Hey folks, have a look and give me your thoughts...low pressures on tandem scrolls
I got sent to replace the tandem scroll compressors in a York RTU (mid-age 10-13 years). One of the compressors in the old pair grounded out.
Recovered, removed old compressors. Cut out old liquid line drier. The old driers (machine has two circuits - four compressors) were installed about 4 inches upstream of the TXV's.
Since we figured there might be goodies in the system from the burnout we wanted to be able to change out the drier easily, so I installed valves before and after the drier and flared the drier in. Remember, one of the brazed joints was very close to the TXV - after cutting and all I'd say that the joint was about 2 inches from the TXV stub.
I used thermal paste between heat and TXV and also wrapped the TXV with a wet rag. In hindsight, I think I should have removed the guts from the thing. Anyway, rana little N2 through while brazing and put the whole thing back together.
Filled with 150psi N2, checked for leaks with bubbles...none found. Evacuated...system held @ 300 microns for an hour.
Weighed in factory charge. Checked compressor phasing and started the thing up.
After several minutes of running, pressures were: 50psi suction, 110 psi discharge. Thought maybe the diagram had the colors of the wires wrong for the compressors and they were running backwards...reversed direction...noisy, very minor differential. Back to original (proper) phasing...same pressures (50 / 110).
Return temp during all this was about 80*F, supply temp (I was only running the new compressors) was 58*F. Good airflow. Noticed some frosting on the distributor tubes.
Hit me with your thoughts....did I overheat the TXV? (it can't open enough anymore) I don't think there's junk in the system...I'm conscientious about reaming, cleaning, and purging with N2 when I braze.
Looking forward to hearing your ideas...
Any other readings like SH and sc, anything and everything you've got will be helpful
Only one circuit running, and a 22* split?. Does the unit have a VFD? If not, possible airflow issue.
Sounds like you did a good job. No mention of ambient. Discharge is low, add more refrigerant and see what happens.
without any other info, i too, am thinking airflow. if your txv wasn't pushing enough refrigerant, you wouldn't have much of a temperature drop across your evaporator coil. frosting distributor tubes only means the air around the distributor tubes is less than 32F and that can be caused by many things.
this may explain why the old compressors failed.
'good airflow' means nothing.
"By Grapthars Hammer....what a savings." Galaxy Quest
If the OP has an 80F return, chances are he has an 80 plus ambient, and he should be running a higher discharge pressure even with reduced evaporator airflow. Assuming the capacity of the new tandem is correct. It is possible an error occurred during the charge weigh
in and there just isn't enough refrigerant in the circuit.
Aaaaaah you guys are right the evaporator is nasty and four rows of funk.
Sorry guys, around 50*F ambient. The evaporators are one atop (not behind) the other in the same air stream and the other one has normal readings, and filters are (supposed to be) regularly changed by a sub contractor, so I wasn't thinking about airflow...
Superheat was around 15 degrees, subcooling was a bit high, around 29 degrees.
Another tidbit I forgot about; when the compressors first started up the suction pressure gently oscillated from about 35psi to 50psi and back a few times before leveling out at 50psi.
I'm certain the correct charge went in, well, I'm certain the 20# 8oz. stamped on the name plate went in, whether that is right or not is an open question. I weighed the tanked a couple times after seeing the low pressures because I thought the same thing..."is there enough juice?"
The 80*F return is because they've got a plenum ceiling return with all kinds of junk (transformers and such) up there adding heat.
Okay well that clears up a bit.
The next question would be head pressure control. 50F is not too friendly for a standard RTU.
You had some load inside, adding some load outside by covering part of the condenser as needed to achieve 100F to 120F condensing temperature would answer your question as to how the unit might run on a summer day.
Yeah there is no head pressure control...good thought there. I'll go back with a garbage bag! Thanks!
Just for what it's worth, something I'm sure you're well aware of but tunnel visioned away from after brazing so close.....but the txv only does one job. It keeps superheat, not suction pressure. If you've got 15 degrees I'd say you definitely didn't toast your txv. It's so often guilty by association but it's doing its thing.
I'm sure realistically it's the low ambient, though to me it looks like low air flow from here.
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Guinness for strength
The valve is working if you have 15*sh. With 50* ambient and low suction, I would have to assume you have low airflow and you need a head pressure control to get your head up. You may have good airflow. But first I would get head up and see what suction does. Are the compressors staged?
I haven't been back tho this thing yet, but here's some more info - it is so hard to provide everything in one go from memory!
The compressors are not staged...the two work together. The machine overall IS staged; there are two sets of tandem compressors - stage two is the one I replaced, stage one works fine AND has more normal looking pressures! @65psi suction and @160 or so discharge from what I recall. The first thing I did when I saw those low pressures was to fire up the other stage for comparison. This is why I have not really been considering OA temp / lack of low ambient controls...
The evaporators are separate, one about 10 inches behind the other and staggered about a foot to balance airlfow through them. I think I said earlier they were one atop the other, but that was not right. The more I think about it, the more it seems like the evaporator....maybe there's something stuck to the backside of it (insulation, plastic bag?).