walk in freezer

[kklobas]

hello, working on wi freezer. coil was icing near txv valve. found that bulb insulation wore through and liquid line was touching top of bulb directly all the time. repaired. check out system and found lpc did not shut off compressor, and the lowest it pulled with ll solenoid closed was 0 psi. would have to expect compressor is bad from running who knows how long with txv overfeeding?
holding 10 below fine for now, but wandering -i should pull lower than o psi so should plan a compressor change soon right?

404a, pressure/temps, box temp good for now. adjusted superheat to 25 at compressor

[2sac]

First off, what is wrong with icing near the txv? Secondly, why are you adjusting a valve based on the line temp at the compressor? Lastly you gave no valuable information for anyone to give any type of recomendation.

[kklobas]

the liquid line was directly touching the sensing bulb, so after fixing that it had to be adjusted. was probably over feeding because a warm liquid line was touching the bulb. anyhow that part is fixed.

my question is with solenoid closed in pump down mode and never pulling below 0 psi that would indicate compressor damage from the overfeeding in the past correct? lp control adjusted so compressor does shut off now correctly.
the reson for adjusting txv is to maintain correct sh at compressor. i adjusted to 25 at compressor.
running about 18 low, 200-275 high (because of fan cycle. outside in wyoming winter) box holding at -10

[2sac]

The superheat should be adjusted to feed the evaporator properly. The SH at the compressor is a byproduct of a properly or improperly running system. A compressor should be able to pull into a vacuum BUT you never mentioned if the valves are holding in a pumpdown. You never mentioned actual amp draw compared to rla. If your box is maintaining -10 and is not short cycling, why just assume it will need a pump?

[kklobas]

its not assuming i need a pump, but im in the middle of nowhere wyoming and they store a few thousand dollars of product in cooler. so more worried that previous floodback along with the fact it only pulled to 0 and also wasn't shutting off until i got lpc control adjusted all make me worry it wont be long before it goes out. I have no idea how long it ran continuously before getting lpc adjusted either. even overnighted parts are usually 2 days out here, so trying to prepare.

did notice a slow rise in pump down of a few psi also.

[Tommy knocker]

Valve it out see if it will pull down and hold. Could be your solenoid valve.

[icemeister]

The old "efficiency" test has its merits, but it's not 100%. If you have a 2 cylinder compressor, you could have one bad suction valve and it'll still pull down OK. Also, if you were pumping down by closing the receiver outlet, it often takes quite a while to pull into a vacuum...even with a good compressor. It's best to close off at the suction service valve.

Anyway...Before condemning the compressor, do an amp draw check against the compressor manufacturer's performance data.

Get the actual supply voltage at the compressor while running, the suction & discharge pressures and the amp draw. It should be within 5% or so of the published amps at that condition. If it's a Copeland, they have a compressor performance program which I have used many times in such circumstances.

[timebuilder]

I think 25 sh at the compressor is high.

[icemeister]

I think 25 sh at the compressor is high.

Why? Typically, the minimum SH at the compressor is 20ºF, so I'd say 25ºF SH is pretty good.

Of course if you go back and re-read everything, it's not absolutely clear the OP was stating that 25ºF as SH or line temperature.

If it's line temperature with a 18# suction pressure (-17ºF SST), then the SH would be 42ºF which is a bit high. Even then, if the discharge temperature is in the neighborhood of 200ºF or less, I don't see a problem.

[cmclifton]

I think 25 sh at the compressor is high.

Copeland wants no lower than 25...

[hvac wiz 79]

Copeland wants no lower than 25...

A friend of mine just went to a Copeland class and they told him 25 max. I've read 20-30 is target. And then seen 20-40 in other documentation. Ice?

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[dangpgt97]

Whats wrong with adjusting superheat to 25 degrees at the compressor? If you have a short suction line you still need a minimum of 20 degrees superheat at compressor. Do the performance curve for the compressor and know for sure if it's performing right.

[timebuilder]

First, from the OP:

the liquid line was directly touching the sensing bulb, so after fixing that it had to be adjusted.

I have to ask, "why did it need to be adjusted?" Would it not have been properly adjusted PRIOR to the line touching the sensing bulb, and only the basic repair was needed?

The superheat should be adjusted to feed the evaporator properly. The SH at the compressor is a byproduct of a properly or improperly running system.

With you on that.

Why? Typically, the minimum SH at the compressor is 20ºF, so I'd say 25ºF SH is pretty good.

I have not seen many systems running 25...

Copeland wants no lower than 25...

Do we know if this is a Copeland?

Now, it the manufacturer of THIS system says it should be 25, then fine. However, I don't think anyone in this thread, myself included, knows what the manufacturer wants to see for a sh value.

[icemeister]

...I have not seen many systems running 25...

This thread isn't about an A/C system. It's a low temp R404A walk-in.

The only walk-in LT systems I ever see running at around 25ºF SH are the ones I've worked on. Others often run at 50-60ºF SH or more.

Here's an old thread that might shed some light on what we're really talking about here. (Check out Basser's answer in post #35):

http://hvac-talk.com/vbb/showthread....ssor-superheat

[ryan1088]

Whats wrong with adjusting superheat to 25 degrees at the compressor?

What if the outdoor conditions change? The heat given off by your accumulator and such wont be the same if it's 100 out as it will at 0.

[ryan1088]

This thread isn't about an A/C system. It's a low temp R404A walk-in.

The only walk-in LT systems I ever see running at around 25ºF SH are the ones I've worked on. Others often run at 50-60ºF SH or more.

Here's an old thread that might shed some light on what we're really talking about here. (Check out Basser's answer in post #35):

http://hvac-talk.com/vbb/showthread....ssor-superheat

True statement. Also that 25 degree superheat is accompanied by a huge ball of ice that is the accumulator. I don't know how many "froze up" calls I've answered.....

[cmclifton]

Do we know if this is a Copeland?

Now, it the manufacturer of THIS system says it should be 25, then fine. However, I don't think anyone in this thread, myself included, knows what the manufacturer wants to see for a sh value.

Don't know if its a Copeland, I was simply trying to point out that 25 superheat isn't high. I probably should have looked up the FAQ on the emerson website instead of throwing a number out there that has been rattling around in my head since my first Copeland class, because clearly I was incorrect. Copeland's minimum is actually 20. There is no maximum superheat, only maximum discharge temp. http://www.emersonclimate.com/en-US/...ompressor.aspx

As Icemeister mentioned, there aren't many low temp systems out there running 25 or even 35° superheat at the compressor...

[cmclifton]

What if the outdoor conditions change? The heat given off by your accumulator and such wont be the same if it's 100 out as it will at 0.

That's the exact reason that they have a minimum superheat number.

[timebuilder]

This thread isn't about an A/C system. It's a low temp R404A walk-in.

That's right. I'm not talking about AC systems. I'm talking about restaurant 404A refrig systems I service.

[dangpgt97]

You set superheat at the evap. If the lineset is short maintaining compressor superheat at 20* minimum is more important than evap superheat. I doubt that ambient would affect 2 ft of suction line that much. Learned it at copeland school actually. Regardless if I set it at 20 on a day that its 100 out I wont have any problems when its -20 out. Maybe that's why copeland says minimum 20 and not 5. Common sense.