HVAC: subcooling + orifice tube

T77911S · 06-24-2015, 04:59 AM

i have a 20 ton AC unit here at work. R22+orifice tube. i know you use subcooling for TXV and superheat for orifice.
my superheat is 10 degrees but my subcooling is about 25 degrees.

it stopped cooling the other day. the low side was lo, about 45psi and the hi side was lo, about 175. superheat was hi. so i added. (i did look for a leak but i did not see any visibale signs.) this did make me reluctant to add but i did.
as exected the low side started coming up, the hi side came up some so i was thinking all was well. it seemed to level off at about 60/200 and SH of 10-15.
i shut it off then started it back up. the low side started to very low and the hi side very hi. the low side came back up but now the hi side was around 300.

long story short; (too late)
i found when both compressors came on the hi side was going very hi and shutting down.
i pulled some r22 out. then i ended up putting a little of it back in with both curcuits runnin. now the pressures are more normal but my subcooling is really hi.
does this matter with an orifice tube?

i have read that with the symptoms i had it could be a blockage and the way the system started back up and along with the hi subcooling i think i could have a blockage but also too much 22 n there.

i know HVAC but i just dont have a lot of experience with it and i dont get to work on it regularly enough.

billybek · billybek's Garage

A few questions.
Manufacturer?
AC or Heat Pump?
Amps on Compressor(s)? Compare this to the compressors RLA.
Any recent work (other than what you did with it today)?
Any recent renovation work in the building?
Airflow OK? Filters clean? Fan moving enough air?

T77911S · 06-25-2015, 04:06 AM

AC only.
did not check amps
no recent work
filters are a little dirty but not bad. waiting on new filters.
good air flow, fan belt ok.
we are having 2 new chillers installed for the rest of the building right now.

i am a little worried ihave too much in with the 25 degrees subcooling even though the superheat is 10. i have never checked the SC on these and thye are only about 7 or 8 yrs old and have not really had any problems until a leak in the condensor a few weeks back on the other unit.

everything i see references SC to a TXV but really nothing with an orifice tube.

billybek · billybek's Garage

Who is the manufacturer?
Do you have a model number handy?
More helpful information would be the temperature drop (air) across the evaporator with one and two compressors running. Single circuit (both compressors on one refrigerant circuit) or dual (individual circ per comp.)?
With cap or orifice tube systems, the suction should track up in pressure with condenser pressure.
Was the refrigerant added previously recovered refrigerant or from a factory cylinder?

A side question, what chillers are going in the building? Size?
Is this a replacement or a new system?
(Enquiring minds want to know

Off to work! 2 days of work until 2 months off! Yay!

T77911S · 06-25-2015, 05:38 AM

the chillers are cooling a seperate building eventhough they are attatched.

they are tranes.

i think i found the problem
one of my condensor fans is turning the worng direction. the one turning the wrong direction is the secondary fan.
here is how i think it should work
the first circuit comes on with the first fan. it looks like there is a small contactor for a temp sensor so that if the outside air is hot, it turns on the second fan. with the first fan on,i could see the secnd fan kind of wiggle back a forth just a little like it is trying to start. (in the right direction). but i think the air being pulled in fmro the first fan is making it hard to start. (i am gueussing there are start windings in the motor that could be bad). with the reverse air that gets it going in the worng direction.
both motors have arrows pointing the same direction and both fan blades are the same, the just turn in opposite directions.

i noticed something strange. i was checking to see if air was being pulled across the condensor. i noticed in some places it was pulling in and some places it was blowing out.
i also noticed with the cover off the side, i was getting a rush of air going into the unit until the other fan cut on, then it stopped.

i checked the cap it was good. IE with power going in and all other wires removed, i had 120 on the other side. both fans are wired the same.

so i assume i have a bad motor

T77911S · 06-25-2015, 06:07 AM

its the cap.

i connected the "bad" fan to the other cap and it ran right. (i also traced the wires back to the wrong cap the first time)

T77911S · 06-25-2015, 07:06 AM

the cap fixed the fan

i just ran the unit in test, no air flow over the coil and pressures were pretty low.
its getting hot on the roof so i will check the pressures monday morning.

the fan would explain why i originally saw low pressures. only one fan was running at first, then when i shut it off and back on both fans ran but the one was backwords so really no cond air flow. thats why my hi side went up to 350.

billybek · billybek's Garage

Cool! Trade pun...
If it is a Trane Voyager RTU, some of them have two condensing coils that are tie wrapped together on the non header end. Sometimes they load up with crap between the two coils. To clean them properly the top of the RTU needs to be removed the coil split on the return bend side and the layer of build up between the coils removed. Coil washing alone will not clean these types of coils out.
Good job finding that.

T77911S · 06-25-2015, 08:20 AM

thanks for the help.

this is a large 20ton package unit. 2 seperate circuits, one evap fan. there really is not much to these units thank goodness.

our old chiller system was from the 80's i think. it had so much crap modded and added that it was really a pain to work on. it was a small r22 chiller system.

i am really curious about the pressures now but i really dont want to get back in the heat right now.

i dont have a lot of experience yet so sometimes i feel stupid that i did not notice the lack of air pulling the cover back on right away. i think as i get older i am missing little things like that more often.

thanks again. i will let you know how it is on monday

T77911S · 06-30-2015, 04:52 AM

i checked pressures this morning. first i checked each circuit by itself. both were to me. like 40/175. then i checked the circuit i was having problems with but with BOTH circuits running.
pressures were a little higher but i thought still low. 45/200. the super heat was really hi, a little over 30 degrees. so i put some in. i brought the lo side up to around 50 but the SH was still very hi. idid not put any more in since both circuits were about the same (when run by themselves). unless the evap has a leak i dont see any signs of a leak and i cant imagaine both circuits having a leak or both having a low charge from the factory. (these are roof top package units).
i would have liked to have seen the low side at least to 60 and the SH down to around 8-10.
the room temp was 67 and the outside temp was maybe 75 and not too humid.

mattdavis11 · 06-30-2015, 06:57 AM

I'm not a residential or commercial a/c expert, but I am with autos. Your high and low are going to rise with a rise in the ambient. I'd charge during the hottest point of the day, and measure vent temperatures, not superheat.

The reason I say this is, I know there is more to be desired. Every home/commercial a/c guy I have talked with uses a rule of 30 degrees. Horsepuckey!

My brother lost some refrigerant (shraeder valve), so we hooked up the gauges, added refrigerant and dropped the inside vent temp from 75 to 45. It was 95 outside. It has been good for 3 years now.

I always charge according vent temperature, and the gauges tell me what might be wrong.