Help! Freezer problem

[hoofbeat]

1 year ago we had a compressor failure on a tecumseh hermetic. R502 system. It was dead from bad valves. We replaced it with new compressor and changed it over to 404A. worked for @ 2 months and slowly lost temp. until it would only keep box temp at 15 degrees. Tried everthing except new compressor.No one would condem the thing. I did and replaced it.I replaced it with a copeland model. my biggest question is , is it possible that a compressor can work to a set point then become inefficient? Super heat at valve is 7 degrees and 21at compressor. Subcooling at 10 degrees.Can a weak copressor maintain what seem to be normal operating temps. and preassures yet not do any cooling below 15 degrees. We always had the box at 10 below and no problems before.Any ideas or comments please? thanks.

[itsiceman]

Is it a pump down?

Sounds like one that never shuts off because LPC is set too low.

[icehouse]

Question ? When system was changed to R-404a was a new TXV installed. Also was lines flushed out. Poly oil is highly susceptible to moisture, and any residual moisture from R-502 will cause problems. Such as "Salts" which will clog TXV screens.

[bob_scheel]

Sounds like the high side valves may be getting damaged by high discharge temperatures. When they leak the compressor's capacity is reduced but system pressures may appear close to normal. Check the compressors amp draw at the measured pressures against the manufacturers chart. If amps are below normal this would point towards the valves.

Possible reasons for valve damage may be too low of a suction pressure and or too high of a head pressure. Is suction pressure saturated temp 10 degrees colder than the evaporator temp? Whats the condition of the evaporator and the condenser?
If both are transfering heat inefficiently due to dirt or corrosion between the tubes and fins then your numbers could be coming up ok but you will get conditions when the ratio between the suction pressure and the discharge pressure is too high. How far away is the compressor from the evaporator? You said you had 7 degrees at the evap and 20 + at the compressor. That seems like a big increase unless the compressor is pretty far away and/or the suction line insulation is poor.

Whats the discharge gas temp when the unit is recovering from defrost on a hot day?

[hoofbeat]

yes it is a pump down and does pump down and cycle off during defrost. It would on temp too if temp ever reached satisfactory limit. TXV was changed and lines were flushed. Amp draws taken at different times and conditions were all within factory specs. Since I changed the compressor we are back to minus 10 but I was wondering about the old compressor and how it could seem like it was performing down to a certain point and then would do no more. Oh yeah, the compressor is remote about 20' away and lines set has new insulation on them and is 1" wall thickness.The thing would maintain proper super heat and subcooling but never pull down temp past 10 degrees.Thanks for posting back.Any more help and comments are always welcolme.

[icehouse]

Sounds like the high side valves may be getting damaged by high discharge temperatures. When they leak the compressor's capacity is reduced but system pressures may appear close to normal. Check the compressors amp draw at the measured pressures against the manufacturers chart. If amps are below normal this would point towards the valves.

Possible reasons for valve damage may be too low of a suction pressure and or too high of a head pressure. Is suction pressure saturated temp 10 degrees colder than the evaporator temp? Whats the condition of the evaporator and the condenser?
If both are transfering heat inefficiently due to dirt or corrosion between the tubes and fins then your numbers could be coming up ok but you will get conditions when the ratio between the suction pressure and the discharge pressure is too high. How far away is the compressor from the evaporator? You said you had 7 degrees at the evap and 20 + at the compressor. That seems like a big increase unless the compressor is pretty far away and/or the suction line insulation is poor.

Whats the discharge gas temp when the unit is recovering from defrost on a hot day?

20*F. - 30*F. Superheat at the compressor is recommended, just look at "Heatcraft's" installation manual. They also suggest insulating suction line with 3/4" wall thickness Armaflex and 1/2" wall thickness Armaflex if in unconditioned area.

[travlingdude]

Simply put, Yes you had a weak suction valve or weak suction. One symptom would be a long time to pump down the system, even though it does pull down to 0-psi. the other symptoms could be lower S.H. at the compressor, slightly higher suction and normal head psi. Did the new compressor pump down faster? this would confirm it. This is also hard to diagnoss as you need to rule out everything else. Most of the time the valves wear evenly and you end up with higher suction and lower head. Also 7 deg S.H is on the high side I would set it to 5 for low temp, 404. Great job.

[bob_scheel]

The question remains, why is the compressor failing so soon/often. Valve damage should not be happening. Is this compressor outdoors? Have a crankcase heater if so? Is the suction line piped to return oil properly? Downsized on risers if necessary? POE oil may not return as readily as mineral oil and accumulate in the piping and evaporator. Then when you do a defrost and the system load goes way up on the restart you get all of the oil back at once and possibly damaging the valves.

[powerup]

`Possible that this system has too much oil. Also, your superheat is at the minimum. Just a thought.

[hoofbeat]

`Possible that this system has too much oil. Also, your superheat is at the minimum. Just a thought.

checked the oil angle. no go

[hoofbeat]

20*F. - 30*F. Superheat at the compressor is recommended, just look at "Heatcraft's" installation manual. They also suggest insulating suction line with 3/4" wall thickness Armaflex and 1/2" wall thickness Armaflex if in unconditioned area.

condenser is 25 feet away. my problem is not getting enough liq. into the coil