MP66 & Compressor Overheating

[Peter_1]

Can you provide us a link to this Honeywell website?
Couldn't find it.

There is a direct correlation between the discharge temperature and the return gas temperature.

This is of course true because but where is the big Sh coming from.

What does the author mean with a 'de-superheating valve'?
I could imagine what he's trying to explain but how must this be doen in practice? You simply can't inject refrigerant in the sution line without taking certain precautions.

If the system is well designed, then you don't need fancy tricks to protect the compressor from high discharge temperatures. http://www.refrigeration-engineer.co...ead.php?t=5203

[Intermez]

Sorry everyone, the thread is not lost. Was called away due to an unexpected death in the family.

Here's where things are at. Replaced the 7/8 suction line with 5/8 with new insulation. Removed the heat exchanger and suction accumulator. New liquid line and suction line filter/dryers installed. New TEV (externally equalized). I do not have a complete set of readings as of yet but will get them today.

I am now able to keep the system running without it going supernova.

With the factory TEV super heat setting I was seeing about 15F of superheat which gradually decreased to around 2F as the system ran for the first 20 minutes, however I was still seeing the discharge temperature climbing up to about 240F (and still climbing) when I shut things down. Suction temperature at the compressor was around 60F. To me, the problem is still there. At this point I assumed that the large amount of superheat added by the suction line could only be due to low flow.

I decided to forget about the superheat setting at the plate outlet. Decided instead to set the superheat only by the superheat at the suction inlet at the compressor. Took a stab, opened up the TEV by 1.5 turns. Suction superheat at compressor now around 20F. Discharge temperature around 210F. At the plate outlet the readings now are: 6psi and temperature of -13F. This equates to -7F superheat at the plate outlet (negative superheat). This was after 30 minutes of running. Plates seem to be pulling down nicely, but this is where I stopped. Will get a detailed set of readings today.

Is this at all normal in cold plate operation? Obviously there is liquid evaporation in the suction line, but I still have low temps at the plate outlet. The amount of heat added by the suction line is down considerably, running about 30F (not the 100F I was seeing before). I'm not yet sure what happens with these settings as the plates pull down. I also don't know yet if the suction superheat will remain stable as the plates pull down (worried about liquid slugging). Should find out today.

Thanks again guys!, Kevin

[kevinmcggg]

possibly not enough refrigerant to do the job?

[Intermez]

PROBLEM SOLVED!!!!!

Peter and Iceman and everyone else, thanks for all your help! I know you all thought I was nuts with the symptons/readings I was getting, but they were real.

It was the contactor. The relay/contactor on this system was a solid state one, not a mechanical one. One of the first things I checked on this system was the voltage at the compressor at startup when the current is the highest. It was around 210 volts, just fine. What was happening is that the solid state contactor was heating up and after 5 or 10 minutes the voltage at the compressor had dropped to 190 volts. All AC motors generate excess heat when operating at lower than normal voltage. This excess heat is what was causing the high exhaust temps. Replaced the contactor with a good old mechanical type and can now adjust things as they should be. It's now working fine. Will have to do some fine tuning but everything is now operating as it should.

By the way, if any of you recall somewhere near the beginning of this thing, I mentioned that the system had both a temperature and suction pressure shut down for some reason. Don't like a temperature shutdown, so I disabled it by setting it to -10F. The system shutdown because with a lot of tweaking I got it down to -10F. When it shut down, I figured I had locked up the compressor, as I had totally forgot about the temp shutdown. Started taking voltage readings to figure out what was going on. That's when I discovered the low voltage problem. So I can take no credit for solving this, it was just dumb luck!

Thanks again everyone, Kevin

[Peter_1]

Kevin, congratulations.
I should never had thought on this propable cause.
This was really a very, very difficult one.
I think that you have learned a lot with this s...y thing.
And this is also something I will try never to forget.

I was wrong with my diagnosis.

A SSR is indeed not the best option for this kind of application. I only use a SSR when there's a need for very fast switching.

Anyway, these service calls will make you every time a better tech then already are.

[US Iceman]

Well, well... If someone were to say we got our knickers in a knot over this project I would have to agree.

It still doesn't explain the high superheat out of the last plate and through the wall though .

If it works, I am not going to kick a sleeping dog.

In any event, I hope all of this was worth something to you Kevin.

Anyway, these service calls will make you every time a better tech then already are.

Amen brother...

[frank]

If it works, I am not going to kick a sleeping dog.

Should this be posted in the other thread ?

[US Iceman]

I'll add that minor contribution to the other thread Frank.

Do you want to guess how many kicks it takes?

[Intermez]

Thanks again everyone!

It still doesn't explain the high superheat out of the last plate and through the wall though .

This problem and all the other weird readings were because of low ***** flow, not enough gas flow to keep the temperatures from rising quickly.......just the way I had to set the TEV to keep things running without the compressor temp going over 250F. All the readings make sense now that I have a compressor that is acting normally. Oh yeah, remember the low ultimate suction with no flow....I'll bet it is higher now that the compressor is running with full voltage.

Finally , Enough school for now, Kevin

[US Iceman]

This problem and all the other weird readings were because of low ***** flow, not enough gas flow to keep the temperatures from rising quickly

I'm not sure if I buy that. The contactor had nothing to do with the mass flow, other than the fact the compressor shut off.

50 degrees of superheat increase within a few feet of pipe is not an answer of low mass flow.

But, if you are happy with the results then I won't argue about it.

[dirk]

it seems the compressor capacity is insufficient or compressor inefficient.

[dirk]

Still sounds like a compressor capacity problem