How to do an AC flush and evacuation/recharge ?

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Ted B. Drude

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Sep 16, 1991, 4:16:53 PM9/16/91
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I am looking for someone who is familiar with AC (air conditioning)
flushing and evacuation/recharging to give me some advice. I have two
cars that need service work on their ACs, and I want to flush out one
system and evacuate/recharge both systems. I have the factory service
manuals for both cars, which explains the general service procedures,
but are lacking in certain specifics.

What I need to know is the following:

1) How do you flush a system after it has been contaminated? My '83 Ford
has an orifice tube that has been clogged again (apparently) due to a broken
desicant bag in the receiver/drier. (That is what I am gathering,
based on my past experience with the car and the diagnostic info found in the
service manual.) Assuming this diagnosis is correct, I know I will need to
replace the orifice tube and receiver/drier, but how do I clean out the hoses,
evaporator, and condenser?

2) One guy recommended that I install a separate filter into the high
pressure line to take care of any residual contaminants that might be in
the system and would want to wipe out my compressor. Is that really
necessary if I flush the system and install a new receiver/drier?
(According to the service manual there is an integral filter inside the
receiver/drier.)

3) I have a basic manifold gauge set. I understand I need a vacuum pump
which I can to connect to the gauges to evacuate a system before
recharging it. Both the service manuals I have recommend evacuating the
systems with a constant specified vacuum for a minimum of 15 minutes
before recharging. Is there an inexpensive (>$50) vacuum pump that I can
find which will work with my existing manifold gauges?

Thanks for your help!

- Ted Drude (drudetb@ingr)

John G. DeArmond

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Sep 18, 1991, 11:23:59 AM9/18/91
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dru...@infonode.ingr.com (Ted B. Drude) writes:

>1) How do you flush a system after it has been contaminated? My '83 Ford
>has an orifice tube that has been clogged again (apparently) due to a broken
>desicant bag in the receiver/drier. (That is what I am gathering,
>based on my past experience with the car and the diagnostic info found in the
>service manual.) Assuming this diagnosis is correct, I know I will need to
>replace the orifice tube and receiver/drier, but how do I clean out the hoses,
>evaporator, and condenser?

That car should have an expansion valve instead of an orfice tube. If
the dryer ruptured, you will find debris in the expansion valve inlet
screen. More common in my experience is when this screen gets clogged
by debris, the cause is improper prior servicing which introduced
water and caused rust. However, I'll assume for this post th at
the dryer did rupture.

If the dryer ruptured and it has been very long ago then it is likely
that silica fines are scattered throughout the system. In addition to
flushing the system, you'll have to remove the compressor, remove the
oil, clean it and replace the oil. Again, if the rupture is very old,
it is likely that abrasive silica fines have worn the compressor.
It would probably be easier at this point to swap the compressor for
a rebuilt unit or get one from a junqueyard.

With the compressor out of the way, we can deal with the rest of the
system. Traditionally, R-11 has been used for flushing. It is liquid
at atmospheric pressure and ambient temperature. But since the enviro-nazis
have succeeded in taxing R-11 using the fraudulent ozone layer argument,
most people have gone to 1,1,1 Tricloroethane ("Trike"). Trike does
a pretty good job but is heavier than R-11 and has a lower vapor pressure
which means it is more difficult to get out of the system.

In practice, the procedure is to open the system and remove all restrictions
such as the expansion valve, the POA valve, any screens and the dryer. You
can clean or replace these items individually. You'll need a circulating
system so that you can pump the trike though the rest of the system.
I have a little magnetic drive centrifugal pump I bought from Grangers for
about $100. The circulating system should also have a filter; I use a
hydraulic system suction line filter. Typically you'll use clear tygon
tubing to connect to the system components so that you can see when the
effluent is clear and to verify circulation. Just circulate the trike
through each component until the flow looks clean. Try to keep the flow
going downhill if possible which will aid in removing solids.

After each part is clean and the circulation system is removed, use
(preferably) dry nitrogen or compressed air to dry the inside of the
part. Reassemble the system, connect a vacuum pump and evacuate for
at least several hours. This will remove the last traces of the trike.
Heating the parts with a heat gun aids driving out the trike. Then
charge the system as normal.

>2) One guy recommended that I install a separate filter into the high
>pressure line to take care of any residual contaminants that might be in
>the system and would want to wipe out my compressor. Is that really
>necessary if I flush the system and install a new receiver/drier?
>(According to the service manual there is an integral filter inside the
>receiver/drier.)

That is correct. a second filter/dryer would be extra and would do nothing.

>3) I have a basic manifold gauge set. I understand I need a vacuum pump
>which I can to connect to the gauges to evacuate a system before
>recharging it. Both the service manuals I have recommend evacuating the
>systems with a constant specified vacuum for a minimum of 15 minutes
>before recharging. Is there an inexpensive (>$50) vacuum pump that I can
>find which will work with my existing manifold gauges?

If the system has been open for any length of time or has ingested air
through a leak, the evacuation requirements are much more strenuous.
The reason is that moisture is attracted into the system and then
held there partially by the everpresent oil film. It is necessary to
cause the water to boil (as opposed to simple evaporation) in order to
remove it. That requires that the pressure be reduced such that
water will boil at ambient temperatures. I don't recall this pressure
off the top of my head but I can tell you that a good dual stage vacuum
pump is required. Refrigeration pumps capable of this task cost in the
range of $300. Even then, it is recommended that the evacuation be maintained
for several hours.

If you cannot produce that temperature, a poor but workable solution is to
pull whatever vacuum you can and then heat all components in order to
drive moisture off. In one extreme instance, I closed the car up in a
very small garage and heated the whole thing to about 150 degrees. A heat
gun works well too.

If you have compressed air at about 4 SCFM at about 90 psi, there is available
through auto parts jobbers a vacuum driven pump that works pretty well.
This is a venturi pump with no moving parts and costs about $80. It cannot
dry a system but it can remove the air. I happen to have such a beast
if you'd be interested.

john

--
John De Armond, WD4OQC | "Purveyors of speed to the Trade" (tm)
Rapid Deployment System, Inc. | Home of the Nidgets (tm)
Marietta, Ga | "It's not a bald spot, its a solar panel for a
j...@dixie.com | a sex machine."

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