sabroe 163 mk 4 issue

[stanthonycold]

Hi,

Why do our lead compressor unload to 10-15% capacity when a certain evap defrosts??It do not happen when our other 2 evaps defrost just this certain evaporator.We use ammonia as our refrigerant.Any ideas on why this is happening???

[josef]

Check for overheating.
Wet steam.

[stanthonycold]

Check for overheating.
Wet steam.


To long defrost you mean?

[Sandro Baptista]

Stanthonycold,

What's the evaporators capacities of each (for the same DT, air temperature, and rh)?

How the defrost is done? How is the system?

Give us more data.

[Grizzly]

Hi,

Why do our lead compressor unload to 10-15% capacity when a certain evap defrosts??It do not happen when our other 2 evaps defrost just this certain evaporator.We use ammonia as our refrigerant.Any ideas on why this is happening???

Hi.
Just a thought and it does depend upon what Model 163 or control strategy you are using?
4-20ma Or as I suspect you have- control on suction.
I am assuming you are using a Unisab Controller MK 2?
To make the compressor unload the software would be looking for a drop in suction pressure.
Which would not be logical in this case.
The other cause that springs to mind is...
A drop in the oil pressure differential.
If the oil pressure differential is to small. There is insufficient oil pressure to overcome the slide valve return spring.
I know when there is not enough "Lift" across the compressor this can happen.
Normally this happens when the condensing pressure is to close to suction. (That's another story!)

I am assuming that a boiling evap could raise the suction pressure, or at least influence the suction pressure sensor. (where is this evap in relation to everything else?)

So if this evap is affecting the system suction pressure reading.
It's just possible this may be dropping the oil pressure diff.
Which is in turn causing the slide valve to return to-wards the unloaded position.

Or something along those lines.
Anyway if you have a 163 rotatune then a lot of the above is rubbish!
looking for issues to do with suction or oil pressures may just help your diagnosis.

Sorry I cannot be more precise, good luck anyway.
Grizzly

[Sandro Baptista]

Stanthonycold,

The problem is during or after the defrost?

If it's during the defrost It could be a normal possibility if the total capacity of the two other evaporators have small capacity compared with this one which is defrosting.

[Nh34life]

Hey are you hot gas defrosting ? check the defrost regulator is not set too high holding up return also, what are the superheats on the evaps that are working/ maybe there starving ?

[Grizzly]

Hi Stanthony!
I have had some sleep and thought some more on the problem!
I may be completely wrong as most of the above guys, seem to favour other options.
My line of thought is that there could be a control issue relating to this specific compressor.
Which of course would depend upon your type of plant control?
Just maybe your have suffered from the curse of the "finger poker!"

I have come across Unisab Controls that have been set up in Auto.
Which makes the comp operate as a stand alone.
Etc.
But having read your post again it's this one Evap? Trying to work out how this is driving the comp?
It's a strange one for sure. (Hence your post I guess!)
Just thinking out loud?
I realise that most of this is basic stuff and you are far more experienced than that!
But sometimes even us experienced guys can forget the basics!
Anyway I know you will keep us all informed.

[stanthonycold]

Hi,

Thanks for the vote of confidence :D but im actually quite new to this Job,just a little over a year in .I will try my best to explain the problem,Ok here's whats happening:We are running 2 sabroe 163 mk 4 compressors ,we have 6 evaporators ...3 in one section of our cold storage and 3 in the other(both cold storages are attached to each other).Both cold storage sections are in the -27 degree celsious range.The remaining 5 evaps are defrosting normally but as of late when our #3 evap goes into defrost as soon as the hot gas starts the lead compressor will unload almost completly!!I put the #3 on defrost today and same thing happened so i put the compressor in manual mode and loaded it up to 90% with no problems,switched compressor back to sequenced mode and it stayed loaded and is now working fine !We are using a M&M computer control system,maybe some other engineer screwed with the computer control but i highly doubt it.Everything is running fine except when this #3 evap goes into a auto defrost the compressor will unload .Any suggestions !

[Sandro Baptista]

1) All evaporators are the same brand and model, right?

2) Ever time you defrost the #3 you maintain the same rate of number evaporators working to one evaporator defrosting?

3) How is the system? Do you have a pumped system with 3 pipes? I mean 1 liquid pumped, 1 wet suction to the drum and 1 hot gas that returns in the wet suction line?

4) Do you have one station of valves independent for each evaporator?

5) As far I understand when the #3 defrost if changed the lead compressor to manual loaded to 90% and switch the UNISAB to automatic and everything is fine, right? And on the next time you must to the same again, right?

6) Just for curiosity...can you see if the #3 tends to form more frost than the others?

[stanthonycold]

1) all evaporators are the same brand and model, right? Yes

2) ever time you defrost the #3 you maintain the same rate of number evaporators working to one evaporator defrosting? Yes

3) how is the system? Do you have a pumped system with 3 pipes? I mean 1 liquid pumped, 1 wet suction to the drum and 1 hot gas that returns in the wet suction line? Yes

4) do you have one station of valves independent for each evaporator? Yes

5) as far i understand when the #3 defrost if changed the lead compressor to manual loaded to 90% and switch the unisab to automatic and everything is fine, right? And on the next time you must to the same again, right? Switch compressor to manual to load then back to sequenced mode not auto

6) just for curiosity...can you see if the #3 tends to form more frost than the others? about the same

[Sandro Baptista]

...hmmm...in fact is very strange...maybe something in PLC that when #3 defrost tells to UNISAB II (?by the PROFIBUS protocol?) to unload the compressor!?

What happens to the other compressor? Does it increases its own capacity?

Does the suction pressures increase? Does the remaining evaporators are with lack of capacity?

[Grizzly]

Hi Stanthony.
It does indeed seem to be a control issue after all!
What happens to your LPR pressure and level when this particular evap.
Goes on defrost?
Also are there any of the control transducers or sensors near this particular evap?
You seem to be a step nearer!
Grizzly

[Sandro Baptista]

Stanthony,

How is working the plant? Still the same problems?

[stanthonycold]

Stanthony,

How is working the plant? Still the same problems?


All is well except for the compressor still unloads when #3 evap goes on defrost.Im pretty sure it is a control or sensor issue but still havent figured 100% whats causing this to happen!!!

[Sandro Baptista]

See what is different on the ladder PLC control defrost #3 evap from the other evaporators and what are the interacting with the UNISAB III. Sure you already have done that...

Keep us informed. I'm curious.

[stanthonycold]

See what is different on the ladder PLC control defrost #3 evap from the other evaporators and what are the interacting with the UNISAB III. Sure you already have done that...

Keep us informed. I'm curious.
all evap control setpoints are the same,,whatever was wrong has now righted itself !! It is working normally once again and we never did anything...still would like to know what happened to cause in the first place.

[Sandro Baptista]

It is a self corrected refrigeration plant :)

[stanthonycold]

It is a self corrected refrigeration plant :)

I sure hope so,would make my life a hell of alot easier!:)

[Grizzly]

Great News stanthonycold.
Could you tell me what type of defrost do you use?
Just a thought!
Grizzly

[stanthonycold]

Great News stanthonycold.
Could you tell me what type of defrost do you use?
Just a thought!
Grizzly


Hot gas defrost!

[Grizzly]

Does the system control on suction pressure.

And do you have Pm valves regulating the hot gas return from the evap back through the suction?

If you do. it's just possible that a solenoid or pilot valve / something was passing pressure back into the suction
line.

At a high enough rate to create a system pressure rise large enough to unload the comp/s?

Just a thought, but the confusing thing. As you say is the fact that it has rectified itself?
All supposition I know but Hey! You never know!
Grizzly

[Sandro Baptista]

Yes Grizzly it's a good possibility. It could be dirty that been stuck on the valve seat and the valve didn't closed well...but then for any reason the dirty has gone to the plant and now it's okay until maybe some day happen again...

[Grizzly]

Sorry my friend I have just realised, I have got it wrong again.
Logically higher suction pressure would cause the comps to load up!

(That's what happens when you try and be clever during the early hours of the morning!)
Just how are the valves for Hot gas configured.
Never ming eh!
You are OK now!
Grizzly

[Sandro Baptista]

Sorry my friend I have just realised, I have got it wrong again.
Logically higher suction pressure would cause the comps to load up!
Grizzly

Yes you're right...I also forgot of the first post...the problem was the compressor would unload. However never had reach me the information about on post that I have sent at 26-08-2010 saying

"...hmmm...in fact is very strange...maybe something in PLC that when #3 defrost tells to UNISAB II (?by the PROFIBUS protocol?) to unload the compressor!?

What happens to the other compressor? Does it increases its own capacity?

Does the suction pressures increase? Does the remaining evaporators are with lack of capacity?"