How long does it take to vacuum down a previously open system

[Dano 500]

Hi all,

So, this system was open for what must have been years. I bought it from someone out of their backyard. It is a 1984 Chevy K20. I've been working on restoring it for about 15 months and now finally got it ready for the road. I ADDED a compressor, & replaced condenser, dryer and orifice tube & I flushed evaporator. I couldn't get it to vacuum down past like -10mm/hg. Let it vacuum for what must have been almost four hours.
As soon as I closed closed the valves on the gauges, I lost all vacuum. From what I have read online, I assume there is a leak in the system. I went to the local parts store and purchased new discharge/suction line and it went down to -20 pretty quickly, but when I closed valves on gauges, lost vacuum again!

So, now I am waiting for liquid line to come in mail, but my main question is how long do you vacuum before you know everything is boiled off inside and it's good to test? If there is still flush in it, or whatever moisture it is, does the gauge go down to -30 right away, or does that take a long time, depending on system?

I've never done a/c work, but I told myself as part of this project, I would get it looking & running like I want to- all by myself. So, I don't want to break with that goal. lol

Thanks in advance,
-Dan

[JohnHere]

I have a lot of questions in mind, but I'll try to address at least some of them here.

Given the situation, I would have replaced everything with new parts, including the the evaporator, because you never know what's lurking inside a system that has been open for years—including gunked-up oil mixed with water, and even insects, that might not have been entirely eliminated by flushing.

If you're evacuating the system and the vacuum fails to hold, it's possible that the leak could be someplace other than in the system. How are your Manifold Gauge Set, hoses, and adapters? Are they relatively new, of good quality, and all seals intact?

Also, what type of vacuum pump are you using? An electric, rotary-vane pump containing fresh oil and good fittings, or an air-operated pump?

The vacuum leak could also be in the evaporator (or anywhere else, for that matter) if you didn't remove it, clean the fins, and carefully inspect it for signs of corrosion or evidence of oil leakage.

Did you replace all the o-rings and/or seals?

As for how long to evacuate, a shop will usually evacuate a system for around one hour at most due to the "time is money" aspect. If it's my own vehicle, and I'm not in a hurry, I'll run the vacuum pump for about 8 hours, or overnight.

Without any leaks in your equipment and system, the low-side gauge should drop almost immediately to around 25 inHg and continue in a few minutes down to about 29.9 inHg if you're located at an elevation of sea level or slightly above. For higher elevations, the gauge reading will be less than 29.9 inHg.

But to achieve the best possible evacuation, a micron gauge will be needed. A good vacuum pump is a given. The goal is to draw-down the system to about 500 microns or below, which might take several tries depending on how much air and moisture need to be removed and how much of the same the system might be outgassing.

Lastly, because the vacuum in this system doesn't hold yet, and you're pulling in additional air and moisture through the leak(s), you'll need to replace the R/D again once you achieve a system that is tight and leak free. The R/D contains a desiccant that becomes saturated with moisture under leakage conditions.

[Cusser]

Lastly, because the vacuum in this system doesn't hold yet, and you're pulling in additional air and moisture through the leak(s), you'll need to replace the R/D again once you achieve a system that is tight and leak free. The R/D contains a desiccant that becomes saturated with moisture under leakage conditions.

Agree

[andrew vanis]

for reference using a 180cfm single stage pump I can get under 500 microns in

Under 3 minutes 1997 Volvo V70 (approx 28 oz refrigerant capacity)
Under 5 min custom system with tons of line length (approx 60 oz refrigerant capacity)
(times from memory and can test/time next time if requested)

I pressure test with old RD and if that holds over 400 psi then:
(I also guess a or fitting union to bypass RD could be constructed)

I vacuum test with RD removed and silicone tapered stoppers (corks) in the fittings to get moisture and what ever out of the system and if that holds the under-500 microns I'm fairly sure the lines and system is good

Then I install new RD and vacuum again under 500 microns and shut off valve between gauge and vac pump - so gauge can still read system. This tests the new RD and re-tests the rest of the system.

If that holds overnight, I fill.

[andrew vanis]

If it's my own vehicle, and I'm not in a hurry, I'll run the vacuum pump for about 8 hours, or overnight.

If system is under 500 microns and holds, is running longer necessary / what benefit does running longer provide?

[Cusser]

I haven't pumped down a system since 2018 when I replaced the compressor/condenser/drier on my 2004 Frontier. But I typically evacuate for about 45 minutes, while I break for lunch !!!

[Tim]

20/5/20.

20 minutes. Vacuum
Shut off gauge valves and pump. Five minutes boil off.
20 Vacuum.

Charge system

[JohnHere]

If system is under 500 microns and holds, is running longer necessary / what benefit does running longer provide?

None, of course. I'm talking if the vacuum reading continues going up (decaying) from, say, 500 microns, you either have a vacuum leak or something is outgassing. If verified to be the latter, I would continue evacuating until the reading becomes fairly stable. Sometimes, that takes several hours to achieve.

[andrew vanis]

If system is under 500 microns and holds, is running longer necessary / what benefit does running longer provide?

None, of course. I'm talking if the vacuum reading continues going up (decaying) from, say, 500 microns, you either have a vacuum leak or something is outgassing. If verified to be the latter, I would continue evacuating until the reading becomes fairly stable. Sometimes, that takes several hours to achieve.

Thanks!

[andrew vanis]

another data point with 180 cfm pump

1997 C/K 1500 Capacity TBD. system may or may not have been opened viewtopic.php?p=29799#p29799

3 min from ambient to 500 micron

another 14 min for a total of 17min 500 micron to lowest 310 micron


Ambient
500m 3min 3min total
380m 2min 5min total
360m 2min 7min total
350m 2min 9min total
340m 2min 11min total
330m 3min 14min total
320m 2min 16min total
310m 2min 17min total