vacuum prep on an auto a/c

As all y'all know, I'm just a talented amateur and while I've been doing prep work on my own, I've had my local, certified, experienced, licensed hvac professionals do the serious stuff.
(all two of them in my area...)
In other words, I just know enough to be dangerous.
which gets me to my question:
Whenever a central unit is installed/serviced, the tech draws down a hard vacuum and keeps it evacuated for a couple of days.
When I've had my car's a/c serviced, they simply evacuate it, do some magic (which in one case required a compressor replacement [a known issue with that unit]), recharge the system, charge me plenty... and send me on my way.
This seems to be the standard time frame in cars.
Would slowing the clock down and keeping the vacuum in place longer make any difference?
Thanks.
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On 8/28/2011 12:21 AM, danny burstein wrote:

Someone already mentioned the possibility of the techs checking for a leak but we use dry nitrogen for that purpose often leaving the line set and evaporator under pressure long before the outdoor condensing unit is installed. We wait to install condensers these days on new construction and unoccupied remodel jobs because of AC theft. I'll leave 100lbs pressure N2 in the lines so I will know if someone put a nail through them or if there is a small leak.
TDD
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responding to http://www.homeownershub.com/hvac/vacuum-prep-on-an-auto-a-c-44911-.htm Jasprt2 wrote: Danny, the short answer is no: it won't make a difference.
RE: The purpose in evacuating home/commercial HVAC units is firstly to remove 100% of moisture from the lines. Moisture in those systems can cause a variety of failures: Core Blockage, Compressor failure, freezing, corrosion etc. and Secondly to maximize the freon/oil charge in the system. No respectable HVAC Tech would knowing charge a system moisture in the lines.
Automotive AC operates at much lower pressures then HVAC units and are far more tolerant of moisture in the lines. In fact - All R134A Refrigerants for automotive use include a moisture absorbing lubricant (dehydrant) in the freon mix. Evacuating the automotive systems is only to maximize the freon/oil charge, not to remove moisture.
There is no performance or maintenance benefits to a longer evac. on an automotive AC.
Don't waste your time... Jasper.
danny burstein wrote:

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Thanks. That was my key question - wondering about the moisture. (it takes much longer, as we all know, for all that water to evaporate even in a very-near vacuum).

Thanks again.

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I am not going to comment on that but would like to know how long you have being doing refrigeration work?

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On 8/30/2011 7:22 AM, Grumpy wrote:

Grumpy, on other planets, the inhabitants use different methods to operate and service refrigeration and air conditioning systems. It's obvious that the land vehicles on his planet are not bothered by any moisture inside the semi-hermetic cooling systems. ^_^
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writes:

Speaking of "land vehicles" and moisture, if you haven't seen it...
National Guard Trucks vs. Irene floods.
Everyone got out alive. Barely.
That _was_ an expensive camera he's holding...

http://www.youtube.com/watch?v
b54bZepoA
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Never mind camera how about those Quarter million dollar's vehicles

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Its seems the norm is for auto techs to pull a vaccuum for less than one hour since the internal volume of the system is so small, after making sure the system is leak free first. However, the longer the better for evacuating a system and if you have the pump and the time, Id say leaving it on for a few hours is good and will cause absolutely no harm to the system . In other words, the lower the Vaccuum Pump pulls down to as measured in microns , the better assurance for an air and moisture free system. The important thing is : When you are ready to recharge the system, to be very careful you dont introduce air back into the a/c system thru your hoses and guage manifold otherwise youve just ruined the effectiveness of pulling a deep vaccuum.
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Its seems the norm is for auto techs to pull a vaccuum for less than one hour since the internal volume of the system is so small, after making sure the system is leak free first. However, the longer the better for evacuating a system and if you have the pump and the time, Id say leaving it on for a few hours is good and will cause absolutely no harm to the system . In other words, the lower the Vaccuum Pump pulls down to as measured in microns , the better assurance for an air and moisture free system. The important thing is : When you are ready to recharge the system, to be very careful you dont introduce air back into the a/c system thru your hoses and guage manifold otherwise youve just ruined the effectiveness of pulling a deep vaccuum.
You have to pull at least 1100 microns to make sure that most all of the moisture is gone. If it won't pull down below 1200 microns, you have water in the system. The next question you have to ask yourself is how deep of a vacuum can you pull before you suck the seals on the compressor shaft inside out......
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Two penguins friends met at a gay penguin bar. First one saz: "Looks like you blew a seal" Second one saz: "No It's just ice cream"
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On 8/29/2011 10:24 AM, snipped-for-privacy@gmail.com wrote:

I do not think pulling a long deep vacuum on an Automotive system is a good idea. A deep vacuum has the tendency to suck out the seals.
In other words, the lower the Vaccuum Pump

Keep in mind automotive systems do have a dryer system that should be replaced if left open too long or open system repairs. Said dryer system seems to negate the necessity for a massively deep vacuum on the A/C.
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