors.

When I put the numbers in that I use it came back with exactly the same answers so very pleased with that agreement.

So at this point I cant yet see where SH improves the COP and as MAD points out if you enter the wrong data into Coolpack it will give the wrong answer. It does not seem to check if what you are trying to do is reasonable. Solkane is also similar in it allows unrealistic SH to be added although the COP remains pretty constant. The basic reason seems to be neither consider the box temperature which in a real scenario is essential.

The main idea of this thread is to find out if SH ever increases COP and is not intended to show up Coolpack or Solkane but as a corrollary to the thread it does appear that care should be exercised in their use.

So my only intention is determine if SH ever improves COP and so far I have not seen any conclusive evidence.

Now there may be many reasons why SH is a good thing but that can be looked at later.

Chef
28-09-2012, 07:14 AM
Right then, Chef. Please set the example/s & lets get cracking.

Ready when you are.

desA
29-09-2012, 07:12 AM
Hi folks,

Apologies for my late entry into the debate. Have been frenetically busy of late. I'll review the post above & make comments along the way. Fascinated to see how Peter's trials work out in practice.

What Peter & Chef's (& hopefully MF's) systems will be able to provide is the following vital information:
1. What happens to Te,sat / Tc,sat / Tc,disch when ONLY evap SH is altered?
2. How does the METHOD of altering SH affect Te,sat / Tc,sat / Tc,disch?
2.1 For instance, if SH is altered by trimming refrigerant mass flow?
2.2 If SH is altered by adjusting evap fan speed?
2.3 If SH is altered by other means?

If Peter / Chef / MF could provide practical, experimental data, then this would be the place to begin the COP calculations.

From my own work in my Laos lab, I noted that as SH was adjusted, that other variables in the system automatically re-set themselves, as the system balanced itself. An adjustment of one part, affects the whole system.

I would also agree with the discussion of 'useful' SH. In other words, the SH rise has to come from useful evaporator effect ie. extraction of energy from atmosphere - NOT heat-ingress along the suction line.

Anyway, I feel that the experimental feedback will be absolutely vital, before going into detailed calcs via Coolpack / Solkane / etc.

[at]Peter : Pam says 'Hi'... Please send our regards to Kristien. :)

Chef
01-10-2012, 05:29 AM
Ah DesA - I have already posted the paramters to use for the comparisons.

Also put up my results for the 3C and 8C SH cases and it pretty well shows the COP falls off with increases in SH.

"Anyway, I feel that the experimental feedback will be absolutely vital, before going into detailed calcs via Coolpack / Solkane / etc."

I think we have to get away from the experimental feedback, the complexity of a test rig to produce results reliable enough to determine COP would be an awesome rig with massive amounts of sensors. SO experimental is out and lets stick to hear say or gut feel.

Detailed calcs in Coolpack/Solkane? How detailed can you get in those programs!

Lets see what you come up with to support your ideas and then we can compare.

Oh and fans on the evaporator are not allowed but changing the mass flowrate or even the charge is fine. Whatever you choose.

desA
02-10-2012, 06:29 AM
Chef,

My thoughts are as follows:
1. Performing a purely academic exercise on a fixed set of values may not be representative of what happens in real life.
2. Obtaining feedback from the real rigs out there - Peter, MF, your systems - will allow us to set more realistic calculation regime, for comparison.

I'm pretty eager to have more solid feedback from the real-world rigs, before we can proclaim a 'cast-in-stone' result of precisely how COP does vary, in reality. Apologies if this was not they way you wanted to go.

Chef
02-10-2012, 07:34 AM
DesA - This is my initially posted question.

"So far I have not been able to show this increase in COP and so ask if anyone knows of a good example/article that describes it."

Really that is all I want and comments and points of view along the way are very illuminating, but your continued insistance on testing is unfounded. I cannot ask anyone to go to the lengths of running real life tests to satisfy such a simple question.

Of course it is an interesting question.

Your comment that performing acedemic calculations would be invalid without testing, seems, presumptuous at best.

If we are not able to calculate, predict and understand theoretical results what qualifications do we have to interpret and make conclusions about test data.

More to the point "if" it was decided to do testing to prove a theory, one must already have a premise to base the test upon, ie we must already have some insight into how the test parameters are to be set to prove the concept either way.

Maybe you could propose such a test schedule and your reasons for such a test.

You never know - with some some promising paramters to work to I may even update my capillary test rig.

mad fridgie
02-10-2012, 08:24 AM
Hi,

"First" "Mads Rig", ah is not longer a test rig, is chunking away nicely cooling milk on a farm.

However I understand both of your concerns, measuring a test, is only any good, if you know what your measuring against, but us in the praactical side, also know that steady state theory does not really apply to any great extent to what really happens.

Also care has to be taken, in understanding what your require from the system. Peters example (my understanding, please correct me if I am wrong), was focusing on the heating side of the loop (even though the cooling is still important).

Back to practical testing, it would be difficult with engineers tools and test gear to accurately see some of the changes you would hope to see. (either the machine needs to be really large, or test equipment needs to be of very high quality (My test gear was better than most ref engineers would have but not up to the equipment that full test labs have.

I think that DesA should throw some calcs into the mix (using his expertise in heat transfer, which may have more of an influence, than just small changes is superheat) I think that could give a starting point, us dummies, then could apply some of our practical knowledge and observations.

desA
02-10-2012, 08:29 AM
[at]Chef : My recommendation for linking the proposed cycle operating points to reality, is that a purely academic review (dead easy to perform) will presume certain parameters to remain constant, whereas, in reality they move as the SH 'raiser/lowerer' will affect the rest of the system. No single variable remains locked as SH is altered, in reality.

Presuming parameters to remain constant, when in fact they change, may (will) lead to incorrect final conclusions.

This was the reasoning behind my suggestions. No problem if this is ignored - feel free - it is your thread, after all. But, don't complain when the final 'comparisons' turn out to be implausible, in practice. :)

Peter_1
03-10-2012, 09:37 PM
It's a pitty I never made this test rig http://www.refrigeration-engineer.com/forums/showthread.php?16504-Polynomial-refrigerants&highlight=cycle+theoretical+sensor
This could have been very useful now. I found after this post someone who could program this alll and send it via ZigBee to a laptop and then project it with a beamer

mad fridgie
03-10-2012, 11:15 PM
It's a pitty I never made this test rig http://www.refrigeration-engineer.com/forums/showthread.php?16504-Polynomial-refrigerants&highlight=cycle+theoretical+sensor
This could have been very useful now. I found after this post someone who could program this alll and send it via ZigBee to a laptop and then project it with a beamer

My rig was very similar, using a SCADA, I did not have all the inputs going into it ( i am thick when comes to electronics)
What i was lacking was high end "energy measurements" and "refrigerant mass flow devices".
So it was excellent for see change, but not good enough to give definitive performance (estimate +/-5%)
I would tie the figures in with the comp manufactures data.

Chef
04-10-2012, 07:28 AM
[at]Chef : My recommendation for linking the proposed cycle operating points to reality, is that a purely academic review (dead easy to perform) will presume certain parameters to remain constant, whereas, in reality they move as the SH 'raiser/lowerer' will affect the rest of the system. No single variable remains locked as SH is altered, in reality.

I do not know what the acedemic reviews presume till I read them But as it is dead easy to perform we hope to see some good stuff posted

Presuming parameters to remain constant, when in fact they change, may (will) lead to incorrect final conclusions.

It is a bit of a circular comment, if parameters remain constant how can they change.

This was the reasoning behind my suggestions. No problem if this is ignored - feel free - it is your thread, after all. But, don't complain when the final 'comparisons' turn out to be implausible, in practice.

Of course your testing idea has merits, lets imagine I want to upgrade to a new gas so you would propose testing all available gases till a good one is found and ignore the massive amount of data provided by the manufacturers/suppliers.

Or better still I have to install a split into a new build in Raynes Drive. So rather than suppliers selection information coupled to weather and solar data I have to buy a house and test various splits over the seasons to make sure I have the right unit. Seems right to me!!!!!
:)

Oh and I wont complain if the results are of the mark but if you can possibly make a contribution it would be appreciated.

desA
04-10-2012, 03:29 PM
Peter mentioned:
It's a pitty I never made this test rig http://www.refrigeration-engineer.co...retical+sensor
This could have been very useful now. I found after this post someone who could program this alll and send it via ZigBee to a laptop and then project it with a beamer

[at]Peter:
When you say 'program this all', do you mean as in generate a computer simulation?

If so, than this may help us to establish the movement of variables with our modification of SH. This could possibly also then assist to settle the COP discussion. Not sure mind you, unless the author manages to correctly model compressor efficiency changes, evap & condensor changes, as the cycle changes.

Would be nice to see in action.

I had also in mind trying to locate a dynamic simulator for an hvac system. This may, however, also suffer from the same inaccuracies.

This is where a simple circuit will be so useful. (My heat-pump lab is currently in moth-balls in Laos. Would have run some hard trials & sorted most of this out by now... :) )

Peter, or MF, are you perhaps able to run some trials with your rig/system/s? It should take one day, at most, to determine what we need - at least, close enough for us to clarify Chef's dilemma.

desA
04-10-2012, 03:39 PM
Chef commented:
Oh and I wont complain if the results are of the mark but if you can possibly make a contribution it would be appreciated.

SH is never controlled directly, but indirectly. Other parameters e.g. TXV, fan speed, etc are controlled, or adjusted directly.

Once we have established the system settling point (Te,sat; Tc,sat; Td,comp; compressor power input etc) (via tweaking of a direct variable), we will be able to compute COP & track it against SH. That will then provide some level of answer to your question. No?

What will also be important is the adjustment methodology:
1. Adjust controlled variable;
2. Leave system to settle into steady-state operating point;
3. Take readings;
4. Repeat, as required.

At that point, I'll be calculating away, you betcha... :)

Chef
06-10-2012, 12:30 AM
I have a little program that pretends it is a fridge. A pretty simple one with the condenser at the back, a plate inside as the evaporator, a capillary tube and compressor. No HX.

The inputs are ambient, box contents, temp setting of the thermostat and Temp cycle (on/off temps)

Other inputs that define the fridge are more or less fixed for that model like compressor characteristics, evaporator and condenser dimensions, heat transfer coefficients, heat leakage rates etc insulation, even convection constants.

The outputs are mainly time based and position based. So with a stable and cycling fridge turning on and off the results show, for instance, box temp v. time or any system variable v. time but more importantly and what I really want is the refrigerant mass in each component.

So as the fridge cycles on and off one sees a nice drop in box temp and in the off cycle a slower rise as heat is lost. Now a snap shot every minute (or whatever) shows where the refrigerant is located and how it migrates around the system during cooldown and warm up.

By changing the thermostats temp cycle for instance from 5C down to 1C shows an increase in the number of cycles and all is as one would expect. At some point the cycle is so short that pressure equalisation cannot be achieved and so starting loads become large.

Throw in 4 litres of warm water and cycle times now extend (after a longer initial cooldown period)

Now in changing system charge, or some of the fridges components sizes and especially the cap tube the SH can be varied and the response of the whole system can be studied, BUT many sources suggest more SH increases the COP and so far all I have seen is a decrease in COP.

So if it is true that COP falls as SH rises it helps to have more confidence in the programs results, but if that assumption is wrong then I need to find out where the problem lies and fix it.

Hence my question

desA
06-10-2012, 10:04 AM
Thanks Chef for a very, very interesting review of your fridge simulator.

If I may ask, does your simulator determine an evap SH / Te,sat / Tc,sat / Tcomp,disch?

If memory serves correctly, you are something of a guru on setting up pretty realistic simulators using EES platform.

I could possibly assist on the evap heat-transfer (condenser, as well), if this would assist in modeliing the Te,sat / SH characteristics of the evap. Once these are in place, then the simulator may be able to give us even deeper insights into the expected system balance points.

I'd say that publishing of the various simulated system parameters for critique, would allow a few folks to assist towards a better understanding of what is really going on. We'd all benefit from the experience, I'm sure.

mad fridgie
06-10-2012, 09:48 PM
I would tend to look at this another way.
Imagine the rig, is steady state, -20C box temp, Te -26C sh 6K (100% useful, no more useful available)
So it must be that a part of the evap is only used superheating.
So this then brings in DesA (you chose a evap), what would be the new Te if the SH was only 1K "the net cooling load remain constant"
Ignore other system changes at this point.
I think this will prove the practical point about SH

Chef
07-10-2012, 12:14 AM
Yes it does Te, SH, Tc Tcompdisch etc plus a few more.

Input and output variables include, Uc, dTc, ambient at condenser, convection constant, condenser dimensions, Qc, dPc, SC, RMc (refrigeration mass cond) heat transfer coeff air side.


Evap variables are similar and also include velocity versus evap position, boiling scheme, Ue V. position, dPe V. position, Qe V. position, RMe V. position ......etc

Other inputs include pressure drops in suction line, valves in comp, discharge line. Line dP can either be input or calculated from system conditions and pipe dimensions and K factors for fittings.

Comp efficiency V. Te/Tc and SH

Cap tube length, diameter, coil size and number of coils, Isenthalpic or adiabatic expansion, Fanno recovery efficiency, # of tubes, K Epsilon vorticity, kinematic relaxation, . Too much output data to list here really.

System wide variables include insulation heat transfer properties, fridge size, thermal mass, box contents and thermal mass and heat transfer properties.....etc

It is a pretty long list really and not sure it will help discover a solution to the SH question.

MAD - With a box of -20C and SH=1C all available used I get Te=-21C but the Qe goes up and so I cant keep the load consatant or the same as with Te=-26
Also have to change the cap tube or it will not even allow that to happen.

Here is a fun plot of the fridge inside temperature distribution, plate is at the top of course and blue is cold.

9487

desA
07-10-2012, 06:29 AM
Very cute.

I would suspect that, in order to answer your original question/s, & check your simulator realism, you'd be better off building a test fridge & comparing variable responses... Anything else may end up being pure speculation - honestly. :)

Chef
08-10-2012, 07:32 AM
DesA - I believe this is an important topic and deserves a proper hearing.

There appear to be many engineers looking for a resolution on this topic.

Please do not keep repeating an old mantra that has been shown to be outside the scope of a discussion thread.

Also there seem to be outstanding questions for you to answer that may go along the right track to progress the answer.

chillerman2006
08-10-2012, 05:18 PM
Hi Chef

you certainly pick interesting topics, beyond me

but from my limited understanding without the calcs

the lower the superheat the higher the cop

as mass flow through the compressor increases with lower superheat

the condenser does less de-superheating, resulting in more subcooling

resulting in more cooling effect through the evaporator

gonna be a long learning curve rewiring my brain if you's now say thats not the case ?

R's chillerman

Chef
08-10-2012, 10:32 PM
Chillerman - lower SH and higher COP is what I see as well so not trying to get you to relearn.

Increased mass flow at lower SH and less desuperheating are two good points. Also there may be increases in compressor efficiency and steeper slopes on the specific entropy curves adding to the mix.

You also mention more SC also adding in there.

There may also be a reduction in pressure losses but I would have to study that more but from intial thoughts dP will fall as SH falls because dP is a function of velocity squared compared to just density.

So we have at the moment

Increase in mass flow
drop in desuperheating
increase in compressor efficiency
decrease in specific entropy
increase in SC
Decrease in pressure drops



Nice one Chillerman, may be fun to which one is more influential

Chef
09-10-2012, 06:46 AM
Here is a snip from a paper that makes it even more complex, it seems it can decrease or increase or not affect the COP.

9488

desA
09-10-2012, 07:31 AM
Chef:
DesA - I believe this is an important topic and deserves a proper hearing.
There appear to be many engineers looking for a resolution on this topic.

Agreed.


Please do not keep repeating an old mantra that has been shown to be outside the scope of a discussion thread.

I suspect that we'll - with respect - have to agree to disagree. :)


Also there seem to be outstanding questions for you to answer that may go along the right track to progress the answer.

There certainly are a lot.

What I'd like to suggest first off is that you reveal a lot more of the software platform you are developing. This can then form the substantive basis for discussion & comparison, in the absence of a robust comparative test platform.

What will also be of major interest in the 'why' of high superheat leading to increased COP is precisely how the condenser operates & how heat is extracted. In this regard, MF can contribute an immense amount of experience, based on his earlier heat-pump projects.

In other words, in raising evap SH - if it leads to 'increased' Tc,sat , will almost certainly have a COP reduction impact. If raising SH, allows a technology 'tweak' to be used such that Tc,sat is lowered, then COP rises.

In order for your simulator to reflect this, you will need to model the condenser in split form.

desA
09-10-2012, 07:36 AM
Here is a snip from a paper that makes it even more complex, it seems it can decrease or increase or not affect the COP.

9488

I suspect that this may be due to the enthalpy (h) response with pressure - more accurately as enthalpy difference response with pressure. The effect at low P (evap side), may be different to the high P (condenser side), affecting COP response - as the cycle migrates over its range of operation.

Interesting perspective...

Chef
09-10-2012, 09:29 PM
DesA - here are the outstanding questions from previous posts that would help, if we keep leap frogging and missing stuff out we will lose vital comments and information.


I would tend to look at this another way.
Imagine the rig, is steady state, -20C box temp, Te -26C sh 6K (100% useful, no more useful available)
So it must be that a part of the evap is only used superheating.
So this then brings in DesA (you chose a evap), what would be the new Te if the SH was only 1K "the net cooling load remain constant"
Ignore other system changes at this point.
I think this will prove the practical point about SH


[at]Chef : My recommendation for linking the proposed cycle operating points to reality, is that a purely academic review (dead easy to perform) will presume certain parameters to remain constant, whereas, in reality they move as the SH 'raiser/lowerer' will affect the rest of the system. No single variable remains locked as SH is altered, in reality.

Chef
09-10-2012, 09:34 PM
I suspect that this may be due to the enthalpy (h) response with pressure - more accurately as enthalpy difference response with pressure. The effect at low P (evap side), may be different to the high P (condenser side), affecting COP response - as the cycle migrates over its range of operation.

Interesting perspective...

I dont really understand what you are saying here, maybe you could rephrase it so we can understand what the article is alluding to.

mad fridgie
09-10-2012, 10:13 PM
I better clear up the evap proposal.
as an example the box requires 500w of cooling (steady state), this being achieved with an evap (simple) which is running at -26C and 6K SH.
We could say that 20% (as an example) of the surface area of the evap is being used for superheating. (in my opinion poorer heat transfer)
So if we reduce the superheat to 1K, then for example only 4% of the surface area is used for superheating. So we now an additional 16% which is wetted and has better heat transfer.
So to maintain our 500w steady state will the evap temperature in increase to reach the 500w equalibrium.
(at this point I have excluded all other parts of the system) So for example the evap temps goes from -26C to -24C (no clacs done)
Now lets look at the rest of the system, to ensure the equalibrium of 500 watts and the new Te, then the system mass flow would reduce (by means of reduction in comp swept volume) Do we know get a better efficiency?
Or if the system remain the same, would we end up with a new equalibrium point, lets say Te being -25C, but we now have a net cooling duty of 520w.
What would these figures do to the system COP.
So it all comes down to what effect SH has on an existing evap performance. Then we can determine if SH on an energy level is beneficial or detrimental.
In depth knowledge on heat transfer coefficients, is not my field. (a little knowledge may be dangerous???)

Gary
10-10-2012, 04:40 AM
I'm not sure I understand the dilemma.

If the load is held steady, then the only way the SH at the evaporator outlet can be increased is to reduce the mass refrigerant flow... thus dropping the saturation temperature and reducing the COP.

moideen
10-10-2012, 05:32 AM
According to the TECHNISOLVE program, as increase the superheat decrease the cop except r12.

Chef
10-10-2012, 05:44 AM
According to the TECHNISOLVE program, as increase the superheat decrease the cop except r12.

Thankyou moideen, a result, what kind of result do you get for R12.

I do not know TECHNISOLVE so by how much and a few conditions.

So far it seems that COP goes down as SH is increased but ther are are also some exceptions. This is exactaly what we are looking for so thanks.

Chef
10-10-2012, 06:03 AM
I better clear up the evap proposal.
as an example the box requires 500w of cooling (steady state), this being achieved with an evap (simple) which is running at -26C and 6K SH.
We could say that 20% (as an example) of the surface area of the evap is being used for superheating. (in my opinion poorer heat transfer)
So if we reduce the superheat to 1K, then for example only 4% of the surface area is used for superheating. So we now an additional 16% which is wetted and has better heat transfer.
So to maintain our 500w steady state will the evap temperature in increase to reach the 500w equalibrium.
(at this point I have excluded all other parts of the system) So for example the evap temps goes from -26C to -24C (no clacs done)
Now lets look at the rest of the system, to ensure the equalibrium of 500 watts and the new Te, then the system mass flow would reduce (by means of reduction in comp swept volume) Do we know get a better efficiency?
Or if the system remain the same, would we end up with a new equalibrium point, lets say Te being -25C, but we now have a net cooling duty of 520w.
What would these figures do to the system COP.
So it all comes down to what effect SH has on an existing evap performance. Then we can determine if SH on an energy level is beneficial or detrimental.
In depth knowledge on heat transfer coefficients, is not my field. (a little knowledge may be dangerous???)


Very clever MAD but having a variable speed compressor does not really fit the original specs for the problem.

OK it was not specifically excluded but it makes it more complex and how are we going to get peer results from other contemporary programs, non of them have variable speed compressors as the input.

I dont know if that can be included - but if it does indeed show a COP increase it would be interesting.

Have you got any more results that show this is possible?

Chef
10-10-2012, 06:49 AM
What I'd like to suggest first off is that you reveal a lot more of the software platform you are developing. This can then form the substantive basis for discussion & comparison, in the absence of a robust comparative test platform.

If you are conversent with macro, finite element methods, finite difference time slices and Newton Raphson derivatives and second order approxamations for N+1 setups it may help.
Eulerian flows in tetrahedral elements would be even beter.

The program is written to discover where the refrigerant mass is located in a cycle and how it moves plus to show efficiency with varying charge, cap tube dimensions etc.

My results so far show COP always decreases with SH increasing which is not in line with some published data and so this is the question. Has anyone got any evidence COP increases with SH.

What will also be of major interest in the 'why' of high superheat leading to increased COP is precisely how the condenser operates & how heat is extracted. In this regard, MF can contribute an immense amount of experience, based on his earlier heat-pump projects.

You have lost me there, the bit about how the condenser operates? Please advise on this one.

In other words, in raising evap SH - if it leads to 'increased' Tc,sat , will almost certainly have a COP reduction impact. If raising SH, allows a technology 'tweak' to be used such that Tc,sat is lowered, then COP rises.

This is a 4 component fridge in its simplest form, no technology tweaks allowed, but if you are saying raising the SH can raise the COP - as you state above - what is the basis for this and some reference would be nice.

In order for your simulator to reflect this, you will need to model the condenser in split form.

What is a split form condenser???????????


Seems you have a way to raise COP with increased SH - sounds interesting.

desA
10-10-2012, 06:55 AM
Chef:
DesA - here are the outstanding questions from previous posts that would help, if we keep leap frogging and missing stuff out we will lose vital comments and information.

??? :eek: :eek:

moideen
10-10-2012, 07:06 AM
Thankyou moideen, a result, what kind of result do you get for R12.

I do not know TECHNISOLVE so by how much and a few conditions.

So far it seems that COP goes down as SH is increased but ther are are also some exceptions. This is exactaly what we are looking for so thanks.

dear chef,

find the attachment.in the technisolve program using r12 the cop increase slightly as increase the SH.

desA
10-10-2012, 08:26 AM
DesA:
What I'd like to suggest first off is that you reveal a lot more of the software platform you are developing. This can then form the substantive basis for discussion & comparison, in the absence of a robust comparative test platform.


Chef:
If you are conversent with macro, finite element methods, finite difference time slices and Newton Raphson derivatives and second order approxamations for N+1 setups it may help.
Eulerian flows in tetrahedral elements would be even beter.

One sincerely doubts that your simulation would require the use of Eulerian flows modeled on a tetrahedral mesh. FE/FD methods are well known - as is Newton Raphson convergence techniques. (Not always the most stable, mind you - fairly dated by now). That is, unless you're attempting to accurately model two phase flow evaporation/condensation regimes. You may want to save some level of energy, by reading up varous legacy books on the topic (e.g Collier/Thome & Thomes' later works).

I suspect that you're attempting to show off a tad... gotcha... :)

Back onto topic, then, dear fellow.


Chef:
The program is written to discover where the refrigerant mass is located in a cycle and how it moves plus to show efficiency with varying charge, cap tube dimensions etc.

Useful. Try experimenting with RefSim (Technisolve). May prove to be a useful back-check. This software will balance the system temps, as various process parameters are adjusted. Even allows real compressors - performance tables & all, to be used. There are ways to tweak the parameters further, in order to study what you are looking for. Allows TXVs.

mad fridgie
10-10-2012, 09:48 AM
Handbags at dawn????????

But back on the subject.

We have to be careful that we are using the superheat for. As Des has stated increased suction superheat can improve the benefits for a heat pump application, where the heat side becomes the priority, but my understanding of the original question was related to the cooling side, but in either case w change on aspect of the cycle then it follows that all other areas change as well. So it still my belief it may not be the software that wrong, but how any data is in putted.
Also we have to consider why require superheat (normally to stop chance of liquid flood back and/or compressor oil dilution, so should the superheat that is required, be useful "direct", non useful (energy directly picked up from out side the refrigerate envelope, or just a pressure drop.

mikeref
10-10-2012, 09:55 AM
Hi Chef

you certainly pick interesting topics, beyond me

but from my limited understanding without the calcs

the lower the superheat the higher the cop

as mass flow through the compressor increases with lower superheat

the condenser does less de-superheating, resulting in more subcooling

resulting in more cooling effect through the evaporator

gonna be a long learning curve rewiring my brain if you's now say thats not the case ?

R's chillerman
Glad you jumped in before me C.M.

Read the comments. "Chef" continues to ask: is there proof that increased (useful or not) superheat increases C.O.P? I don't qualify for this level of discussion however, any amount of superheat, say 15K on a cooling cycle cannot increase C.O.P. as there is less duty performed by the volume of saturated refrigerant in the evaporator to start with. Increase S.H = less work done and longer running cycles. Negates C.O.P. During any duty cycle.

Maybe you are thinking, if the increased discharge temperature is raised with higher superheat, then the transfer of heat through the condenser is maximised, increasing performance. (Oversized condenser)

If one were to use increased superheat in a reverse cycle situation, then my answer is yes.

Just my humble response guys.

desA
10-10-2012, 09:58 AM
Handbags at dawn????????

Slide-rules... ;)


MF:
So it still my belief it may not be the software that wrong, but how any data is in putted.

Agreed.

mad fridgie
10-10-2012, 10:43 AM
Glad you jumped in before me C.M.

Read the comments. "Chef" continues to ask: is there proof that increased (useful or not) superheat increases C.O.P? I don't qualify for this level of discussion however, any amount of superheat, say 15K on a cooling cycle cannot increase C.O.P. as there is less duty performed by the volume of saturated refrigerant in the evaporator to start with. Increase S.H = less work done and longer running cycles. Negates C.O.P. During any duty cycle.

Maybe you are thinking, if the increased discharge temperature is raised with higher superheat, then the transfer of heat through the condenser is maximised, increasing performance. (Oversized condenser)

If one were to use increased superheat in a reverse cycle situation, then my answer is yes.

Just my humble response guys.

The increased discharge temp will increase the amount of heat rejected on the de-superheating part of the cond, but when phase change is occurring "condensing in this case" greater energy is transferred, but saying that at what point is increase temp better than phase change.

Get carried away, makes us all think!!!!!!!!!

desA
10-10-2012, 12:20 PM
MF:
at what point is increase temp better than phase change.

This will be a function of the type of heat-exchanger surface. For instance, a plate unit will respond differently to a tube-in-tube, or shell-&-tube unit.

Gary
10-10-2012, 08:47 PM
Are we talking about evaporator outlet SH?... or compressor inlet SH?

It makes a big difference.

mad fridgie
10-10-2012, 09:18 PM
This will be a function of the type of heat-exchanger surface. For instance, a plate unit will respond differently to a tube-in-tube, or shell-&-tube unit.
Agree, when comes down to the specific of heat exchangers, more knowledge is required than what i have.
This why i though you could do a small simulation on an evap, and see how the SH effects the performance.

mad fridgie
10-10-2012, 09:24 PM
Are we talking about evaporator outlet SH?... or compressor inlet SH?

It makes a big difference.

Agree.

I believe, chef is talking about evap superheat, but increasing evap SH will increase discharge SH, and is there then a benefit to the system as a whole.
If we have a fixed simple system any change within the system must also effect to some degree the rest of the system.
One thing that is also confusing, if you look at the nominal performance of a compressor (not design detail), the rating always shows high suction superheat, 100% useful, which in many cases is outside the working envelope and there is no practical way of getting this SH as useful.

Gary
10-10-2012, 10:13 PM
As I see it, if we raise SH by lowering the saturation temp (reduced mass flow), there will be a reduction in the COP. The discharge heat is just sensible heat.

If there is increased load, there will be a higher saturation temp, higher SH... and higher COP.

If the load is held stable and the evap outlet SH is held stable, but we add heat to the suction line... then the added heat is a bonus and the COP will rise.

The answer to the question is... maybe.

Gary
10-10-2012, 10:17 PM
Outside the working envelope? In discussing COP, the entire low side is the working envelope.

Chef
11-10-2012, 01:10 AM
One sincerely doubts that your simulation would require the use of Eulerian flows modeled on a tetrahedral mesh. FE/FD methods are well known - as is Newton Raphson convergence techniques. (Not always the most stable, mind you - fairly dated by now). That is, unless you're attempting to accurately model two phase flow evaporation/condensation regimes. You may want to save some level of energy, by reading up varous legacy books on the topic (e.g Collier/Thome & Thomes' later works).

You asked me to reveal a lot more about the software platform - so I did.

In a coiled capillary tube one cannot use cubic meshes (they dont go around bends) and so tetrahedral is the best choice. For each mesh one solves the conservation of mass plus the momentum equations. Now add to that the energy equation and Oh gosh, bless my soul, it is called the Eulerian formulation.

And strangely enough as Eulerian maths involves derivatives the Newton Raphson is the recommended first approach to a solution. And this is exactly what I do in my cap tube and evaporator.

Come on DesA - any first year student would know that. That would of course be in an engineering discipline and not psychotic behaviour

So if you ask a question - read and respect the answer.

So now that you are done throwing your toys out of the pram maybe, as you say old boy, we can back to the task at hand.

Chef
11-10-2012, 01:24 AM
If the load is held stable and the evap outlet SH is held stable, but we add heat to the suction line... then the added heat is a bonus and the COP will rise.

The answer to the question is... maybe.

So if the added heat to the suction line can be described as SH you saying the COP will rise. Where do you get the heat from.

desA
11-10-2012, 07:41 AM
desA
One sincerely doubts that your simulation would require the use of Eulerian flows modeled on a tetrahedral mesh. FE/FD methods are well known - as is Newton Raphson convergence techniques. (Not always the most stable, mind you - fairly dated by now). That is, unless you're attempting to accurately model two phase flow evaporation/condensation regimes. You may want to save some level of energy, by reading up varous legacy books on the topic (e.g Collier/Thome & Thomes' later works).


Chef
You asked me to reveal a lot more about the software platform - so I did.

In a coiled capillary tube one cannot use cubic meshes (they dont go around bends) and so tetrahedral is the best choice. For each mesh one solves the conservation of mass plus the momentum equations. Now add to that the energy equation and Oh gosh, bless my soul, it is called the Eulerian formulation.

And strangely enough as Eulerian maths involves derivatives the Newton Raphson is the recommended first approach to a solution. And this is exactly what I do in my cap tube and evaporator.

Come on DesA - any first year student would know that. That would of course be in an engineering discipline and not psychotic behaviour

So if you ask a question - read and respect the answer.

So now that you are done throwing your toys out of the pram maybe, as you say old boy, we can back to the task at hand.

Dear Chef,

Many cap tube simulators will tend to use a 1D model. Refer here to the Indian lecture notes you mentioned some years ago. With the length-to-diameter ratio for a cap tube being so large, the radial/centrifical effects would/could generally be neglected. The numeric scheme for such 1D model could use a number of discretisation approaches - some more stable than others - 1st order, 2nd order, etc. A description of one approach is offered on the Technisolve website - that program offers 3 different solutions to the cap tube length. Without being able to dissect your methodology, I would have to take your word that you have a new, novel approach to solving the matter.

The reference to Eulerian flows & tetrahedral meshes would, in most engineering academic circles, infer the use of a CFD approach to problem solution (FV/FEM/FD...). Given the current lack of theoretical knowledge in the two-phase area, using such an elaborate approach would have been both overkill, & in all likelihood, very inaccurate. Still a lot to be learned in this area. Packages like Elmer may be of use to you here. The name Euler has been attached to many, many areas of maths/science.

I will stand my ground academically against you, as I've been in the simulation & heat-transfer arena all my working career. I am prepared to argue all the way through defence of the Navier Stokes & Energy equation sets, if needs be. This is my stock-in-trade, if you will.

My preferred approach, however, is to engage in a healthy debate, via this forum & the wonderfully helpful contributors & colleagues. We can all stand to learn a huge amount from each other, even if we don't each, individually have each & every answer. The collective approach is both fun & informative - we all learn a huge amount. In many aspects of this business, much can be said about it being part art-form, experience-based, & academic.

On a public-domain web-forum, the 'shoot the messenger' approach, generally leads to tears & discomfort for the other forum members reading the 'kill-shots' & 'return-shots'. Perhaps you would, in future, be so kind as to take this kind of attack offline via PM? I'd certainly be most happy to discuss further with you off-line.

Should we agree to disagree & get back to the topic at hand? I'll bet that we are both spoiling the thread by our bison head-butting approach.

Take care,
desA

desA
11-10-2012, 07:52 AM
Gary:
As I see it, if we raise SH by lowering the saturation temp (reduced mass flow), there will be a reduction in the COP. The discharge heat is just sensible heat.

Makes sense - if Tc,sat remains substantially the same. Does it?


Gary:
If there is increased load, there will be a higher saturation temp, higher SH... and higher COP.

Please expand on this. How is the load increased, for instance? Fan speed increase? More meat in the freezer?


Gary:
If the load is held stable and the evap outlet SH is held stable, but we add heat to the suction line... then the added heat is a bonus and the COP will rise.

Can we show this, in practice? What happens to the compressor operation as the suction line flow is heated - reduced density - higher volumetric flowrate - higher flow velocity - higher line pressure drop...?

mad fridgie
11-10-2012, 11:05 AM
Outside the working envelope? In discussing COP, the entire low side is the working envelope.
No it is not, all compressors have an operating range which includes maximum suction superheat temps, so adding superheat useful/non useful has its practical limits. Even though there is quite a bit of theory, there still has to be some level of practical application

desA
11-10-2012, 01:46 PM
RefSim feedback

This process simulator allows a designer to tweak various system parameters & see how others change. I've found it very useful over the years.

RefSim package relies on the evap program DXC, which fixes SH at a normative value of 7.5K. It cannot be adjusted further.

There is no way to adjust suction line heat addition, or removal. I was hoping to coerce RefSim via this route. Didn't work.

RefSim can export to a Mollier chart program, with a snapshot of the last system balance. In Mollier, the SH can be adjusted, See an earlier reference in post #77. The problem with this is that Mollier takes, as given, the values of Te,sat / Tc,sat / SH / SC - it does not re-balance via RefSim to compute the new system balance point - if SH varied within Mollier itself.

Basically, I expect Te,sat to lower as SH is raised off a fixed operating condition. For the condenser, I expect the increased SH to raise Tcomp,disch. For single condenser, this would then take up a little more of the condenser surface area with de-superheating, displacing some condensing area. With reduced condensing area, Tc,sat would also rise a little to compensate. This assumes that the reduction in refrigerant mass-flow around the system (with increased SH achieved by throttling TXV a tad), does not auto-compensate in the condenser, so re-balancing available condensing area.

Haven't thought through the compressor efficiency implications yet.

So, if my logic is correct, with unchanged compressor efficiency, a rise in SH leads to Te,sat reduction / Tc,sat rise / COP reduction. (Please holler folks if I'm missed something obvious).

COP reduction can be compensated for in the condenser by oversizing the surface area, forcing a reduction in Tc,sat. Alternatively, use a de-superheater/condenser combination.

These logic twists, assumptions & guessing, is why I honestly advocate performing some runs on a real system.

mad fridgie
11-10-2012, 09:03 PM
I do not know of a design program that does circular reference calculations.

So your example is good and what you would expect, where manual determination of steady state points is the only method of calculation.

The problem with testing is that the rig has to be big enough that low grade test equipment can see the difference or the requirement for high end test equipment/chamber (calorific meters and so on)

Chef
11-10-2012, 11:12 PM
DesA wrote
Basically, I expect Te,sat to lower as SH is raised off a fixed operating condition. For the condenser, I expect the increased SH to raise Tcomp,disch. For single condenser, this would then take up a little more of the condenser surface area with de-superheating, displacing some condensing area. With reduced condensing area, Tc,sat would also rise a little to compensate. This assumes that the reduction in refrigerant mass-flow around the system (with increased SH achieved by throttling TXV a tad), does not auto-compensate in the condenser, so re-balancing available condensing area.

At the moment the general concensus of views seem to be along these lines. But it is probably the most promising area to try to show if there are (strange) conditions where SH can increase COP. If for instance the condenser did compensate more in improving COP than the evaporator/suction reduces the COP for a given rise in SH.

The design of such a condensor may be implausible but for the discussion imagine a condensor operating with a very large dT - say 40C and now the SH is increased, Te drops and mass flow drops, now Tc falls (maybe if the reduction in mass flow has a greater effect than increase in disch temp) and more than compensates for the SH increase. It also sounds like it would be unstable but a line worth thinking about.

Also your synopsis of the solution methods for cap tubes is noted and the two phase flow part is of course key and it seems the method I have adopted is working just fine. However as stimulating this topic is may be it should be addressed elsewhere.

desA
12-10-2012, 05:11 AM
In the condenser, I believe that the main variables to investigate for each gas type would be the latent heat & specific heat. The relative values of these could/would play a role in how the condenser surface reacts to changing Tcomp,d (via raised SH) & changing refrigerant mass flow.

In other words specific heat would affect the de-superheating portion & latent heat the condensing portion. For now I've neglected the sub-cooling impact.

As mentioned earlier, the type of condenser e.g. plate, shell-&-tube, tube-in-tube - will also react slightly differently to the de-superheating/latent heat loads.

The thing to watch when adding condenser surface is not to force a 'temperature pinch' where the secondary cooling fluid (water, air etc) temperature somewhere inside the condenser reaches the same temperature as the refrigerant Te,sat. At that point, the condenser dynamics change & Tc,sat will literally jump upwards - losing effective high-side control (very, very non-linear). I've seen this with compact plate condensers.

mikeref
12-10-2012, 09:58 AM
The increased discharge temp will increase the amount of heat rejected on the de-superheating part of the cond, but when phase change is occurring "condensing in this case" greater energy is transferred, but saying that at what point is increase temp better than phase change.

Get carried away, makes us all think!!!!!!!!!

There is a limit to how high discharge temperature can go before gas, oil and compressor break down. That much is obvious so enter liquid injection. Looking at volumetric efficiency and preserving the operating system with the trade off being less work done by the evaporator, but regulated injection is able to make better use of compressor generated heat than what could be obtained from the evaporator..:cool:

Counter the loss of refrigerant going to the evaporator with an increase in refrigerant quantity. More refrigerant circulating means more Phase change.
Regulate the head pressure by bleeding off high pressure vapour,(from a receiver) into the suction line without raising suction, or lowering head pressure significantly..
Lower head, lower current draw, increased superheat in suction line, higher C.O.P...
Everyone is happy. ;) Please, pick my theory to pieces but don't ask me to do the maths. :rolleyes:

desA
12-10-2012, 10:24 AM
Mikeref:
There is a limit to how high discharge temperature can go before gas, oil and compressor break down. That much is obvious so enter liquid injection.

Liquid injection into compressor suction line, or compressor where liquid injected internally?

desA
12-10-2012, 04:49 PM
A few useful simulation links are listed below. If anyone has links to other useful refrigeration simulators, where the Te,sat / Tc,sat variables are derived, as various process parameters are manipulated, these would be most useful.

RefSim explained
http://www.coolit.co.za/refsim/index.htm
http://www.coolit.co.za/refsim/articles/cyclesim/refsim.htm

A Maple approach
http://www.engr.mun.ca/~yuri/Courses/MechanicalSystems/Refrigeration.pdf

Peter_1
12-10-2012, 06:53 PM
[at]Peter:
When you say 'program this all', do you mean as in generate a computer simulation?


He could 'translate' all these values and plot the real-life cycle on the log p/h graph

Peter_1
12-10-2012, 06:58 PM
[at
If so, than this may help us to establish the movement of variables with our modification of SH. This could possibly also then assist to settle the COP discussion. Not sure mind you, unless the author manages to correctly model compressor efficiency changes, evap & condensor changes, as the cycle changes.

Would be nice to see in action.

I had also in mind trying to locate a dynamic simulator for an hvac system. This may, however, also suffer from the same inaccuracies.

This is where a simple circuit will be so useful. (My heat-pump lab is currently in moth-balls in Laos. Would have run some hard trials & sorted most of this out by now... :) )

Peter, or MF, are you perhaps able to run some trials with your rig/system/s? It should take one day, at most, to determine what we need - at least, close enough for us to clarify Chef's dilemma.

The plan was also measuring current and voltage and then calculating what the actual refrigeration power was (via the Bitzer software) and compared this with actual electrical power.
We had a polynomial for the Bitzer compressor as well.
Problem is I have the next 3 months no free minute at all. Even now, I 'm typing this while making a quote.
We take all the work we can for the moment because I'm expecting the same problem s my colleagues have: lack of jobs.

desA
12-10-2012, 08:16 PM
[at]Peter,

Thanks so much for your input. Your proposed system sounds incredible. When you have this system up & running, I'd love to collaborate with you when everything is stable. (Would be an opportunity to visit Brugge :) )

To work first - not easy times for everyone at the moment.