Strange Suction Issues

[hvac-lv]

OK,

Got another Am. Std 25 ton issue. This is another unit that the same co. came out and bypassed the HP cutofflast summer. It's been tripping the breaker on amps, just like the other one. This unit I found to be overcharged by 4#, and the condenser fans were in need of changing. All that's been dealt with. I recharged the first circuit w/ the factory charge from the data plate (17#). Now, on start up, the head pressure goes right to where it should be, today it was operating around 260 psi and it was about 90 deg. where the unit was at, give or take.

Now, here's the tricky part, and the reason I suspect it was overcharged to begin with. The suction pressure takes a while to go down on start up (7-10 secs or so), it then drops all the way to the LP cutout point, hovering for a few seconds just above it. What does that sound like to you guys? Also, when I cut the main power to stop the test cycle, the compressor shut down sounds like there might be some liquid getting back to it; it sounds like if you were to make a fake machine gun noise with your tongue on the roof of your mouth - I'm guessing that's not a good thing.

I've just cleaned the evap coil, both sides, 2 days ago, the belt for the fan is tight and the filters need changing, but they're not bad enough to cause that big of a drop. Also, I ran the second circuit with the first one off and the pressures were perfect, so I can't see the problem being related to airflow or the evap coil. With both circuits running, I still have the problem w/ circuit #1.

Metering devices on this unit are fixed orifices.

Please, please, please, send me your thoughts! thanks a ton!

[newoldtech]

It sounds like a possible restriction. Maybe a plugged drier or orifice. Measure your superheat, it will be of great help.
Subcooling and Delt T would help too.

[hvac-lv]

That drier could be the issue, I suspected it, but wasn't sure. Honestly I don't think the compressor will stay on long enough to measure superheat. I noticed that the max working psi is 500 for these driers, which I know was exceeded when the unit was overcharged and I had my gauges on, only a few seconds each time, but much longer during the times the unit ran before we noticed any problems.

With the repl. of the drier I need to recover, replace, vacuum, recharge and repeat after 2 weeks, correct? Anything else I should to do to help get that lineset good to go?

[newoldtech]

Originally posted by hvac-lv
That drier could be the issue, I suspected it, but wasn't sure. Honestly I don't think the compressor will stay on long enough to measure superheat. I noticed that the max working psi is 500 for these driers, which I know was exceeded when the unit was overcharged and I had my gauges on, only a few seconds each time, but much longer during the times the unit ran before we noticed any problems.

With the repl. of the drier I need to recover, replace, vacuum, recharge and repeat after 2 weeks, correct? Anything else I should to do to help get that lineset good to go?




Doing it once properly should be enough.

I'm not sure I understand what you mean by the lineset? I assumed this was a package unit. If its a split, and you weighed in the charge you might have to add more depending on the distance of the coils.

[hvac-lv]

sorry, I used the term too broadly, it is a package unit. So replace it once and if it keeps acting up, two times should do it?

[newoldtech]

Do a proper drier change and pumpdown. It should only take one time. The only time I do it twice is if I have a compressor burnout that requires me to change the suction drier as well. That should take care of your problems. Keep checking back, maybe someone else will add an idea or two. Good luck!

[hvac-lv]

definitely will do all of the above, thanks again. Now, I just need to find the invoices from the last few companies to touch these units, and go check any other units they may have touched :-)

[duke of earl]

Don't forget to purge nitrogen while brazing! If you cycle the unit on in cooling and disable the blower, the cap tubes should frost immediately. If some don't then they are plugged.

[berg2666]

When you change the drier check the evaporator ditributors for restriction. We have had a lot of these same prblems with voyagers here.

[core]

As far as the sound goes. I think that unit has scroll compressors and it is possible that the discharge check valve is getting stuck open.

When this happens on shut down the scroll spins backwards due to the higher pressure on top of the scroll and makes a wonderful sound.

It will not damage the compressor.

Core

[matt8085]

Hey Duke-of-Earl, have you ever seen this in an Intellipak?
I mean the plugged cap tubes. I have a unit now where after the third compressor in the circuit starts, the head goes up to, like 360, for a couples minute or so and then comes back down to normal. The problem is that sometimes one of the compressors trips its breaker, shutting the other two off on error. The coil is spotless clean, all condensor fans operating.

[duke of earl]

Matt,

I have not seen it with Intellipaks but I do know that if it were plugged cap tubes, the head would not go up like that. The restriction would have to be in the condenser in most cases. I have seen where the drier media has broken away from the shell and ended up at the TXV causing low suction and high head.Maybe that circuit is a little overcharged. Measure OAT and subcooling once that circuit stabilizes.On a hot day you shouldn't see more than about 10-12 degrees of subcooling. Also check pressure drop across the condenser coil (there should be a discharge tap and a liquid line tap for this).An excessive pressure drop will reveal blockage between the gages.Let me know what you find.

[hvac-lv]

Hey great advice everyone, thanks so much.

I don't get the chance to do a lot of HVAC around here, with all the other issues that come up in the gen. maint. field. I will definitely put all that info to good use. Yes, they are scroll compressors so that thank you for that insight, makes me feel a little bit better about the overall state of my big boys (25 tonners).

When you said purge Nitro. while brazing did you mean to have Ntro flowing through the system with the schrader valves removed, or just fill the lines up with nitro while keeping the valves in place? Just needed a little clarity on that.

Also, I currently have oxy/acet. for a welding setup, but I've had a few techs tell me that they use air/acet, and run nitro due to the fact that it virtually eliminates the chance of oxidation of the copper if you happen to get your flame wrong. What do you guys use? I was thinking about using MAPP gas just to avoid the possiblity of ruining the copper around the filter/drier, I know it will take a little longer, but I've got plenty extra, is this a good option, or should I just pick up a tank of air when I refill my nitro tank?

[esdd]

There is a service bulliten for these units, the filter dryer pluggs, the orifices on the lquid distributor manafold become restricted, the compressors overheat due to a lack of return cooling, and eventually fail. Check with your trane distributor, they can get copies of the tsb, and the parts kit to convert the unit to txv's. I am currently waiting on two of the kits, to convert two package units. I don't know how it is handled but Trane is providing the kits and c/c heaters at no charge.

[berg2666]

What is the model numbers of the affected units? Does anyone know this. Does it also pertain to Voyager and Precedent units?

[esdd]

As far as I know the affected units are YCD/TCD300 series units and their Amer. Std. sisters. The 240 units are not affected.