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View Full Version : Can I leave a system in a vacuum overnite?







kengineering
05-04-2006, 12:04 AM
In my shop it has been a common practice to leave new self contained units in a vacuum overnite if we have run out of time for the day or if we suspect a very small leak. We come in he next day check to make sure the vacuum held, (no leaks) then proceed to charge the system with the appropiate refrigerant and finish the project. I have now been told that if left in a vacuum overnite moisture will permeate through the pores of the manifold hoses and contaminate the system.

Can I get some feed back on this issue?

Thanks, Ken

Dan
05-04-2006, 12:32 AM
If you turn the pump off and you maintain a vacuum below 500 microns overnight, you don't have any problems with the hoses. Now, if you are leaving the pump on... I would have to rethink my answer.:)

Dan
05-04-2006, 12:37 AM
For example, you have a low side hose with a small amount of leakage, but the hose has no leakage on the high side. If you are pulling a vacuum with a pump that can maintain a vacuum while the one hose permeates or leaks, then you could well be drawing moisture in some small proportion throughout the system, if you are leaving the pump on.

If you only have one connection, then the hoses won't be a problem, but you do have the consideration of an internal leak in the piping.

I don't think that maintaining a vacuum with the pump on is a good practice. Use positive shutoff valves to isolate the pump and insure your vacuum.

kengineering
05-04-2006, 02:19 AM
Ok, I can see I need to clarify and redirect the question as;

If I have the system hooked up overnite with a micron gauge connnected but isolated from the pump, which is off, it is acceptable if the micron rise is within 500 microns.

Dan
05-04-2006, 11:32 PM
it is acceptable if the micron rise is within 500 microns.

If the vacuum remains below 500 microns, yes it is acceptable. Not if it rises 500 microns.

US Iceman
06-04-2006, 02:41 AM
Ken, One thing I might suggest is the use of copper tubing for pulling a vacuum. Not 1/4" tubing, but probably 3/8" or 1/2". The connections can be any size you want, but I would recommend the larger tubing to reduce the pressure drop.

There are also two types of hoses for the manifold gauges. The regular ones which are normally color coded, red, blue, and yellow. Or, you can get heavy duty hoses that are solid black (at least mine are).

I always suspected the regular hoses of being permeable, so I started to use the heavy duty charging hoses for everything, except for extended vacuums for dehydration of systems. Then I used copper tubing.

rbartlett
06-04-2006, 07:20 AM
I once lent my vac pump to a guy working Heathrow a.p
He hooked it up on vac and went over to the other side for a quick look at another job. He the went on a 2 week holiday!!

My poor vac pump was a moltern mess which he never had the courage to show me...

Cheers

Richard

Ireland
06-04-2006, 10:26 PM
A guy I worked alongside a number of years ago who always left a new plant on vacuum for a couple of days, ideally over a weekend.

He reckoned back then that it was the way to go and, to be fair, he never got much trouble from his installation and, from what I hear, he still does this and it's working well for him.

weeksy
07-04-2006, 11:13 AM
Quote: have now been told that if left in a vacuum overnite moisture will permeate through the pores of the manifold hoses and contaminate the system.

Does this realy happen???

I'm struggling to understand how. my gauge lines are spec'd to withstand pressures up to 500 psi.....

...If they can hold that sort of pressure, i can't see a little vacuum causing any issues.

IMHO, the person who told you this either a) made it up or b) has the worlds worst set of gauge lines.:cool:

NoNickName
07-04-2006, 12:32 PM
There is no problem in leaving the vacuum overnight and over a fortnight as well.

Andy
07-04-2006, 08:42 PM
Hi:)

depends on the system size. A domestic probably won't need this, but an industrial system will need a number of days, dependant on ambient, what moisture is in there and what vacumn you feel is required to be held when the pump is off. Ammonia will not need the same vacumn as a small domestic.

Gauges are fine for small jobs, I have made up lines a bit like hydralic (with a plastic inner tube) for slightly larger systems, and a 1/2" hydralic hose for larger systems.

Kind Regards Andy.

US Iceman
07-04-2006, 09:16 PM
I have worked on several large R-22 systems. The vacuum pumps were left on for weeks to dry out the system.

Multiple pumps and cold traps galore.

Just make sure to check the oil levels regularly.;)

austech
01-05-2006, 12:17 PM
I frequently leave systems on vac over night or for a few days, with the vac pump running. This is after I am confident that there are no leaks by performing pressurisation checks and using the electronic leak detector in the system as there is no sense in sucking in moisture and other contaminants. I usually leak check the hoses, connections and gauge manifold before leaving it on vac. This applies to large refrigeration systems and those that have been contaminated. :D

curious123
10-05-2006, 03:43 AM
I once lent my vac pump to a guy working Heathrow a.p
He hooked it up on vac and went over to the other side for a quick look at another job. He the went on a 2 week holiday!!

My poor vac pump was a moltern mess which he never had the courage to show me...

Cheers

Richard

On an electron microscope the vacuum pump runs continusly and only shuts down for an oilchange twice a year and last many years. Your pump most likly was contaminated and this wrecked it.

The MG Pony
10-05-2006, 08:42 AM
No it probly ran out of oil, not all pump systems are the same on how they deal with oil, vacuum pumps that I've seen consume the oil as they go needing refilling every X houres of run time. A closed circuit oil system will obviously rarly need any oil toped up of which I'd assume your E-Micro scope is using such a system.

Harris Daniel
17-05-2006, 03:03 PM
are you sure that there is no leak at all. If you are not sure, then it is not a wise thing to leave the unit on vacuum overnight...as the air may enter the unit and moisture may come in contact with lubricating oil of compressor ...which is not advisable.
if at all you want to verify leak...it is better to leave the unit pressurised with Nitrogen and note the standing pressure....any drop in pressure next day will indicate a leak in the system.
Thanks
Harris Daniel.
Box 4311. Dubai. United Arab Emirates.