High superheat is caused by three things. 1 low charge, 2 restrictrion in piping, and 3 poorly adjusted, or non functional txv. I would guess your low on gas. On txv sytems you need to charge by subcooling, and verify txv operation with superheat reading. If you need more explanation, I will give it to you.

 

Also the charge can only be adjusted at full load.

 

Keep it simple

First of all, it has a TXV, you have to charge with sub-cooling.

However, I don't think your low on refrigerant.

Low suction & high superheat, your not moving refrigerant.

To rule out low refrigerant, use it like a tool. For example, if the circuit holds 40 lbs, adding 10 lbs should improve the situation if your low, if you see no improvement, TXV.

I didn't see pressures for other circuit in your post, its ikely healthy but in a struggle because of the heavy load, just a thought.

For future reference, if you have a VAV system & you feel like it will comprimise your ability to charge it correctly, you can always weigh it in.

 

Is superheat overrated?

heating man - thanks for the reply - My gut tells me its low on charge as I checked circuit 1 txv and compared stem position vs circuit 2 txv and they dont appear to have been touched. However, I was told that on a vav intellipak that superheat will fluctuat high and low and that I can't go by superheat to tell me how the unit is performing and the only true way to charge the system was using subcooling. Superheat was shoved down my throat as a true indicator of performance. I have little experience so this is a great learning test!

 

txv problem

Hey chiller thanks - I did check pressures on circuit 1 - suction at 70lbs with head at 310lbs -(dirty coils!) superheat was around 19 - again didnt take subcooling- (however, starting to understand its importance!) Return air was being sensed at 76 - but it was a full load calling for all 4 stages. I didnt understand no condensate on the suction line - the tubes going into the evap were condenses but the suction line coming back was dry. All fans were running. Filters were dirty - so I removed them to get full airflow. Disabled compressor 2c (sounds like its on its last leg) and was still running high sh and lower suction pressure with just the 2 compressors running.

 

The Txv will fluctuate some, but between say 5 and 25 deg. 50 is too high, it means the refigerant is boiling off very early in the evap coil. You need to know if the txv is geting a full column of liquid to it. We know this from subcooling. But before tou go and recharge the unit, you need to replace the filters before the coils plug, and clean the coils before you can accurately diagnose.

 

I was taught early on that you have troubleshoot by the numbers. Other than the dirty condenser, without getting real serious, the other circuit sounds healthy. I wouldn't be concerned with wether the line is sweating or not, I would just go by the numbers.

There is no doubt that a VAV system can effect the performance of a unit. As more boxes pinch down & the vanes close or the VFD ramps down (in some cases, you may have neither) clearly, less air flow will reduce the load on the evap which can cause very tight superheat in some cases. However, your working on a unit that is already in trouble. Since its not fall, spring or winter, chances are the unit is under a heavy load & I would be suprised if very many of the boxes are pinched down, unless it over sized for the application.

You may want to look at the oil level, if its low you will want to top it off. It may help or the damage could already be done. If its not working correctly you should turn it off anyway until you can get back to it.

 

chiller mek

Good point, I sometimes forget to explain knowledge I assume others share. Sweating is not a good diagnostic tool, the numbers are (presures and temperatures, ans cfms and so forth)

 

Your bearings from the bad compressor are plugging the drier and causing the low suction pressure/high superheat.Also sweating on the suction line is meaningless.

 

Thats sound like a leap, its possible, but other things have to be checked first.
swann- what kind od suction was there before stage four.

 

Maybe a leap but I have changed many and seen lots of failed compressors.

 

Thanks guys - filter drier only has a 2 degree drop across it - my suction pressure when only 1c comes on for 2nd stage is 70 lbs with a superheat that was above 30 - sight glass right before txv was bubbling extensively - from watching circuit 1 - I gathered there would be some bubbles until all 3 compressors were running - but even with all 3 running there were extensive bubbles - no receiver in system. I did check the oil level, with 1 compressor oil was ok - they have an equalizer line between them and with all 3 running, the oil seemed to leave the compressors and go out fast but did not return fast. Picked up an oil pump and will be going back this morning - I am going to wash the coils thoroughly, take pressures and then pull out the charge and weigh it back in using subcooling. I will let you know what happens and if it was low or not.

thanks for all your input -

 

2 degree drop across drier is acceptable (1-3 degrees in most cases) I-packs run about 17-20 degrees subcool. Superheat should always be monitored, but do not charge by superheat. Do not rely on your site glass to determine charge. You can use it as reference, but a clear site glass does not mean clean bill of health. Are you leaking any where? You'll need to get the condenser clean before determining charge. These compressors are in tandem. Be very careful about adding oil, just because the site glass is low don't mean you need to put oil in it. Oil return is based compressor load. Since you can not unload a scroll yet, you have tandem apps. If one compressor is running on two comp. system you'll get less oil back. However, kick on the other comp for that stage you will get more oil back. If you are leaking you'll lose oil & refrigerant, low charge means less compressor capacity so the less oil return you'll get. Oil migrates to the coldest part of the system (evaporator) Condenser fans should be based on liquid line pressure 250-270 to push refigerant thru the evap at a higher velocity aiding in oil return. By your first post you said that your unit was running IGV @ 60 % w/ a 65 degree supply air temp. That tells me that the unit is not fully loaded. Some of your VAVs are closed off. The unit should supply what ever the desired setpoint is at the Human Interface. Sounds like the space served is satisfied at 65 !? Are the vav boxes pneumatic? Sounds like you may have an air flow issue or something. What's your return air temp? and are you controlling duct static or building static? Post back hopefully this helps.

 

Update

Ok - just got back from cleaning the coils and checking the system. Journeyman met me out there - the coils were pretty bad.

Measured subcooling - running @ 19 - superheat is still around 30 - Return air on the system is measured at 76.7 - that is from a wild return and that is what the return air temp was all of yesterday. The unit runs off of building pressure, which regulates the fan vanes. Unfortunately, I do not know the system all that well as this is my first go round at the site. It appears the building is set up all through intellipaks controls which go to seperate t-stats for zones throughout the building.

We let the unit run through the night with the 3rd compressor disabled from the 2nd circuit and it ran ok but yesterday could only get a 65 supply temp today supply air is 57 - of course this mornings temp was 63 and the high is only going to get to 74. Compared to yesterdays temps of 75 in the morning and 88 in the afternoon. Unit only called for 2nd stage of cooling today which was fine yesterday - its the fouth stage that was the problem. Arranged to come back on a hotter day to verify ops - and am going to qoute the compressor replacement, including changing the oil.

Still unsure about the system and the unusually high superheat from yesterday - not sure if it was a combination of the large load and also the bad compressor which could have increased the temp of the refrigerant in the LL - which caused a higher temp in the evap thus resulting in the suction line not sweating???

Hopefully I am not confusing everyone. Boss thinks the system just had a bad compressor, however, I am still not certain there isnt another problem. Its running ok now but the real test comes on a hot day. The saga continues - Thanks again for all the replies - I will definately take subcooling for all troubleshooting from now on. Will keep you all updated!

 

Swann

Remove the sensing bulb from the suction line of the circuit that's giving you problems, jump the low pressure switch, put all the compressors in that circuit online using test mode. Your suction pressure should be high and might be flooding liquid back to your compressors(so use caution). If not, then change the powerhead on the txv. 290 hi 30 low sounds like a restriction to me.


And get some help out there. Seriously.


-JB

 

subcooling

always check superheat always!
never add oil until you have attempted to bring oil back first.
sub cooling is good to know but low superheat shells compressors!
make sure you have 10-12 degrees evaporator superheat about 80-100 degrees discharge superheat, or at least discharge line temp below 225 your amp draw is o.k then check sub cooling.
it is proven that at 9 degrees superheat, liquid still exists.
I usually charge the unit until compressor superheat is about 35 degrees and my amp draw is getting close then I start checking the entire system. Including sub cooling.
Frank

 

Try removing some oil from the circuit your problems might just go away!
Heatingman should have a service bulletin about too much oil causing problems on scroll
units.