Venturi vacuum pumps?

[wheels 543]

my 'Wheels XLS Special' vane pump isn't good with volume but once the bags down i8t has no problem holding it there.

The volume a pump can remove is a difficult one to measure. So "free air" displacement is often used for what any pump, pressure or vacuum, can move. That is the amount of Air it an move at atmosphere. i.e. no pressure, no vacuum. The amount of air is determined by Displacement x Speed - Efficiency. A positive displacement type pump are the ones such as Vane or Piston. These have a set volume of displacement for every stroke or revolution. The chamber moving the air is sealed and the optimum is to have a little air as possible seep past any form of seal. These kind of pumps move lower volumes or air, but provide the greatest vacuum/pressures. They also produce the greatest amount of heat due to friction of the contacting surfaces and the Air being squeezed through places.

No displacement pumps can move huge volumes or air quickly, but have the least in the way of Vacuum/or pressure. They use centrifugal force to move the air and require multiple stages to increase either states of pressure/vacuum.

The most Vacuum that can ever be created results in 1 atmosphere, or about 14.7 PSI of outside pressure pushing against the surface of (in the case of vacuum bagging) the bag.

For any pump being used as a Vacuum pump, there is increasing difficulty in removing the Air as it approaches full vacuum. It is called rarefaction or in other words, as the air is removed, the molecules inside the bag become rare to find and as each molecule is removed, there are less and less to find and remove. One really big plus with vacuum bagging is that the container is ever increasingly shrinking till no more space exists. For a containment vessel, the ability of removing all the air becomes increasingly more difficult.

Interestingly as the air is removed, the larger molecules tend to be removed rhe most easily and the very small molecules become the more common toward the end of the process, leaving the last molecules in any vessel as being Hydrogen and Hellium.

[Guest]

Thanks fellas. I thought it was OK as I did a couple of panels for a mate who knows the subject better than I and he was happy with the results. The chat about the differing units had me a little confused, something that's easier to do these days

 

I should have the gauge today so I can calibrate the one on it and have a good idea just what suck it has, which I'm pretty sure is good.

[Dusty 0]

I am currently using a small portable 4.2cu ft two stage pump which has proven to be more than enough to laminate new deck and side tank panels for a Finn I am restoring. I was able to achieve 29.5 inches hg but we spent a lot of time making sure there was no leaks in the bag. we used 170gram carbon each side of 5 mm 100kg density Corecell M foam. Super lightweight panels that needed bugger all prep for painting.

We had a valve in the line so shut off the line and pump and watched the vacuum gauge, after three hours the vacuum hadn't changed but by then the 38 degree temperature had the resin kicked off.

I wouldn't recommend using a pump that size on larger projects (say over 18ft) For your average 30 footer I would not go below 11cuft and use multiple vacuum ports and if using resin infusion would probably look at 15 cu ft.