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Air Powered A/C Vacuum Pumps


Question:  Can I use an air powered vacuum pump instead of the more expensive electric vacuum pump? What's the difference.

The simple answer is yes, provided you have an air- compressor with enough rated CFM to operate a venturi pump.  Check with the manufacturer of the pump for their recommendations.  Typical requirement for a venturi pump is around 4.5 CFM at 90 PSI.  

In comparison, a conventional electric pump has many benefits over the venturi counterpart. 


Ease of Transport

An electric pump has no compressed air requirement, so there is no need to have any additional equipment.  Something to consider if you plan on using your pump in multiple locations and don't feel like lugging some big compressor around with you.  

Noise Levels

Between the steady sound of rushing air, and the constant noise of a straining compressor, you might end up with something that sounds like a full blown construction site.  Worth considering if you have neighbors and plan on doing late night work.

Moisture Removal

The purpose of a vacuum pump is to remove air and moisture from the A/C system. If the goal is to remove moisture, you might have a hard time with an air powered venturi vacuum pump.  As a test, we used a large commercial compressor dialed in at over 150 PSI., and we were easily able to maintain the suggested air requirements.  The vacuum produced by the venturi pump  wasn't even enough to register on our Yellow Jacket micron gauge.  That means we were not able to reach any vacuum above the LCD vacuum gauge starting point.  That's something most electric pumps can do in very short order.

... A little more on moisture removal

At sea level, water begins to boil and change into a vapor state at 212 degrees Fahrenheit. If we increases the pressure we can raise the boiling point of water. An example of this would be the typical automotive cooling system which uses a 15 lb. radiator cap to maintain a design pressure. In that system, the idea is to raise the pressure and therefore raise the temperature at which the cooling liquid will boil. If we wish to lower the boiling point of liquid, we simply remove the pressure that's on top of that liquid. That's how we boil water out of an air conditioning system. We use a vacuum pump to bring the system to a level of near perfect vacuum so the water will boil off and be carried away as a vapor. It's important to note that ambient temperature has much to do with the point at which liquids will boil under vacuum. The greater the temperature, the fewer microns of vacuum will be required to start the boiling process.  If you've been keeping note, you know that non condensables (air) and moisture are two things you definitely don't want in your a/c system. 

The chart below shows how temperature plays a role in the level of vacuum needed to boil water. 

Inches of Mercury

Boiling Point of Water F

26.45

120

27.32

110

27.99

100

28.50

90

28.89

80

29.18

70

29.40

60

29.66

50

29.71

40

29.76

30

29.82

20

29.86

10

All values are at sea level.  Subtract 1 inch for each 1000 ft. above sea level

Opinion...

Venturi pumps are certainly popular with the do-it-yourself mechanic since cost is often times the biggest factor.  I'll give these pumps some credit since they can remove a substantial amount of air from the system if you have a decent compressor.  Pulling a vacuum on the system is what's most important in terms of end result to the DIY'er. 

Regardless of sales literature, I believe you won't be able to boil much water out of an automotive system with the vacuum level produced by a venturi pump. I was able to see that by using my micron gauge. I believe air powered a/c vacuum pumps aren't very well suited for use in an automotive service shop unless you plan on hacking these things apart to make other cool air powered vacuum devices.   We have several very useful items in the shop that derive their power from the guts of a venturi pump.


Summary

If you plan on servicing A/C systems, and want professional results, invest in a good electric vacuum pump.  It's not uncommon to get ten or more years of service from a good electric pump.  As an option for  the do-it-yourself mechanic, some tool rental companies now rent electric vacuum pumps on a daily or hourly basis.  That may be a better option versus overworking your air compressor for an hour or more, just to get mediocre results.










Visitor Comments
  1. Comment #1 (Posted by mike)
    thanks good info. just wondering if air vac was good to use. will look for better pump instead. any suggestions?????
     
  2. Comment #2 (Posted by JT)
    I thought vacuum is vacuum. So if the venturi based pump can get the vacuum down as close to 30 inches and is able to maintain it for an hour it would seem that it is doing the job. I don't understand what the difference is between the vacuum provided by a venturi versus a mechanical vacuum pump.
     
  3. Comment #3 (Posted by Steve)
    If the vacuum level is not low enough the water will not boil off and be left in the system.
     
  4. Comment #4 (Posted by mpl)
    I have to agree with JT vacuum is Vacuum I have a sears singel stage compressor it held 28 inch for over an hour. So why are you trying to sell elec. pumps or high end compressor. this add is miss leading but did point out to make sure to hold vacuum.
     
  5. Comment #5 (Posted by GC)
    what they are saying is that you need an very big, powerful compressor to do the same job an electric pump would do
     
  6. Comment #6 (Posted by Jaguarjoe)
    Those cheapie air operated jobs only pull down to 28" or so. No water will boil off unless its over 100 degrees. An electric job will pull down to 29.5" or so causing the water to boil out at a much colder temperature.
     
  7. Comment #7 (Posted by David)
    I certainly agree with JT and question whether there is some sort of motive behind the sentiment expressed by the author here, or perhaps, just a bit of ignorance. I have used a Mac Tool air venturi vacuum for many years, because it is rarely used more than a few times a year. The tool appears to have been made by Robinair and is currently available as model 34970 from Robinair (and its suppliers). The current model claims to draw vacuum to 29 inches /Hg with air driving the vacuum at a rate of 75 PSI, or greater. My unit was rated similarly, at over 29 inches/Hg, and I have always achieved what is considered as close to complete vacuum as one is likely to achieve with a vacuum pump. In Austin, Texas, I always reached between 29 and 29.5 inches/Hg, and now in the Denver metro area (more than 5000 feet above sea level), I consistently achieve between 24 and 24.5 inches/Hg. In case the latter figure sounds low to any of you low landers, simply consider the conversion mentioned above and fully understood within the industry where one inch mercury is subtracted for each thousand feet above sea level. My location now, at just over 5300 feet above sea level, means I am still consistently drawing the equivalent of 29.5 inches/Hg with the Mac/Robinair air venturi vacuum pump. My air compressor is a mid-level 3.5 HP Craftsman unit with 15 gallon tank. It is adjusted to provide 80 to 85 PSI typically, and it works great for what this task calls for. Is there some other explanation about how or why full vacuum would differ from one vacuum source to another, or is this simply some strange bias shown by the author? For a typical automotive application, the equivalent of 29 inches vacuum can be achieved in the system in under five minutes. Of course, the point about boiing moisture out is well taken and needs to be understood by potential service personnel, which is why I normally apply that level of vacuum for between 20 and 30 minutes. Feel free to consider using the air venturi vacuum if 29 inches/Hg plus draw is acceptable for your air conditioning service project. If it is not, I'm at a loss as to where you're going to reach a significantly deeper vaccum.
     
  8. Comment #8 (Posted by Tom54)
    Reading the posted comments set me to thinking, Why not use water instead of compressed air in the air vacuum pumps. Water has more mass and line pressure should be more than enough to get the job done and you can water the lawn at the same time. This is just theory right now but a $10 Harbor Freight unit would be a start on a prototype
     
  9. Comment #9 (Posted by Joe)
    I'm an engineer, and spent years specifying vacuum equipment for industrial use. This "vacuum is vacuum" stuff is wrong. You guys don't understand what this is about. There's no doubt that an electric vacuum pump will produce much lower, outer-space-like pressures than the venturi does (I know because I specify this stuff). What that does for you is it sucks the moisture out of your drier, making it like new each time. The drier really hangs on to moisture, so this table of boiling points is useless. You want a lot more vacuum that what's shown here. Sadly, whoever wrote this article didn't bother to measure the vacuum that the air-operated pump was able to reach. It would have been interesting.
     
  10. Comment #10 (Posted by Joe)
    I'm an engineer, and spent years specifying vacuum equipment for industrial use. This "vacuum is vacuum" stuff is wrong. You guys don't understand what this is about. There's no doubt that an electric vacuum pump will produce much lower, outer-space-like pressures than the venturi does (I know because I specify this stuff). What that does for you is it sucks the moisture out of your drier, making it like new each time. The drier really hangs on to moisture, so this table of boiling points is useless. You want a lot more vacuum that what's shown here. Sadly, whoever wrote this article didn't bother to measure the vacuum that the air-operated pump was able to reach. It would have been interesting.
     
  11. Comment #11 (Posted by Daniel)
    Vacuum is vacuum regardless of what causes the absence of atmosphere. I can by a multi use air compressor for about the same price I can by a single use vacuum pump. I've used the venturi for years on my autos and have never had a problem. Keep in mind that I'm in south Louisiana and finding an 80degree day isn't difficult.
     
  12. Comment #12 (Posted by Mike)
    I have compaired air vacuum pumps to electric pumps. I used a baby food jar with a service fitting atached to the lid. Water in the jar with a layer of refrideration oil on top. The air pump was unable to boil the water. The electric pump caused a boil in aproximately 1 minute.Both pumps were Robinair. Any moisture left in the system will combine with the refridgerant and oil and become acidic.
     
  13. Comment #13 (Posted by phil)
    Vaccum is not vaccum yes venturi will remove the air from the system but not moisture unless the level of vaccum is equal to or grater than 6 torr or 29.7Hg it is the lat 2 didits that realy make the differance when carrying out vaccum and water test you can actually see the pressure rise this would indicat either a leak or moisture is present, whilst 99% of the time moisture may not be present in the systems and using a air venturi may be perfectly acceptable but do bear in mind a slightly higher discharge pressure will be present,if moisture is within the system this will cause copper plating of internal components,acid can also be formed within the system this will cause constant burn outs on hermetic and semi hermetic compressors. be safe invest in a electric vaccum pump.
     
  14. Comment #14 (Posted by Paul)
    There's another way about this. We are working with two variables... temp and vacuum. If we are limited in vacuum we can achieve, we simply need to increase temp. Not too hard, the radiator is right there. You could easily get the condenser and desiccant to 150+ degrees by running the motor (not A/C!) while you evacuate the system.
     
  15. Comment #15 (Posted by Mark LaTorre)
    Comment #14 (Paul) has a very good point we have several variables that can be used to our advantage here. My local elevation is around 500 ft above sea level that means that if I want to boil out the water from the system on an 80 degree day, I'll need about 1/2" less vacuum than the chart above requires. Or there is another way. The additional help of elevating the temp in the engine bay and the interior cabin of the vehical by turning the heater on low and closing all the windows, also if you connect your ac gauge hoses so the hood can be completely lowered the engine and radiator will heat all of the exterior AC componets to 150 degrees+ also, remember the heater core is inside the same air box as the AC evaporator will also be at 150 degrees+ guys with everything at or above 150 degrees Its not going to take a real suffisticated vacuum pump to remove all traces of moisture. Using the chart I estimate that obtaining only 25" will be necessary to boil off the moisture. I'm not saying that the high dollar vacuum pumps are not great they provide the ability to vacuum a system with out any of the extra fuss that I have prevoisly described.
     
  16. Comment #16 (Posted by Tom Greenleaf)
    Note that this like other charts is discussing "boiling" of water. We forget that water also evaporates to vapor which with a vacuum less than the chart shows would take out the water as a vapor tons faster then just letting water sit in dry air at an ambient temp. The time a vacuum is held is also important as well. 30 minutes or more is a good idea to hold the vacuum noting no loss of Hgs for the duration....
     
  17. Comment #17 (Posted by John)
    Its recommended to replace the drier anyway anytime that atmospheric air has been introduced into the system
     
  18. Comment #18 (Posted by Cody)
    I don't know the conditions of the test preformed in this article but I own such a vacuum pump. I use a belt driven 4 hp 30 gallon compressor and it can bring the manifold gauge down to 29 inches in about 10 seconds and holds it for 30 minutes or more. I'm sure more expensive pumps can go further but so long as it's above 80 degrees outside pulling a 29 inch vacuum will be enough to boil water. I agree that an air vacuum pump is not suitable for shop use due to compressor strain and such. But, for the home mechanic like me that works on a/c systems maybe once a year or so they do just fine. I believe the author of this article has a bias for whatever reason and I doubt to validity of his results until further information is given. I know for a fact these pumps boil water. When I first got mine a few years ago I amazed my kids by hooking it up to a clear container filled with water and let them watch it boil. It's also worth mentioning that you can also pull a strong enough vacuum to boil water using a $10 tap on a water faucet. I remember my science teacher doing this way back in grade school.
     
  19. Comment #19 (Posted by Russ)
    Silly question! Why can't I hook up a fitting to the inlet of my compressor that draws the air into the motor, this should pull a nice vacuum. My compressor puts out 4.3 SCFM @ 115 PSI
     
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