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
@ 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
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...
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...
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.
Before you pull it apart, have a look at the whole system.
Rapidly changing load or flows can really mess with you.
God Bless the USA
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
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?)
Originally Posted by chiller rob
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.
Originally Posted by Karl2008
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.
Microtech readings should not be relied on, accurate measurements are reqd.
Water flow rates should be verified.
Last edited by ptsac; 02-25-2010 at 03:11 PM.