Finding a refrigerant leak

[atgcpaul]

I've got a -20C cold room at work on the 2nd floor that has developed a refrigerant leak. The condenser unit is on the roof which is separated by offices on the 3rd floor.

First service call was 6 weeks ago. Unit was slowly warming up. Tech didn't bring his leak detector but did check a few exposed joints near the expansion valve and on the condenser with a bubble solution and 2 hours later, couldn't find a leak, but added 16lbs of 404a. I think it holds 35lbs.

2 weeks after that, I call in again. Same symptoms, different tech. He has his detector and tried to trace it out even getting as far into the chase as possible. 6+ hrs of checking and only finds a loose fitting near the expansion valve. It was wrapped in foam so that's why Tech 1 didn't find it. He tightens it down, adds another 16lbs gas and goes.

2 weeks go by and like clockwork, Tech 2 is out again. Still no obvious leak and he shows me an intermittent leak on one of the compressors which tightens down but it doesn't explain how much gas we're losing. This time he has to add 25lbs.

These visits aren't cheap so I'm trying to get their boss to find it for real. I know they have bigger jobs and as I see it, it's steady work. It's surprisingly hard to find someone else to take this system on.

Anyway, what's the recommended path forward? I'm prepared to let the cold box warm up to room temp. The contents are stable for a while. From what I've researched, once the system is off, all the refrigerant is recovered. The system is pressurized to 300psi with dry nitrogen and an added tracer gas. The leak is hunted down--just with a leak detector or will the leak audibly "whistle"?--and repaired. The nitrogen is evacuated again and either pressure tested again and/or held under vacuum for 24hrs+. If it passes that, the system is charged up again.

Thanks for the advice
Paul

[blackhat]

What is your max down time?

[Woodenfish]

Those electronic refrigerant testers are horrible. IME, most electronic leak detectors couldn't find a leak unless it was visibly gushing out. UV/tracer dye is the most reliable method of detection.

It wouldn't surprise me to hear that prior loose fitting is the source again. If a component is loose and the pressurized gas is leaking the chances of the seal being compromised/distorted is great. Just securing it is often a poor solution. I always replaced the components seal in those cases so I was confident it was repaired.

[atgcpaul]

blackhat said:

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What is your max down time?

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At this point, until the leak is found and fixed. Short term the samples will be OK if they go through a thaw. We can still access our samples. I would like to minimize it to a week, though.

Woodenfish said:

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Those electronic refrigerant testers are horrible. IME, most electronic leak detectors couldn't find a leak unless it was visibly gushing out. UV/tracer dye is the most reliable method of detection.

It wouldn't surprise me to hear that prior loose fitting is the source again. If a component is loose and the pressurized gas is leaking the chances of the seal being compromised/distorted is great. Just securing it is often a poor solution. I always replaced the components seal in those cases so I was confident it was repaired.

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So do you simply add a bit of tracer dye into the system as you would the gas? Meaning, you wouldn't recovery all the gas and pressurize it with nitrogen plus the tracer gas? I guess this is done with car ACs, right? I'm not saying I would do this, but whoever is doing the service would.

My main concern is that the leak is somewhere between the 2nd floor and the roof in addition to the couplings in a ceiling (suspended so not horrible) or behind a wall. Is there any advantage using just the nitrogen over the dye? Is the leak audible behind dry wall?

Thanks,
Paul

[blackhat]

Forget dye. It won't help you with this. You need the tech to isolate the system into sections, preferably evap, lines and condensing unit and pressure test all three. This isn't a pinhole. I would likely use an ultrasonic detector to sweep whichever section fails the pressure test. Exact configuration would determine my approach from there. Find it, fix it and retest. Then reconnect, vac and recharge.

[atgcpaul]

That's what they used. I bought one for the lab, too. It took quite a while to find it but it was at one of the brazing joints covered by insulation. Fortunately it was in the drop ceiling and not in the chase.