Venturi Vacuum Pump?

[mannyskid]

So, I am looking at getting into vacuum bagging and have one question. I have looked at a few vacuum bagging "starter sets", and I keep seeing something called a venturi vacuum, it is a pump that uses an air compressor to push air out of an area. the starter set that I want to buy is the one from Soller Composites (where I also buy my carbon from). It is the top item on this page:

https://www.sollerpaddles.com/composites/tools.html#materials

So I was wondering if anyone has any expirience with this type of vacuum pump? It would certainly be cheaper for me since I have a large air compressor at my dad's shop that would be up to the task. Should I buy it or should I invest in a regular vacuum pump?

Manny

 

[Ethan]

We used to use one for propellant, it pulled about 27.5 inch Hg (full vacuum is ~29.9 inch Hg)
Served us well for many years, your mileage may vary...but I'd imagine it'd work quite well for bagging.

 

[degreaser]

The downside of a venturi is that your compressor needs to be running and it won't pull that hard of a vacuum. The upside is that it's cheap. Personally, I invested in a Gast pump.

Here's a really good site for bagging and the various types of systems, including venturis:

https://joewoodworker.com/veneering/visitorspress.htm

 

[mannyskid]

We used to use one for propellant, it pulled about 27.5 inch Hg (full vacuum is ~29.9 inch Hg)
Served us well for many years, your mileage may vary...but I'd imagine it'd work quite well for bagging.

Oh, that's good, I need a vacuum for degassing propellant as well.

 

[degreaser]

Oh, that's good, I need a vacuum for degassing propellant as well.

Yes, and the Gast seals are not rated for the solvents used for propellents (I called them and asked). Good call on that.

 

[bobkrech]

Since I design and built vacuum and space simulation systems I going to chime in here.

A venturi vacuum is fine for vacuum bagging where the purpose is to apply pressure to a layup to squeeze out excess epoxy. The pump is basically a Tee pipe wth a venturi orifice perpendicular to the flow.The high pressure gas expands and sucks air from the side of the Tee creating a vacuum. You can buy venturi pumps for as little as $15, but you also have to have a $100+ high pressure, high capacity air compressor to power it, and it must run continuously to maintain the vacuum.

Atmospheric pressure is 14.7 psi, 760 torr or 29.92 inches of mercury. Pulling a perfect vacuum on a bag creates a 14.7 psi force on the laminate in the bag so it squeezes excess resin from the composite. Venturi vacuum pumps develop moderate vacuums down to 40 torr. Evacuating a bag to this moderate vacuum level applies a 13.9 psi force on the composite so for this application, a venturi vacuum pump works well. But so is a Gast type diaphram pump, and it doesn't require a compressor.

Degassing or dehydrating a propellant or epoxy is another issue. Neither a venturi pump or a Gast type pump will develop a good enough vacuum to optimally degas propellant or epoxy resin. The fine powders used in propellant have very large surface areas that attract water and trap air in very small pores. The vapor pressure of water is about 20 torr at room temperature, so a venturi or Gast pump won't remove water or pull much air out of a propellant. Air and moisture adsorbed on the solid surfaces or trapped in internal voids prevents particle wetting and will produce lower density propellant than you can obtain with a better (lower) vacuum. Single stage oil sealed rotary vane vacuum pumps will develop vacuums lower than 0.1 torr and 2-stage oil sealed rotary vane vacuum pumps will develop vacuums lower than 0.01 torr. These are the vacuum levels used to obtain professional density propellant, or to degas epoxy to aerospace mnanufacturing standards.

Examples of inexpensive single and dual stage vacuum pumps and venturi pumps can be found at Harbor Freight.

My 2 cents.

Bob

 

[Ethan]

We were obtaining densities of approximately 97-98% of theoretical using a venturi, which is more than sufficient for any amateur propellant. We were, however, mixing under vacuum which makes a significant difference. With our current pump (PVR EM12) we can reach 99% +.

So while it may not be ideal, it can be made to work...

 

[CarVac]

At my school's club we use one for vacuum bagging. It works, but that's because there's this monster giganto air compressor in the room across the hallway that supplies constant air. That makes a racket, but the venturi itself just makes a moderately loud wshh. I've experimented using body tubes as mufflers, but it's white noise so I need to find some acoustic foam...

Incidentally, the big air compressor's duty cycle doesn't seem to increase noticeably when we're pulling a vacuum; perhaps the leakage in the compressed air piping throughout the building is significantly greater than the airflow through the pump...