Unisab logic

[frigofer]

Hi Guys!


first of all i am not a refrigeration engineer but a programmer, therefore please understand that my question MIGHT seem basic for you. secondly my english is not the best..

here is my problem: i need to understand based on what logic the unisab controls the inverter and the slide. my understanding tells me that since the COP of the inverter is better than that of the slide, the control SHOULD first turn down the inverter, once the inverter is at the minimum it should regulate down the slide. so first reduce the RPM and then the slide. but this is just my understanding.. i have seen some data where the rpm was at max and the slide at 90/92%..

why?

shouldnt the slide be at 100% and the RPM at 90/92%? the COP would be higher in that case...

thank you

best regards

Frigo

[RANGER1]

Frigo,
A screw is most efficient at 100% load, so as you have described is correct.

Most screws are minimum 1440 rpm.

[frigofer]

Hi ranger!

thanks for your answer,

but why is then the unisap controlling the RPM at max and the slide at 90% and not inversed?
what logic makes and explains this decision?

thanks

[RANGER1]

frigo,
Is this compressor an auto or manual Vi?
Frick, Sabroe?

[frigofer]

auto sabroe

[Segei]

What was Vi at that time?
Assume that minimum Vi for this compressor is 2.2. When suction pressure is high and discharge pressure is low, optimum Vi can be lower than 2.2. Assume that optimum Vi for current operating conditions is 1.8. So compressor over compress the refrigerant when it is loaded to 100%. Slightly unloading compressor will reduce internal Vi to 1.8 and efficiency will improve(hopefully). However, this is just my logic.

[Josip]

Hi, frigofer

auto sabroe

roatatune with auto Vi? .... SAB128/163 ?

Best regards, Josip

[Grizzly]

Hi Josip.

The 163 comps we look after are fixed vi on account they are variable speed.


Grizzly

[Sandro Baptista]

Josep,

The rotatune doesn't has slide valve.

[Sandro Baptista]

What was Vi at that time?
Assume that minimum Vi for this compressor is 2.2. When suction pressure is high and discharge pressure is low, optimum Vi can be lower than 2.2. Assume that optimum Vi for current operating conditions is 1.8. So compressor over compress the refrigerant when it is loaded to 100%. Slightly unloading compressor will reduce internal Vi to 1.8 and efficiency will improve(hopefully). However, this is just my logic.

Why do you say the Vi decrease if the slide valve go forward (as approaching to the discharge port)?

Maybe what frigofer says it happen it may because the compressor motor is a little overload and the UNISAB decrease capacity by the slide valve. Decrease the rpms it not an option because the shaft torque absorbed by the compressor remains the same...if the condensing pressure and suction pressure remain the same.

[Segei]

Why do you say the Vi decrease if the slide valve go forward (as approaching to the discharge port)?

Maybe what frigofer says it happen it may because the compressor motor is a little overload and the UNISAB decrease capacity by the slide valve. Decrease the rpms it not an option because the shaft torque absorbed by the compressor remains the same...if the condensing pressure and suction pressure remain the same.

Major reason of poor part load operation of screw compressors is when they unload internal Vi don't march external (system) Vi or pressure ratio. Compression will initiate at later stage and under compression is happen.

[Sandro Baptista]

I thought it was the opposite...I mean as the slide valve go forward also the Vi increases and the final effect was over compression, despite the fact the compression initiate later with part load.

[RANGER1]

frigo,
Maybe potentiometer is not calibrated properly & 92% is 100% load.
Try to force load solenoiod & see if it loads any more, if no movement 100% load in real terms.

With variable auto Vi slide valve travel is changed each time secondary slide valve adjusts to suit conditions.
I would have thought slide valve would be calibrated at 100% load at maximum Vi 5 or 5.8 whatever it is.
Then as Vi adjusts below that, main slide cannot travel as much.

Does slide valve still pulse to load at the condition you describe?

I have a Unisab manual but not handy right now so will check.

[Josip]

Hi, Grizzly

Hi Josip.

The 163 comps we look after are fixed vi on account they are variable speed.


Grizzly

Yes, I know that (Vi is manually fixed according to system pressures), but OP said it is ''auto sabroe" what confused me and I was thinking there is some special rotatune execution with auto Vi !?! ... this one is not original rotatune 128/163 .... seems he has some other SAB-Frick type with inverter .... where we can have auto Vi (changing in steps) and slide for capacity control plus inverter to change rpms, but that is not original rotatune ....

... not sure if I guess right? must wait for OP for answer ...

Best regards, Josip

[Josip]

Hi, Sandro

Josep,

The rotatune doesn't has slide valve.

Thanks, I know that, but valid only for original rotatune SAB 128/163 .... and not for some other SAB compressors with inverter, with auto Vi and slide, agree


Best regards, Josip

[Grizzly]

Hey Josip.
Sorry for questioning you, I missed your question mark when i read your post first time around.

You were obviously thinking the same as me that the rota-tune models that were being questioned.
Don't have auto vi as many others have also said.
Basically frigofer after only 4 posts wants to be told how to write software to control the bespoke Sabroe software.

frigofer, if someone is willing to go into that sort of detail good luck.
Basically the control options are 4 to 20ma or control on suction pressure.
Obviously there are lots of other parameters to consider, which if set wrong could be a very expensive experiment.
Who would hold the warranty, not Sabroe for sure!
Grizzly

[Josip]

Hi, Grizzly

Hey Josip.
Sorry for questioning you, I missed your question mark when i read your post first time around.

You were obviously thinking the same as me that the rota-tune models that were being questioned.
Don't have auto vi as many others have also said.
Basically frigofer after only 4 posts wants to be told how to write software to control the bespoke Sabroe software.

frigofer, if someone is willing to go into that sort of detail good luck.
Basically the control options are 4 to 20ma or control on suction pressure.
Obviously there are lots of other parameters to consider, which if set wrong could be a very expensive experiment.
Who would hold the warranty, not Sabroe for sure!
Grizzly

not problem at all, anyone can make mistake in this or another way ... this is multinational and multilingual (back home) forum and sometimes it is easy to lost something in translation or during reading, thus easy to make a kind of mistake ... and I agree to warn me in that case, won't harm me because, for a long time I knew my limits ...


Funny, .... reading the first post again, OP there mentioned only inverter and that was indication, unit in question is not rotatune, but I saw that toooo late , anyhow, we must wait for OP to find out what unit is in question ...

Personally, I believe setup of Unisab III is not correct ... it is very powerful control system and uncompleted setup is coming back with undesirable response/s ...

Best regards, Josip

[frigofer]

Hi guys!

first of all thanks for all your answers. let me explain a little bit better what i am trying to do and what is going on here:

as stated in the first post, my knowledge of refrigeration is approximately 1%.. i know just the basics, carnot, dew point, condensation, evaporation, molier chart and few other things. i know veeeery basically how a compressor logic works. what i have to do is write a program that does not have to be 100% accurate, it just has to be an approximation. we want to simulate a system running with and without inverters. to compare COP's and show them to the customer. if the calculation startup logic is off it does not matter too much since the simulation will be off for both cases, with and without inverter. since i am not fisically near any machine, what i didwas connect remotely to a system and downloaded the data. on this system (SAB 193 S with UNISAB III) the slide is allways below 92% except 2 situations where it goes to 97%. and ita lso goes all the way down to -1.2%. so i guess that the measurement system is some % off sclae. but since it went up to 97% some times there must be something else slightly off.

what is important for me now is to understand why the slide is so low, because based on that i know how to code my program and what to tell others for fixing it. since i am not from this sector i have no idea if this slide "problem" is indeed a problem or if it is "normal"..

hope that this clearified a bit the situation..

thank you very much!

[RANGER1]

frogfer,
The Unisab starts compressor & is fully unloaded, also Vi is in lowest ratio or minimum condition.
The Vi slide stays at minimum until slide is 100% loaded, then it can adjust to the optimal Vi ratio automatically.
At this point both Vi & slide valve move together to optimal Vi ratio.
Unless calibration is out or a forced unload for whatever reason, I would suggest that at 97%the Vi is close to minimum setting which is calculated by pressure difference across compressor.
This can still mean compressor is 100% loaded, but Vi/slide valve is at a different position.

To be sure Vi & slide valve could be recalibrated.

[Segei]

I thought it was the opposite...I mean as the slide valve go forward also the Vi increases and the final effect was over compression, despite the fact the compression initiate later with part load.

This is simplified example. 100% loaded compressor will compress ammonia from 0 bar to 10 bar. Condensing pressure is 10 bar. No part load losses. 90% loaded from 0 bar to 9 bar. Under compression is 1 bar. 70% loaded from 0 bar to 7 bar. Under compression is 3 bar and losses will increase. And so on...

[Josip]

Hi, frigofer

Hi guys!

first of all thanks for all your answers. let me explain a little bit better what i am trying to do and what is going on here:

as stated in the first post, my knowledge of refrigeration is approximately 1%.. i know just the basics, carnot, dew point, condensation, evaporation, molier chart and few other things. i know veeeery basically how a compressor logic works. what i have to do is write a program that does not have to be 100% accurate, it just has to be an approximation. we want to simulate a system running with and without inverters. to compare COP's and show them to the customer. if the calculation startup logic is off it does not matter too much since the simulation will be off for both cases, with and without inverter. since i am not fisically near any machine, what i didwas connect remotely to a system and downloaded the data. on this system (SAB 193 S with UNISAB III) the slide is allways below 92% except 2 situations where it goes to 97%. and ita lso goes all the way down to -1.2%. so i guess that the measurement system is some % off sclae. but since it went up to 97% some times there must be something else slightly off.

what is important for me now is to understand why the slide is so low, because based on that i know how to code my program and what to tell others for fixing it. since i am not from this sector i have no idea if this slide "problem" is indeed a problem or if it is "normal"..

hope that this clearified a bit the situation..

thank you very much!

You can setup your UNISAB III to measure COP, then you can setup your compressor to run constantly at some speed/frequency (3000rpm) and then play with the slide i.e. part load ... not sure in complete procedure .... never did it, only normal start up and parameters tune ... but maybe you can contact people from Sabroe how to proceed in your case ... I believe it is possible to use UNISAB III to show COP in both cases and I think that is the best way .... but I can be wrong either

for your above problem (in bold) I think they need to adjust capacity slide and Vi slide (agree with RANGER1) or maybe compressor is working with active limiters and not allowed to reach 100% capacity... motor current, suction ramp, separator velocity etc....



Best regards, Josip

[Sandro Baptista]

This is simplified example. 100% loaded compressor will compress ammonia from 0 bar to 10 bar. Condensing pressure is 10 bar. No part load losses. 90% loaded from 0 bar to 9 bar. Under compression is 1 bar. 70% loaded from 0 bar to 7 bar. Under compression is 3 bar and losses will increase. And so on...

Segei,

I don't see it exactly as you said. Notice that at the part load the slide valve also approach the discharge port and so the final compressor gas volume also is smaller.

[Segei]

Segei,

I don't see it exactly as you said. Notice that at the part load the slide valve also approach the discharge port and so the final compressor gas volume also is smaller.

You are right when we talk about increasing Vi. However, at certain point discharge volume will achieve minimum and we have maximum Vi. After that no reduction of volume on discharge side and unloading will lead to under compression.