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ferret82
19-01-2012, 03:03 PM
Well as the title states I am having trouble with a air-conditioning system on the Oil Rig that I work on. The system is quite large and has two AHU per compressor which supply a top floor and bottom floor. Each AHU has its own manifold with two TxV's on the pipework into the evaporator.

The problem is that the compressor unit is a fair distance away from the AHUs so there is alot of pipework, I have had one guy before I got here have a go and thought that it required more and more refrigerant, to which I found that the compressor was cold round the base which I though that it may have been getting liquid back to the compressor.

I am a little confused as to where I should take my readings to calculate the super heat and sub cooling of the system. The system uses R22 and is sea water cooled. I have noticed that the site glass is about 1/2 full but the Discharge Temp is 45dC and suction temp 0dC. The suction valve is sweating/icey but still the AHU I dont think are removing enough heat as compare to the other side it is warmer. Mind you this units pipe work is much shorter. Also in the AHU there is alot of water coming off the evaporator coils which I think is excessive.

When I put a temp gun on the coil it is reading about 15dC Is there any thing I should know about taking the take the super heat and sub cooling reading with 2 AHUs in the system? What I would like to know is where abouts should I take the super heat reading from the out let of the evaporator if there is alot of pipe work back to the compressor. If I cant take the pressure where the bulb is located what should I do then? Do I take the temperature I find on the suction line(somewhere after the evaporator outlet) - the bulb temperature. If so then how to you finely tune each system to have 8dC superheat. Is this done though shutting off valves. Hopefully I have explained my self well enough, if you need more information let me know Thanks for you time and help

The Viking
19-01-2012, 09:03 PM
First off,
Has this system ever operated properly?

Then,
for sub-cooling and super-heat calculations we do need the pressures, ideally taken at the same point as the temperatures but if that not is possible by the compressor is normally OK as long as the system isn't too far from "good".

By the sound of things one or more of the TEVs are not properly set up and your system might be short of refrigerant. There might also be an issue of too much seawater passing through the condensor.
We just don't know without all the data (like pressures)

Now a reflection,
This system consists of 4 individual evaporators fed via TEV/TXVs, if any one of them isn't correctly set up the system will fail.
This is not the system to learn on, what you need is an engineer that is experienced in setting this type of systems up. You will probably not find that by calling in just any A/C guy (you might but...) your best bet is most likely to find someone experienced in setting up and troubleshooting supermarket systems or commercial walk in refrigerators.

Sorry.

:cool:

r.bartlett
19-01-2012, 10:57 PM
The system is flooding. One or more of the TEV's requires closing down or replacing.


temp gunning the coil is meaningless. take the temp of each suction as it leaves the ahu the coldest is most likely where the problem lies.

But more importantly: as Viking suggests it needs a qualified tech with gauges and experience to move this on to conclusion

ferret82
20-01-2012, 02:46 PM
Thanks for your reply Viking & r.bartlett

I do understand when you say to get a engineer in who can do all the calculations properly and get the system running as it should but unfortunately being an hour chopper ride of the coast in the middle of the ocean I don't have this at my disposal. If only we could transport though the internet......One day.....

So taking on board what you said I started from the basic. I went around and checked all the TxV's and where the bulbs were located and if they had a good surface. I did find that all of them were loose which wouldn't have helped. Another thing which I found was the AHUs were pulling air in from everywhere but the supply duct becuase the panels had been ripped of and not put back on and sealed properly. They tell me this has been a problem for a while now and no one could figure out why.

So on to the pressure reading,
At the compressor suction valve the LS = 52psig (-2.2dC), HS = 215psig(38dC)
Now At a guess the Compressor room to the AHU is approx 60metres
At the AHU, there is a Bottom floor and top floor,
On the bottom floor I was able to take 1 pressure reading because of a serice port fitted on the outlet of the evaporator which was 65psig (3.3dC) the other I took with a temp gun and this is were the bulb was located. The temp reading was 24dC.
The top AHU the temp readings at the bulb were 19dC and 24dC.

Now I am not 100% sure how should I add anything to the LS pressure reading to compensate for the 60m run back to the compressor?

So working out the Superheat of each TvX i get
Bottom
3.3 - -2.2dC = 5.5dC
24 - -2.2 = 26.2dC
Top
19 - -2.2=21.2
24--2.2 = 26.2

The sight glass on the system is a little way over half full and now the compressor is running hot instead of cold.

Again I appreciate your time and knowledge, I know this is not he best system to learn on, and if I could have it my way I would have someone come out and fix it.

The Viking
20-01-2012, 07:53 PM
OK,

One thing that stands out is the pressure drop, as you got a reading of 65psi at one of the AHU coils my suggestion is that we take that as the true reading of pressure for the superheat calculations.
This gives one coil with no superheat what so ever...

But before we start adjusting things, what is the liquid line temperature by the sight glass?

And, any chance of you using a thermometer with a flat probe (or a bead taped to the pipe) to take the temp readings? The IR "gun" type thermometers aren't known for their accurate measurements on cu-pipe...

Now, some thoughts, if the refrigerant is flowing more or less freely through one of your evaporators this might cause the system to look like if it is short of charge and will also starve the other evaporators of liquid.
To be able to give you better advice, what brand and model are the TEVs?

Gary
20-01-2012, 08:54 PM
First thing first. What are the air on/air off temperatures for the AHU?

Gun type thermometers have a use, but this ain't it. Get a real thermometer.

ferret82
21-01-2012, 08:45 AM
"Gun type thermometers have a use, but this ain't it. Get a real thermometer." point taken...

So the temperature that were found were using a fluke thermocouple which I have wrapped around the pipework and secured.

The reading were as followed

At the compressor the virtually the same reading

suction valve the LS = 52psig (-2.2C), HS = 215psig(38C)
Now At a guess the Compressor room to the AHU is approx 60metres

Top AHU

Outlet air = 16C (61F)
Return Air = 25C (77F)

The temperature on evaporator discharge 1 = 6.4C (42.6F)
The temperature on evaporator discharge 2 = 4.7C (40.6F)

The TEV's are XB-1019 HW100-1B

Bottom AHU

Outlet air = 15C (60F)
Return Air = 23C (74F)

The pressure on evaporator discharge 1 = 66psi 3.3C (38F)
The temperature on evaporator discharge 2 = 6C (60.3F)

The TEV's for the bottom AHUs are XB-1019 HW-2B.

The temperature before the sight glass is 28.2C (82.9F)
Sight glass 1/2 full

Seawater inlet temp 33.3C (92F)
Seawater outlet temp 40.2C (104F)


Hopefully now we are heading in the right direction. Thanks for your help.

Kayne

RANGER1
21-01-2012, 09:58 AM
What sea is 33.3 C, sound a bit warm ?

It seems you fixed something with putting the torn off panels back on.
This is obviously critical as well as clean coils & filters.

Shortage of refigerant is vital as well, so whether you have a leak that another issue for it
to perform correctly.
Maybe you have a blocked filter drier, who knows when it was changed last?

You also mention a lot of water off evaporators, which i think is good as it means its doing work.

It is mentioned about pressure drop in suction line of one unit, but I'm not sure if you can measure the pressure at the evaporator, like on other unit, so it might mean something else, like poor airflow or dirty air filters etc.

I could be a bit thick, but I can't determine what superheat you really have!

So what is going on with operation now as it stands?

The Viking
21-01-2012, 11:48 AM
Kayne,
You are in business. Delta Ts of 8 and 9 degrees respectively over the coils in AHUs ain't bad.

With a sub cooling of 10K there is plenty of refrigerant in the system.

You haven't given us the suction line temperature for the 1st coil on the bottom AHU but assuming it's not far off the others, there isn't much more to be done.

You sorted it!
Well done.

ferret82
21-01-2012, 12:29 PM
RANGER1

We are located about 200KM off shore Ex mouth in WA. The water temperature could be somewhat inaccurate because I did use the same method of wrapping the thermocouple around the pipework which is steel. From where the suction is for the salt water the units are probably another 30m above that so it could be picking heat up else where before getting to the Condensers. I haven't ventured that fair yet.



"So what is going on with operation now as it stands?" - it seems that the outlet temperature now of 16C used to be about 10C(50F), so if this is the case then there still needs a little more work to be done on it.

The panels that needed repair has help the system and also allowed easier access into the room because it is not trying to suck the door in.

I am not sure when the filter/dryer was changed last but I will investigate it and might just change it so I have a base point.

The Viking

Well that is a relief that something is heading in the right direction.......

My question now is how do I lower the outlet temperature further as it has been lower then this in the past.

I will head around again and take all the temperature again to see if anything has changed and then put them back up here.

Well the temperatures that I have found now do differ from the ones that i found today, and I believe that these readings are correct, as I have a new meter and bench tested it.

At the compressor the virtually the same reading

suction valve the LS = 54psig (-1.1C), HS = 220psig(41C)

Top AHU

Outlet air = 16C (61F)
Return Air = 25C (77F)

The temperature on evaporator discharge 1 = 11.6C (53F) - (-1.1) = 12C superheat
The temperature on evaporator discharge 2 = 8.8C (47.8F) - (-1.1) =9C superheat


Bottom AHU

Outlet air = 15C (60F)
Return Air = 23C (74F)

The pressure on evaporator discharge 1 = 66psi 3.3C

The temperature on evaporator discharge 1 = 22C (72F) - (-1.1) = 23C superheat
The temperature on evaporator discharge 2 = 16.4C (62F) - (-1.1) = 17C superheat


The temperature before the sight glass is 32C (90F)
Sight glass 1/2 full

I think that the superheat readings are a better representation of what the system is doing because I think that it could be colder.

Gary
21-01-2012, 07:36 PM
My question now is how do I lower the outlet temperature further as it has been lower then this in the past.


Air in and air out temperatures should be measured as close to the coil as possible. The ideal is to have 11C/20F difference between the air in and air out temps (delta-T aka dT). This ideal dT gives us not only the right room temperature, but also the right room humidity.

When the dT is too high, there is insufficient air flow through the coil. When the dT is too low, this can indicate too much air flow (highly unlikely) or not enough refrigerant flow through the coil (indicated by high coil outlet superheat). Clearly, the superheat is high.

Similarly, we can judge the water flow through the condenser by the dT across it. Reduced water flow raises the dT, telling us when there is a water flow problem.

Air and water flows are absolutely essential for proper operation. Once we have determined that these flows are sufficient, we move on to refrigerant flows. Starting with the subcooling (difference between saturated condensing temp and liquid line temp).

Gary
21-01-2012, 07:51 PM
HS = 220psig(41C)

The temperature before the sight glass is 32C (90F)
Sight glass 1/2 full


41C - 32C = 9K subcooling... yet the sight glass is only half full?

I am assuming this is an inline sight glass, located near the receiver or condenser outlet? Are there any devices between the outlet and the sight glass? Exactly where is the pressure reading taken? Best would be at the receiver or condenser outlet. Is there a receiver?

ferret82
22-01-2012, 06:04 PM
Yes Gary that is correct the sight glass is a little over half full. The sight glass is located a short distance from the outelt of the Reciever. The only other equipment that is in that line is a filter/dryer with a valve either siide and a by pass valve. Also there is a solenoid which I am guessing is for pumping the unit down.

The sightglass is not part of the main liquid line but is smaller pipe welded onto it.
When you say "Exactly where is the pressure reading taken" is this HS=220psi = 41? If so this is from the gauges on the discharge valve port at the Compressor.

There are no other service ports in the system from the discharge side of the compressor through the condensor - Reciever then up into the ceiling... So this pressure at these point can not be found.

The AHU Air on/Air off are Top 23C on / 18C off dT = 5K

The Top temperatures on the discharge of the evap are
evaporator discharge 1 = 16.5C
evaporator discharge 2 = 13C

Bottom 24C on / 17C off dT=7K

The Bottom temperature on the discharge of the evap
evaporator discharge 1 = 25.4C
evaporator discharge 2 = 21.3C

The LS pressure when all the above measurments were taken was 54psi = -1.1C

The attachment will give you an idea of what I was explaining above. Thanks again for your help,

Gary
23-01-2012, 06:45 AM
Evap dT is not excessive.
Cond dT is not excessive.
Subcooling is not excessive.
Superheat is high.

At this point it's looking like the system needs more refrigerant. I would add refrigerant until the sight glass is clear, then check the superheats. Given the very long lines more refrigerant may be needed, but let's see what clearing the sight glass does.

RANGER1
23-01-2012, 10:49 AM
Evap dT is not excessive.
Cond dT is not excessive.
Subcooling is not excessive.
Superheat is high.

At this point it's looking like the system needs more refrigerant. I would add refrigerant until the sight glass is clear, then check the superheats. Given the very long lines more refrigerant may be needed, but let's see what clearing the sight glass does.




Better do a leak test first & eliminate root cause of problem, as its the right thing to do before adding refrigerant, otherwise problem will keep coming back.

If its a BP rig dont worry about it!

ferret82
24-01-2012, 06:55 AM
I think that I may have found the problem. I went though all the discharges on the evaporators and found one that was slightly higher then the rest. With this little information I thought " what the hec" I will adjust the TEV. Anyway I started to adjust and nothing, clockwise and anticlockwise. So my understanding of this TEV is that it is wide open and allowing to much refrigerant thought. Also I charged to a full sight glass and nothing really changed greatly. I appreciate all the time and knowledge that you have provided me with this situation and also the time within you replied to my questions. I will change this out and then see what happens to the system. Thanks kayne

Gary
24-01-2012, 08:16 AM
The TEV controls the flow of liquid refrigerant into the evaporator. If there is no liquid at it's inlet, it has nothing to control and adjusting it will accomplish nothing. You need to keep adding refrigerant until the superheat starts to drop. Given the very long lines, the best way to get it right would be to install a sight glass at the inlet of the furthest TEV and charge it until that sight glass is clear.

Gary
24-01-2012, 08:41 AM
So working out the Superheat of each TvX i get
Bottom
3.3 - -2.2dC = 5.5dC
24 - -2.2 = 26.2dC
Top
19 - -2.2=21.2
24--2.2 = 26.2


This is wrong. Use the pressure reading at the evap outlet. i.e. 65psi/3.3C

24 - 3.3 = 20.7K SH
19 - 3.3 = 15.7K SH
24 - 3.3 = 20.7K SH

ferret82
24-01-2012, 11:52 AM
So I checked the system over where I could for leaks to find none. Can you tell me if the fluro dye you put into systems is any good because this might be the next step.

So added refrigerant until the sight glass was full.

Then took the following temp and pressures.
LS compressor 53psi -1.1C
HS compressor 220psi 41C

Inlet to condensor 54C
Outlet condensor 42C. So 12k subcooling

At the sight glass is 40C

The bottom AHU appears to be working as it should I did not when taking the temperature off the discharge pipes that every now and then the temperature would rise up to 20C then fall back down again. There is alot of water falling off the evaporator also.

Now this is where I am a little lost if this evaporator has 10K across it then how come the outlet air temperature has not changed since I started and to prove this I used the temp probe to confirm. I would have thought that this would have decreased

Bottom Ahu Temp discharge pipe 1 16C.Super 12K
Temp dis pipe 2 10C super 6K

The pressure from discharge pipe 1 68 psi 4.4C

Air on evap 27C
Air off evap 16.8C. dt 10K

Top Ahu on the other hand has nothing across the coil yet the temperatures of the discharge pipe say otherwise.

Temp at discharge pipe 1 11C super 7K using 68psi or 10K using LS pressure
Temp at discharge pipe 2 12C super 8K using 68psi or 11K using LS pressure

I am not sure if you can use the pressure off the Bottom unit for the top units? This is the reason for 2 Superheats.

Air on evap 26C
Air off evap 19C dT 5K

It looks to me that the suerheat is good yet no difference across the Evap coil??

Gary
24-01-2012, 04:08 PM
Then took the following temp and pressures.
LS compressor 53psi -1.1C
HS compressor 220psi 41C

Inlet to condensor 54C
Outlet condensor 42C. So 12k subcooling

At the sight glass is 40C


That's wrong. The subcooling (SC) is the difference between the saturated condensing temperature (SCT) and the liquid line temperature (LL).

41C SCT - 40C LL = 1K SC

Gary
24-01-2012, 04:32 PM
The bottom AHU appears to be working as it should I did not when taking the temperature off the discharge pipes that every now and then the temperature would rise up to 20C then fall back down again. There is alot of water falling off the evaporator also.


The occasional rise of the evaporator outlet temperature means the system needs more refrigerant. The TEV controls the flow of liquid. The rise means it is getting vapor instead of liquid. As stated in an earlier post, the best way to get it right is to install a sight glass at the inlet of the furthest TEV and add refrigerant until that sight glass is clear.

The water falling off the evaporator means the air is being dehumidified. That's a good thing.

Gary
24-01-2012, 04:47 PM
I am not sure if you can use the pressure off the Bottom unit for the top units? This is the reason for 2 Superheats.


The suction pressure at the top and bottom are the same and this is much more accurate than using the pressure at the other end of that 60 meter pipe. Use the pressure at the bottom evaporator for all of your superheat calculations.

ferret82
24-01-2012, 04:52 PM
Thanks Gary,

I have already put the plans in motion for putting a sight glass into the system.

Thanks again

Gary
24-01-2012, 04:56 PM
It looks to me that the suerheat is good yet no difference across the Evap coil??

The dT is the end product. It is not going to be right until everything in the system is right AND the room temperature and humidity are also right.

At this point it is enough to know that the dT is not excessive, which would indicate an airflow problem.

Add more refrigerant, wait for the room to get down to design temperature and design humidity, then let's see how it all looks.

Gary
24-01-2012, 05:02 PM
In order to avoid confusion, we need to get some terminology straight.

The piping between the compressor discharge and the condenser inlet is called the "discharge line".

The piping between the receiver outlet and the TEV inlet is called the "liquid line".

The piping between the evaporator outlet and the compressor inlet is called the "suction line".

Proper terminology assures that we are all on the same page.

Gary
24-01-2012, 05:24 PM
So I checked the system over where I could for leaks to find none.

If the compressor is open drive, the shaft seal is a likely spot for leaks, more likely to leak when the compressor is off, and far more likely to leak if the compressor is off for a long period of time, e.g. shut down for the winter months. It is not unusual to lose the refrigerant charge over the winter on open drive systems.

And then there is the nightmare scenario where the condenser is leaking internally. This generally results in having water in the system, but I see no indications of such. Do your sight glasses have moisture indicators on them? Always a good thing for water cooled systems.

RANGER1
24-01-2012, 08:52 PM
Apparently the sea tempreture in your area can reach 30 degC & as Gary mentions can be a good place for a leak to develop.
As part of preventative maintenance it should be cleaned & leak tested, probably annually.

I guess a sacraficial annode should be in there somewhere as well, to be renewed regularily.

If you do leak test condensor always ensure refrigerant pressure in it is above water circuit pressure
to avoid any water entering fridge circuit.
You also have a risk if you lose complete refrigerant charge, water can enter fridge circuit if leak as well.
Shell & tube condensors also usually have a divider plate & if corroded water can bypass through it reducing condensor performance.


Also Gary is a very patient man to go through all your readings, as I looked at it & it was way to confusing to me.
Try & space things out for each statement etc

mad fridgie
24-01-2012, 10:33 PM
Has this always been a problem. or is this a new problem? Or does it come and go?
If this a 60 meter pipe run in high ambient conditions, then with pressure drop and heat infiltration into the liquid line, you are likely to flash gas, at the TEV inlets. It maybe that at where the liquid line splits, that the liquid and vapour are separating, liquid to bottom AHU and Vapour (simplified) to the Top AHU.
Can you isolate the bottom unit, and just see if the top one works independently. (if not then it looks like a TEV problem) If yes then we can look into if further.
You may have an internal seeping expansion valve (on the equalizing line) which could be giving a false low evaporator outlet temp.

ferret82
29-01-2012, 09:33 AM
Hi,

Well sorry for the delay in responding to the thread the rig got evacuated becuase of a cyclone. In responce to the the thread above


If the compressor is open drive, the shaft seal is a likely spot for leaks, more likely to leak when the compressor is off, and far more likely to leak if the compressor is off for a long period of time This will now be a good time to see if this seal is leaking becuase the unit is a open drive compressor. It was shut down before we left and it now have been a few days so this is something that I will check.




Do your sight glasses have moisture indicators on them?
No they dont they are just a clear sightglass


Has this always been a problem. or is this a new problem? I am lead to believe that there has not been an issue with the system, yet they have been told from other contractors in the past that the water temperature and the piping run is a problem which makes it hard to get this system right. This problem has only occured this time I have arrived to do my 4 week hitch.

There are a few people onboard who like to fiddle and tinker with TEV and capacity controlers and just add gas becuase the system is not producing cold air. I think that these people now understand that they cant be doing this. In saying this though I am not sure what has been done to the system now.


It maybe that at where the liquid line splits, that the liquid and vapour are separating, liquid to bottom AHU and Vapour (simplified) to the Top AHU.
Can you isolate the bottom unit, and just see if the top one works independently. (if not then it looks like a TEV problem) If yes then we can look into if further.

Yes there is an isolation value both unit. I have to do a little more searching to find out where the pipe work goes. If I can not find this then I will close the values and see what effects it has on the system.


As part of preventative maintenance it should be cleaned & leak tested, probably annually.

This was only done 3 months ago (before I got to the rig). The ship was in singapore and a company up there did this PM on the condensors and provided tests certificates.


Also Gary is a very patient man to go through all your readings, as I looked at it & it was way to confusing to me. Try & space things out for each statement etc

Ranger I totally agree with you, I will spend a little more time on the formatting before submitting. Gary I do appricate everything that you have told me and helped me with over the past few weeks, along with the information from everyone else. You can trust me when I say that its not just going in one ear and out the other. I appricate what you are telling me, as you have learnt over many years of being in the trade.

As I am heading home for my break in the coming days I will not be able to work on the system. I have though got the plans in moton to get sight glasses installed to actually see what the system is doing at the furthest point.

Thanks again for your time and help

Gary
29-01-2012, 11:16 PM
No they dont they are just a clear sightglass

I am lead to believe that there has not been an issue with the system, yet they have been told from other contractors in the past that the water temperature and the piping run is a problem which makes it hard to get this system right. This problem has only occured this time I have arrived to do my 4 week hitch.

There are a few people onboard who like to fiddle and tinker with TEV and capacity controlers and just add gas becuase the system is not producing cold air. I think that these people now understand that they cant be doing this. In saying this though I am not sure what has been done to the system now.


When the sight glass is installed at the inlet of the furthest TEV, I highly recommend it have a moisture indicator. That sight glass is going to make your job much easier.

The condenser dT is borderline high, indicating barely sufficient water flow. Since the condenser has been recently cleaned and presumably the strainer has also been cleaned, there may not be anything you can reasonably do about it. This is something you will need to keep an eye on.

The long piping runs do indeed make the system difficult to set up properly. The new sight glass will help a lot.

The very last thing anyone should be tinkering with is the TEV... more often than not this will cause more problems than it solves. To accurately adjust a TEV, the airflow must be right AND there must be a solid column of refrigerant at it's inlet AND the room must be down close to design temperature.

What many fail to understand is that the TEV superheat should be expected to be high when the room temperature is above design temperature... and in fact it MUST be high in order to open up and provide increased refrigerant flow to match the heavy load. It is the high superheat that opens it up.

Once the TEV is properly adjusted, it will NEVER need to be adjusted again. They do NOT go out of adjustment.