- Thread starter JD720
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MarkB_MI
Well-known Member
- Location
- Motown USA
A good vacuum pump will pull down to 50 microns or less. You won't find any of the venturi units rated in microns, because it would be over ten thousand microns. A better bet would be a cheapy pump from Harbor Freight. For less than a hundred bucks you can get a pump which HF claims will pull down to 75 microns. Even if it can only pull 500 microns, that's far better than a venturi unit and more than adequate for your AC system.
Geo-TH,In
Well-known Member
Don't know much about an air operated vacuum pump. I have seen people solder a brass fitting on the suction line of an old refrig compressor and use it as a vacuum pump.
An old friend made my vacuum pump, about 40 years ago, by using a vacuum pump off an old diesel engine. Belt drive and it has a 1/4 hp motor. I think the belt vacuum pump may have been used for the power brakes, not really sure. It worked well for me years ago when I needed to suck down an AC.
George
An old friend made my vacuum pump, about 40 years ago, by using a vacuum pump off an old diesel engine. Belt drive and it has a 1/4 hp motor. I think the belt vacuum pump may have been used for the power brakes, not really sure. It worked well for me years ago when I needed to suck down an AC.
George
MarkB_MI
Well-known Member
- Location
- Motown USA
I find the "works good" comments humorous. HOW WOULD YOU KNOW? If you won't spring a hundred bucks for a Harbor Freight vacuum pump, it's a safe bet you're not going to spend 200 bucks for a micron gauge. You can't tell the difference between 25 microns and 25,000 microns on a regular AC gauge set. And you need to get down around 1000 microns to get the water out of your system, which is the whole point of pulling a vacuum.
I have done AC work off and on for over 25 years and never had a problem with a system evacuated with my air operated pump. The water will boil in a vacuum as long as you are not doing a repair in extremely cold weather it will boil out. Do a good repair by flushing system and replacing dryer/accumulator then run your pump long enough there should be no problems. One of the biggest problems I have seen is people pull the vacuum on them and call it good, they don't leave it on there long enough to boil out any moisture.
Got an air operated vacuum pump.
My compressor would not put out enough air, so I spent under $100 on an electric vacuum pump at HF and have been very happy.
Much cheaper than buying a $500 air compressor.
Plus I can vac down a tractor AC system or whatever anywhere I can get electricity no large compressor needed.
My compressor would not put out enough air, so I spent under $100 on an electric vacuum pump at HF and have been very happy.
Much cheaper than buying a $500 air compressor.
Plus I can vac down a tractor AC system or whatever anywhere I can get electricity no large compressor needed.
MarkB_MI
Well-known Member
- Location
- Motown USA
Tom, I agree that if you wait long enough, even one of the venturi vacuum pumps will dry out a system. But who really wants to have their air compressor running non-stop for the four or five hours it will take?
Let's assume your venturi will pull down to 28.5 inches/30,000 microns (which is probably generous). The boiling point of water is about 90 degrees F at that temperature. If it's a hot day, that venturi will do a reasonable job of drying the system. Not great, but good enough. But if it's a cool day, it is simply not going to work. To boil water at 70 degrees, you need to get below 20,000 microns, and it's absurd to think you can do that with a venturi. Given that even a garbage HF vacuum pump will pull down to 100 microns, it's really not worth it to use a venturi. The pump will do a better job faster, and use much less electricity.
Let's assume your venturi will pull down to 28.5 inches/30,000 microns (which is probably generous). The boiling point of water is about 90 degrees F at that temperature. If it's a hot day, that venturi will do a reasonable job of drying the system. Not great, but good enough. But if it's a cool day, it is simply not going to work. To boil water at 70 degrees, you need to get below 20,000 microns, and it's absurd to think you can do that with a venturi. Given that even a garbage HF vacuum pump will pull down to 100 microns, it's really not worth it to use a venturi. The pump will do a better job faster, and use much less electricity.
Mark, I agree if I was going to do more AC work I would buy one of the HF pumps. My MAC pulls over 29" on the gauge. If I have replaced the dryer and flushed the system once it gets done to the lowest reading I generally run a hour or less, my AC training only suggested 30 minutes. Like you said there would be a big difference in electricity usage, even a HF pump would pay for itself instead of hiring out a job if a person is capable of doing the job.
I have one. Don't know how good it worked because I got a bad set of gauges.
It will really wear your compressor out. Instructions said to pull a vacuum with the pump for 30 minutes. Seemed to me if it takes 30 minutes it really doesn't have much draw.
On the bright side I did complete the A/C repair I was doing and all was fine.
Someone will get a good deal on a lightly used pump at the next garage sale.
It will really wear your compressor out. Instructions said to pull a vacuum with the pump for 30 minutes. Seemed to me if it takes 30 minutes it really doesn't have much draw.
On the bright side I did complete the A/C repair I was doing and all was fine.
Someone will get a good deal on a lightly used pump at the next garage sale.
john in la
Well-known Member
You use a vacuum pump to boil off moisture in the A/C system not just to vacuum air out the system.
So lets pick on Harbor Freight. They sell a air operated vacuum pump for less than $20. Item #96677 Per the web site it has a vacuum level of 28.3" of mercury at sea level.
Now lets look at when water will boil.
Water boils at 212 deg at sea level.
If we increase the pressure on the water we can raise that number such as in a car radiator system with a pressure cap.
If we lower the pressure to a vacuum we can lower that boiling point pressure.
At a vacuum level of 28.3" of mercury the air temperature would have to be about 95 deg for water to boil.
So even if you have a big compressor that will put out enough air; and even if the vacuum pump works as good as they say in the owners manual; the outside air temperature would have to be 95 deg or more for it to pull a big enough vacuum to make the moisture boil. If the water does not boil you are wasting your time vacuuming out the system.
So lets pick on Harbor Freight. They sell a air operated vacuum pump for less than $20. Item #96677 Per the web site it has a vacuum level of 28.3" of mercury at sea level.
Now lets look at when water will boil.
Water boils at 212 deg at sea level.
If we increase the pressure on the water we can raise that number such as in a car radiator system with a pressure cap.
If we lower the pressure to a vacuum we can lower that boiling point pressure.
At a vacuum level of 28.3" of mercury the air temperature would have to be about 95 deg for water to boil.
So even if you have a big compressor that will put out enough air; and even if the vacuum pump works as good as they say in the owners manual; the outside air temperature would have to be 95 deg or more for it to pull a big enough vacuum to make the moisture boil. If the water does not boil you are wasting your time vacuuming out the system.
MarkB_MI
Well-known Member
- Location
- Motown USA
I'll add that anyone serious about A/C work should own not only a vacuum pump but also a micron gauge. You'll find some "interesting" things. For example, do you know that most gauge hoses won't hold a good vacuum? I thought my pump was bad because it wouldn't pull below 1000 microns. There was nothing wrong with the pump: I hooked it up directly to the A/C unit with copper tubing and it pulled all the way down to 45 microns. This was a brand-new gauge set from a well-known manufacturer.
I have one I use on my service truck all of the time. I am not worried about it not getting all of the moisture as I always use a AC flush that contains alcohol. I blow it through the lines first. The alcohol will help the vacuum get any water out. I have used it for twenty years and never had an issue.
I like the "experts" saying this and that will not work. It is kind of like the R12 and R134 issue. When they first changed we went to great lengths to flush the systems and change the compressor oil. Now I just replace the re-dryer and vac the system and charge them on these old tractors. Not had any issues there either. I bet that I have done a hundred that way.
I like the "experts" saying this and that will not work. It is kind of like the R12 and R134 issue. When they first changed we went to great lengths to flush the systems and change the compressor oil. Now I just replace the re-dryer and vac the system and charge them on these old tractors. Not had any issues there either. I bet that I have done a hundred that way.
MarkB_MI
Well-known Member
- Location
- Motown USA
JD, help me to understand how flushing with alcohol is going to remove moisture. After all, you can only distill isopropyl alcohol or ethanol to around 90-96 percent pure. Flushing with alcohol is going to ADD water to the system, not remove it.
Backing up a bit, how does water get into the system in the first place? You opened the system, and humid air entered it. OK, so you displace the air with your flush, which evaporates, leaving you with a system full of vapor that's 4 to 10 percent water. How do you get THAT out? My guess is you blow it out with compressed air, which leaves you right where you started: an A/C system full of wet air. (Now if you purge it with dry nitrogen, then that's another story. But the flush would still have not aided the process.)
Fact is, if you're not drying the system by pulling a solid vacuum (1000 microns or less), you're betting that the dryer will do it for you. Which you can probably get away with, IF you replace the dryer.
Backing up a bit, how does water get into the system in the first place? You opened the system, and humid air entered it. OK, so you displace the air with your flush, which evaporates, leaving you with a system full of vapor that's 4 to 10 percent water. How do you get THAT out? My guess is you blow it out with compressed air, which leaves you right where you started: an A/C system full of wet air. (Now if you purge it with dry nitrogen, then that's another story. But the flush would still have not aided the process.)
Fact is, if you're not drying the system by pulling a solid vacuum (1000 microns or less), you're betting that the dryer will do it for you. Which you can probably get away with, IF you replace the dryer.
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