Post a reply to the thread: Suction superheat is abnormal,Defective TXV?
You may choose an icon for your message from this list
Please enter the name by which you would like to log-in and be known on this site.
Please enter a password for your user account. Note that passwords are case-sensitive.
Please enter a valid email address for yourself.
Will turn www.example.com into [URL]http://www.example.com[/URL].
this is agreat informative thread before the "pros " section
Originally Posted by ehsx The pilot txv is an oem liquid charged valve adjusted to maintain 1*f ssh( confirm by checking dsch sh, ~15 at full load. Main valve could be sticking also. With the machine operating you should be able to manually load & unload the chiller while monitoring amps & waters dt. The controls you mentioned are not supported by Daikin/McQuay so i have no idea how loading is controlled. Correct I can manually load and unload it. I believe they are using amperage and time to calculate position and we also fitted an vane open switch so it can prove either way. At the beginning of this I was thinking the IGV was stuck but my vane open switch proves otherwise. Nor do I get any change when I manually drive open the vanes when the condition hits. They are open.
The link below may help you with understanding the operation of the pilot and main TXV operation. I myself am leaning towards your main valve sticking intermittently. They used to rebuild or replace them but my personal preference is to change them out to an EXV. Daikin makes a kit for most of their machines but without OEM controls I’m not convinced it’s your best alternative. Daikin uses a very complex algorithm to drive their EXV’s and I have no idea if MCS has that capability. https://m.youtube.com/watch?v=dXiV5YzTZQ4
The pilot txv is an oem liquid charged valve adjusted to maintain 1*f ssh( confirm by checking dsch sh, ~15 at full load. Main valve could be sticking also. With the machine operating you should be able to manually load & unload the chiller while monitoring amps & waters dt. The controls you mentioned are not supported by Daikin/McQuay so i have no idea how loading is controlled.
I've just dug this up as I have a similar scenario going on with a WSC087 in which I question my TXV. I would love to rengage the above members if possible. There is a ton of good info in this thread that I truly appreciate. In my case, I have an intermittent issue with our machine which is running MCS controls (magnum) with a Yask drive. We retrofitted the controls and drive of 2 chillers last year and one of the machines is now losing the load at random. Superheat and subcool go up sometimes into double digits and I lose any mere sign of liquid in my evap glass. RLA starts to drop while controls remain constant. As stated the chiller loses it's load meaning the building loop starts climbing. If not already, the IGVs go full open and compressor goes to max speed. During this you can watch RLA drop from 380ish if running well and loaded, down to as low as 260. Again we are at max speed with our IGVs at 100%. Eventually the machine will begin to surge and controls kick in and we start all over again. The above typically results in facilities switching chillers and requesting service for the machine. When we arrive and start the unit will typically start up and run fine for quite a while. After 2 of the above scenarios, I camped out waiting for the problem and wasn't let down, though it took a while to rear its ugly head. After a 12hr day I am leaning towards the TXV intermittently closing and eventually opening back up. I have monitored the temps on the TXVs of both of these chillers and have found when my trouble condition rises, the outlet of the pilot TXV is fixed around 55 degrees. Main valve outlet is around 48. The working chiller has a steady swing in the temperature out of the pilot valve. It rises slowly about ten degrees which I assume the valve closing off slightly and then it starts slowly cooling back down and I assume we are slowly opening back up and repeat. The trouble chiller work this same way most of the time...when trouble arises, my pilot TXV outlet is nailed and does not move. The pilot valves do not have access fittings installed for me to monitor pressure. I am not a huge Mcquay guy or I prolly wouldn't be typing right now...Can anyone tell me if my understanding of this expansion valve is correct or am I way off base???? It wouldn't be the first time and surely not the last. I would love to hear what any McQuay tech may say as well as the usual suspects above. Please keep in mind I did not plan this post while working so I have very little hard data to post. I try to fix my own stuff and not depend on the magical intrawebs of cyberspace but this one has me guessing a bit. We do have submittal data from the start up and I have verified all flow data is acceptable. The condenser flow is just acceptble, but within spec. I am back on that site tomorrow for other work and should have some time to get additional data if needed and the ugly beast rises while I'm there. Our company doesn't have current McQuay credentials to get good info from them. Though it sounds like they are going to send me to Staunton in the near future. After the retro fit we found a small refrigerant leak on the machine and were having trouble getting the IGVs to prove. We tore down the unit and re-oringed the vanes, repaired the leak and put in a fresh charge. I was not part of the IGV work and was a bit upset about that at the time. As I stated, I'm no McQuay specialist and the guy that did the vanes was but he is no longer with us.
Originally Posted by Karl2008 Hi, Phil, Could you please post some data of your units. What I'm very concerned are subcooling and suction superheat under different chiller capacity. Thank you! I do not have that data to hand as it was many years ago. However it was evident that there was no superheat control monitored with accurate test instruments. Subcooling at low loads will be a low value as most of the liquid refrigerant will be in the evaporator just simmering over the top tubes, exp. valve will be hardly open and refrigerant flow low as vanes shut. If you have a condenser sightglass you will see a low liquid level, less tubes in contact with liquid and therefore less subcooling. At high loads the vanes will be open, refrigerant flow will be full, the action in the evaporator will be like a fast boil and liquid refrigerant will be lining up behind the expansion valve, more open than before, but still metering, more condenser tubes will be in contact with the liquid hence more subcooling. Evap superheat should be pretty constant throughout all loads, but getting an accurate measurement for this value (0-2F) is difficult. Discharge superheat with liquid injection off at full load is much easier to measure at 14-16F for R134a Thats the way that I picture it, maybe someone can describe more scientifically. Most important. Microtech readings should not be relied on, accurate measurements are reqd. Water flow rates should be verified.
Hi, Rob, Thanks very much for your reply! In the mean while I also should say thanks to Klove, Stickrhead. Thank you guys, I learned a lot from you guys. Rob, for the subcooling, I don't think it is low. I think it's a little higher. I have the submittals of the units. The manufacture test data shows that subcooling is only 8.1F at full load condition. At 80% load condition, the subcooling is 6.8 F. So I doubt there's a little overcharge for this unit. For the suction superheat, when chiller run at lower condition, the TXV should open wider with decreasing compressor capacity to flood evap tubes with refrigerant. However, I was wondering whether TXV can not operate under lower load condition (it can not open wide to flood the evap?) Thanks! Originally Posted by chiller rob Karl2008 thanks for taking the time to present this info... i think you do have a machine problem and yes it is costing effeciency... you have some really good help here... Klove you are the man... Stickerhead is also one of the best centrif mechanics in the country... i had to take a minuite and go back to the log... Here are the key points for me... @ 50% FLA Log: Evap Delta T 2.3 Evap Approach 5.6 Suction SH 11.1 Cond Delta T 4.1 Cond Approach 2.7 Discharge SH 29.9 Subcooling 4.5 @ 80% FLA Log: Evap Delta T 5.1 Evap Approach 8.0 Suction SH 0.8 Cond Delta T 7.2 Cond Approach 5.4 Discharge SH 16.7 Subcooling 9.0 Comments Low Load Log: superheat is way to high for a centrif at any load... water flow looks low at 2.3 f evap approach looks a bit to high... subcooling looks a bit to low... Comments High Load Log: superheat is were it should be... evap water flow still looks low... evap approach still looks to high subcooling looks to a bit to low... Analysis... I totally agree with eveything klove is saying here... you need the submittal if this machine were designed as a series unit with a 7 deg evap that would be one thing but as a conventional chiller your chilled delta t is way to low... i would suspect excess water flow thru the chiller or dirty tubes... Check your evap pressure drops and confirm flow thru the evap. Punch the evap tubes... (opportunity for eddy current?) your subcooling as well as your evap approach indicates to me that your machine charge may be to low... i think your condenser delta t is ok but your approach is getting out there and your subcooling is a bit low... i cant really use discharge superheat for any real data in this case as i dont know if you have liquid injeciton active or not... (see stickerhead) Ultimatly you may have to pull the charge and see where your at... you would charge short of nameplate and trim to stickerheads comments... as for the high suction superheat at low loads... it would be nice to know what the time duration between the 50% and 80% logs are but i am thinking that a refrigerant sample from the evap checking HBR (heat boiling residue) may confirm that you have excessive oil in the evap... you may have oil return problems due to the low charge and the oil return circuits are not pulling back...or worse... you have a cut oring and your loosing oil at certain positions of the igv piston... its cheap information... pull a sample... a conversation comes to mind that i had with one of our technicians a while back... he had a machine that was running kinda like this one and he pulled the charge and found it was way low (like less than 50%) and still was running with some degree of perfromance... i would not even start thinking about the expansion valve right now... if it is maintaining a constant 0.8F at 80 percent FLA load for any duration its working ok... just a few ideas and comments... good luck... Chiller Rob
Hi, Phil, Could you please post some data of your units. What I'm very concerned are subcooling and suction superheat under different chiller capacity. Thank you! Originally Posted by ptsac Hello Karl, I have had a similiar scenario to yours and found that the expansion valve was affected by condensing pressure. It was found that the main expansion valve teflon rings were scored and liquid line pressure was bypassing to pilot valve control side. To check whether this is causing your problem is fairly simple. Fit guage to pilot valve outlet to monitor pilot valve pressure, shut pilot valve feed and main expansion valve should start to shut. If valve doesnt move you should see no change on your guage. I had two units the same and both were Italian assembly and vessels. I am assuming that this is an European unit, I found swarf in the line. Worth a quick test, may not be your problem. Phil
Before you pull it apart, have a look at the whole system. Rapidly changing load or flows can really mess with you.
Hello Karl, I have had a similiar scenario to yours and found that the expansion valve was affected by condensing pressure. It was found that the main expansion valve teflon rings were scored and liquid line pressure was bypassing to pilot valve control side. To check whether this is causing your problem is fairly simple. Fit guage to pilot valve outlet to monitor pilot valve pressure, shut pilot valve feed and main expansion valve should start to shut. If valve doesnt move you should see no change on your guage. I had two units the same and both were Italian assembly and vessels. I am assuming that this is an European unit, I found swarf in the line. Worth a quick test, may not be your problem. Phil
Karl2008 thanks for taking the time to present this info... i think you do have a machine problem and yes it is costing effeciency... you have some really good help here... Klove you are the man... Stickerhead is also one of the best centrif mechanics in the country... i had to take a minuite and go back to the log... Here are the key points for me... @ 50% FLA Log: Evap Delta T 2.3 Evap Approach 5.6 Suction SH 11.1 Cond Delta T 4.1 Cond Approach 2.7 Discharge SH 29.9 Subcooling 4.5 @ 80% FLA Log: Evap Delta T 5.1 Evap Approach 8.0 Suction SH 0.8 Cond Delta T 7.2 Cond Approach 5.4 Discharge SH 16.7 Subcooling 9.0 Comments Low Load Log: superheat is way to high for a centrif at any load... water flow looks low at 2.3 f evap approach looks a bit to high... subcooling looks a bit to low... Comments High Load Log: superheat is were it should be... evap water flow still looks low... evap approach still looks to high subcooling looks to a bit to low... Analysis... I totally agree with eveything klove is saying here... you need the submittal if this machine were designed as a series unit with a 7 deg evap that would be one thing but as a conventional chiller your chilled delta t is way to low... i would suspect excess water flow thru the chiller or dirty tubes... Check your evap pressure drops and confirm flow thru the evap. Punch the evap tubes... (opportunity for eddy current?) your subcooling as well as your evap approach indicates to me that your machine charge may be to low... i think your condenser delta t is ok but your approach is getting out there and your subcooling is a bit low... i cant really use discharge superheat for any real data in this case as i dont know if you have liquid injeciton active or not... (see stickerhead) Ultimatly you may have to pull the charge and see where your at... you would charge short of nameplate and trim to stickerheads comments... as for the high suction superheat at low loads... it would be nice to know what the time duration between the 50% and 80% logs are but i am thinking that a refrigerant sample from the evap checking HBR (heat boiling residue) may confirm that you have excessive oil in the evap... you may have oil return problems due to the low charge and the oil return circuits are not pulling back...or worse... you have a cut oring and your loosing oil at certain positions of the igv piston... its cheap information... pull a sample... a conversation comes to mind that i had with one of our technicians a while back... he had a machine that was running kinda like this one and he pulled the charge and found it was way low (like less than 50%) and still was running with some degree of perfromance... i would not even start thinking about the expansion valve right now... if it is maintaining a constant 0.8F at 80 percent FLA load for any duration its working ok... just a few ideas and comments... good luck... Chiller Rob
Originally Posted by Karl2008 The ideal superheat should be 0.6 *. How is the impact on the system if superheat increase to 6 *, can it be quantified? You can plot it out on a p/h chart for R134A and do the calculations, but if you don't get your flows correct and make sure you have good heat transfer first, you're chasin' your tail. If your SH is going up due to uncovering tubes, then your efficiency is going down, if it's going up due to some other dynamic that unloading the machine is amplifying and the tubes are covered, then it's just going up. Ideal may be 0.6*, but I would worry a lot more about that as it loaded. And remember when you start chasing 6/10ths of 1 degree (especially on refrigerant gas temperature), it may be VERY hard to catch - that's a setpoint, and this is a 540TR chiller with boiling refrigerant in it. Things are rarely exact.
The ideal superheat should be 0.6 *. How is the impact on the system if superheat increase to 6 *, can it be quantified? Originally Posted by klove The issue here is that the superheat is going down as you load. If the TXV were feeding too little and starving the evap, then the SH would be going up as you load and uncover the tubes. As has been said, a 6* variation is a bit much, but your conditions aren't necessarily adding up to a bad expansion valve.
Originally Posted by Karl2008 Thank you guys for the reply. Yes I agree with you that the dirty evaporator maybe the major reason caused the capacity loss. What I really want to make sure is whether the TXV is bad. 0~6 ℃ change of suction superheat is so weird. What's the variation range is normal for a flooded evaporator under different load conditions. I think 0~3℃ does make sense. What's your experience on this? Thanks! The issue here is that the superheat is going down as you load. If the TXV were feeding too little and starving the evap, then the SH would be going up as you load and uncover the tubes. As has been said, a 6* variation is a bit much, but your conditions aren't necessarily adding up to a bad expansion valve.
Thank you guys for the reply. Yes I agree with you that the dirty evaporator maybe the major reason caused the capacity loss. What I really want to make sure is whether the TXV is bad. 0~6 ℃ change of suction superheat is so weird. What's the variation range is normal for a flooded evaporator under different load conditions. I think 0~3℃ does make sense. What's your experience on this? Thanks!
As long as you have good subcooling, your discharge gas temp should be at 16 degrees at full load with the liquid injection turned off. With that said, your suction superheat will be around 1 - 2 degrees F, If not, look at the evap tubes or divider like Klove said.
Originally Posted by Karl2008 I convert it as below: Lower load condition: COMPRLA%: 50% cond water inlet temp: 24.39℃ -75.90F cond water outlet temp: 26.67℃ -80.01F condensing temp: 27.67℃ -81.81F cond liquid line temp: 25.17 -77.31F comp discharge temp: 44.28℃ -111.7F evap water inlet temp: 8.28℃ -46.90F evap water outlet temp: 7℃ -44.60F evaporating temp: 3.89℃ -39F suction temp: 10.06℃ -50.11F Higher load condition: COMPRLA%: 80% cond water inlet temp: 26.28℃ -79.3F cond water outlet temp: 30.28℃ -86.5F condensing temp: 32.33℃ -90.19F cond liquid line temp: 27.28℃ -81.1F comp discharge temp: 41.56℃ -106.81F evap water inlet temp: 9.89℃ -49.8F evap water outlet temp: 7.06℃ -44.71F evaporating temp: 2.61℃ -36.7F suction temp: 3.06℃ -37.51F The designed cond water flow rate is: 104.3 l/s The designed evap water flow rate is: 91.6 l/s The refrigerant is R134A %RLA is drawn by the compressor. The sensor should be no problem. Fouled or obstructed evaporator tubes is the first thing I'd look for based on your numbers. If it's a 2 pass evaporator, you may also have a divider plate gasket leaking a significant amount of flow past it back into the leaving water. Either of these two things, or you have a very small tube bundle by design and the numbers you have are normal (I wouldn't suspect this). What I'm looking at is the fact that your evap approach at low load is 6*F, which is high already. At 80%RLA the evap approach goes to 8*F, but the suction superheat drops off dramatically. The real puzzling thing is that your discharge superheat is no lower than it is. Almost as if you have a sensor reading incorrectly. I have had a machine or two that had fouled tubes and low refrigerant charge at the same time and gave exactly these symptoms. You've given the design flow rates - do you have a chart from Mcquay in an IOM that will give you the pressure drop at the correct flow - and do you have the design temperature differentials on the cond and evap?
I convert it as below: Lower load condition: COMPRLA%: 50% cond water inlet temp: 24.39℃ -75.90F cond water outlet temp: 26.67℃ -80.01F condensing temp: 27.67℃ -81.81F cond liquid line temp: 25.17 -77.31F comp discharge temp: 44.28℃ -111.7F evap water inlet temp: 8.28℃ -46.90F evap water outlet temp: 7℃ -44.60F evaporating temp: 3.89℃ -39F suction temp: 10.06℃ -50.11F Higher load condition: COMPRLA%: 80% cond water inlet temp: 26.28℃ -79.3F cond water outlet temp: 30.28℃ -86.5F condensing temp: 32.33℃ -90.19F cond liquid line temp: 27.28℃ -81.1F comp discharge temp: 41.56℃ -106.81F evap water inlet temp: 9.89℃ -49.8F evap water outlet temp: 7.06℃ -44.71F evaporating temp: 2.61℃ -36.7F suction temp: 3.06℃ -37.51F The designed cond water flow rate is: 104.3 l/s The designed evap water flow rate is: 91.6 l/s The refrigerant is R134A %RLA is drawn by the compressor. The sensor should be no problem. Originally Posted by klove I hate to do this to you, Karl, but you have to look at where I live. I can't convert metric in my head, and I don't have a conversion chart handy for temps and flows. If you could do that over to US, I'll be glad to give an opinion. Also, your two water flow rates are both saying condenser. I assume the lower one is for the evap? And what type refrigerant is being used? Have all your sensors been tested for accuracy, and does your %RLA actually match what is being drawn by the compressor motor?
Originally Posted by Karl2008 Thanks very much for your reply. Here is the detailed information of the unit: Unit code: WSC087MAV508/E3012-BE-2*A/C2612-CLYY-2*AYYY/R134-BAABR Lower load condition: COMPRLA%: 50% cond water inlet temp: 24.39℃ cond water outlet temp: 26.67℃ condensing temp: 27.67℃ cond liquid line temp: 25.17 comp discharge temp: 44.28℃ evap water inlet temp: 8.28℃ evap water outlet temp: 7℃ evaporating temp: 3.89℃ suction temp: 10.06℃ Higher load condition: COMPRLA%: 80% cond water inlet temp: 26.28℃ cond water outlet temp: 30.28℃ condensing temp: 32.33℃ cond liquid line temp: 27.28℃ comp discharge temp: 41.56℃ evap water inlet temp: 9.89℃ evap water outlet temp: 7.06℃ evaporating temp: 2.61℃ suction temp: 3.06℃ The designed cond water flow rate is: 104.3 l/s The designed cond water flow rate is: 91.6 l/s So the suction superheat is very low at higher load condition. I know there should be some increase when the chiller run at lower load, but the variation of this machine is so big. Does this make sense? I hate to do this to you, Karl, but you have to look at where I live. I can't convert metric in my head, and I don't have a conversion chart handy for temps and flows. If you could do that over to US, I'll be glad to give an opinion. Also, your two water flow rates are both saying condenser. I assume the lower one is for the evap? And what type refrigerant is being used? Have all your sensors been tested for accuracy, and does your %RLA actually match what is being drawn by the compressor motor?
Thanks very much for your reply. Here is the detailed information of the unit: Unit code: WSC087MAV508/E3012-BE-2*A/C2612-CLYY-2*AYYY/R134-BAABR Lower load condition: COMPRLA%: 50% cond water inlet temp: 24.39℃ cond water outlet temp: 26.67℃ condensing temp: 27.67℃ cond liquid line temp: 25.17 comp discharge temp: 44.28℃ evap water inlet temp: 8.28℃ evap water outlet temp: 7℃ evaporating temp: 3.89℃ suction temp: 10.06℃ Higher load condition: COMPRLA%: 80% cond water inlet temp: 26.28℃ cond water outlet temp: 30.28℃ condensing temp: 32.33℃ cond liquid line temp: 27.28℃ comp discharge temp: 41.56℃ evap water inlet temp: 9.89℃ evap water outlet temp: 7.06℃ evaporating temp: 2.61℃ suction temp: 3.06℃ The designed cond water flow rate is: 104.3 l/s The designed cond water flow rate is: 91.6 l/s So the suction superheat is very low at higher load condition. I know there should be some increase when the chiller run at lower load, but the variation of this machine is so big. Does this make sense? Originally Posted by klove It will lead to capacity loss, but before you can determine that sort of thing, it's imperative that you have the design data for the machine. E.G.: High water flow = low temp differential, even if the machine is working as advertised from a capacity standpoint. Some things can make you think capacity loss when in fact that's not an issue. The place to start is with the distributor or the factory, and the machine model and serial. Get the actual design machine specs, or everything you do will be based on a swag. There's enough Kentucky windage that has to be used at low loads because capacity and RLA/FLA don't work on a straight line but on a curve, so to get reasonably good data to go by for troubleshooting, you have to know what it's supposed to do at full load and interpolate at the lower loads (unless you can get the part load testing points, assuming they even exist). I'm just speculating, but your whole issue could be caused by control imposed limits to refgt level in the evap at low load conditions, and there may not be any problem at all. Again, that's speculation at this point. You also need to post the full model/serial and the type of control panel you have on this thread, along with a full set of readings on water temps and refgt side, so that all who respond will know what they're talking about. Some of Mc's guys are members here, but they need to know what you have to be able to tell you what to look at. You seem to be bent on a TXV problem, and that may be what you have, but I would advise to stop leaning that way until you have some better data. Floodeds do some odd things at low loads depending on a lot of factors, and if your superheat is dropping with higher capacities on the machine, that doesn't sound like a TXV issue.
Forum Rules
