evacuation&dehydration

[nrd]

if you can help, what is the difference between a single deep evacuation and a triple evacuation?

[troyorr]

The difference is on is three times more than the other

[jayguy]

a single evacuation is taking a piece of equipment down to a final micron reading in one step.

a triple evacuation (or some other number if you wish), is to take a piece of equipment, evacuate it down to as low of a micron value as you can, raise the pressure up to atmosphere (or close to) usually using nitrogen, and re-evacuate it down again, as low as you can go. each time you re-evacuate it, you should be able to drop the micron level down lower.

you would typically do this procedure if you had moisture or other contaminants, a large volume of piping or vessels relative to your vacuum pump size, or a combination of both. since you will never be able to get out all molecules, it is better to have dry nitrogen in there rather than moisture, air, burned refrigerant, etc.

[tex~n~oz]

A single evacuation is where you generally take the vacuum down to 500 microns (generic manufacturers suggestion).
A triple evacuation is where you evacuate to 1000 microns and break the vacuum each time with dry nitrogen with a final evacuation to 1000 microns.

I personally think it's pox to do either of these to a system and prefer a different schedule.

The system I use is:
First evacuation to 500 microns as this is the beginning point of sublimation for moisture.
Second, I break the vacuum with dry nitrogen and let the system set for a few minutes to thaw any ice that may have formed and become insulated.
Third, I evacuate a second time to at LEAST 150 microns and I prefer a 75 micron vacuum if time permits.

For medium and low temp applications go below 75 microns or it can come back to bite you.

If you have a sublimation chart you will see that water turns to ice and sublimates at around 450 to 415 microns (if I remember right.. Don't quote me on this).
Personally I think they use 500 and 1000 as it's realistic for most technicians to attain, but not good enough in my opinion. Patience and attention to detail are a lost art.

An evacuation will not clean up a contaminated system.
Metal oxides don't evaporate.

All you can hope for is a postponed recycle.

[heatingman]

Hey tex, what vacuum pump are you using? If it'l go that low, i want one.

[gasnowman]

I seen my pump pull 35 microns after an oil change. Most pumps out of the box will pull 50 microns or lower. It maybe time for a earl change heat.

[tex~n~oz]

A decent 2-stage vac will pull 20 to 65 microns brand new out of the box. I have one that is over 10 years old and will pull 40 microns still.
Long pump life is guaranteed by changing the oil after every use, not leaving acidic oil and moisture hang around in the sump. Proper use of the gas ballast will help prevent condensation forming in the pump oil as well.
Don't buy the "keg O' Oil" and keep it sitting around months between use. Buy sealed, single use oilers and toss what you don't use. Moisture in the pump oil is detrimental to a decent vacuum. Keep your pump plugged and capped at all times when not in use.
BTW I pulled a 240KWh chiller down to 30 microns last week with a 3-phase vac pump that I think is as old as I am..
It did take 2 days but I can rest assured that there was no free water in the system..

It only takes one Oh-Sh!t to ruin a good pump and ONE DROP of free moisture to kill a chiller.

Cheers

[hvac_superman]

Wow, those are good vacuum pumps. I was told by a manufacture that if you go that low you can boil the oil off. Is this true? anyone?

[rimek]

Originally posted by hvac_superman
Wow, those are good vacuum pumps. I was told by a manufacture that if you go that low you can boil the oil off. Is this true? anyone?

no-

[tex~n~oz]

Hey Hvac-superman,
In a nutshell.. NO, you can't boil off the refrigerant oil.
The vacuum pump oil has very similar characteristics to the mineral oil used in 400 or 300 refrig oil. Because it provides the vapour seal, it wouldn't allow it.

Because mineral oil is probably a Carbon-21 oil, it's boiling point at atm pressure would probably be around the 360C+ mark. Oils have a much higher intermolecular attraction than water and are harder to bring out of liquid state.

I would hazard to say that your manufacturer was concerned about seperating something else out of the oil and I cannot answer for that, but to say "boil" the oil off? I might even think slight evaporation is possible at those deep vacuums but that would be about it unless you added copious amounts of heat.

I've looked into the oil level glass of many deeply evacuated compressors and never once witnessed boiling oil.

From my best calculations from the information that I have available, mineral oil boils at 1 microns (10-6 Torr) @ 75F.

Anyone?

[esornivram]

most new vacumm oil does have large macroscopic cross section for water, however if you canged and use low pressure chiller oil that will noy be an problem, i use trane 22 chiller oil, we usually have some left over from service work, it works well, we pulled an 3 day vacumm on an wet chiller, the oil never clouded up, the moisture goes out with pump discharge

[heatingman]

I change my oil after every use, I use jb oil, and in a pinch I've used agricultural vacuum oil. My pump will go down around 150 or atleast thats when I shut it off, but most of the time I don't have the time to go deeper than 400, and on the larger equiptment thats not brand new, I'm lucky to see 900 microns, and it holds. Fermilab once had me pull a vacuum on a Rowald one off machine that held 350 lbs of 407c for a week and never saw 1000 microns, but it held. What a piece of junk that chiller was. Six txv's that were cycled, liquid injection cooled copeland screw complete with a volume indexing slide valve, head pressure regulators, and bypass valves, miss-piped hot gas bypass (after the evaperator), 2 paralell liquid to suction subcoolers, and a subcooler with cooling water fed from a different chiller 32 deg water fed to it, and since there was so much subcooling, the refrigerant in the suction line condensed from contact with the liquid line, so there were expansion valves to feed back to the compressor. I Digress. Point is what vacuum level do you guys normally get on heavy tonage equiptment. By the way the screw locked up after 3 months of service due to poor design.

[emcoasthvacr]

Holding 250 microns is not economical, unless the Stars, Moon, & Sun are aligned.

Originally posted by heatingman
Hey tex, what vacuum pump are you using? If it'l go that low, i want one.

[emcoasthvacr]

I hope your pump last it's warranty period. Pulling 35 microns is ridiculous, unless your father works for the EPA.

Originally posted by gasnowman
I seen my pump pull 35 microns after an oil change. Most pumps out of the box will pull 50 microns or lower. It maybe time for a earl change heat.

[r2]

so where are you all taking the used oil?

[refrige-nate]

back on the original topic...most of you have already posted complete answers but i thought i might add that triple evac. is used to remove as much moisture from the oil as possible. refilling with nitrogen agitates the oil and breaks the surface tension of the oil, this allows the moisture to be removed. this is a common precedure in cascade refrigeration systems.

[tex~n~oz]

emcoasthvacr - Pulling a hard vacuum isn't uneconomical. It only takes time (that the tech doesn't have to be there for)and it doesn't hurt the vac pump one bit.
The things that kill vac pumps are not deep vacuums, but contaminated oil from failure to ballast initial vacuum, not keeping the vac pump closed up during inactivity, using new oil that has been exposed to air, not changing oil immediately after use or at set intervals during use.

heatingman - The larger the volume, the longer it takes to pull a deep vacuum. As the pressure reduces in a vacuum, the efficiency of the pump reduces as well, therefore, a pump that produces an effect of 6 CFM at 1 atm will do just a slight fraction of a ml/m @ 500 microns. Hence, it pulls a vacuum very fast at first, then slows to a crawl once it hits the later stages of the vacuum. Once you've done a generic calculation on the volume, you can interpolate the amount of time it might take, but many variables exist. First, non-condensables come off quick while condensibles might experience a phase change at the lower temperatures of the vacuum and hold it at say 400 microns for hours and hours. This is where the dry nitrogen break comes in handy. It allows the pressure to rise above the freezing point of those condensables AND through evaporation pulls them into vapour phase. Given enough time for these two things to take place, the second phase of the evacuation will happen much quicker.
There is really no limit as to how low a vacuum you can pull on any system that is completely sealed, it's just a matter of how long are you willing to wait. Last year I suddenly fell ill and ended up in the hospital for 5 days. My trusty apprentice, knowing not to assume anything on my behalf, just changed the vacuum pump oil every-other day until I was well enough to get back to work. Seven days later, This massive chiller was sitting pretty at 18 microns.
Did it hurt anything? Not at all, just assured that the system was super evacuated. Would I intentionally do it? Nope. Not unless I was working on the primary or secondary of a neon cryogenics cascade system.
Personally, I feel there is a problem if I can't pull a system to 150 microns in 24 hours. I have either moisture issues, entrainment, or something....
150 microns is the maximum I'll let float. Overkill? I don't think so, it's just the way I work. In all my years of being this way, I've lost an incredibly small number of compressors to acid.


r2 - I'm not sure what the boss does with it here, but in the past we've used the oils in used oil heaters at the shops. Living in the tropics means we don't use heaters here.