oil recovery problem in ***** overfeed system

[COLD WANG]

hi everyone ,i had a question :
how to solve oil return problem ,especially the oil return in low pressure reciever.

[US Iceman]

The oil will float in a miscible layer on top of the liquid R-22 in the vessel.

You will need some type of oil recovery system to ensure oil return. This is normally done with a method called a "skimmer".

Imagine if you will some material floating on the surface of the water in a lake. If you skim (or remove) the material from the top of the water, that is the same principle as removing the oil from the vessel.

Almost all of the systems like this I've worked on used a venturi to suck the oil off the liquid refrigerant layer. The oil is recovered and distilled before the oil is returned to the compressor(s).

[COLD WANG]

thanks iceman,
how the skimming system worked? by many pipes in the drum?how can it samples the oil rich layer? with many holes on drum surface? -40c temp the oil is separated from the R22? HOW ABOUT THE MISCIBILITY AT -40DEGC?THE MIXES CAN BLEED UPTO SUCTION LINE?depend on THE PRESSUR DROP between the liquid surface and suction lines?the pressur drop is so small! it can assume the liquid rise to suction?

[US Iceman]

I normally use three pipe connections on the vessel. The middle connection is at the controlled liquid level. One connection slightly above the controlled liquid level, and one connection slightly below the controlled liquid level. Each connection has a metering valve to adjust the flow for various operating conditions. All three of these lines are then connected into one pipe. This common pipe is the suction connection to the venturi.

The venturi is gas powered by high pressure discharge gas from the compressors. With the venturi, the discharge gas flowing through the venturi creates a suction on these three connections.

The suction helps to pull the oil off of the liquid level.

The outlet pressure of the venturi is sufficient to transport the oil/liquid R-22 mixture back into the compressor suction.

If the systems are adjusted properly, you will get very little if any liquid back into the suction of the compressors (screw compressors only please).

If you are using reciprocating compressors, it's best if you use a heated oil still to warm up the oil and boil off any liquid R-22, before it returns to the compressor crankcase.

You are correct about the -40C miscibility problems. The oil has to have a low pour point in order to flow and still have sufficient viscosity for the compressor lubrication.

[COLD WANG]

hi iceman ! thanks!
but in our country,a lot of people tell me they dont need the venturi.they simply sample the oil pipe and rise up to the suction line above ?are you sure that plan is right?this method can bring the oil up to the higher suction line ?offcouse they need a heater .and from the ashrea 98 handbook chapter 2,page 2.23,we can find a flooded evaporator pipe figure like this way.

[Peter_1]

Interesting USIceman.Do you have some schematics about this?
What happens to the pipe that's slightly below the controlled level? Is this tube constantly overflown with pure liquid then?

The skimmer system is the same principle they use in swimming pools to remove the floating debris on the water.

Ideally should be if the skimmer could float on the liquid level so that the suction side of the skimmer is allways at the correct height, sliding over the central suction tube.
I saw this once in a swimming pool but can't remeber anymore how it excatly worked.

[binman54]

From the article:
Trouble Shooting ACR Systems - Issue Oct-Dec 1998

“All flooded systems are generally designed for liquid refrigerant levels of 60 - 80% in the chiller shell. The oil in R-22 systems is not heavy and does not get collected at the bottom of the shell. The oil is not so light as to float on top of the surface either. The concentration of oil in the refrigerant is much higher at a level somewhat below the upper boiling surface of the refrigerant. Since the liquid level in the chiller fluctuates within limits as per the load, it is customary to provide two or three outlets from the shell, to tap a concentrated mixture of oil and refrigerant. Depending on the actual level of the refrigerant, an appropriate hand shut off valve is opened while the remaining two valves remain closed and the mixture is taken to a vessel, which is heated either by an electric heater or by hot discharge gas from the compressor. The refrigerant in the mixture boils and is returned back to the suction, whereas the oil which remains in liquid form, is returned to the crankcase. Oil retrieval for all flooded chillers has to be put into operation continuously so that oil enrichment does not take place in the chiller. The oil return problem in flooded systems becomes more severe as evaporating temperatures become lower. This problem of oil return is flooded R-22 chillers is common not only for reciprocating compressors but also for screw compressors.”

There is a diagram in the article that shows a typical piping arangement. It was in a PDF format that I could not copy and paste here.

We generally left all three valves open a little so that an (hopefully) oil rich mixture was always bled into the oil pot. When there was frost on the oil pot it had liquid refrigerant in it. When all the frost was gone the oil pot was full of oil. This oil was then bled off into the compressors. By heating the oil in the pot the pressure was higher than the compressor crankcase pressure so it was easy to move the oil from the pot to the compressor.
Most systems were manual in that someone had to look at the oil pot, determine when it was full of oil and open or close valves to move that oil into the compressors.

binman54

[US Iceman]

What happens to the pipe that's slightly below the controlled level? Is this tube constantly overflown with pure liquid then?

You are correct Peter. If the metering valves are not adjusted properly, one of the connections may pull gas, while another pulls all liquid. It is not very exact to set this up. It's more of a good guess really.

Some of the liquid will atomize as it goes through the venturi, but there is always some small amount of liquid that leaves the venturi outlet I belive.

I will have to look for some drawings. I might have something...

[Peter_1]

What do you think about the floating skimmer I once saw in the swimming pool?
Is this ever made in refrigeration to recover the oil?
Gave it in google under pictures and 650 results for it.

[Peter_1]

http://www.zip.com.au/~dragon/HOMEx.html

[US Iceman]

I have a friend of mine who has done this before on a pumped recirculation vessel. Inside of the vessel he had a short piece if tubing with a float attached to the open end. The external connections were piped to suck the oil through the tubing.

As the liquid level raised and lowered, the float followed the liquid level so that the same amount was drawn off all of the time.

The float he used was shaped like one of those rubber ducks kids like to play with in the bath tub.

One thing to be careful of is... the oil does not form a separate layer. It forms in a mixed layer that could be thick. If the vessel pressure is reduced you might see some foam/bubbles that would raise the float out of the oil rich layer.

So, my thought would be to have the tubing weighted down somehow so that the end of the tubing was always slightly below the actual liquid level of the refrigerant.

[COLD WANG]

hi,everyone,i had a question:
normally the flooded evap or low pressur recieve is bellow the suction line to the compressor , without The venturi the mixes can rise up to the suction line? the line is normally two meters higher than the liquid lever? if without venturi we can find another way to solve the problem? a heater?a double risers?

[US Iceman]

...without The venturi the mixes can rise up to the suction line? the line is normally two meters higher than the liquid lever?

How would the mixture rise without any assistance?

This is why the venturi is used. First it creates a suction to reover the oil, and the outlet pressure of the venturi is greater than the suciton pressure so you can lift the mixture.

[COLD WANG]

hi iceman.
i had a zip file that i will send to you,but it is too large to upload.could you tell me your email adress?the file is a picture:
the ashrea 98 handbook chapter 2,page 2.23,we can find a flooded evaporator pipe figure like this way without a venture

[Peter_1]

you can upload it for free on www.megaupload.com so that we all can see it.

[COLD WANG]

hi peter1,hi iceman,
i upload success.please give some advise to me.

[NH3LVR]

There are two other methods used which no one has mentioned.
The first was used by a Manufacturer building continuous freezers. He built Spiral Freezers using R-22 (Flooded with Surge Drum) and no apparent means of oil removal. He installed Frick Screw Compressors. The oil removal method was to pump the Accumulator out overnight once a month and drain the oil from the drop leg. It would be less than a gallon a month. This is not a good method for a system that pumps any amount of oil.
This would certainly not be a good idea in a cold storage, but it worked fine in this application.

The other is used by North Star Flake Icemakers. http://www.northstarice.com/FlakeIceMaker.aspx?lang=2 The Ammonia and ***** models are interchangeable. With NH3 you have to deoil by pumping down,injecting hot gas, and letting the oil and condensed Nh3 flow back down the suction line to a suction trap. Usually this is a once a month procedure.
With ***** it is a bit different. Using the same drain valve, (controlled by a solenoid), you maintain a continuous bleed back into the suction. This keeps the oil percentage down and works well. You may have to close the suction stop valve on the icemaker a bit to keep a slight differential pressure.
Again these are not for all applications but can work well.

[Tycho]

Here's the kind we use on most ***** systems, it's a simple heatexchanger.

semi static level of cold liquid ***** on one side, and every time the pilotvalve opens, warm liquid on the other side.

also note the placement of the return pipe connected to the compressor suction... it is placed so that if there are two compressors and one is not running, the oilreturn to the standby compressor goes back into the suction line and drains back to the liquid tank.

works like a charm, no moving parts except for the reg.valve...

[COLD WANG]

thanks tycho!
its very simple,this kind you use is diffrent with iceman,the refrigentant in your system isnt R22?because the bleed is on the bottom ,not on the top of the liquid level,the liquid static pressure can help the mixes uprise to the suction line, but with R22,we cant do it .

[Tycho]

thanks tycho!
its very simple,this kind you use is diffrent with iceman,the refrigentant in your system isnt R22?because the bleed is on the bottom ,not on the top of the liquid level,the liquid static pressure can help the mixes uprise to the suction line, but with R22,we cant do it .

I beg to differ

The refrigerant is R22, in a standstill liquid reservoir, oil will seperate from the ***** and float on top (even then you have a small amount of oil in the *****) but as long as the pumps are running, oil will be mixed into the *****.

the main return from the "consumers" is pointed downwards in the tank and and this will mix the ***** and oil.

We've used this kind of oilreturn for well over 20 years now, and it works

I know some take their oil return from the pump pressure line, using a TE12 expansion valve to regulate the amount of cold liquid entering the oilboiler, I cant say I like it, cause the moment there is something wrong with the TE valve you will be pumping liquid directly into the compressor suction.
(I'll draw it up when I get home, in a few hours, if it was unclear

-cheers

Tycho

[COLD WANG]

thanks tycho for your reply!
how much degc temperatur in the tank ,if lower to -40DEGC,the miscibility is very low with R22 and oil,if temp is above 0DEGC,i believe you can do it, but at -40DEGC,or even lower temp ,this kind you suggested
can do ?

[Tycho]

thanks tycho for your reply!
how much degc temperatur in the tank ,if lower to -40DEGC,the miscibility is very low with R22 and oil,if temp is above 0DEGC,i believe you can do it, but at -40DEGC,or even lower temp ,this kind you suggested
can do ?

We use this method on both -40C freezing plants and flooded sea water chillers.

[COLD WANG]

thanks tycho ! thanks for your reply!
but the oil is floating on the top of the liquid,us iceman told us.there is fewer oil in the bottom .in the ashrea handbook :

“Refrigerants 22 have a separate oil-rich phase floating

on a refrigerant-rich layer. . When R-22 are used with
mineral oil, the bleed line is usually taken off the shell just slightly
below the liquid level, or there may be more than one valved bleed
connection at slightly different levels so that the optimum point can
be selected during operation. With alkyl benzene lubricants, oilrefrigerant
miscibility may be high enough that the oil bleed connection
can be anywhere below the liquid level. The solubility charts
in Chapter 7 give specific information.
Where the flooded cooler design requires an external surge drum
to separate the liquid carryover from the suction gas off the tube
bundle, the richest oil concentration may or may not be in the cooler.
In some cases, the surge drum will have the highest concentration of
oil. Here, the refrigerant and oil bleed connection is taken from the
surge drum. The refrigerant and oil bleed from the cooler by gravity.
The bleed sometimes drains into the suction line so that the oil can
be returned to the compressor with the suction gas after the accompanying
liquid refrigerant is vaporized in a liquid-suction heat interchanger.
A better method is to drain the refrigerant-oil bleed into a
heated receiver that boils the refrigerant off to the suction line and
drains the oil back to the compressor.”

the oil you used isnt mineral oil?

best regards

[Tycho]

we use this on plants with both kinds of oil and have never had any problems with it.

Only "problem" I've seen with one of these is if the ***** level is to low, you will get poor oilreturn, but that is kinda obvious concidering the design

[COLD WANG]

thanks tycho!
i see.i have looked at the book “ industrial refreration handbook” chapt15,this chapter is same as you metioned.but differant with ashrea handbook .i cant upload these pdf file ,if your have a email adress ,i can send to you .my frind,i would like enjoying your advice.YOUR OPITION IS VERY HELPFULL TO ME!
Merry Christmas
best reagards
cold wang

[COLD WANG]

hi ,us iceman.

i upload part of the industrial refrigeration handbook.

in both two kind method(two figure) ,the connection of bleed mixture is all in the bottom of the tank ,not in the top? as same as what tycho told me.

hi , anyone can explained ?

[Tycho]

Well, I cant give you a definite answer to why it is done this way, but my take on it is that as the warm liquid passes through the heatexchanger, ***** is boiled off and you are after a while left with a high concentration of oil on the cold side.

This layer of oil will stay on top of the liquid as it's a higher concentration that whats left in the liquid seperator. when the warm liquid passes through, it will cause the cold liquid to boil and thus lift the oil up towards the outlet of the exchanger and with the amount of gas created, the velocity is high enough near the outlet so that oil is pulled out and into the compressor suction...


(disclaimer: The above is just my take on it )

on some plants where the length of the oilreturn pipe is very long, we let the connection to the suction go about 3-4 cm inside the suction pipe so it creates a "injector" effect to ensure proper oil return. if it's too much, we can throttle the reg valve.

The oil return should alway be connected before the compressor suction valve, so that if someone inadvertedly close it they wont drown the compressor with liquid

Also, I just remembered another problem I had with these, if the amount of oil in the ***** gets too high, you wont get enough boiling in the exchanger to lift the oil high enough.

This can be solved by placing a small (10 - 20 mm) pipe from before the reg valve on the oilreturn to after the suction valve, so that an engineer/service personel can manually return oil by closing the regvalve, opening the bypass and throttle the suctionvalve. this should not be done by anyone who does not know what they are doing

[NoNickName]

I'm resurrecting this thread as I'm asked to provide information on how to connect our compressor to such a system, having flooded evaporator and overfeed liquid.
I read the thread, but I'm unable to definitely understand why this is preferred instead of the usual separation of oil on the discharge pipe.
How common is this method?

[NoNickName]

anyone would like to answer?

[jason09]

hi nonick,
i did nt read all posts but the skimming type of oil return is fairly common on packaged liquid chillers but if i was you i would use a float off the discharge line ,its simple and effective

[NoNickName]

hi nonick,
i did nt read all posts but the skimming type of oil return is fairly common on packaged liquid chillers but if i was you i would use a float off the discharge line ,its simple and effective

That's what I do in the majority of cases, in fact.

[NH3LVR]

I read the thread, but I'm unable to definitely understand why this is preferred instead of the usual separation of oil on the discharge pipe.
How common is this method?

The problem is that no matter how efficient the discharge oil separator some oil will get through.
As I mentioned in a earlier post on this thread this may not cause a problem in some systems. See my comments on North Star Icemakers.
In others it can be a major problem. These systems are those where there is no return and the evap cannot be shut down on a regular basis for deoiling.

[jason09]

i admit that im from an air conditioning back ground and have little experience with low temp application but i dont think you will ever stop a small amount of oil circulating past the separator but it should catch and return a most of it.

[smpsmp45]

Dear Tycho,

You had posted an diagram which is self explanatory. One of our customers has R 404A based Ice plant ( star Design) & that is used on Flooded Evaporator. he too has similar oil recovery issue. I have suggested him your technique for the system. I shall get back to this once we modify & start using the system

[VagAn]

Hello And nice to meet you.
We seem to have a problem with oil not returnig to the heater. could it be because of changing R22 with R507 or should R507 behave tha same way the R22 did?