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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.










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