Refrigerant Flow

[PLEH2EREH]

Possible reasons a refrigerant level would start rising in a flash economizer if system is pretty close to design pressures/temperatures? Components are: Twin screw compressor, oil separator, water cooled condenser, what I call a seal pot, flash economizer, flooded evaporator, accumulator and refrigerant 134a. I know more detail is needed and will try to provide. This level increase starts being noticed when the capacity slide valve gets about 96% and continues up to 100% (gradual increase). System pressures: Discharge/131, Condenser/126, econo/80, suction/43 (psig). I know the system has a pretty good load on it, but one would think the dp between the econo and evap is enough for the refrigerant to flow, not stack (if you will). There are control valves...and they are 100% open...

[RANGER1]

Probably need
type of level control on both economizer & flooded evaporator,
also how do you know they are 100% open.

Sounds like a critical charge system?

Confirm oil is not a problem, as it could give false levels.
If probe used is it calibrated or reading correct.

Confirm economizer & surge drum/compressor suction pressure.

If all above ok can you raise economizer pressure see if level lowers level.

[mbc]

to give right answer in short time . send copy of drawing plan you have ,

[PLEH2EREH]

Instead of maintaining a level in the evaporator they maintain a level in the economizer (not my idea). So, the control valve between the evaporator an economizer has a set value (%) to maintain that level. The control valve between the economizer and the seal pot maintains the level in the seal pot. The valves are 100% open not only because the PLC indicates 100 % open, but the valve stem indicator (visual) indicates them to be open. I've witnessed these valves operate so pretty much convinced they are what they indicate (other than unflanging them and looking inside). There is a control valve between the economizer and the compressor side port, which maintains 80 psig. Raising that pressure didn't make any difference. The 80 psig is maintained very well. Pressures confirmed. There is a level indicator on the evaporator, and it indicates 100%...which I believe means the tubes are covered 100%, or should I say, the chiller barrel is 50% full, if that makes since. And yes, I've considered the oil issue.......Don't believe it is going to be oil in evap problem though, skimmers and oil still working and always has worked well.. now as far as false levels in the seal pot or economizer, maybe.... would just have to talk them into letting me eliminate those.

[PLEH2EREH]

Forgot to mention, factory spec indicates at full load these readings: 41 psig suction, 150 psig discharge and of course the 80 psig econo. My thoughts were, if it is running 43 psig suction, 80 econo, and only 130 discharge, that perhaps the discharge pressure needs to be raised. Of course this will make motor amps increase and the such, but was curious if the higher discharge would help "force" the liquid on through??

[PLEH2EREH]

Charge is 4,000 lbs R-134a, 1,000 ton rated....producing right at 900 tons

[RANGER1]

Does the control valve between seal pot & economiser close when economizer at level, or does seal pot dump into economizer to maintain level in seal pot?

[RANGER1]

Any filters or strainers

[PLEH2EREH]

Partial flow through drier(s) chambers, hand valve bypass (100% open). The seal pot dumps into economizer to maintain level in seal pot. That in between cv maintains a 50 % level in the pot.

[RANGER1]

So, economizer feeds flooded evaporator through a control valve that has adjustable opening degree, but is set.
In liquid line from economizer to evaporator liquid pressure is 80 psi
Is there any points to measure & verify that pressure.
Is there, or could there be any orifice or hand regulating valve?

How is evaporator configured?
Does it have a surge drum, if so where located in relation to economizer height, distance etc

Is it a new system or has it worked in the past?

Why there is a line under everything I write, I'm not sure!

[PLEH2EREH]

Look right up here ^ . If that U is highlighted in blue then everything will be underlined!!
Yes, the liquid fed to the flooded evaporator is ~80 psig (it fluctuates but within .75 psig max). Evaporator pressure runs 44.9 ~ 45.8 psig. No orifices.. The only hand valve being adjusted at this time is the one around the drier cores. There are 5 housings, paralleled, with 4 or 5 cores each (don't remember the #). Suppose to have a "slip stream" through those to maintain a clear sight-glass downstream of the cores. Hand valve can be used either as a bypass for replacing cores while system is running and or to allow more flow. Has been tried 100% open and less. I believe the evaporator is a 2 pass. Water in the tubes, refrigerant shell side. This system has an accumulator, not a surge drum (unless your description is the same). It is located above the economizer, higher than the evaporator. The condenser is located above the evaporator, the economizer same level as the evaporator, the accumulator is about the same height as the condenser, maybe a little higher but not much. System is app 6 years old. Never has been ran this "loaded" so this scenario is new, so it's just something I got to figure out. There are 8 skimmers on the evaporator providing liquid refrigerant/oil to the oil still downstairs. Actually had more time to "tinker* with the system yesterday. One question answered would help me put aside one theory: System is designed to provide 1,000 tons at a suction pressure of 41 psig, discharge 151 psig. If it is running at 45 and 131 psig, does the discharge pressure need to be increased? 67.1 f to 51.8 f or 15 degree drop across evaporator (designed 18 drop). So the approach on the evaporator is 1.8 degrees f

[RANGER1]

Can't see why discharge pressure increase would help, but try & see.
Do the skimmers work, is accumulator at correct level?

[PLEH2EREH]

Should it have a level? And if so, please explain why...

[Segei]

I would try to increase pressure in economizer to 90 psig, 100 psig.

[RANGER1]

Should it have a level? And if so, please explain why...

If you have a flooded accumulator or surge drum & skimmers, the level has to reach skimmers to return oil.
No level no oil return, eventually oil logged evaporator.

[PLEH2EREH]

If the accumulator was flooded I would be getting liquid refrigerant back to the compressor (in this application). The skimmers on the flooded evaporator (*****=shell and water=tube) are (all 8) adjusted in such a manner that each provides a "sample" to the oil still. The oil still and oil return are working. The oil level in the oil separator is the same so I don't think an oil logged evaporator is causing the refrigerant liquid level increase in the economizer.

[RANGER1]

So with the high level in the economizer, does this alarm or shut plant down.
Does this high level in economizer flood back to compressor?
If suction pressure is a bit higher than design, can you check slide valve is at 100% or double check calibration to get most out of compressor.
Some have potentiometer, micro switches or linear position indicator.

[PLEH2EREH]

Hi level alarm, hihi level s/d. yes. Slide was calibrated during the last downtime. linear.

[Segei]

Liquid supply to evaporator should be balanced with refrigeration load. Assume that compressor capacity is constant. Suction pressure increase from 40 psig to 45 psig will increase load by 10%. Liquid supply will be reduced by 5% because lower pressure difference between economizer and evaporator(was 40 psig , now 35 psig). Imbalance is 15%. If compressor capacity increased by 10%, total imbalance will be 25%. Control valve should open additional 25%. If it will open 20% and it is fully open, misalliance will be 5% and level in economizer will go up until high level tripped.
Solution Increase pressure in economizer. Economizer pressure of 100 psig will increase supply by 15%, compare to 80 psig. Monitor liquid supply from condenser to economizer. May be condensing pressure should be increase if pressure difference between condenser and economizer is too low.

[PLEH2EREH]

Segei! I really do thank you 4 replying: and I do appreciate all. This is an awesome site and the people that manage it are top level. I will have to get back to you later; got grand-kids 2night !!!! I will reply later... if i survive !!LOL!!

[mbc]

why I asked to put drawing is.
for people it is easy to understand and see all system and pressure ,
especial for people, English is second language

[RANGER1]

Look right up here ^ . If that U is highlighted in blue then everything will be underlined!!
Yes, the liquid fed to the flooded evaporator is ~80 psig (it fluctuates but within .75 psig max). Evaporator pressure runs 44.9 ~ 45.8 psig. No orifices.. The only hand valve being adjusted at this time is the one around the drier cores. There are 5 housings, paralleled, with 4 or 5 cores each (don't remember the #). Suppose to have a "slip stream" through those to maintain a clear sight-glass downstream of the cores. Hand valve can be used either as a bypass for replacing cores while system is running and or to allow more flow. Has been tried 100% open and less. I believe the evaporator is a 2 pass. Water in the tubes, refrigerant shell side. This system has an accumulator, not a surge drum (unless your description is the same). It is located above the economizer, higher than the evaporator. The condenser is located above the evaporator, the economizer same level as the evaporator, the accumulator is about the same height as the condenser, maybe a little higher but not much. System is app 6 years old. Never has been ran this "loaded" so this scenario is new, so it's just something I got to figure out. There are 8 skimmers on the evaporator providing liquid refrigerant/oil to the oil still downstairs. Actually had more time to "tinker* with the system yesterday. One question answered would help me put aside one theory: System is designed to provide 1,000 tons at a suction pressure of 41 psig, discharge 151 psig. If it is running at 45 and 131 psig, does the discharge pressure need to be increased? 67.1 f to 51.8 f or 15 degree drop across evaporator (designed 18 drop). So the approach on the evaporator is 1.8 degrees f

Maybe flow through S&T chiller is to high giving lower than expected td & load.
This may also effect higher than designed suction pressure.
You could also try to reduce flow to give design td & in turn may lower suction pressure.

Higher economizer pressure will reduce overall capacity of system.
Raising condensing pressure, although below design will also reduce capacity.

Unless ss already done changing filter driers & cleaning filters in them.

Anyway only suggestions

[PLEH2EREH]

Sorry MBC about not providing drawing; the company has restrictions/policies about such information. AND, if I tried to draw it... LOL ! You would think??? ok, which one is what? LOL !! However, I may make an attempt at it and attach it to a post...just don't laugh at it!!! RANGER1, yes the flow through the the evaporator is greater than design...about 300 gpm more; but they have no control of that.... what it is is what it is!! And yes I agree on that with you, and I di tell them that. I also agree on the reduced capacity....but if condensing pressure is too low, or not as "designed", it could result in oil issues. SEGEI, I will try increasing econo pressure and observe the results. I think this was tried before (like a year or 2 ago), and it didn't help...but I can't remember what the actual circumstances were at that time. ***Suction pressure increase from 40 psig to 45 psig will increase load by 10%. If you don't mind, please explain.. ***Solution Increase pressure in economizer. Economizer pressure of 100 psig will increase supply by 15%, compare to 80 psig. *** Once again, please explain the math...Gotta ask questions to learn, right! Thanks!!

[RANGER1]

No valves on discharge of pump or other reason to not be able to reduce flow?
I am purely guessing but why even have this economizer for such a high temp application.

So so when plant is on lower load level in accumulator must be higher, right!

[Segei]

Higher economizer pressure slightly reduce capacity of the system. However, at the same time it will supply more liquid into the evaporator and suction pressure will go up a few psig. This pressure increase will compensate capacity reduction by higher economizer pressure.
Most likely he should not increase condensing pressure. When pressure difference between condenser and economizer will be low, liquid refrigerant start to collect in the bottom of condenser and it will become subcooled. This subcooling will increase flow of the refrigerant from condenser to economizer. Self regulating system.

[Magoo]

After reading all the above.
Have you checked the liquid fill valve to open flash economiser, even solenoid valve pre, if fitted. Just that I had a pilot operated control solenoid that needed re-kitting because it was passing slightly during off cycle, a damaged diaphragm..

[Segei]

Sorry MBC about not providing drawing; the company has restrictions/policies about such information. AND, if I tried to draw it... LOL ! You would think??? ok, which one is what? LOL !! However, I may make an attempt at it and attach it to a post...just don't laugh at it!!! RANGER1, yes the flow through the the evaporator is greater than design...about 300 gpm more; but they have no control of that.... what it is is what it is!! And yes I agree on that with you, and I di tell them that. I also agree on the reduced capacity....but if condensing pressure is too low, or not as "designed", it could result in oil issues. SEGEI, I will try increasing econo pressure and observe the results. I think this was tried before (like a year or 2 ago), and it didn't help...but I can't remember what the actual circumstances were at that time. ***Suction pressure increase from 40 psig to 45 psig will increase load by 10%. If you don't mind, please explain.. ***Solution Increase pressure in economizer. Economizer pressure of 100 psig will increase supply by 15%, compare to 80 psig. *** Once again, please explain the math...Gotta ask questions to learn, right! Thanks!!

Assume that compressor displacement is 1000 cfm. However, refrigerant density is 10% greater at 45 psig than at 40 psig. It means that mass flow and load at 45 psig will be 10% greater. Compressor manufacturer has engineering data that show capacity and power of the compressor at different suction and discharge pressures. I took this ratio for ammonia but for R134A it should be similar.
Refrigerant flow through orifice is proportional to square root of pressure difference before and after this orifice. 80 - 45=35 Square root is 5.9. 100-45=55 Square root is 7.4. 7.4/5.9=1.25 My initial estimation was wrong. Supply will increase by 25%.

[PLEH2EREH]

7.4/5.9= 1.254 *x 100* =125.4% so how will supply increase 25%? I'm confused.

[PLEH2EREH]

If I can I will get compressor specs today and post later.

[Segei]

7.4/5.9= 1.254 *x 100* =125.4% so how will supply increase 25%? I'm confused.

Current pressure difference give us 5.9 or 100%. New pressure difference will give us 7.4 or 125.4%.
125.4 - 100=25.4% increase.

[PLEH2EREH]

OK. I was butt backwards on that one! Thanks!!

[sandybapat]

From the description given by PLEH2EREH, it is similar to having high side floats to control the liquid refrigerant flow so that the liquid level in the seal pot and economizer is maintained.
Since suction pressure is remaining high, possibility of oil accumulation in evaporator is lower.
The condenser selection and design ambient conditions need to be checked. The discharge pressure is 130 psig as against 150 psig. There may be two reasons; viz. lower ambient temperatures or much larger condenser. If the actual ambient WB temp is lower than design WB temp by 20 deg F then I would expect discharge pressure around 110 psig. In that case, since both suction and discharge pressures are higher, I would conclude that the system is overcharged.
With present pressure conditions the compressor will give 10% more capacity (4 deg F higher evaporation temp) and 10% less shaft power (20 psig 9 deg F lower condensing temperature).
PLEH2EREH did mention about economizer flooding, but has not indicated whether they are getting the desired process temperature or not.

[PLEH2EREH]

Discharge pressure isn't higher. Matter of fact, I had them to raise the condenser set point from 120 to 130 psig (R-134A). The condenser water vale is controlling well and has yet to open 100% to maintain set point. Funny you mention it, but I do believe the condenser was (slightly) over-sized, but not so much as to create problems.

[sandybapat]

What is design ambient wet bulb and actual wet bulb temperature?

[PLEH2EREH]

Sorry it took awhile for me to reply.... things change, and this and that, and ....well, most have been there. and then........everything is OK. Did remove some ***** and some oil but..... gotta wait till load demand comes back.

My thoughts, for what they are worth.....the chiller was being operated beyond factory spec and that was the biggest problem. I do appreciate all the replies and advice...Tks again.

[sandybapat]

PLEH2EREH - Would appreciate if you could share what remedial actions you have taken and what improvement you have observed for the benefit of all who have contributed to this thread. This way there will be enrichment of knowledge / experience and will give encouragement for more participation.

[PLEH2EREH]

Sorry for the delay.....after removing app. 250 lbs refrigerant and some oil things started looking a little better then,,,, load decreased because demand decreased....then plant went down and now....it isn't in operation. Can't wait till it gets back on line and see how it operates this summer! To be continued...........

[sterl]

Many compressor control arrangements do not permit vapor flow out of the economizer until the slide valve has loaded to a certain load rate...Otherwise the side port pressure and the suction pressure are very close or the same and the advantages of subcooling via the side port is the same as conventionally expanding the liquid....

Does your economizer arrangement maintain 80 or 85 Psig actual shell pressure irrespective of the machine's load rate? Is there anything like a solenoid on the economizer vapor side valve train?