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Thread: killing our scrolls? help
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27-06-2011, 10:42 PM #1
killing our scrolls? help
i really need help with a 4 year old modular chiller set up that has killed 4 out of 8 scroll compressors. r-22 . air cooled condenser and chiller not from same company. high side runs 350 to 380 psi while low side is mid to low 50's psi. 375psi vapor into condenser 345psi leaving condenser going into an 80lb receiver . any ideas.
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27-06-2011, 10:55 PM #2
Re: killing our scrolls? help
how long is the delay from stopping to starting,should be at least 3mins to allow for pressures to equalise.-is it a mercury climatic? by any chance?
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27-06-2011, 11:09 PM #3
Re: killing our scrolls? help
High side pressures seem overly high, what is the ambient temperature?
What is the state of the condenser?Brian - Newton Abbot, Devon, UK
Retired March 2015
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27-06-2011, 11:10 PM #4
Re: killing our scrolls? help
Simply your head pressure is to high, and is a good chance is outside the operating range of the compressor. I bet the discharge line is glowing red (joke) but is very hot.
Why is it so high, what is your air "on" temp to the cond, what are cooling.
Making a guess, i would say that your system has air in it.
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27-06-2011, 11:21 PM #5
Re: killing our scrolls? help
as the other guys have said yr head way to high for a scroll to handle on 22 they will run but fail very quick check for non condensables and get yr failed units replaced so it can handle the load check the rotation of the comps if they going wrong way they sound like a bag of spanners and dont work very well then overheat best way to see if you had phase rotation see if yr condenser fans running the correct way an check yr oil levels that buggers them up as well good luck with that one
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28-06-2011, 06:20 AM #6
Re: killing our scrolls? help
Other faults that may give the symptoms you have.
Overcharged?
Chocked liquid Strainer?
Moisture in the system?
Given the issues you mention.
I would reclaim the refrigerant and recharge with fresh or at least . Do a pressure temp comparison to confirm you have R22 in the system and that it is not contaminated.
Grizzly
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28-06-2011, 12:57 PM #7
Re: killing our scrolls? help
it does have 3min delay timer.
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28-06-2011, 01:03 PM #8
Re: killing our scrolls? help
rotation is correct. we have made shure there is no non-condensables, by starting over on one of them ,pulling a good long vaccume on system all over (each side of check valves) .not shure about chocked liquid strainer? we have put in new filter dryer just before expantion valve.outside temp under 100f
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28-06-2011, 02:24 PM #9
Re: killing our scrolls? help
another thing to check if system cycling down ok ,ht scrolls throw out alot(all) of oil as they approach vacuum
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28-06-2011, 06:01 PM #10
Re: killing our scrolls? help
yes we have had that problem with short on and off times when cold out and oil leaving compressor. but we are not going to run them under 40 degrees anymore and we also have better control of condenser fans. we also have a very low delta-t at about 4 degrees f. im told it needs to be around 10 . so does that mean refridgeration valve is staying almost closed making low pressure? rember also i have 30# pressure drop on high side through condenser.
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28-06-2011, 06:56 PM #11
Re: killing our scrolls? help
Do they pump down when they stop? If yes, do not let them go below 20psi.
The pressure should not change through the condenser. Sounds like it is partially clogged.
Are there NRV's on each discharge?
Check expansion device, high head pressure can indicate problems with it at times.
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28-06-2011, 08:59 PM #12
Re: killing our scrolls? help
Is the high head pressure a new problem or has it always been there?
Mostly found in Oxfordshire, UK :)
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28-06-2011, 09:15 PM #13
Re: killing our scrolls? help
At 350-380psi, your saturated condensing temp (SCT) is 144-150F. SCT should be no more than 30-35F above ambient temp. In other words, if the ambient temp is 100F, the SCT should be no more than 130-135F, which translates to 296-315psi. 350-380psi is way too high.
If it isn't a condenser airflow problem, then the system is very probably overcharged.
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28-06-2011, 09:26 PM #14
Re: killing our scrolls? help
Start with air flow and water flow. What is the temperature of the air entering the condenser and the air leaving the condenser? What is the temperature of the water entering the evaporator and the water leaving the evaporator?
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28-06-2011, 09:58 PM #15
Re: killing our scrolls? help
the compressors do not pump down. there is a 24volt solinoid that opens and closes directly with compressor contactor. they are 25 ton scrolls. this modular chiller set up has never been right. i remember the start up guys from each company (one with the chiller and the other with the condenser) fighting during start up. one was saying he did not like the pressures at all ,he said it looks like you have non condensables.
since then we have had two different factory reps and several different chiller guys here and try to get set up right. obviously from most of the response iv been getting 350-380 psi is too much for a scroll. i agree. i will get more temp readings across condenser. entering water is 46deg f and leaving is 42degf thanks for the help i really hope we can get to the bottom of this
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28-06-2011, 10:36 PM #16
Re: killing our scrolls? help
Measure the temp of the refrigerant leaving the cond and the temp leaving the reciever.
Measure the liquid pressure leaving the reciever.
One has to presume that the cond has been sized correctlly
Are you cond fans 3 phase, check rotation of each fan, also check that all fans have the same blade and are fitted correctlly.
If your head pressure is so high, I can not see how you would know if your cond fans set up is OK (what pressures are these set to?), unless this is set up with the wrong pressures.
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28-06-2011, 11:22 PM #17
Re: killing our scrolls? help
On the evaporator, the water should enter the bottom and exit the top. The refrigerant should enter the top and exit the bottom.
At 4F, the delta-T is low. It should be about 10F. Either the evap outlet superheat is too high or there is too much water flow.
I'm thinking there is a combination of several problems... and we need to track them down one at a time.
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28-06-2011, 11:27 PM #18
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29-06-2011, 12:14 AM #19
Re: killing our scrolls? help
Given the low side pressure, excessive water flow is unlikely. Once the high side pressure is reduced (probable overcharge), the low side pressure is going to drop even further. This all adds up to high evap outlet superheat, which points to liquid restriction. Given the fact that it was like this new, I would suspect undersized TXV orifice and/or undersized liquid line, drier, etc... but we need more information to confirm all of this.
Last edited by Gary; 29-06-2011 at 12:19 AM.
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29-06-2011, 12:43 AM #20
Re: killing our scrolls? help
Reading between the lines, the OP said that there had been a few engineers on site and from different companies, It would suprise me that at least one of the engineers would have picked up the basic problem. Could it be that the cond has just been incorrectly sized, and someone/company is just trying to protect their arse
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29-06-2011, 01:03 AM #21
Re: killing our scrolls? help
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29-06-2011, 01:07 AM #22
Re: killing our scrolls? help
If the cond was undersized and 100% cond was not achieved (vapour and liquid in the liquid line), then you would have lower suction, due to the a reduced mass flow through the expansion device, you normally see high superheat, but on rare occassions you do not because of the increased pressure drop through the evap, where little heat transfer occurs at the beginning of the evap.
Last edited by mad fridgie; 29-06-2011 at 01:13 AM.
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29-06-2011, 09:45 AM #23
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Re: killing our scrolls? help
What is compressor make and model?
What is chiller make and model?
What is condenser make and model?Last edited by nike123; 29-06-2011 at 10:41 AM.
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29-06-2011, 01:35 PM #24
Re: killing our scrolls? help
the part i just read about vapor and liquid in the liquid line is what is hapening. the sight glass just before the expantion valve is full of bubbles. i mean FULL of bubbles! i heard that in some systems a little bit of bubbles is ok but im shure not like we have. story goes like this when start up was done they filled system until sight glass was clear. when it stared to get hot outside the discharge elbows right off the compressors started breaking so they sent up a different person and he took out enought r-22 from a full unit to re-fill one that lost charge from broke elbow. Im told original charge was 100# of r-22. these chillers have an 80# receiver just before the expansion valve. right now the one we are working on has a new compressor and 50# of r-22 it was arround 70deg outside and i set fans to 30hertz and the pressere just off compressor was 275 and the same 275psi right before going into condenser. just leaving condensor we had 220psi the low side just before going into compressor was maby 50psi . about this condenser i have looked at other ones and they all have two big long tubes on the end that the small tubes conect too and the incoming vapor hooks too one and the leaving liquid hooks to the other one. our condenser has the two big tubes and one realy short one at bottom the vapor hooks to one big tube and the leaving liquid hooks to that short tube not the other gig tube . im not shure what that is i read somewhere about a sbcooling circuit? if that is what it is is it piped corectly? we only have two pipes coming from chiller to condenser.
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29-06-2011, 10:31 PM #25
Re: killing our scrolls? help
Good info,
It sounds like your reciever is piped incorrectly, it sounds like your reciever is piped for a free draining cond, and not a cond with a dedicated sub cooling circuit.
Possible fix. (change from flow reciever to surge reciever)
The liquid drain from the cond, needs to feed the liquid line directly. (an not to the reciever inlet) The liquid line is still connected to the reciever outlet and open. The reciever inlet is not in use (blanked off).
Your fan speed is still to low (head pressure still to high), at 70F ambient you should be looking at pressure of around 200 psi.
This does depend upon on your expansion device and your compressor capacity control method.
You also need to look at your control loop for the fan speed control, "PID"Last edited by mad fridgie; 29-06-2011 at 10:49 PM.
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29-06-2011, 11:44 PM #26
Re: killing our scrolls? help
I have read about condensers eing piped wrong, the only way the would have liquid exiting them was if they had filled right up with liquid.
If i was you i would stand by the chiller and imagine i was some refrigerant and follow the path i would take all round the system as if the system was running, noting what state i would be in in various parts of the system as i went, and see if i could make that journey (path not blocked or restricted) and be in the right state at the right time (superheated vapour, saturated vapour, saturated liquid, subcooled liquid, saturated liquid, saturated vapour, superheated vapour, etc) take temperature and pressure readings if needed and see if this shows up anything.
JonMostly found in Oxfordshire, UK :)
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30-06-2011, 03:44 AM #27
Re: killing our scrolls? help
sorry you lost me on the flow reciever vs surge receiver. it shure looks like there is a ton of knowledge out there i will get any information you guys need to figure this out its probly simple to someone if i can just get them the info they need please help and thanks to all for getting involved.
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30-06-2011, 03:48 AM #28
Re: killing our scrolls? help
Did you get the condenser air in and air out temperatures, yet?
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30-06-2011, 03:55 AM #29
Re: killing our scrolls? help
sorry not yet i will get that tomorrow anything else that might be helpful it is suposed to be in the upper 90s here tomorrow
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30-06-2011, 03:58 AM #30
Re: killing our scrolls? help
A normal reciever all the refrigerant flows through the reciever, a surge reciever is like an expansion tank on a water system, The main flow effectively by-passes the reciever, when things change as they do, the level of the refrigerant in the reciever goes up and down.
Your problem is more than likely to be simple, but somes times you "can not see the wood for the trees"
Do you know another engineer who has NOT worked on the plant, if so I would bring him in for a quick look. Some fresh eyes and thinking
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30-06-2011, 04:01 AM #31
Re: killing our scrolls? help
Draw a picture of the system and indicate all working pressures and temps, also note the load if possible and the number of compressors running along with speed of the cond fan
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30-06-2011, 04:11 AM #32
Re: killing our scrolls? help
can you explain a little more on the reciever im still trying to understand how that works sounds interesting im just a little slow.
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30-06-2011, 04:15 AM #33
Re: killing our scrolls? help
keeping it simple, a normal reciever ALL the flow for arguments sake enters the top and leaves the bottom. OK so far?
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30-06-2011, 04:17 AM #34
Re: killing our scrolls? help
ok i understand that
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30-06-2011, 04:22 AM #35
Re: killing our scrolls? help
Now lets remove the reciever completely, liquid form cond goes straight to TEV and evap. (very common and often called a critical charge system) In your case the liquid/vapour mix leave the cond and enter the sub cooling circuit.
At this stage any supercooled vapour and liquid should get sub cooled further, and you should have basically a clear sight glass at the TEV. OK so far?
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30-06-2011, 04:24 AM #36
Re: killing our scrolls? help
ok im with you so far
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30-06-2011, 04:26 AM #37
Re: killing our scrolls? help
So the above is great good condensing and proper sub cooling, but what if the system has massive changes (load, mass flow ambient) The density of the refrigerant would change and where its location would change with in the system, agree? OK
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30-06-2011, 04:31 AM #38
Re: killing our scrolls? help
ok i think im with ya
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30-06-2011, 04:35 AM #39
Re: killing our scrolls? help
So we do not want to flood the condensor with refrigerant (drives up head pressure) and we still proper subcooling.
So we need to be able to store some refrigerant to allow for the changes in a system. (simple again) If I inserted a "T" in the liquid line with a pipe raising up and blanked at the top. When I charged the system I would fill until this pipe was half full. When TEV closes the pipe will fill and when the TEV opens the pipe will empty (this happens before chages occur in the cond/sub cooler). Only the changes moves through the pipe, not all the refrigerant. OK
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30-06-2011, 04:38 AM #40
Re: killing our scrolls? help
If we change this pipe into a vessel, we have greater room for change "surge reciever"
It is the dedicated sub cooling circuit which determines this method or a subcooler can be used after a standard reciever, but before the TEV
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30-06-2011, 04:44 AM #41
Re: killing our scrolls? help
It might be very helpful to know the surface temperatures at various points along the liquid line, from the condenser outlet to the TXV inlet, particularly the entering and leaving temperature of each component in that line.
If this is in fact a subcooling circuit, then the receiver is not needed at all, and could simply be eliminated.
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30-06-2011, 04:55 AM #42
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30-06-2011, 05:40 AM #43
Re: killing our scrolls? help
ok im listening but dont get it why is my sight glass so full of vapor.
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30-06-2011, 06:05 AM #44
Re: killing our scrolls? help
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30-06-2011, 07:42 AM #45
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30-06-2011, 12:45 PM #46
Re: killing our scrolls? help
what do you guys think about all the bubbles at expantion valve
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30-06-2011, 06:31 PM #47
Re: killing our scrolls? help
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30-06-2011, 06:39 PM #48
Re: killing our scrolls? help
Is there a difference in elevation between the condenser and the receiver? Is the condenser higher than the receiver? Is the condenser lower than the receiver?
Is the condenser outdoors and the receiver indoors?... or are they both outdoors?
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30-06-2011, 11:07 PM #49
Re: killing our scrolls? help
I have to agree with gary, information is required.
You may also have more than one problem. (one masking the other)
For example lets say the drier is partially blocked, reducing flow, so you add more refrigerant to increase the pressure, to drive more refrigerant through the restriction. As the cond becomes full of liquid, more sub cooling occurs, so less flashing would occur through the restriction and the sight glass would look clear. You still have high head pressure. The flashing in the sightglass is an affect not the cause.
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01-07-2011, 03:47 AM #50
Re: killing our scrolls? help
Hey Gary, something here troubles me. O.P. says he has 30 psi drop through condenser and with 345 psi leaving the condenser, would you not consider the possibility of undersized condenser or short cycling air even with 50 psi suction pressure? Taking into account that he does not have liquid to tx but liquid/ vapour mix, this would allow valve to starve enough to keep suction pressure low? I see we don't have air on/off temps yet.. Mike.
To the optimist, the glass is half full. To the pessimist, the glass is half empty.