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knkreb
31-03-2008, 12:38 PM
Okay, here it goes. I've exhausted everything I can think of on this system.

Specs:
Carrier 38AK044 (40ton A/C unit) Split system
R22
Witt LVL25 Air Handler (approx 30 ton capacity according to manufacturer)

Originally set up to to have Alco 22 ton twin TXV's on both circuits.

The condenser coil was a mess when I first got to this unit. Over half of the coils were bent over, dirty and not allowing good air flow. After straightening coils, and cleaning it, it required a good amount of refrigerant to be added to it to clear the site glass. It only runs in the summer time, and is in a very dirty environment, so I wasn't real concerned about winter/cold operation, so I kept the charge light enough in case the coils clogged up early.

The unit ran amazing well after that, but would trip off on oil pressure every so often. It would flood at start up. Found also that the solenoids were not wired properly and were "open" any time that the thermostat called for cooling. Carrier provides a relay to only open the solenoids while compressor is running, but it was not used. I corrected this, and thought that I had it.... turns out it keeps doing it.

That's when I called the manufacturer, to find that the coil was only rated for 30 tons. A little bit of math for summer time operation turns out that 22 ton valves have nearly 30 tons of capacity (approximately). So, I changed the expansion valves to two OVE-15GA's. These would have about a max capacity of 21 tons each when the head pressure is up at it's highest.

Still it tripped oil pressure. It was a mild day that it did this, and has tripped several times on mild days. So, next I added time delays to the solenoid valves. Now it will: open hot gas valve immedately, stage 1 solenoid at 30 seconds, and stage 2 at 5 minutes. This would provide a "pump out" of any liquid in the system, and only bring on one valve at a time. In my thoughts, this would reduce the possibility of flooding from both valves bringing down the coil to temp at the same time. Oil in the site glass is running extactly where it needs to be at while compressor is operating.

It still tripped oil pressure even after this. So, at the last visit, I charged according to the chart, and will be checking in to see if it's still running.

One other note, the compressor valves are leaking back, so it's equalizing on the off cycle. I was hoping the "pump out" would help reduce this issue somewhat, but don't know yet how much headway I've made.

One last note, the hot gas valve is a 6 ton valve bypassed into an aux side connector on stage 1 distributor.

Thoughts of anything left to look at?

andywill
01-04-2008, 08:46 PM
Hi
You dont mention weather you have checked the oil pressure with a gauge against the suction pressure or that you have checked the oil filter/strainer in the compressor. What is the make of the oil pressure switch and is the compressor charged with the correct oil for R22!!!

PaulZ
01-04-2008, 09:42 PM
Hi knkreb Some other things to check would be the way the oil failure switch is wired. If it is not last in the safety circuit it could still have power to it when the compressor shuts off after pump down. Is there a crankcase heater fitted if so does it work. If the compressor valves leak you could be getting liquid condensing in the oil during shutdown. Also do the solenoids hold. Let us know what you find. Paul

knkreb
02-04-2008, 11:57 AM
Thanks guys.

Solenoids do hold. It is wired as a "first" safety in the line of defense, however, it's tied to the contactor coil wire, so it doesn't keep timing in case of low pressure shut down.

Net oil pressure is (if I remember correctly) approx 30 PSIG. There is handwritting on the inside of the unit for several years of other companies writing down "new compressor" and "net oil pressure." The last few "net oil pressures" written by other companies were 12 -17 PSIG.

I did just find that the crankcase heater wasn't working yesterday.

Danfoss made the oil pressure switch.

Thanks for the ideas guys, keep 'em coming. It's nice to have some fresh out look on the problem, as now I'm starting to overlook the obvious stuff.

dear.mr.davidov
02-04-2008, 12:10 PM
hi knkreb
if you see oil in the site glass extactly may be yuor problem is oil filter or oil pump dont build oil pressure. if oil pump dont build good pressure you may to change rotation direction of compressor sometimes this is help

Gary
02-04-2008, 04:30 PM
... it's tied to the contactor coil wire, so it doesn't keep timing in case of low pressure shut down.

It still may be possible that the control keeps timing when the compressor kicks out on internal overload. Then the internal overload resets, but the oil control does not.

Check the voltage at the compressor terminals with the compressor running. If the voltage is low, look for burnt contacts and/or loose connections... or undersized wiring.

andywill
02-04-2008, 07:38 PM
hi knkreb

when you say 30PSi is that 30PSi higher than the suction pressure. I may be mistaken but on a danfoss oil pressure switch I believe that the oil pressure needs to be about 9-10PSi higher than suction at least, the higher the better. If its the same switch I'm thinking of these used to be adjustable by turning the cog inside the switch.Hmmmm previous visits by other engineers could have lowered the diff..Just a thought. You could remove the cover and observe the arm on the diaphram, when you start the compressor this should rise from the bottom to the top and stay there assuming the diff is set correctly. If so and the arm rises and drops off this could indicate that you have a restricted internal oil strainer or compressor bearing wear. Another thought, is the system cutting out above 0PSi when it's pumping out...

Hope you get closer to the problem. Intermittent faults like these can be a right pain. If only it could happen when your watching it!!!!

Pooh
02-04-2008, 11:18 PM
Is the compressor star delta or part wind start? I have had intermittent oil fails due to faulty contactors, star delta timers were the oil switch has received a single that the comp is running but in actual fact it has not been. Also had a similar fault on a Danfoss soft start unit were the cooling fan was faulty causing the soft start to trip and the oil switch was still receiving a run signal.
Oil failures are not always due to oil or system problems, as you have stated that it trips on mild days and that is when the unit shuts down check the above its worth a look.

Ian