Denco D12A problem

[Daz man]

Hi all,

I have an issue with a Denco close control AC cabinet in a server room.

The story was, a few weeks ago I had a restricted liquid line drier as the top of the drier (outlet end) was freezing all the way up the liquid line.

After reclaiming the 7.5kg charge I changed for a new one and evacuated all the way down to 3 torr ish and recharged. Left working fine, no further issues, working well.

Then I received a service call back a couple of days ago to say the same cabinet was showing a compressor LP trip (LP alarm). After connecting the gauges I was getting the following readings -

Back pressure 36psi
Head pressure 250psi
Super heat 28k
Sub cooling 20+k

As above, the SC and SH readings are miles out. I thought the best action to take was to change the ext eql TEZ2 expansion valve, as no matter what I did to the adjustment screw on the valve the readings didn't alter!

This is where I think I've messed up! After changing the valve and recharging again, exactly the same issue is happening, but I did use the existing orifice (number 5) from the old valve, I didn't fit a new orifice into the new valve. Could this be the cause of my issues?

The distributor passes (or the octopus legs as the apprentice calls them!) are freezing over during operation and also the body of the exp valve very slightly is freezing over!

Any other possibilities? Blocked evap coil or blocked distributor?

I'm hoping this new replacement orifice I have collected tonight will hopefully sort it out tomorrow?

Any help welcome, thanks

[al]

Did you clean the orifice filter?

[install monkey]

blame the apprentice!

[Daz man]

I didn't clean it as such, but it didn't look blocked put it that way

[marc5180]

I didn't clean it as such, but it didn't look blocked put it that way

Firstly, a couple of questions.
What refrigerant is it on? I assume R407c??
Why did you only vac it down to 3torr? Did you carry out a rise test?

Where did you take the superheat? At the evaporator outlet or at the compressor?

Are the air filters clean and is the airflow correct? (Coil not blocked, fan spinning wrong way or cabinet door left off when taking readings etc)

The adjustment screw isn't the way to test a TXV. You should hold the bulb which if working will flood the coil, increase the suction pressure and decrease the superheat. Did you try this.
The orrifice won't be the cause of the problem, on the TEZ2 they are the bullet type ones that slide in.

Are all of the distributor tubes freezing evenly or just one or two of them?

[Daz man]

Hi,

yes it's on r407c. I carried out a full leak test with ofn and it held over a 24 hour period.

The air filters are clean (new) and the air flow is correct.

yes all the distributor tubes are freezing evenly! I recorded the super heat at the compressor.

Thanks for for your feedback.

[marc5180]

I doubt the difference between the Evap outlet and the compressor inlet is much but even so it's best to take the superheat at the Evap outlet or near to the TXV bulb on the suction line because this is where the TXV takes it's reading from.

Does this system have a hot gas bypass? Is this working correctly?

[Daz man]

Hi

No there is no hot gas bypass on this machine.

[Neddy]

Where did you take your readings for Subcooling? At the inlet of the expansion valve or leaving the Condensor? Did you fix the expansion valve bulb in the same position as the one you took out?

[Daz man]

To be honest I always remember years ago installing one of these cabinets (was cream in colour back then) and then watching a Denco engineer come to site and commission it up for me. I basically have always taken my sub cooling reading from where he did.

I connected my Fluke K-couple probe to the liquid line pipe (1/2") just before the sight glass/exp valve, top right hand corner of the single door cabinet as you open it. Then obviously deducted that reading from my discharge pressure/temp reading.

As previously posted, today I installed a brand new orifice number 5 into the exp valve, fired it up and this time my readings plummeted even further (as if the unit was pumping down).

After further investigation my new liquid line drier appears to be either restricted or more likely blocked again! (see pic).20140328_112602.jpg20140328_112607.jpg

Is it possible there is something floating about in the system, moisture, non condensables or sludge from where the pipes weren't purged with ofn when brazing during installation? Thus causing the new drier to block up again? Bare in mind this cabinet was installed way back in 2006.

Tomorrow (Sat) I am going back to site with another 1/2" liquid line drier and whilst the gas is out another expansion valve just to eliminate the possibility of a faulty one which is in now.

Strange, very strange. Thanks blokes.

[Daz man]

yes, you can see the brown tape (attached pics from previous post) on the inside of the scroll fan with an arrow drawn on it, to prove correct air flow!

[marc5180]

Hi, can you take the readings with the cabinet door closed, I'm concerned about the airflow over the Evap which is giving you wrong readings?

That being said, where you measured the Subcooling is fine because you are looking for 100% liquid at the inlet of the TXV but you should also check the Subcooling at the outlet of the receiver/condenser and compare them both. You may well have a restriction or flash gas in the liquid line so it's best to rule both of these out.

Is the drier the correct way or is it a bi flow drier?

[Grizzly]

10 out of 10 for not giving up!
Grizzly

[NewmanRef]

From the pictures in your post it's quite evident you have a restriction at the drier. I had the a similar problem on an Airedale chiller years ago and it would literally pump down where the drier was blocked so much. We changed drier 3 or 4 times before the system ran properly. We later cut open the driers and found that carbon from brazing had mixed with comp oil to form a very thick sludge which had almost set hard within the drier. This install was about 5 years old and we found it very odd that it took this long to present a problem... But it did.

[r.bartlett]

To be honest I always remember years ago installing one of these cabinets (was cream in colour back then) and then watching a Denco engineer come to site and commission it up for me. I basically have always taken my sub cooling reading from where he did.

I connected my Fluke K-couple probe to the liquid line pipe (1/2") just before the sight glass/exp valve, top right hand corner of the single door cabinet as you open it. Then obviously deducted that reading from my discharge pressure/temp reading.

As previously posted, today I installed a brand new orifice number 5 into the exp valve, fired it up and this time my readings plummeted even further (as if the unit was pumping down).

After further investigation my new liquid line drier appears to be either restricted or more likely blocked again! (see pic).20140328_112602.jpg20140328_112607.jpg

Is it possible there is something floating about in the system, moisture, non condensables or sludge from where the pipes weren't purged with ofn when brazing during installation? Thus causing the new drier to block up again? Bare in mind this cabinet was installed way back in 2006.

Tomorrow (Sat) I am going back to site with another 1/2" liquid line drier and whilst the gas is out another expansion valve just to eliminate the possibility of a faulty one which is in now.

Strange, very strange. Thanks blokes.

I think you need to reclaim the gas, change the drier and put it on vac for a couple of days. com back and put in an all new charge and see what happens. That drier just isn't right

[r.bartlett]

From the pictures in your post it's quite evident you have a restriction at the drier. I had the a similar problem on an Airedale chiller years ago and it would literally pump down where the drier was blocked so much. We changed drier 3 or 4 times before the system ran properly. We later cut open the driers and found that carbon from brazing had mixed with comp oil to form a very thick sludge which had almost set hard within the drier. This install was about 5 years old and we found it very odd that it took this long to present a problem... But it did.

had a similar issue on a oil separator return to a pack. the std driers wouldn't last more than a day or two so we paralleled up 2 of the 12" driers and they lasted months. Good call though on this one and worth thinking about esp if he's back on site tomorrow he can change the drier and cut the old one whilst there.

[Daz man]

Guys R.Bartlett & Newmanref really appreciate your feedback, thanks blokes.

will leave the vac pump on over night Saturday and all day Sunday. Then will recharge the 7.5kg of Virgin r407c on Monday morning!

[install monkey]

change ur vac pump oil too- and get a solonoid coil lifter

[Daz man]

Hi, can you take the readings with the cabinet door closed, I'm concerned about the airflow over the Evap which is giving you wrong readings?

That being said, where you measured the Subcooling is fine because you are looking for 100% liquid at the inlet of the TXV but you should also check the Subcooling at the outlet of the receiver/condenser and compare them both. You may well have a restriction or flash gas in the liquid line so it's best to rule both of these out.

Is the drier the correct way or is it a bi flow drier?

hi marc5180 - yeah, these readings are with the cabinet door shut! There is a slight gap at the bottom of the door just to allow my gauge lines through but nothing that should alter readings by a lot. Also the drier is in the right way (I hope the arrow sticker isn't the wrong way!) and no it's not a bi flow drier. Thanks for posting - I will get there with it. Perseverance is key I reckon!!

[Daz man]

change ur vac pump oil too- and get a solonoid coil lifter

Dude good call on the vac pump oil. But the solenoid lifter I have and is already primed and ready!

[install monkey]

put ur torr gauge at the furthest point from your gauges- will show true vac on system

[marc5180]

Try blowing it through with nitrogen once the drier has been taken out to check for any blockages or crap in the pipe.
Also look at the drier once it's out to see if there is any sludge type stuff inside it.

[Daz man]

Removed the drier this morning! OMG! See pic. What is that, oil/sludge? New drier fitted and also whilst the refrigerant was out a new externally equalised expansion valve (just in case) but this time flare, flare rather than flare solder - to try and keep oxidisation to a minimum. Ifing and R'ing now wether to vac out for a couple to three hours or leave on vac over weekend? (Tightness test fine)

[marc5180]

Have you carried out an acid test on the oil? Acid inside the system can mix with the refrigeration oil and form 'sludge'
By the looks of it a full system clean up and a deep vacuum will be required.

Did you fit ball valves either side of the drier?

[r.bartlett]

a vacuum won't help this situation. You either work through it by changing the drier once a day for a week upgrade to much larger driers to reduce the amount of changing

or ideally remove the oil from the compressor flush the pipework with endoflush or similar. fresh oil after -and put in a drain point tap so you can change the oil easily again if necessary

That would be the better option as then you'd be left with a relatively clean system and the ability to change the oil occasionally

[NewmanRef]

That's the same crap! Definately worth an acid test on your oil. We did this and there was no acid, but definately worth checking. Out of interest, how long is your pipe run to condenser? When we had this problem the pipe run was approx 60m. Obviously the more braze joints the more carbon potential. We also had about a 30m double riser in the system with traps every 5m. We wondered whether the sludge had built up in one or some of the traps before eventually being forced round the system and glueing up the drier.

[Gideon Beddows]

When you come to set it back up, set the Head Pressure to control at 265 psi (18.2b) then add the refrigerant until you get about 5 K subcooling. Then set up your superheat. Anything from 4-10K will do you , unless you are trying to keep the humidity low in which case try to keep as close to 4K as you can get.

Never seen such a ****ty drier on a Denco system. Looks like you should be changing them at least weekly until clean again?

Good luck !!