I need help tuning a 2 stager

[Xenogias]

Hello. I hope someone out there will be able to provide me with some good advice. I have a chilly1 2-stage cascade with 2 NF9FX compressors, coil-in-coil HX, and CPEV on the 2nd stage. Gasses are R507 on the 1st stage and r1150 on the 2nd stage. Unit is tuned for variable load (due to the cpev I think). Static pressure is 60psi on the 1st stage and 140psi on the 2nd stage. I am reading about -33C on the HX and -90c on the evap with no load. The unit used to get colder (into the -100c to -110C range unloaded) but I don't remember if the static pressures have changed.... I was wondering if these look like ideal static pressures and if either the high or low stage could use some more gas.

Also, just to make sure...screwing the knob on the CPEV "in" will increase the amount of refrigerant going to the evap and thus allow it accommodate greater loads correct? And screwing it "out" will decrease the refrigerant but lower temps on a lighter load? Another silly question but I'm assuming its safe to screw the CPEV knob all the way in and out when testing...no chance of it coming out or causing a blockage/pressure buildup, right? (I'm assuming thats what the CP part of CPEV means).

Finally, a question of maintenance. I vaguely remember being told something about cycling the unit. Does it need to sit for a certain period of time between sessions? Is there anything I need to do for general maintenance? I would ask the man himself but Chilly1 doesn't check his PMs all the time.

The unit has been in storage for over a year and I kinda forgot how to deal with it in the interval so don't want to take any chances while I'm starting to use it again. Assistance greatly appreciated.

[tim-]

how you explain the function of cpev is correct. screw it clockwise = increase flow. counter-clockwise decrease.

you should let unit equalize hi and low side pressure atleast so the delta isn't more then 2-3bar before fire it up again as it tear on compressor to start with a big difference in pressure between high and low pressure side.

140psi sounds like undercharged for me. atleast on the edge to be undercharged. normally it's a bit higher static @ regular room temp ( ~20-22c).

if it have been in storage for over a year it is possible it have leak a little, especially when you have cpevs.. it's flare connections right?

regards
Tim

static pressure vary very much depending on tempture.

[Xenogias]

yes, flare I believe. Should I check around them with soap+water? is it at all possible there could be a leak at the adjustment screw of the cpev?

What would you recommend I do about that? Also, how does my 1st stage static pressure sound? 60psi seems low to me also. Ambient temperatures is 23C.

I just fired it up after about an 8 hour break and am having a problem getting the 2nd stage to pull down. It did this earlier too. It seems like some kind of blockage maybe... 2nd stage pulls down to about +20C (measured at the evap) and then hangs there, adjusting cpev does nothing (yes I let the 1st stage pull down to about -30c before turning 2nd stage on). Also the CPEV adjustment knob develops frost on it, but the evap head is just slightly cool to the touch. At this time my 1st stage pressure is about 5psi and 2nd stage pressure is about 25-30Hg. Yipe! I know 2nd stage compressor is working because it gets warm/hot. Could this too be a symptom of undercharge?

[Xenogias]

I'm planning on taking this to a HVAC professional that has R1150 and R507 on hand tomorrow or Wednesday (if I can find one) and would like to know what to tell them. Thoughts? Also, the 25-30Hg about is inHg, not mmHg.

[DetroitAC]

Sounds like both stages are very badly undercharged.

1st stage with R507 should be at ~160 psig @23C

2nd stage only has enough charge to produce liquid to cool the CPEV head? Ha! that's not much at all.

The good news is that it seems the leak is big enough, maybe it was made bigger by running it? so that the performance has seriously degraded since your first post. Maybe your HVAC guy can find that leak? That's the first thing to tell him, you have leaks on both stages, no point in going further until those are fixed.

[tim-]

normally flare connections is what's leaking if something is leaking. it's not that common a brazed joint leaks.

I would check the flare connections. Ofcourse you should strip both stages down and pressuretest it and search for leaks anyway.

I havn't splitted a cpev in part so I havn't seen it i pieces irl. but ofcourse there is a possibility the valve is leaking. But my suggestion would be to chech flare connections first as it's what I would guess is leaking especielly when it's leak slow. those are often hardest to get 100% leakproof. I do prefer expansion valves with brazed connections.

some loctite 542 or something simular would be good to seal flare connections if it's leaking.

regards
Tim

[Xenogias]

not having much luck with local hvac shops. Anyone know where I can get an ethylene charge nearish western North Carolina?

[tim-]

a hvac shop with ethylene will be hard. that's not a very common refrigrant.

I would try find someone who have experience of cascades here on xs instead as I think it is waste of time to try to find a local hvac shop with ethylene.

regards
Tim

[Xenogias]

Sounds like both stages are very badly undercharged.

1st stage with R507 should be at ~160 psig @23C

2nd stage only has enough charge to produce liquid to cool the CPEV head? Ha! that's not much at all.

The good news is that it seems the leak is big enough, maybe it was made bigger by running it? so that the performance has seriously degraded since your first post. Maybe your HVAC guy can find that leak? That's the first thing to tell him, you have leaks on both stages, no point in going further until those are fixed.

I'm not convinced the 1st stage has a leak as I remember it being charged to that before. It's currently at 80psi static... Are you sure 160 is more normal?

I was hoping to avoid tearing off all that insulation but I'm going to do it on the 2nd stage so I might as well on the first.

Also, I just pulled some of the insulation off of the line from HX-->CPEV and from CPEV-->liquid line of evap and there is a fine white powder over the inside of the foam--like talcum powder. Is this normal of some insulation foam or have i found evidence for the leak?

Also, for correctness sake the fittings on the CPEV are brazed, not flare...

[DetroitAC]

I'm not convinced the 1st stage has a leak as I remember it being charged to that before. It's currently at 80psi static... Are you sure 160 is more normal?


The part I'm sure about is that at 23C, R507 will have a saturation pressure of 162 psig. The saturation pressures of refrigerants aren't the sort of things one has an "opinion" about, they are published data and I'm just looking at published R507 data.

If that stage is correctly charged, it should have some liquid present, so it should be at the saturation pressure. I can't imagine an R507 system that could be all vapor at ambient temp and work correctly, well I suppose it's possible, but honestly I've never seen something like that.

The part I'm not sure about is R507, I'm just going off what you've said. 80psig is more in the correct range for R-134a. Are you sure it's R507 and not some random blend of gasses?

[Xenogias]

well when the system is off, the high and low sides equalize pressure (almost) so then you wouldn't have both liquid and gas anymore just one or the other, right?

I was wrong in my reading before. Its actually 75psi (was looking at the gauge at a weird angle earlier). I did check my documentation and it is r507 on the 1st stage.

so....the 75psi reading was taken off the low-side for what its worth and when it's running it pulls down to -36c at the HX under the heat load of the 2nd stage (so it's working somewhat). At that time the pressure is 0psi on the low-side. It could be colder but I can't imagine it doing even that if the R507 loop was so drastically undercharged. I don't mean to debate the point with you because my knowledge of this isn't complete and I do think it could stand more charge and I just want to make sure.

I completely agree about the 2nd stage...undercharged and leaking. What do you make of that powder?

[DetroitAC]

Ah, I think I might see the problem. Leave the system off for a long time, like check it tomorrow morning. If you are measuring the pressure after it has been running, the HX might still be cold.

I don't know what to say about some unknown powder substance...

When in doubt about an unknown substance, I taste it just in case it's pie.:up:

[[XC] gomeler]

rofl detroicAC. I could kill for some pumpkin pie right now *sigh*. White powder makes no sense to me either, green copper would indicate an insulation leak but white? Never seen that form on anything. Maybe it's a leftover from the construction.

[tim-]

I was thinking.. it could be flux or something. any oil or something on the pipes?

if you get us some pictures it would be good too.

[boshuter]

The white powder stuff is just what you said it is.... a talcum like substance used to coat the inside of the tubing. All the tubing insulation I get from the local HVAC supplier has that substance on the inside of the tube (I've never seen it on the split foam insulation, just the solid tube insulation).

BTW; I have a single stage sitting here that is fairly well tuned for 200w and it's at 100psi static with r507, so yours is pretty close, maybe a little low.

[_HL4E_HalfLife_]

The powder is there to make it ez for HVAC tech's to slide the insulation over the copper lines.

[SoddemFX]

The part I'm sure about is that at 23C, R507 will have a saturation pressure of 162 psig. The saturation pressures of refrigerants aren't the sort of things one has an "opinion" about, they are published data and I'm just looking at published R507 data.

If that stage is correctly charged, it should have some liquid present, so it should be at the saturation pressure. I can't imagine an R507 system that could be all vapor at ambient temp and work correctly, well I suppose it's possible, but honestly I've never seen something like that.

The part I'm not sure about is R507, I'm just going off what you've said. 80psig is more in the correct range for R-134a. Are you sure it's R507 and not some random blend of gasses?

Hi Erik,

Depending on the volume in the compressor shell and heat exchanger type and general stuff the static charge might not be in saturation. A lot of the recip single stages (r404a or r507a or r402a) round here have a static charge in the region of 60PSIg. Off the top of my head my little cascades r404a stage is about 80PSIg, that's a coaxial heat exchanger fed by capillary tube the same as this :)

Tom

[DetroitAC]

Thanks for clearing that up for me Tom

Sorry I added confusion to your problem Xenogias!