Potential Root Causes? Holds pressure but doesn't hold vacuum

[Inclag]

Hi,

New forum member so please be gentle with me....

I'm looking for theories from those more experienced than myself as to what might be causing an odd behavior in a refrigeration system that my team is working on. We've conducted pressure hold tests using nitrogen gas for 24hrs within our system at ~100 psi and have observed no discernable leak from the system. This system has electronic pressure sensors throughout so we've been able to log pressure data overnight at a frequency of every 30 seconds and are not seeing any signs of a pressure drop. Secondly we've been applying leak detector to a known problem area and this is also exhibiting no signs of a leak under positive pressure.

However, when preparing to fill the system and pulling vacuum, we are not seeing the micron level stabilize when capping the system. The unit was capped over the weekend, and the micron gauge has risen to >60000 microns. Considering that the maximum pressure differential created under vacuum is ~14.7psi, I'm at a loss as to how we can observe a >1 psi pressure shift under vacuum, but when under far greater pos. pressure differential we are not observing any shift.

Some additional color
- This system was left open in an office environment for a few months. Has anyone seen issues similar to this as a result of a filter dryer being spent?

Anyway, I'm not an HVAC expert and I'm just trying to get a sense for those that are, if you've observed similarly confusing situations before and if so, what the problem was to aid myself and team in trying to hunt down our issue.

Thanks!

[Hurst11]

What kinda system are you working on and what is your evacuation setup? Are you pulling through a manifold or do you have an actual vacuum set up.

[cmdunn]

I've had issues with filter driers holding on to moisture or refrigerant and causing evacuations to take a lot longer. It could be slowly off gassing and raising your pressure while it's in a vacuum but you wouldn't see anything while it's pressurized. I would definitely change the driers and probably the oil too if the system had been open a while, especially if it's POE or another synthetic.

[Inclag]

So unfortunately I can't divulge much about the system due the nature of what it is. What I can share is that no manifold is being used and vacuum and filling is being accessed via a core removal tool. This system does not include any oil since the refrigerant circulation method is 'non-traditional' and does not use a compressor.

We've been pulling vacuum down to the ~60 micron level for probably 24+ combined hours now, spread over a ~6 day period and we're still seeing the same issue. Honestly, due to the intended use and expected life of the system, I'm pretty close to just filling the darn thing with refrigerant. Not being an expert in the space, my limited understanding is that there can be a number of factors that can contribute to a rising vacuum pressure. I believe we've ruled out the presence of a leak due to the positive pressure testing, so I'm curious if anyone has experienced any 'gotcha' situations where a positive pressure test over a time scale of significance, yielded a false positive leak pass/fail result that a vacuum test was able to catch.

[Elfshadow]

So unfortunately I can't divulge much about the system due the nature of what it is. What I can share is that no manifold is being used and vacuum and filling is being accessed via a core removal tool. This system does not include any oil since the refrigerant circulation method is 'non-traditional' and does not use a compressor.

We've been pulling vacuum down to the ~60 micron level for probably 24+ combined hours now, spread over a ~6 day period and we're still seeing the same issue. Honestly, due to the intended use and expected life of the system, I'm pretty close to just filling the darn thing with refrigerant. Not being an expert in the space, my limited understanding is that there can be a number of factors that can contribute to a rising vacuum pressure. I believe we've ruled out the presence of a leak due to the positive pressure testing, so I'm curious if anyone has experienced any 'gotcha' situations where a positive pressure test over a time scale of significance, yielded a false positive leak pass/fail result that a vacuum test was able to catch.

So..... you can actually pull a vacuum to fast....

It's rare but if you pull a vacuum to fast you can cause ice to form.

Your microns will rise as the ice melts.

[Elfshadow]

So unfortunately I can't divulge much about the system due the nature of what it is. What I can share is that no manifold is being used and vacuum and filling is being accessed via a core removal tool. This system does not include any oil since the refrigerant circulation method is 'non-traditional' and does not use a compressor.

We've been pulling vacuum down to the ~60 micron level for probably 24+ combined hours now, spread over a ~6 day period and we're still seeing the same issue. Honestly, due to the intended use and expected life of the system, I'm pretty close to just filling the darn thing with refrigerant. Not being an expert in the space, my limited understanding is that there can be a number of factors that can contribute to a rising vacuum pressure. I believe we've ruled out the presence of a leak due to the positive pressure testing, so I'm curious if anyone has experienced any 'gotcha' situations where a positive pressure test over a time scale of significance, yielded a false positive leak pass/fail result that a vacuum test was able to catch.

sounds like your working on a heat pipe style system.

[Inclag]

So..... you can actually pull a vacuum to fast....

It's rare but if you pull a vacuum to fast you can cause ice to form.

Your microns will rise as the ice melts.

Thank you. This is beneficial and could be a potential cause. We are generating a sub 500 micron vacuum in extremely short order. Probably easily under 30 seconds.

Are there methods/modes/tools of regulating the vacuum pull to more like ~15,000 or 12,000 microns? This would presumably boil off any moisture in due time I would imagine. I recognize that purging/replacing the drier is probably the 'correct' solution, but this system that we are working on only has a single access port due to the nature of what it is. If I had my druthers there would be some significant changes in order to increase the workablity of the system, but my team is between a rock and a hard place and a modality of boiling off potential moisture even in light of perhaps the time needed to do this, would be more preferable than pulling things apart. Pulling apart is going to incur risk and human cost/time which isn't something I can tolerate at the moment. A passive mode of connecting a regulated vacuum that incurs sitting and waiting for potential moisture to boil off is far more preferable.

I'd be willing to give the latter option, if doable, a shot before filling our system.

[Elfshadow]

Thank you. This is beneficial and could be a potential cause. We are generating a sub 500 micron vacuum in extremely short order. Probably easily under 30 seconds.

Are there methods/modes/tools of regulating the vacuum pull to more like ~15,000 or 12,000 microns? This would presumably boil off any moisture in due time I would imagine. I recognize that purging/replacing the drier is probably the 'correct' solution, but this system that we are working on only has a single access port due to the nature of what it is. If I had my druthers there would be some significant changes in order to increase the workablity of the system, but my team is between a rock and a hard place and a modality of boiling off potential moisture even in light of perhaps the time needed to do this, would be more preferable than pulling things apart. Pulling apart is going to incur risk and human cost/time which isn't something I can tolerate at the moment. A passive mode of connecting a regulated vacuum that incurs sitting and waiting for potential moisture to boil off is far more preferable.

I'd be willing to give the latter option, if doable, a shot before filling our system.

Triple evacuation and sweep with nitrogen sweeps. If you only have a single port.... you can't really flow nitrogen though. If you have pressure transducers can you pull one out to let you flow nitrogen through.

Might want to google triple evacuation.

This a heatpipe or convection system?? Probably won't answer that.

The triple evacuation is meant to be a means of dehydrating refrigerant tubing in a system that has high water content before refrigerant is introduced. This was and is still used on systems that have the potential for water to freeze while vacuuming.

[ehsx]

“>60000 microns”. ? Not water or moisture at that level.
If that high, my guess would be a shaft seal or relief valve. System model & type would help……

[Elfshadow]

“>60000 microns”. ? Not water or moisture at that level.
If that high, my guess would be a shaft seal or relief valve. System model & type would help……

Based on what hes talking this is something at a college or company.

non-compressor based system that still uses refrigerent.

Im guessing something using convection. There was a ton of noise around convection based CPU cooling a few years ago.

Thinking something like this....

Getting below 500 micro in 60 seconds indicates a very small volume to.

[Scoobie]

I would verify your “set-up” is not leaking.

Maybe take a pice of 3/4 tubing 10ft long, braze both ends closed, add a service port. Pull that down and make sure it will hold.

I’ve fought the same battle you are dealing with. Interested to hear if you get it resolved. We replace the little rubber thingys in the hoses as they leak under vacuum but not positive pressure sometimes.

[Restaurant Mech]

For doing a long decay test you need to actually get all of the moisture out of the system.
It's virtually impossible to just vacuum to 500 microns and blank it off for a day without having an extremely deep vacuum pulled in the first place.
I have performed long blank off tests. Most of the time in order to legitimately pass the test my pump would need to pull that vacuum for days before blanking off.
So a legitimate 125 micron reading at blank off time would typically read 350 microns after 72 hours.
There is no such thing as a quick vacuum for a blank off test longer than 30 minutes

[pacnw]

As stated, is this a small volume system?

If so, that could be the issue in itself.

[Fla.HP]

I have seen this problem once. It was a pop off. Would seal under pressure, not so much under vacuum.
And a poorly brazed connection can seal under pressure, but leak under vacuum.

Sent from my phone, using that talk thingie.

[Hurst11]

I have seen this problem once. It was a pop off. Would seal under pressure, not so much under vacuum.
And a poorly brazed connection can seal under pressure, but leak under vacuum.

Sent from my phone, using that talk thingie.

I’ve never saw that, how does that happen? The braze hold under pressure but leak when not?

[Fla.HP]

Just think about what happens to copper joints under stress. From vacuum to full pressure...and the expansion and contractions they go through

Sent from my phone, using that talk thingie.

[gustavhorna]

You are doing something wrong! if you measure pressure this will vary with ambient temperature. You can't have it CONST!
By the gas equation pV = nRT your pressure will vary over the course of the day, as T does!

In contrast if you see vacuum decay you HAVE a leak!

[yg1box]

Have you checked your hoses and core tool. Try a little nylog. Most usually my vacuum leaks are at my added connections and not my systems.

Sent from my Pixel 2 XL using Tapatalk

[idontgetit]

Hmm well single access port is going to make it pretty hard to say. I have seen Schrader leak vac that would hold pressure but sounds like you have none.

Very low volume systems are hard to micron test because the tiny bit of leakage makes a huge impact.

Core removal tool might be a cause nylon the O rings, the vac gage itself, nylon it's o ring.

Sorry if it's insulting but if your team is qualified enough to be developing such a system they should be able to handle such a problem with ease.

[idontgetit]

Nylog not nylon