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Interesting article from reliable respected source, don't believe it 100% though, especially from it's description of compressor unloaded for extended periods.
If fully loaded, yes believe.
Vi port not in play when compressor unloaded say 80 to 90% or less on lower Vi ratio on all major brands, except S80 Sabroe.
The booster compressor mentioned possibly close to 100% loaded, as slide stop to far forward not allowing main slide to travel enough.
https://irc.wisc.edu/export.php?ID=423
Too many numbers :)
I like the explanation of Vi in this one much better :)
http://www.bowmanrefrigeration.com/u...compressed.pdf
I've only worked with fixed Vi compressors, except one Sabroe compressor.
Don't remember what model it was, maybe a 128 if that is a Sabroe number :).
it was installed next to a Howden WRV 255 to handle the remaining load of a blast freezer once the 255 had unloaded and stopped.
They would run side by side, the WRV conrolled by inhouse PLC from my previous employer, the Sab on the Unisab II.
As long as they were side by side the Sab worked fine, but when the 255 stopped and there was a slight increase in suction pressure (0.1-0.2 bar) the unisab would go mental and start adjusting the Vi and slidevalve seemingly without any purpose... the slidevalve indicator would show that the compressor was decreasing capacity, but the amps went up then the amps would drop and the slidevalve would go back up rinse and repeat.
I like to think of myself as well versed with the Unisab II, but I couldn't figure it out, and neither could the guys from Johnson Controls, so I take that as a win.
In the end, after three years I think, they chucked out the Sab unit and put in another WRV 255 unit :)
Variable VI is only useful in some situations. If you run at the same parameters all the time it is of no real advantage.
If you have a compressor which can you run in both low stage and high stage modes it is valuable.
we have one of the two or three guys that has actually gotten a PHD in engineering degree or what it's called, in Refrigeration in Norway working in our company... he's like an encyclopedia when it comes to refrigeration... an a generall frikking good guy all around...Quote:
Originally Posted by NH3LVR
He has tons of work to do, but if you are waiting far the vacuum to come down, just call him up and you'll be on the phone for 2 hours just shooting the ****, because you get sidetracked easily and have stuff to talk about :))
And he says that when it comes to variable Vi "depends on who is holding the eye and using it to look"
I get it as in "it's all in the eye of the beholder" and all that
our systems are either -40C or -8C, so we run with fixed Vi, because the systems quickly settle into proper running conditions...
And we don't need the Unisab II to go mental and adjusting everything up and down and sideways and causing the compressor to go haywire like YEah, I'm decreasing the load, but the amps ar going up... but watch this, I'm going to increase the slide and now the amps are going down, but just to freak you out I am going to move the Vi. and that is going to make the Amps just go POOOF YEAY High!, and then to counteract that, I will decrease the slide!
Why am I doing this? nobody knows, but it's fun!
I just love how he would rather say "welp, depends on who is holding the eye and using it to look" instead of saying "It's all in the eye of the beholder"
I was on the phone with the guy that had developet the program for the Unisab RT once, he had retired, But I got his number of a Johnson control guy that couldnt help me...Man that guy loved to talk about programming... and it was a delight to listen to him talking.
He was like "yeah I remember the algorithm i put into the Unisab Rt, it was very similar to what we used in the F-16 to make it fly, actually we developed the F-16 flight algorithm first and then we used the same software to develop the Unisab RT" then he stopped and I asked "uuhm did you say F-16?" and he was like "The f-16 was along time ago, you probably heard wrong, it has nothing to do with the unisab RT" and it really doesn't...
So he continued "and a lot of the functions we put into the F-16 to make it fly were also used in the Unisab RT, not to make it fly, but to make it calculate the pressures converted to temperatures etc etc...
so that is a highlight in my career, I have spoken to the guy that developed the software for the first Unisab, who also worked on developing the system that made the F-16 fly!
I was trying to make a Unisab RT talk to a Unisab II.
The Unisab II had a default baud rate of 9600 and the Unisab RT had a maximum Baud rate of 4800
So all I had to do was to go into the Unisab II and set the baud rate down to 4800... and then it worked like a peach.
But the old guy kept me on the phone troubleshooting for 2 hours just to keep me talking...
What he didn't know was that I LOVED EVERY SECOND OF IT! :)
Thanks Tycho for interesting story.
To me it’s a sales pitch thing, but know some reasoning behind it.
Vi is controlled by position of port under rotors in slide itself.
Axial port is in discharge end housing.
If machine unloaded only fixed axial port in play.
That’s why I think article is not 100% true.
Tod I’m sure would be very interesting to talk to as well, who knows he might answer.
I think it depends of compressor manufacturer. For example. I read Frick claims that their compressors control Vi until capacity 40-50%. However, any claim should be checked. For example. You set manually capacity to 60% and try manually to change Vi from 2.2 to 5 and back. If you have side port, it should be closed. Monitor load of the motor. I have done some tests and loads didn't changed. It means that at 60% capacity variable Vi doesn't affect energy use(efficiency) of this compressor.
Segei,Quote:
Originally Posted by Segei
I would have thought with above method compressor would do more work if slide at 60% & Vi slide stop moved towards higher Vi.
It would be reducing amount of bypassed gas.
Grasso do it this way for LT compressor so Vi port is in correct position sooner, then slide stop is moved from low Vi towards slide valve 9complicated if using your own PLC programme.
Thought Tycho would enjoy this.
https://youtu.be/DiezpAq-XWM
oh my... :DQuote:
Originally Posted by RANGER1
shame they didn't show the part where they shaved off a piece of the teflon ring on the piston :)
I think that slide stop works only if it touches the slide valve. They move together form low Vi to high Vi and back. However, range of slide valve movement is greater than range of slide stop movement. As soon as they disengage slide stop become useless. It will happen at certain low capacity.
I like this paper, as it agrees with what I think:) I think!
https://pdfs.semanticscholar.org/2aa2/55f0838e69b91281a8b99339e3f7b0e87378.pdf
Several years ago I replaced a Mycom Screw. The original was fixed VI. The replacement was a manually adjustable VI. That save on inventory I guess. Unfortunately neither the manual or Mycom could tell me which direction to turn the adjustment to set to the higher VI.
NH3LVR,Quote:
Originally Posted by NH3LVR
Tycho post with this link, would have helped, but a bit late now I guess.
Pages 9,10 & 11
http://www.bowmanrefrigeration.com/u...compressed.pdf
This is good article but it was written 30+ years ago. Probably geometry of the screw compressor has changed since then. This article suggests that to unload compressor at Vi 2.0 this Vi should increase to 5.0 with following unloading of slide valve. In our days unloading will be initiated without increase of Vi.Quote:
Originally Posted by RANGER1
Segei,Quote:
Originally Posted by Segei
Re-read & not sure on that, as I took it as showing 2.0 to 5.0 Vi as an example.
Rotor profile may have changed, but that does not change Vi theory in my opinion.
The only real thing that has changed in last 30 years is VSD drive affordability & controls.
Interestingly that interest is dropping off, as clients want 2-3 years payback, which is not always achievable.
They are great for keeping at full load, perfect suction pressure control, less wear & tear, depending cannot be justified unfortunately.
Plant room flexibility not what it used to be as well with different size machines, zero redundancy.
Hard to work out what is best sometimes
Frick really rely on Vi changing a lot for minute to minute. If Vi to high amps go through the roof.
Mycom "J" like to do it real slow,
Grasso LT unloads slide stop first
Sab 80 alters through mechanical means only with differential pressure in discharge port(sadly out of production).
Don't know if this is helpful to the conversation, but here is some York software that shows the Vi and cap slide in a Sab193 - 283 unit. where you can interact ant manipulate them both to se how they work
https://drive.google.com/open?id=1z2...SABpOfx1xeQlEk
It also shows the function and flow in the entire unit, and the animated function of various valves.
I have found it quite helpful in educating apprentices.
It's an .EXE file. so to download it, you will have to take my word for it not being malignant.
Do we really need to change Vi very fast?Quote:
Originally Posted by RANGER1
I think that if refrigeration plant designed and operate properly Vi should be changed just from time to time.
Vi depends of suction pressure and discharge pressure. Suction pressure usually constant, discharge pressure depends of ambient conditions and these conditions don't change fast. Let's have a look at what should be change to improve plant operation and efficiency.
1. Typically low pressure vessels using solenoids to fill these vessels up. HEV wide open and when solenoid open a lot of flash gas create huge load. Simple valve with stepper will keep required level without load fluctuation.
2. Majority of the plants have steady refrigeration loads. Why do compressors continuously load and unload? I mentioned about the vessels. Holding freezer has 4 evaporators. Load is very steady. When temperature is satisfied all 4 evaporators off. Why all 4 off? Don't we have load any more? Switch one evaporator and keep run 3 of them.
3. Typically evaporators have BPR for the defrosting. At the end of the defrosting a lot of gas blow by the coil and this create huge parasitic load. Drain float will solve the issue.
4. Control of many screw compressors continue to send signals for loading when compressor is 100% loaded. Do they want to achieve 110%? Millions loading signals. Why?
These are just few of examples. Probably, you can tell many more.
Majority of fluctuation of refrigeration loads and changing Vi based on poor plant design and operation.
Thanks Tycho,Quote:
Originally Posted by Tycho
Good tool for learning.
It also proves my point in first post about average of 60% average load & incorrect Vi secondary slide valve.
Although secondary slide stop was in incorrect position, that does not mean it wasted power in that case.
Of course it should be in correct position & rectified, as if loaded up more then it would effect it more & more if set at higher than required Vi.
Segei,Quote:
Originally Posted by Segei
Agree in general, but not all plants perfect that’s for sure.
Some process plants load can change dramatically like abattoirs, blast freezing etc.
Not many plants have VSD drives on screws, probably 30%.
Some ORM control panels want to change Vi at critical differential pressure points that some systems hover at.