Over the road A/C - is 22 inches of vacuum enough?

Brian Diehl · July 13, 2012, 07:28:00 AM

My vacuum pump will only pull 22 inches of vacuum on my A/C system. It will not pull more vacuum even if I have is just vacuuming the gauge hose line. I pulled a 22 inch vacuum on my A/C system last night and it was still at 22 inches this morning. Is it okay to fill it now with R134a or does it need to be pulled down to 28 inches of vacuum?

Reference this post for why I'm needing to do this: http://www.busconversions.com/bbs/index.php?topic=23930.0

zukmancdr · July 13, 2012, 09:06:58 AM

In my experience, if it holds all night you should be good.

Sent from my DROID BIONIC using Tapatalk 2

rv_safetyman · July 13, 2012, 09:07:31 AM

Brian, I assume you are using an AC gauge set that is connected to both the vacuum pump and the AC 134 adapters.

As you know, a part of the process is draw down the vacuum and then shut off the system and see if the vacuum holds. If it does not, then you have a leak and that might be the cause for not being able to draw down more vacuum.

Also, check your gauges with all ports open (not hooked to the system). My gauge dial on the low side has a bit of an offset (does not seem to be adjustable). If you factor in the offset on my gauges my pump draws down fairly close to 28 inches.

If you have no leak and your gauges are "zeroed" properly (actually only getting 22 inches) then it will be interesting to see what others say. My answer would be to simply run the pump for maybe two to three times longer than the recommended time (I would run it for at least an hour).

Jim

Brian Diehl · July 13, 2012, 10:02:00 AM

I am able to adjust the guage to zero. I have done that. The vacuum pump itself doesn't want to draw down past 22" even without being connected to the bus A/C. That is why I ask if I have to draw the system down to 28" or if I can refill now that it has held the 22" vacuum without issue all night.

luvrbus · July 13, 2012, 10:15:11 AM

Buy you a cheap air operated pump from HF they will pull a tight system down to 30lbs have you checked your pump off the system the 134A is getting a little pricey to chance 25 lbs I have had systems hold 20+ overnight then leak with high head pressure

I am fighting that now on MH system

Oonrahnjay · July 13, 2012, 10:55:15 AM

Quote from: rv_safetyman on July 13, 2012, 09:07:31 AM(snip) If you have no leak and your gauges are "zeroed" properly (actually only getting 22 inches) then it will be interesting to see what others say. My answer would be to simply run the pump for maybe two to three times longer than the recommended time (I would run it for at least an hour).
Jim

Yes. Brian, as you know, there are two reasons for this procedure -- to get air molecules (oxygen is the most reactive but nitrogen is also a pollutant that will react with the oil and coolant ("Freon") out of what's meant to be a closed system, and also to assure that there are no leaks. I think Jim is right here - if you're not getting exactly the right vacuum reading, you may have trouble getting all the air out and extra pump time should take care of this. To assure that the problem isn't a leak that's causing the low number, I'd also let the system sit for at least 4 hours to be sure it's not leaking *at all*.

I'd really like to see a number that implies full evacuation (i.e. 28") but if you can't get that, I'd say that the extra pump time and a perfect leak test should be OK.

Brian Diehl · July 13, 2012, 11:08:11 AM

Quote from: Oonrahnjay on July 13, 2012, 10:55:15 AM
To assure that the problem isn't a leak that's causing the low number, I'd also let the system sit for at least 4 hours to be sure it's not leaking *at all*.

The system sat at 22" from 9:30 PM last night until 6:50 AM this morning when I checked just before heading off to work.

Quote from: luvrbus on July 13, 2012, 10:15:11 AM
Buy you a cheap air operated pump from HF they will pull a tight system down to 30lbs have you checked your pump off the system the 134A is getting a little pricey to chance 25 lbs I have had systems hold 20+ overnight then leak with high head pressure

I am fighting that now on MH system

I have the 3CFM two stage pump from HF.

Uglydog56 · July 13, 2012, 11:14:52 AM

It depends on the boiling point of the freon. On a non-bus related refrigeration plant I used to be responsible for, it would sit at 25" for a few hours while it boiled off and evacuated the remaining freon (these were big plants). The pull down to 28" was to ensure you had gotten all the residual stuff evacuated. Even though holding at 22" is a good sign, I would check the specs on 134 which I don't have in front of me, because I think you are going to have to pull it down to 28".

Brian Diehl · July 13, 2012, 11:51:05 AM

Boiling point at atmospheric pressure is -15 according to these guys: http://www.engineeringtoolbox.com/refrigerants-d_902.html

stevet903 · July 13, 2012, 01:24:28 PM

The reason you want to pull a deep vacuum is to remove any air and boil off any moisture in the system. At 28 inches water boils at about 100 degrees ambient, it takes about 29.1 inches to boil at 70 degrees ambient. Carrier recommends evacuating down to 500 microns - 22 inches isn't really doing anything for you. However, most gauge sets are not very accurate on the vacuum side. They make special gauges just to measure the vacuum. Can you close off the gauge so that the pump is just pulling down the gauge and hose and see if it will pull more of a vacuum? You could also try changing the vacuum pump oil - it needs to be changed regularly since it gets contaminated with water from the system. Napa carries it. That pump should be able to pump to 25 microns with fresh oil.
Sometimes a system won't leak under vacuum, but will under pressure. You can use nitrogen at 2 or 300 psi to check the system - you can definitely hear a leak at that pressure. I just use filtered shop air at 150 psi since I don't have a nitrogen tank and make sure the pressure doesn't drop overnight. If it does, it's time to break out the soapy water and go on a leak hunt. People tell me I'm crazy not to use dry nitrogen, but if the system has been opened or if all the freon has leaked out, it has moisture in it anyway, a little more isn't going to hurt it. I always replace the drier as the last step in closing the system, then pull a deep vacuum to get rid of the moisture. I also use Nylog Blue label to seal the o-rings and fittings. Here is a good read on evacuating AC systems - www.transportaircon.carrier.com/Files/Bus/Local/US-en/TACtalkJune06.pdf

gus · July 13, 2012, 04:07:54 PM

Steve,

That is a good point about using air press. If it leaks under vacumn it also sucks air in anyway.

One question, is 150psi enough since the system is routinely run at 250-400psi?

Nitrogen tank pressures spook me, I really don't like messing with them.

oldmansax · July 14, 2012, 05:43:01 AM

Quote from: stevet903 on July 13, 2012, 01:24:28 PM
The reason you want to pull a deep vacuum is to remove any air and boil off any moisture in the system. At 28 inches water boils at about 100 degrees ambient, it takes about 29.1 inches to boil at 70 degrees ambient. Carrier recommends evacuating down to 500 microns - 22 inches isn't really doing anything for you. However, most gauge sets are not very accurate on the vacuum side. They make special gauges just to measure the vacuum. Can you close off the gauge so that the pump is just pulling down the gauge and hose and see if it will pull more of a vacuum? You could also try changing the vacuum pump oil - it needs to be changed regularly since it gets contaminated with water from the system. Napa carries it. That pump should be able to pump to 25 microns with fresh oil.
Sometimes a system won't leak under vacuum, but will under pressure. You can use nitrogen at 2 or 300 psi to check the system - you can definitely hear a leak at that pressure. I just use filtered shop air at 150 psi since I don't have a nitrogen tank and make sure the pressure doesn't drop overnight. If it does, it's time to break out the soapy water and go on a leak hunt. People tell me I'm crazy not to use dry nitrogen, but if the system has been opened or if all the freon has leaked out, it has moisture in it anyway, a little more isn't going to hurt it. I always replace the drier as the last step in closing the system, then pull a deep vacuum to get rid of the moisture. I also use Nylog Blue label to seal the o-rings and fittings. Here is a good read on evacuating AC systems - www.transportaircon.carrier.com/Files/Bus/Local/US-en/TACtalkJune06.pdf

This is the way it's done. Except the part about using shop air... LOL! The reason we in the business use nitrogen (besides not lugging an air compressor around) is it's DRY. Every molecule of water you put in a system has to be boiled out via a vacuum pump. Also, the new oils (POE, PAG, etc.) have an affinity for moisture & it can not be removed by a vacuum pump. Only a new drier will remove the moisture. Since the capacity of the drier is finite, you may have to change the drier more than once to remove the moisture. FYI, one drop of water will severely hinder performance. It will freeze in the expansion valve & stop cooling. Then the ice will thaw and it will start cooling again, until the water freezes again. The system will just cycle over & over. Since the OTR air is usually overkill on our buses, you may not notice the drop in performance until it's REALLY hot or you have a lot of people on board.

TOM

Brian Diehl · July 14, 2012, 06:12:06 AM

I'm taking good advice and heading up to David Benck's place today. Hopefully all goes well and I'll be comfortably cool on the way home. :-)

Brian Diehl · July 14, 2012, 07:14:57 PM

Almost two hours on the 7cfm vacuum pump and we were getting 29" of vacuum. Wow! There was a lot of moisture in the system still. We did not end up needing to add any oil as the system still had enough oil in it. It also took 28 lbs of R134a to fill the system properly instead of the 24lbs the book says it should take. I had a nice comfortable cool ride home. After running solid for about an hour I was able to cycle the system on and off it got so chilly in the front of the bus. Thank you David! I appreciate your time and expertise and really enjoyed the tour around your place.

gus · July 14, 2012, 09:54:42 PM

Well, there goes my air leak check system!!