vacumming AC system question

[comet240]

hi i made a vacumm pump from a old fridge compressor made adapter to hook onto my cars ac for a 2003 sunfire cuz when i bought it it had no rad for the AC so i put it in i tested it with a vacumm gauge and just sucked on itself and i get -27 HG is that good enough as i read u suposed to get -29 HG does it really matter and i i read i would run the vacumm for 1 hour is that true someone also told me i run it for 8 hours that seems long

[shorod]

I've always run the vacuum for 30 minutes from the time it reaches -29 in-Hg, then close the valve and shut off the vacuum pump and monitor it for 30 minutes. If after 30 minutes you're still at -29 in-Hg, you can be reasonably sure you don't have any sizable leaks.

-Rod

[comet240]

ah ok what are u using to get -29hg for 30 min

is -27 reasonable pull or do i absoulutly need -29 as this fridge compressor cant pull anymore then that

[old_master]

The maximum vacuum you can obtain is -2 atmospheres or, 29.4" Hg. The reason for evacuating AC is to remove moisture. When the system is at 29.4"Hg, any water or moisture that is in the system boils and turns to a vapor. Only then can it be removed by the pump. Rule of thumb in automotive HVAC is: Once the system achieves 29.4"Hg, if you're not going to leave the pump run for a minimum of one hour, don't bother. I usually let 'em suck for a couple of hours. It's not uncommon for larger commercial building HVAC systems to be evacuated for 24 or more hours.

With clutch cycling systems, the drier is inside, (part of) the accumulator. The drier is made of a dessicant material which is kinda like a sponge; it attracts and holds only a certain amount of water/moisture. When it has reached it's capacity, it's time wring it out. If an HVAC system has been exposed to atmosphere, (all refrigerant has leaked out) for more than 30 minutes in high humidity conditions, the drier will absorb humidity, become saturated, and must be replaced. Evacuating the system will not remove it.

[comet240]

so what kinda of pump do u use to get -29 so then me running at -27 for an hour is a waste of time then? and how would u know if the dryer is saturated or not by humidity since it wasnt running and how much is one of htose things?

[shorod]

Thanks, old master, for the details. Looks like I'll be changing my ways and pull the vacuum for at least an hour.

Comet240, if you're only going to be doing this a time or two (assuming you do bother to fix the leaks rather than go through this process once a year or more frequently) you might consider picking up a pump like this one. Also, don't forget to pick up a manifold gauge set.

-Rod

[comet240]

ah ok so -27 is not good enough then and do u have to pull a vacum on both sides or just pulling on the low side good enough here i figured pull -27 was good ennough then i had my ac dyrier fluid the sealent and then the refrigerent


that pump doesnnt state how much it can suck ad the gauges will they work for a 502 system as i have a 502 ice cream machie to be fixed and i have the old r12 gauges

[jdmccright]

You need the deep vacuum (-29 in-Hg) in order to get the water in the system to boil off, and the longer you can pull the vacuum the better. As mentioned, an hour is usually sufficient to remove water from the system. The vacuum check with the valve closed is to test for leaks or residual water.

You are better off opening both sides to vacuum...most gauge sets allow this. But worst case, you should pull vacuum on the side where the drier/accumulator is.

If the system was open to the atmosphere or there was a leak such that the freon had all escaped, then you shoud worry about having a saturated dessicant, depending on how long it had sat unpressurized. You'd probably be better off replacing the drier/accumulator since it would take you forever and a day to pull it down to full vacuum (the trapped water would continually evaporate).

But as long as there is any pressure left in the system, that would be enough to keep the air from entering from the outside, keeping water contamination to a minimum.

[comet240]

ah ok and how much is the accumlator i take it that is the drier?

[old_master]

Wow! Looks like I was at work too long today... most all of your questions were answered! Your refrigerator compressor should do the job, or as mentioned, the one from Harbor Freight will work also. The main thing is the CFM, the higher it is, the faster it will achieve max vacuum. Refrigerator compressors, depending on how big they are, will usually flow somewhere around 2 CFM, the HF evacuator is rated at 2.5 CFM. Both are fine for home DIY use. Your reading of 27" might just be gauge error, don't lose too much sleep over it. As far as the accumulator/drier goes, what year and make vehicle are you working on?

[jdmccright]

Don't know the prices where you live, but an accumulator for a 2003 Sunfire is about $30 here. Yes, the accumulator is the equivalent of the drier...it also holds the dessicant.

I will note that your altitude relative to sea level is also a factor in how low you can pull the vacuum. Heck, the day's weather will be a factor, too. But I wouldn't worry about that so much unless you know a big storm or cold front is coming...that can change barometric pressure significantly. Here is an excellent table showing the ideal barometric pressure at different altitudes:

http://www.engineeringtoolbox.com/ai...ure-d_462.html

You can interpolate for your location. I would say as a non-quantitative rule, you should pull down within +0.5" Hg of your location's altitude.

[old_master]

Great table, I think I'll print it out! Thanks!

The 29.4" Hg I was referring to is atmospheric pressure at sea level: 14.696, rounded to 14.7, (hmmm, optimum fuel mixture too, wonder if that's a coincidence...NOT) and then doubled.

[comet240]

ok ill look and i live in Ontario Canada 2 hours east or Toronto

[comet240]

lol i figured on that site i could enter in my location and tell me my sea level i wonder if there is a place to find that ill look after unless u guys see it before then post me i at work so i just checking site lol

but ya i live near Peterborough Ontario Canada 2 hours east of toronto

[comet240]

i guess im at a level around 624 + so i guess i still need a pull of 29 doh lol