AC evacuation question - Page 2 - PeachParts Mercedes-Benz Forum

#16 04-21-2010, 11:35 AM

I got by for many years with a pump made from an old Frigidaire refrigerator compressor. This makes a wet vane pump and will pull a GREAT vacuum! It pulled as hard as I've ever seen from any of my wet vane pumps. It CERTAINLY pulls a MUCH better vacuum than a silly venturi pump.

My recovery/recycle units have a vacuum pump that looks very much like that old compressor.

vstech · #17 04-21-2010, 11:43 AM

oh, there's no question that a piston recip compressor is capable of much better and faster evacuation than a venturi VP is. it's just that without proper gauges, there's no way for someone to know if the pump is pulling what's required.
good to see you on here again Larry!
change it hot and often!

#18 04-21-2010, 12:14 PM

Well, the same can be said regardless of the compressor type. You are right, a micron gauge is the Cat's A$$. I've been beefing up my equipment over the last few years; recovery/recycle machines, one for 134, one for 12, an Inficon sniffer, a charging scale, an oil injector, etc. My next purchase will be a micron gauge.

I had the compressor I described that I made a handle for and brazed on an inverted flare fitting for the inlet. It yanked down a vacuum very fast and with nothing more than manifold gauges worked as good as anything I saw. If there was a down side, it would have to be the weight and bulk, but for the money it's pretty tough to beat. The other down side is that you can't change the oil. It's the ideal poor boy's vacuum pump.

leathermang · #19 04-21-2010, 01:44 PM

Quote:

Originally Posted by vstech

not if there is a leak.
or if significant moisture is present.
or if the vacuum pump is not working properly.
nothing beats a micron vacuum gauge.
best practice is to evacuate to 2000microns, break with nitrogen, and re'evacuate to sub 1000 microns then charge with correct volume of refrigerant.
sub 500 microns is even better, but not needed, unless working with POE refrigerant oil.

I am not at all arguing against the nicety of having any tools one can afford to have... the more tools the better...
I do not have one...so I have to rely on whether it will hold for many hours with no movement .... but totally agree that leak checking is only viable with PRESSURE.... so leaks should have been taken care of before vacuum is being applied anyway....

Matt L · #20 04-21-2010, 07:54 PM

Quote:

Originally Posted by LarryBible

I had the compressor I described that I made a handle for and brazed on an inverted flare fitting for the inlet. It yanked down a vacuum very fast and with nothing more than manifold gauges worked as good as anything I saw.

Well, with my manifold gauge, I can't detect ANY movement of the needle after the first ten minutes under the vacuum when connected to the car. The micron gauge says otherwise. Way otherwise.

Quote:

If there was a down side, it would have to be the weight and bulk, but for the money it's pretty tough to beat. The other down side is that you can't change the oil. It's the ideal poor boy's vacuum pump.

You could solder a bung hole in there.

leathermang · #21 04-21-2010, 08:02 PM

Quote:

Originally Posted by Matt L

...You could solder a bung hole in there.

I do not agree with using an old refrigerant pump...but you seem to know enough about the physics of the system to answer these "questions" :

Why was there not a ' bung hole ' there in the first place ?

What happens to the system , the oil, etc long range if you install a bung hole?

Matt L · #22 04-21-2010, 08:35 PM

Quote:

Originally Posted by leathermang

I do not agree with using an old refrigerant pump...but you seem to know enough about the physics of the system to answer these "questions" :

Why was there not a ' bung hole ' there in the first place ?

What happens to the system , the oil, etc long range if you install a bung hole?

There is no bung hole because there is no reason to change the oil in a refrigerator compressor. Most of them run without service for their useful life.

Soldering a bung hole to the case takes the same care as soldering onto the lines. You should fill the system with nitrogen so that the material on the inside does not oxidize.

leathermang · #23 04-21-2010, 09:48 PM

Quote:

Originally Posted by Matt L

There is no bung hole because there is no reason to change the oil in a refrigerator compressor. Most of them run without service for their useful life.

Soldering a bung hole to the case takes the same care as soldering onto the lines. You should fill the system with nitrogen so that the material on the inside does not oxidize.

AH HA !! I was right ! you do know the physics of the situation..

Now, these people who install a bung hole...

Where should that be placed ?

Will that allow the oil to be changed out for each AC job ( if they want to follow the rules most AC professionals follow ) ?

Should a second bung hole be installed ? Would that allow changing out the oil when needed or wanted ? Or , feel free to correct the question or assumption if needed to explain the situation to those wanting to use the old refrigerator compressor....

Matt L · #24 04-21-2010, 09:51 PM

Well, one should be enough, supposing that you can lift and tilt the thing to drain the oil.

But I'm happy with a standard rotary-vane pump. I'm sure that I'll buy another when this one gives out.

leathermang · #25 04-21-2010, 10:10 PM

I have a brand new HF two stage pump ready to be used...
but I have seen people suggesting using the old refrigerator compressors for decades... which I have read have serious conceptual problems with using...
So I was just wanting to know what might be done to ' overcome' those and use them ....

vstech · #26 04-21-2010, 10:11 PM

... you could always turn it upside down and drain out the oil from the port you connect the hoses to. you could even pressurize it to force the oil out faster... heck, you should drain out the 3g oil anyway, put in some quality vacuum pump oil.

#27 04-22-2010, 08:39 AM

Yes, the hermetically sealed compressors were sealed for a reason and for the purpose they were designed for worked quite well for a long, long time.

I don't disagree that if a Micron gauge is used, the old compressor might not make the vac of a commercial vac pump, it will get you by and be better than a venturi pump that so many DIY'ers use.

The bung is a great idea for the person on such a tight budget that they don't have other alternatives. I've been that guy with not an extra dime to work with! I wish I had thought of that when I was using that old compressor. Drain the oil and put in some fresh vacuum pump oil and it probably would have come close to the performance of a small wet vane commercial pump.

Using a Micron gauge will prove to you that putting fresh oil in a vac pump can make a big difference in it's performance. That would be one of the limiting factors with an old refrigerator compressor.

leathermang · #28 04-22-2010, 08:53 AM

I agree totally with what Larry just said.

Matt L · #29 04-22-2010, 08:57 AM

Same here.

Venturi pumps are a joke for AC work. You'll get about 25" of vacuum. That's about 100,000 microns. Not so good.

jcyuhn · #30 04-22-2010, 10:44 AM

Minor threadjack with a question here. Once you pull a vacuum deep enough to boil water at the ambient temperature, how much additional value is there in pulling down to a small number of microns? I understand that removing all the air/water from the system eliminates some dead load from the compressor which increases efficiency. But practically speaking, is there any difference between achieving 29.5" and an essentially perfect vacuum?

I have Mastercool single stage wet vane pump which doesn't pull down nearly as far as a two stage pump. Best I can recall it's rated at 100 microns, whereas the two stage pumps are rated at 25 microns. In the real world, is one of these really better than the other?

I haven't actually had to perform an a/c repair in 5+ years, knock on wood...

- JimY