Clean Condenser Running too hot??

[Chseybold]

Hi guys, I'm new to the forumn but have been around refrigeration for a while. I've been stumped by a problem and hope someone can give me a hand. We commisioned a blast freezer in Suriname and it has been giving us heaps of trouble. The system is using bitzer 2 stage 30hp compressors with a water cooled shell and tube condenser. There are 2 seperate refrigerant circuits per blast room, with each loop having a dedicated evaporator and condenser.
I triple checked all the calculations and everything matches, right size txv valve and condenser has more than 100,000 btuh extra capacity over the point where we want to run it.
We recentely cleaned the condenser and we are still not able to get the system to work how it is supposed to. There is good water flow. The system uses 404a and we are running it at 325psi and still can't get the site glass to clear up and the liquid is over 120F.
There is a liquid reciever installed as well and this has an equalizer line connected to the receiver. To allow good liquid flow from the condeser. I've read all my books and have racked my brain on this but can't figure it out. Any suggestions?

[123-steve909]

is that with product in the blast chiller or is it empty?

[123-steve909]

any idea what your superheat is??

[Gary]

Is the water fed into the condenser from the bottom up or the top down?

What are the water in and out temperatures?

[123-steve909]

good point, i have seen someone pipe a w/c condenser up wrong and it did the same

[Gary]

We also need to know if the system was operating properly before the condenser was cleaned. It is important that the end bells and gaskets were re-installed properly.

[Gary]

good point, i have seen someone pipe a w/c condenser up wrong and it did the same

Indeed, pumping water from the bottom up fills every little nook and cranny, while top down can leave large air pockets.

[123-steve909]

hi gary,
i see what your saying, i have seen a coaxil condenser with the water and refrigerand both going in at the same point and both leaving at the same point so the water contact with the refrigerant was small. the head pressure was really high and once changed the system worked as it should

im interested in your books, where has the link gone?

[Chseybold]

The water is being fed through the bottom of the condenser, there is no product in the machine. The system never really worked right from the beggining as each room has 60hp of 2 stage compressors so it should be cooling a lot better then it is. So it leads me to believe the manufaturer did something wrong in the assembly, I just don't know what and unfortunately they are out of business. The latest test I did, the unit was running around 15 F room temperature with about 25F superheat. Could it be noncondesables in the condenser?

[Chseybold]

The water in is 78F and the out is 90F. The condensers are getting at least 40gpm each. The shell part of the condenser is very hot, maybe I'm not getting a good liquid seal on the bottom? But as u can see the water side seams ok.

[123-steve909]

could be non-condensables in the system, i would be taking a close look at the condenser side of things as it seems to be something wrong there. is the water supply counter flow to the refrigerant?

[Chseybold]

By hot I mean, 135F shell temperature. With 170F hot gas temp. The compressor seams ok and is not running hot.

[Chseybold]

It's shell and tube, so it should be. I just don't understand why I can't add more Refrigerant into the system without the highside pressure going up.

[Gary]

hi gary,
i see what your saying, i have seen a coaxil condenser with the water and refrigerand both going in at the same point and both leaving at the same point so the water contact with the refrigerant was small. the head pressure was really high and once changed the system worked as it should

im interested in your books, where has the link gone?

The link is in my profile under contact info.

[bowral fridgy]

you won't get a clear sight glass with 404A without sufficient subcooling on liquid line. It is very prone to flashing off

[Chseybold]

See attached

[Chseybold]

I would think the condenser should be able to subcool at least a few degrees, or no?

[Gary]

At 122F SCT minus 90F water out temp = 32F approach temp. This is an indicator of heat transfer between the refrigerant and the water and should be no more than 20F.

There is definitely a heat transfer problem in the condenser.

[Gary]

You said the liquid was over 120F. Is that a liquid line temperature measurement?

[Chseybold]

yea after the liquid filter the line is 115F.

[Chseybold]

Could the temperatures be a result of the hot gas going by the liquid seal on the condeser? the condenser is Standard Refrigeration HSE-25a and at 40gpm and 20td it should reject 243,000 btuh when fouled. which is way above what my compressor and circuit need. So what gives?? It also important to note that there is a liquid reciever in the system and the line from the condenser goes up and then down into the reciever and that is why i put a equalizer line in, to get a good flow to the reciever.

[Chseybold]

I know there is an answer but I'm just stumped on this one.

[Gary]

If there were air in the system, then the SCT would be considerably higher than the liquid line temp.

[Gary]

Could the temperatures be a result of the hot gas going by the liquid seal on the condeser? the condenser is Standard Refrigeration HSE-25a and at 40gpm and 20td it should reject 243,000 btuh when fouled. which is way above what my compressor and circuit need. So what gives?? It also important to note that there is a liquid reciever in the system and the line from the condenser goes up and then down into the reciever and that is why i put a equalizer line in, to get a good flow to the reciever.

It's even worse than that. Your TD is 45F. The HSE-25a is rated at about 500,000 btu for that TD.

[Gary]

I found a service manual for this condenser:

http://www.stanref.com/pdfs/Service.pdf

Maybe we can find the answer somewhere in there.

[Gary]

Hmmm... On page 18, your front gasket is #1723. That horizontal bar is very important. If it were missing, part of the water would bypass the condenser. Could this be the case?

Your rear gasket is #2953. If you swapped the two gaskets this could cause big problems.

[Gary]

Here is a link to the product manual:

http://www.stanref.com/pdfs/107_SR.pdf

On page 8 there is a pictorial diagram. Is your condenser piped accordingly? Refrigerant in the top and out the bottom as shown? Water in the bottom and out the top as shown?

[Gary]

I don't understand the reason for the receiver equalizer line. Is there some way to shut it off?

[Gary]

Could the temperatures be a result of the hot gas going by the liquid seal on the condeser? the condenser is Standard Refrigeration HSE-25a and at 40gpm and 20td it should reject 243,000 btuh when fouled. which is way above what my compressor and circuit need. So what gives?? It also important to note that there is a liquid reciever in the system and the line from the condenser goes up and then down into the reciever and that is why i put a equalizer line in, to get a good flow to the reciever.

It seems to me that if the receiver is above the condenser, then an equalizer line would have the opposite effect, eliminating the pressure differential that moves the liquid from the condenser up to the receiver. If the pressures are equal, then the only force left to move the liquid is gravity... and gravity doesn't push upwards.

[Magoo]

Did you clean the pre-pump strainers and basket strainers at tower when cleaning condenser tubes, good water flow does not necessarily mean correct water flow. The manufacturer will publish water pressure differential info across condenser at correct flow rates which includes a fouling factor.
magoo

[Chseybold]

These are all good suggestions, I really appreciate the help guys. I think I'm going to have the following approach. I'm going to shut one of the units down while having pump continue running. After a while I will check the high side pressure and see if it matches with the water temp. This way I can elliminate the possibility of non-condesables. In the loop.

[Chseybold]

It seems to me that if the receiver is above the condenser, then an equalizer line would have the opposite effect, eliminating the pressure differential that moves the liquid from the condenser up to the receiver. If the pressures are equal, then the only force left to move the liquid is gravity... and gravity doesn't push upwards.

Gary, I'm thinking this has a lot do with it. The liquid exits the bottom of the condenser then goes up to a level higher then the shell of the condenser and then down into the top of the reciever and from there it has to exit through a dip tube in the reciever. The reciever and the condenser are at the same level and as you can see no gravity drain. The equalizer was put in between the hot gas line and through a T fitting at the pressure relief port on the reciever. The problem is that with and without the equalizer the system doesn't work right. So eliminate the reciever?

[Chseybold]

Did you clean the pre-pump strainers and basket strainers at tower when cleaning condenser tubes, good water flow does not necessarily mean correct water flow. The manufacturer will publish water pressure differential info across condenser at correct flow rates which includes a fouling factor.
magoo

yea all of these are clean

[Gary]

Gary, I'm thinking this has a lot do with it. The liquid exits the bottom of the condenser then goes up to a level higher then the shell of the condenser and then down into the top of the reciever and from there it has to exit through a dip tube in the reciever. The reciever and the condenser are at the same level and as you can see no gravity drain. The equalizer was put in between the hot gas line and through a T fitting at the pressure relief port on the reciever. The problem is that with and without the equalizer the system doesn't work right. So eliminate the reciever?

Does the sight glass clear without the equalizer line?

I would eliminate the equalizer line, take a new set of pressure and temperature readings, then decide where to go from there.

[Chseybold]

Does the sight glass clear without the equalizer line?

I would eliminate the equalizer line, take a new set of pressure and temperature readings, then decide where to go from there.

The sight glass does clear without the equalizer line but the system still doesn't run right. We have 2 Bitzer S6F-30.2 Compressors on each room, and the room has never gone below -10F. (This is without any load by the way and running for 24hrs) Without the equalizer the unit starts to run on high amprage and it seem the condenser fills with liquid and loses it's capacity. The other problem is that the unit is far away and we'll have to relay the steps for the local tech to do. There are 2 rooms by the way for a total of 4 circuits and all of them have similar symptons.

[Gary]

The sight glass does clear without the equalizer line but the system still doesn't run right...

Without the equalizer the unit starts to run on high amprage and it seem the condenser fills with liquid and loses it's capacity.

Which tells you that the unit is grossly overcharged. Eliminate the equalizer line, then remove refrigerant until the sight glass bubbles... then add a little refrigerant back in to eliminate the bubbles.

[Gary]

An equalizer line is useful when the condenser is cooler than the receiver AND the condenser is above the receiver.

For example: If the condenser is on the rooftop and the receiver indoors, then cold ambient can drop the condenser temperature/pressure, while the receiver is at the higher indoor temperature/pressure.

Equalizing the pressures allows gravity to drain the liquid from the condenser down to the receiver.

Equalizer lines must always have a check valve to prevent reverse equalization when the condenser becomes warmer than the receiver.

[Chseybold]

Which tells you that the unit is grossly overcharged. Eliminate the equalizer line, then remove refrigerant until the sight glass bubbles... then add a little refrigerant back in to eliminate the bubbles.

That is how it was done when we first started the unit. But it still had problems. But you are right about the equalizer. I just read up on it now, it's for assisting gravity flow. But here it is not the case. So the equalizer is wrong then, it just seems there is something wrong with the reciever/condenser set-up. Or have u seen a shell and tube condenser fuction with such a set-up where the liquid has to flow upwards into another vessel where the liquid and gas can seperate. How is the gas removed from the reciever, and what happens when the txv valve closes down?

[Chseybold]

I'm having trouble visualizing the flow

[Gary]

1 psi differential can lift liquid 2.43 ft.

Most condensing units lift the liquid from the bottom of the condenser up to the receiver. The added psi is inconsequential.

[Magoo]

The system is in overload, the enclosure should be at design before any product is introduced. No different than a spiral freezer, all spiral manufacturers stipulate that enclosure has to be at design before product enters. If otherwise you want to pull-down and freeze product at the same time the condenser.., heat of rejection., would be hugely greater than at design conditions. With compounded recips., outside of design pull-down you risk burning them out with high discharge pressures/ temps. the inter-cooler stage has no chance of de-suerheating low stage, so high stage pressures and temps.
magoo

[Gary]

That is how it was done when we first started the unit. But it still had problems.

I don't doubt that the system has other problems, but first we need to deal with the known problem, eliminating the equalizer and adjusting the charge.

Then we need to look at the temps and pressures. These would include:

Low and high side pressures.
Condenser water in and out temps.
Evap air in and out temps.
Liquid line and suction line temps.

[Gary]

Anything new happening with this system?

[Chseybold]

Nothing new, we're in the process of changing the condesate drain line as the going up and down of the line isn't a good arrangement, and I think there might be a gas lock in this area. There is also an intersage take off on this line that shouldn't be there. The pipe size is much larger as well 1 3/8 compared to the 7/8 liquid. I also think that there is no baffel plate at the bottom of the condenser so there is no subcooling in the condser.

[Gary]

Any chance of getting the temps and pressures I asked for earlier?

[Gary]

Nothing new, we're in the process of changing the condesate drain line as the going up and down of the line isn't a good arrangement, and I think there might be a gas lock in this area.


Are you referring to the drip line from the condenser to the receiver?

[Gary]

There is also an intersage take off on this line that shouldn't be there.

Is this referring to the liquid line for the interstage metering device?

[Chseybold]

Are you referring to the drip line from the condenser to the receiver?
Yes this line. The unit is running at 15 degree superheat, 20psi suction, 300psi high side, 75F entering water / 85F leaving water.

[Chseybold]

Is this referring to the liquid line for the interstage metering device?
Yes this line. A whole is drilled/burned into the line at the higher level and it pulls from here. I think that this tube is pulling from around the middle of the line (think of the cross section of the pipe) and it is flashing. So that is one of the reasons I think there is a gas lock in this line. Usually this interstage line is connected to the liquid line (after the reciever in my case)

[Gary]

Yes this line. The unit is running at 15 degree superheat, 20psi suction, 300psi high side, 75F entering water / 85F leaving water.

300psi gives us SCT of 117 - 85 = 32F approach. Same as before. We still have a heat transfer problem in the condenser. Not having the liquid line temp, we can't tell if this is an overcharge problem or not.

[Gary]

Yes this line. A whole is drilled/burned into the line at the higher level and it pulls from here. I think that this tube is pulling from around the middle of the line (think of the cross section of the pipe) and it is flashing. So that is one of the reasons I think there is a gas lock in this line. Usually this interstage line is connected to the liquid line (after the reciever in my case)

I agree. Not a gas lock, but possibly vapor feeding the interstage TXV.

What is the interstage superheat?

[Gary]

The water in is 78F and the out is 90F. The condensers are getting at least 40gpm each.

Hmmm... it occurs to me that with SCT of 122F and approach of 32F, there should not be enough heat transfer to get this dT of 12F.

I recall there is a formula for calculating BTU output given dT and GPM. Anyone have that formula handy?

I'm thinking that the actual water flow may not in fact be 40GPM or even close, or the dT would be much lower given the high approach.

[Gary]

I'm wondering if this could be a case of 60hz pump motors running on 50hz power supply... or the pumps rotating in the wrong direction... or undersized piping... or reduced flow through valves/fittings?

[Gary]

Correct me if I'm wrong here:

BTU = GPM x dT x 500

40 x 12 x 500 = 240,000 BTU

So... is this system moving enough BTU to do the job?... OR is the GPM considerably less than 40?

[Chseybold]

The flow is ok, the minimum flow for the cooling tower water distributer to rotate is 80gpm. It rotates and the pump is sized ok. 60hz power

[Chseybold]

Also if there wasn't enough water flow, wouldn't the water temp differential be much higher then 10?

[Chseybold]

Unfortunately the client is getting very nervous as this system is not working properly. Finally upon doing some research I think the problem is with the liquid seal on the bottom of the condenser, I posted another question about this in the fundamentals section. If there isn't a proper seal no matter what I do I won't be able to subcool the refrigerant...

[Chseybold]

and supply liquid without flash gas in it. Thanks for all the help by the way. I think I might just take a crap shoot and send him another condenser to test.

[Gary]

Also if there wasn't enough water flow, wouldn't the water temp differential be much higher then 10?

Yes it would... if the heat were getting transferred to the water.

You stated that trying to clear the sightglass causes to SCT to rise. This would transfer more heat to the water, which would give us a higher differential, which would seem to confirm insufficient water flow.

Given a clear sightglass, perhaps we could look at all of the temps/pressures and figure out the underlying problems.

I would add refrigerant until the subcooling is 15F, then take an entire new set of pressure and temperature measurements.

[Peter_1]

Just joining this thread: I should recover all the refrigerant and vacuum the whole system. This is a job for only some hours.

You're then sure having no non-condensables (NC) in the system.

Check temperature with pressure of your recovered refrigerant and determine if refrigerant was eventually contaminated with NC.

Recharge the refrigerant in a liquid phase and leave some lbs in the bottle to be sure that eventual NC remains in the bottle. Calculate roughly how much you may charge in the system.

I doubt the condenser is fault, you should have then 2 faulty condensers.

[Chseybold]

Given a clear sightglass, perhaps we could look at all of the temps/pressures and figure out the underlying problems.

This is one of the problems, the sight glass can't clear!! Not even with an overcharge!?

[Chseybold]

Peter the problem is that there are not 2 but 4 of the same condensers!! So the only thing I can think is that they are defective or piped wrong (which I do not think is the problem) I'll tell them to test for non-condensables again, because that is a strong possibility but due to how many of the machines there are you would think that at least 1 of the 4 would work!

[Gary]

Let's look at this from a different point of view:

40GPM x 10dT x 500 = 200,000BTU

If we assume that the pump is indeed moving 40GPM, then it is delivering 200,000BTU with the rest of system not working properly.

What if the rest of the system were working properly? Just taking a wild guess, the output would be something more like 300,000BTU... in which case we would need a 60GPM pump in order to maintain the 10F dT.

What is the heatload?

What is the capacity of the compressor?

At the current SST is it indeed outputting 200,000?

Do you have performance charts for the compressor?

[Gary]

Given a clear sightglass, perhaps we could look at all of the temps/pressures and figure out the underlying problems.


This is one of the problems, the sight glass can't clear!! Not even with an overcharge!?

Of course the sight glass can be cleared... and charging to 15F SC is NOT an overcharge.

[Gary]

Here is my current theory: The evaporator isn't working properly because there is not enough refrigerant to clear the sightglass and feed liquid to the evap.

You haven't cleared the sightglass because the head pressure goes too high.

This brings us to the big question. Why does the head pressure go too high when the sight glass is cleared? There are only a few possible answers:

A problem with the condenser.

A problem with the pump.

Both.

I would add refrigerant until the SC is 15F.

If the head pressure and dT are then excessive, I would install a 60GPM pump.

Then see what the numbers tell me about the condenser.

[Gary]

Finally upon doing some research I think the problem is with the liquid seal on the bottom of the condenser...

That's just another way of saying that there is not enough refrigerant in the system... it means the same thing. When the sight glass is clear, there is a liquid seal at the bottom of the condenser.

[Gary]

Next question: You say the head pressure goes too high when you try to clear the sightglass, but you don't tell us what the pressure went to. How high is too high?

And you say the sight glass cannot be cleared even with an overcharge. How do you know it is an overcharge? What is your definition of overcharge?

[Gary]

If there isn't a proper seal no matter what I do I won't be able to subcool the refrigerant...

This makes no sense. Can you explain your reasoning on this? Maybe I am missing something.

[Peter_1]

Which Bitzers are you using? I suppose a SGF30.2Y?
Those are giving 35 kW at -35°C. ;) together with the absorbed power is this +/- 60 kW= +/- 204.000 BTU

The pump is giving 40gpm, is this a free flowing flow or is this with a certain back pressure?

[Peter_1]

Do you have a technical file of the used pump? flow/head pressure?

[Gary]

Please correct me if I am wrong.

As I understand it, the equalizer line and receiver have been removed.

These are major changes, so we need to know the new temps and pressures.

Without information, all we can do is offer wild guesses.

[Gary]

With incoming water at 78F:

If the dT is 10F and the approach is 32F, the SCT should be about 120F and the high side pressure should be about 310psi.

[indian]

Hiya guys have spent a little time reading your threads. May be wrong but to my mind there are possibly two problems here.

Firstly I would say that the equalization line must go. I think this may have allowed discharge gas to flow into the receiver mixing with the liquid causing flashing (was there a check valve? suspect not). Even if there was a valve in place the high water flow and big condenser with low load would allow liquid to form in the bottom of the condenser until the point where pressure forced it into the receiver along with discharge gas causing a mix in the liquid line i.e. flashing. Yet if the liquid built up in the condenser enough, as more charge was added to stop sight glass flashing, this would reduce the surface area of the condenser which would eventually cause lack of condensing and high head pressure as was noted originally. If you read second point will explain why I dont think there was a check valve on the equalisation line.
Removing the equalation line would stop this.

But then if the receiver is still in place we get a second problem. No one has mentioned head pressure control or water regulating valves. As has been said the condenser should easily handle the system duty at full load, at minimum load it is way too big with a great flow of cooling water flowing through it. Therefore the refrigerant is condensing in the condenser - as it should - but due to the uphill run it cannot get into the receiver so it fills up the condenser. Eventually the surface area reduces the system pressure builds it pushes liquid into receiver and fills it. Then once more as the system gets more refrigerant the condenser fills with liquid reducing the condenser area again and pressure builds but we now have a clear sight glass a full receiver and a shed load of refrigerant and an overcharge.

So for me get rid of equalisation line and receiver, decant and recharge just until sight glass stops flashing and possibly fit some for of head pressure control for low loads.

Possibly a complete load of rubbish but makes some sense in my strange world:confused:.

[Gary]

Hiya guys have spent a little time reading your threads. May be wrong but to my mind there are possibly two problems here.

Firstly I would say that the equalization line must go. I think this may have allowed discharge gas to flow into the receiver mixing with the liquid causing flashing (was there a check valve? suspect not). Even if there was a valve in place the high water flow and big condenser with low load would allow liquid to form in the bottom of the condenser until the point where pressure forced it into the receiver along with discharge gas causing a mix in the liquid line i.e. flashing. Yet if the liquid built up in the condenser enough, as more charge was added to stop sight glass flashing, this would reduce the surface area of the condenser which would eventually cause lack of condensing and high head pressure as was noted originally. If you read second point will explain why I dont think there was a check valve on the equalisation line.
Removing the equalation line would stop this.

But then if the receiver is still in place we get a second problem. No one has mentioned head pressure control or water regulating valves. As has been said the condenser should easily handle the system duty at full load, at minimum load it is way too big with a great flow of cooling water flowing through it. Therefore the refrigerant is condensing in the condenser - as it should - but due to the uphill run it cannot get into the receiver so it fills up the condenser. Eventually the surface area reduces the system pressure builds it pushes liquid into receiver and fills it. Then once more as the system gets more refrigerant the condenser fills with liquid reducing the condenser area again and pressure builds but we now have a clear sight glass a full receiver and a shed load of refrigerant and an overcharge.

So for me get rid of equalisation line and receiver, decant and recharge just until sight glass stops flashing and possibly fit some for of head pressure control for low loads.

Possibly a complete load of rubbish but makes some sense in my strange world:confused:.

I agree your comments on the equalizer line, but as I understand it, this line has been removed.

As to your comments on liquid flowing up to the receiver:

1 psi of differential can push a column of liquid up 2.43 feet.

When there is sufficient liquid at the bottom of the condenser to form a liquid seal, the needed pressure differential will be immediately established and if there is sufficient liquid in the condenser to maintain that seal the pressure differential will remain and the liquid will continue to flow.

The sight glass will be clear, there will be a slight difference between the condenser pressure and the receiver pressure and the liquid level in the condenser will be just enough to maintain a liquid seal. Similarly, the liquid level in the receiver will be just enough to provide a liquid seal.

Most small condensing units have a vertical receiver mounted on the same base plate as the condenser, with the liquid flowing from the bottom of the condenser up to the top of the receiver. The added pressure differential is inconsequential.

But this is a moot point, because the OP has informed me that the receiver has also been removed.

[indian]

The theory there may be fine and well dont intend to kid you that I am fluent enough in math to argue the point.

Actually totally agree about liquid locking of the condenser to receiver, have seen hundreds of small condensing units where condenser outlet below receiver and every one works - but not seen too many that had a condenser that was, what - say double the size it needed to be.

The only thing is from my experience I know that I can take a supermarket pack system which has way too much refrigerant in it to pump down normally. Yet if I take the condenser and force the fans to run full speed I can back up all the liquid into the condenser without tripping the hp until its pretty damn full of refrigerant. Have done similar with compressor receiver sets emptying the receiver into the condenser simply by closing the condensate return and running with condenser on full speed.

In doing this all I was doing was simulating a low load ie reducing need or availability of liquid to system combined with (I know some of you guys dont like the term) over condensing the liquid - or excessively sub cooling it. In this way the liquid backs up into the condenser.

Even with the receiver removed this confirms my point if we have a low load ie valve is closing down (like pump down) combined with maximum condensing ability you will backup liquid into the condenser which will eventually hit a point where the reduced surface area in the condenser until head pressure shoots up and we have a problem - over charge. Think there may even be a bit of math in there regarding the volume of refrigerant if it is excessively sub cooled think its volume may shrink as well so you can get more in.

By the way am on your side have read other posts re ODP and GWP have studied Paleoclimatology, Geology at University and all the facts as most people belleive them are not FACTS.

[Gary]

In my previous post I was discussing a system that is properly charged.

When a system is overcharged, the excess refrigerant liquid backs up into the receiver and the condenser simultaneously while still maintaining the needed pressure differential. Being that liquid is backing up into the condenser, the active surface area is reduced and the pressure does indeed rise.

Subcooling tells the story, but we don't have the temperature and pressure measurements to evaluate this system properly. We can't see it from here and we don't have the measurements, so all we can do is guess.

[Gary]

Hmmmm... just another thought:

It has been a very long time since I worked on a 2 stage system, but it seems to me that the liquid is heavily subcooled at the interstage and thus the liquid line from the interstage to the evap needs to be insulated. Please correct me if I am wrong on this.

[indian]

Did'nt think it was two stage system just a two stage compressor so stages internal within the compressor just suction and discharge to condenser - at least thats what I took it to mean.

[Gary]

I am thinking you are probably right.

Pictures might be helpful.

[Gary]

Are you making any progress with this system?

[Chseybold]

I'm sorry for not following up with you guys. Unfortunately I never had the chance to go back down to South America to do the suggestions. But I think indian described the situation in his post. I wanted to remove the receiver completely and pipe it direct. But I never got the chance as the client has financial troubles.
Thanks gents for all the good advice, wish I could have implemented them and gave more data.

[Brian_UK]

Thanks for coming back and giving the update, that's life I suppose.