Slide-rules... ;)Quote:
Handbags at dawn????????
Agreed.Quote:
MF:
So it still my belief it may not be the software that wrong, but how any data is in putted.
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Slide-rules... ;)Quote:
Handbags at dawn????????
Agreed.Quote:
MF:
So it still my belief it may not be the software that wrong, but how any data is in putted.
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!!!!!!!!!
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.Quote:
MF:
at what point is increase temp better than phase change.
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.
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.
Outside the working envelope? In discussing COP, the entire low side is the working envelope.
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.
Quote:
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).
Dear Chef,Quote:
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.
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
Makes sense - if Tc,sat remains substantially the same. Does it?Quote:
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.
Please expand on this. How is the load increased, for instance? Fan speed increase? More meat in the freezer?Quote:
Gary:
If there is increased load, there will be a higher saturation temp, higher SH... and higher COP.
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...?Quote:
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.
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
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.
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)
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.
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.
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:
Liquid injection into compressor suction line, or compressor where liquid injected internally?Quote:
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.
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/artic...sim/refsim.htm
A Maple approach
http://www.engr.mun.ca/~yuri/Courses...rigeration.pdf
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.
[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.