I don't think youve missed anything. If the situation calls for it, I see no problem. I had a service mgr once that was of the same mindset ( don't run it overnight) but gave no explanation as to why. Never had a prob nor heard of anyone having a prob running overnight.

 

You are right.

Pump isn't going to boil oil out of a compressor.

Sounds like he needs a bit more guidance.

 

"Not knowing all the facts". Where have I heard that before?. The blanket statement of leaving a vacuum pump on over night does not mean a whole lot. If the vacuum pump oil is dirty there is no threat of "pulling too deep of vacuum". You really did not give any details. Is the compressor the only component being evacuated, or is it the whole system? One could assume either way. Does the tech have a micron gauge/meter to measure vacuum with. Is the system an airconditioner or low temp freezer, again not stated. Would it hurt the oil if left overnight? No, a better question is would it help? If evacuating just the compressor ( both service valves closed). The vacuum pump oil is fresh and clean and is able to pull to 50 micron or close, and the compressor oil is clean, no leaks on fittings, gauges, hoses and hoses are vacuum rated and clean, then you are ready to evacuate and it should not take long at all, in fact no need to leave it overnight. But if you must, the compressor oil will be fine.

 

I would be more concerned with leaving the pump unattended and there was a power loss. Pump oil could be pulled into the system. Any mechanical device can fail at the most inopportune time.

 

Happened to me, once by a GFCI and another by an electrician that said our extension cord was not safe and unplugged it and cut the ends. Results= vacuum oil in the piping. Besides, with a micron gauge, you may not need to run it all night.

 

Older parrafin oil, could be a problem.
Newer oils, no problem.

 

I do it regularly, but only on systems that are huge. I don't like the idea of leaving it unattended either... but I take a few precautions. Make sure to throw some rags under the pump (incase it wants to get rid of it's oil), I try to set it lower than where I'm hooked up too just incase it shuts off. Also I make sure to put a GFI on the cord too.

You might be able to boil the oil out, but if it's a larger system, I think it would take forever to even start.

 

I think some of the pump manufacturers have or used to have this warning in their literature.

 

Was just remembering... they do make a solenoid you can put in the vac line, if you loose power it shuts off to prevent oil from being sucked into the system.

 

There is no preset amount of time to evacuate a system. Its done when the micron gauge says it is done.
As other have said, you won't "boil" the oil out of the compressor.
A good clean vacuum pump pull a system down to the vapor pressure of mineral oil, but it doesn't really "boil off" like moisture does, it just off gasses oil molecules at a very low rate. It would take a very long time, like months or years, to remove enough oil to matter.



Some pumps have a built in check valve to prevent pump oil from being sucked into a system. My Yellow Jacket 6 CFM pump has that feature.

 

also be aware of what type of surface you have the pump set on. A while back there was a thread here about your worst screw ups. A guy posted that he left his vac pump on overnight and it caught fire and it burned the building down

 

and i left my pump on today for monday morning on a bed of cardboard and rags to sop up any oil

 

The only issue I would have with leaving it over night and starting the vacuum at the end of the day,would be if it was a chiller with a busted barrel,almost always I get a lot of water displacing the oil in the first few hours and wind up doing serveral oil changes on my vacuum pump during that period.

 

I dont see the purpose in leaving the pump on overnight. Heat the evaporator, condensor coil and lines, break with nitro, and be on your way..If you pull down to a low micron level, let it sit for a little while and it holds at the same level, your done..

 

I had to do it once. Had a small unit that uses a hot gas coil to evaporate condensate. Coil corroded and the unit was left, unrepaired for a while. Big time moisture problem. I tried driers with no improvement.

Pulled the charge, piped the pump in with 1/4" copper and let it run for 2 days with an oil change in the middle. Cut a C-303 drier in and let that run for a week. Came back, pulled both driers and cut a C-083 in and left it run.

Haven't heard a peep out of it in the years since.

 

The only problem with leaving on over night besides power loss is what is the neighborhood like. Remember crackheads will steal any thing of value.

 

using a cold trap?

 

I think he talking about the statement that oil can start to boil a 200 microns

I never heard that it will boil off thought.