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Hi MF
Please excuse my late entry - I have not long returned from overseas travel.
Gary is definitely on track. Perhaps an adjustable ejector is required for the dynamics of the system for prototyping. This would allow for faster system stable state and varying loads. It would enable the sweet spot to be found. I know this will complicate the system and introduce a dual control loop. Understanding dynamic fluid flow and what is happening with all three dimensions (mechanically) and liquid/vapor phases. Viktor Schauberger's work in Living Energies by Callum Coats covers fluid flow dynamics with the effects of geometry and temperature in simple terms. Roy dossat also covers how fluid flows and shows calculations.
The concept as I understand it is achieving energy savings by increasing the potential energy transfer through the evap by precooling the refrigerant which reduces flash gas at start of evap. So thinking about it, the fluid in the evap is moving with a certain type of flow, we know the turbulence is reduced at the start, liquid would be swirling around the out side of the tube with vapor forming in the center of the tube more towards the end. The return bends in evap would reintroduce a more turbulent flow.
We could use a calculation of what might be happening at the ejector where the two gasses meet. We have 31% mass flow through the ejector with the remainder from the evap. This leaves the velocity as a variable and the quality of the refrigerant mixes. What are your thoughts on this - have I gone off an a tangent?
The evap issue, is to be considered but later, (but in short, the reduction in flash gas can detrate the evap or it can also increase the evap duty. but this is a story on its own)
Try to to think that the system build up to its performance.
Keeping it simple, the prime objective is to increase the inlet pressure to the compressor (higher than the evap inlet)
This in its self would increase the comp mass flow.
It has been presumed that I have choosen the correct ejector (one was adjustable), but there are many different tpye styles and designs. So even though garys suggestions are extermly valid (as one would expect), I believe that i fundementally chosen the wrong type, but the only ones I could find, that would seem to suit my purpose. But with help forms other (thanks to you all) the supply of others may be easier, or to the point that a design for manufacture maybe another option.
Like most things is lack of riskable resources that has stopped the R&D process.
(if I was looking at many megawatt systems then there are standard units avaliable, then would best suit the process, normally used in steam systems)
Targeting the compressor inlet pressure versus evap inlet pressure adds all of the evap variables to confuse the issue. Targeting compressor inlet pressure versus evap outlet pressure narrows the focus down to the ejector performance, making ejector troubleshooting/adjustment easier.
What factors would cause the system to build up to it's performance?
In reading through various papers, it is my (possibly flawed) understanding that high motive/low suction is more stable, but less productive... while low motive/high suction is more productive, but less stable. Seems there needs to be some form of flow proportion control for optimum operation.
I'm thinking this might be achieved by setting the AEV to a fixed optimum value and varying the evap flow for fluctuating conditions. This would require the EEV to sense and control evap flow, thus flow proportions, according to ejector output.
Hmmm... in addition to everything else, it seems there is a maximum discharge pressure, which when exceeded causes the system to become unstable, resulting in backflow through the ejector.
The issues with the ejector are many fold, as you have noted.
It would seem that there are possible designs, that may not have an optium compared to others, but are more consistent over a range? which would be required to a refrigeration system. If the widget could be built is has to be easy to install and commssion. I am practical and was looking at achieving a % of peak performance, the 80-20 rule.
It seems that you have a feel for the system, in your opinion, without worring about the details "is the fundemental concept correct"
thanks for your input
Mad
I suspect that the system may be flipping between multiple steady, or quasi-steady operational points.
If an additional 'controlled' motive gas were to be injected into the ejector e.g. via hot-gas bypass, until system begins to stabilise in the required flow directions - then this HGB is slowly backed off until system stabilises as desired.
A few thoughts for you to ponder MF.
The flipping of the system is i think a dead cert at the moment.
I do have a number of by-passes, but not the HGB.
This would be stealing some mass flow from condensor outlet, but should not be disregarded as a method of achieving some of of stabilty.
I am sure that the concept has merit, but need to ensure that we have a theoretical positive steady result, then look what a happens when we move away from design.
When this proved beyound a doubt as theorectical possibilty, then a descion has to be made about employing a specailist(s) for the design of individual components, which a can be applied to a test rig.
I believe that the present rigg has a possiblity of proving concept, but I think that the proof point is a very critical point (which could take for ever to find) "I am feeling lucky punk" NO
I'm wondering if there is sufficient liquid pressure to feed the evap coil?
This had to be considered, when designing the rigg, I chose a large EEV!
I am able to control the superheat right down to 1.5 bar difference, then the evap does become starved.
How ever heat transfer co-efficients may be effected. (how ever I did attempt to simulate this during standard tests)
Thanks again gary for your thoughts
combining your thoughts here gents, is it possible to run the discharge pipe & seperator outlet through a small heat exchanger, ensuring the motive force is just vapour & maybe then allowing for a lower superheat evap outlet ???
Hi Gary
liquid injection agreed, but the minimal discharge superheat required prior to condensor to ensure no liquid to fwidget, surely makes sense, the problem I have is I can not get my head around the quasistatic process required to fathom equilibrium, best left to the real brains of the industry, as not many will, unless you can do us the honour of explaining the process in detail with all your calcs,
R's CM
Calcs? You are confusing me with someone who does math. :confused:
Ok, mate we shall have to hope Chef, DesA or Peter_1 does us the honour of explaining it one day & you can add that to the other 99.9999% of the industry you already have tucked away, thanks anyway mate
Hi mf
Just thinking outside the square - is it possible to calculate roughly when it is not working? with graph gradients or measurements verses time. Say on start up it could be that the sweet spot is not found till after x seconds and then check for additional times. There may be some common parameter values at start up which equal definite failure of experiment. You could be looking at a resonant response for success.
All tests were completed on a 20 min cycle, with only one change per made at any single time.
So about 12 tests a day.
Plus short term tests when a major change is made. (some times you can see when things are just wrong)
As with any refrig system, time is taken to reach equilibrium, even though this section was ignored for the performance results, it was all ways monitored.
Good thoughts
Tesla
CM; your point also has merit, but as Gary says we are introducing extra energy, what you should of considered is using the liquid line (how ever this has its own issues "less flash gas"
Individual parts can be made to work with satisfaction, but refrigeration is not a group of parts, it is a system!
Post removed - as again have miss-understood...not a designer...just mr nuts & bolts !
:)