Date: Tue, 5 Jul 2011 00:22:52 -0400
Reply-To: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Subject: Re: A/C - what it the purpose of the vacuum check
In-Reply-To: <4E12324B.30405@charter.net>
Content-Type: text/plain; charset="US-ASCII"
I'll apologize up front for coming across a bit harsh but this question
smacks of reasons refrigerants are regulated and why the EPA seems it
necessary to require anyone servicing AC or refrigeration systems to have
some certification.
The whole concept of mechanical refrigeration relies on the "latent heat of
vaporization". In other words it is the change of state from liquid to gas
and back that makes this possible. In order for this work the refrigerant
has to bas pure as possible. Non condensable stuff like "air" or liquids
that do not vaporize like refrigeration oils all reduce the efficiency of
the system. Also, any air or moisture in the system will cause internal
corrosion and enough water vapor can cause ice crystals that can clog parts
such as the expansion valve which can cause a blockage dead heading the
compressor and causing immediate damage. Part of charging an AC system is
to pull a deep vacuum so that only pure refrigerant is in the system. Of
course a new filter-dryer helps as the desiccant can deal with some
moisture.
As for leak testing, since atmospheric pressure is only 1 bar, (14.1 psi) a
vacuum is really a poor leak test. It will only find the big ones and AC
gauges are not that responsive in the vacuum range. A properly equipped AC
shop will pressurize the system with nitrogen or a dry air source say to 300
psi and make sure everything holds. Then after this test is successful, the
vacuum is pulled. For a system that has been open this should be allowed to
go at least 4 hours or more. This will boil all the contaminants out
including what is absorbed in the refrigerant oil. Stepping back if using
oil from an open container, put the required amount in before the vacuum.
Now the system can be charged, by weight, do not rely on pressure to
determine fill, especially with R-134a. Relying on pressure will result in
over fill most every time. For most Vanagons you need~1,300 grams. For
R-134a you use ~85% of the amount required for R-12.
Now here is where things can get ugly. As or after filling you find that you
do still have a leak or you find that another part needs repair or
replacement. You have no means to recover the stuff in the system. So here
you let leak out or release it yourself to get rid of the pressure. The
intentional release or charging a known leaking system are violations at the
federal level. While individuals get away with this any regulated shop may
not. You need the recovery equipment to service AC systems.
Dennis
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
John Rodgers
Sent: Monday, July 04, 2011 5:36 PM
To: vanagon@GERRY.VANAGON.COM
Subject: A/C - what it the purpose of the vacuum check
What does this vacuum check do for the system in terms of servicing the
A/C system.
When I put my A/C system back together next week, the pressure in the system
will be ambient. Is the only purpose in pulling a vacuum check to enable the
refrigerant to enter the system more easily? Or a check for leaks? And do
both sides of the system et checked?
Thanks,
John
--
John Rodgers
Clayartist and Moldmaker
88'GL VW Bus Driver
Chelsea, AL
Http://www.moldhaus.com
