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Venturi vacuum pumps?


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Quick question for the learned.

Has anyone used one? Any good?

Sounds like I might be able to borrow a proper pump, but having my own venturi would give me a bit more flexibility / time frame.

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Vacuum bagging some small bulkheads, foam/glass/epoxy, and a cedar cored centreboard. Probably marginal if vac is justified, but am keen to Give it a go.

 

Main concern is just how noisy they can be. Will be working in the garage under the house in evenings, could be v unpopular with the family if it makes a racket all night.

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What sorts mine Wheels? I'm thinking the word 'vane' for some reason.

 

In mm, cm or whatever what's your 'small bulkhead' and the other bits? Roughly is close enough.

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Not big!

Sunburst, 2 inside the side seats, and the main one, 1.5x0.5?

A centreboard case, the floor behind it, and something to go under compression strut.

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Yes it's a vane pump KM.

Station, not sure you have the right description perhaps. A venturi works by a large volume or air being forced through a Tube and the Tube has a narrowing where the Air speeds up. Into that narrowing is a side entry tube. The higher speed of air causes a low pressure and thus a suction on that side tube, pulling whatever that side tube is connected to. In your case, it just sucks air and so you can have a vacuum. but it is not much of a vacuum and I doubt it would be enough to be of use to what you are wanting to do. You would be better using your Vacuum cleaner perhaps. You also don't need to pull a vacuum all night. You only need it long enough to draw through the resin and at most till it starts to go off.

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You should never give advice as to how long it will take for resin to go off. It definitely needs to reach "B" stage of cure prior to releasing the vacuum, not 'starting to go off'.

 

Personally I would use the proper venturi over a vacuum cleaner, but if by chance you are able to borrow a real pump go for that instead every time.

 

if you are going to use the venturi you are going to want to have a reasonably good compressor, as it could need to run constantly for 8/10 hours or more being winter.

 

I would recommend that you do a sample of your laminate on an off cut so you can play with that to see if it has gone off enough prior to disengaging the vacuum system.

 

For example if you get a good vacuum you could be applying near on 14lbs per square inch to your job.The palm of your hand would be roughly 12sq inches if pushing down, so 168lb of pressure, or 75kg. You turn the vacuum off too soon and it's like letting go of a rung out sponge

 

I should be doing a vac layup on friday if your anywhere near Aucks north shore, if you wanted to see the set up - just pm me

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Venturi's are a great way to take a lot of air out of the bag before you apply a proper vacuum pump to a job.

As FNG says they require a lot of compressed air, as a guide a portable single phase "15' with 280 litres per minute free air delivery will not keep up with the air required on a fairly small job for more than a few hours.

 

A vacuum cleaner is also a good way to evacuate air from the bag but will not pull an effective vacuum.

 

How the long the vacuum needs to be on for basically has to do with the temperature of the epoxy.

As epoxy starts to cure it creates heat.

The vacuum must be left on until the heat has dissipated, and the part has returned to room temperature.

Otherwise the part can distort while it is cooling.

 

A company called VABS hires small vacuum pumps and could be a very good option for you.

They are very quiet and can pull a true vacuum close to 1 bar.

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2 questions for our more learned posters -

 

1 - Can you have too much of a vacuum or is there an ideal vacuum or do you just suck hard as the pump will go?

 

2 - Once the excess air is out of the bag does it matter whats sucking as long as you can suck the required amount of suck? Just asking as my 'Wheels XLS Special' vane pump isn't good with volume but once the bags down i8t has no problem holding it there.

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2 questions for our more learned posters -

 

1 - Can you have too much of a vacuum or is there an ideal vacuum or do you just suck hard as the pump will go?

 

2 - Once the excess air is out of the bag does it matter whats sucking as long as you can suck the required amount of suck? Just asking as my 'Wheels XLS Special' vane pump isn't good with volume but once the bags down i8t has no problem holding it there.

 

a Rotary Vane pump (which most single phase pumps will be) are oil cooled and require a pretty good vacuum before the oil starts to circulate properly.

These pumps can overheat when evacuating large volumes of air from big bags.

 

Its also true that a better vacuum (as close as possible to 1 bar) will produce the best results and result in the lowest void content in the laminate.

 

The only time you back off on a vacuum is if you have a water cooled pump and the water starts to boil.

 

Not also that autoclaving is an extension of vacuuming.

An autoclave increases the pressure that the laminate sees and produces even better results than a vacuum on its own.

 

It is a misconception that vacuuming is to draw out resin from a laminate.

The main purpose is to consolidate the laminate to remove voids and this is totally achievable without removing any resin from the job (if you have applied the correct amount of resin in the first place, or used prepreg with the correct resin content).

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I'll stay out of the actual resin part as I no nothing.

a Rotary Vane pump (which most single phase pumps will be) are oil cooled and require a pretty good vacuum before the oil starts to circulate properly.

These pumps can overheat when evacuating large volumes of air from big bags.

But that isn't totally correct. The following is general info, not aimed at you Tim or anyone else.

There are many different designs of Vacuum pump, just like many kinds of any other pump type. The one I used for KM's system is a single phase, oil less pump, specially designed for Resin Vacuuming and continuous rated. It gets hot, too hot to hold your hand on, but is designed to run like that and sits at it's rated 60deg operating temp.

You can also get Diaphragm Vacuum Pumps and centrifugal Vacuum Pumps.

There are also Vacuum Cleaners available that have a "By Pass" motor, which means it's cooling air is not the same air that is being sucked up.It's cooling air is completely separate and these will suck continuously. How much Suck comes down to quality, but even cheap ones still have enough suck to be used for most Resin applications. Well that is if the Vacuum pressure is what KM and I were told to use by a professional which was no where near full 1Bar vacuum. In fact for me, it was disappointingly very little after all the effort I went to making a system that could suck so well.

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Just out of interest what were you suggested to pull ( in vacuum ) and did they say why.

 

Have they done it day in day out for many years like Tim and myself may have. And at the varying levels of products/boats?

 

I have a saying - real knowledge is limited to ones own ignorance - not meaning to be ignorant here just very interested in their reasoning's

 

Cheers

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Just out of interest what were you suggested to pull ( in vacuum ) and did they say why.

 

Have they done it day in day out for many years like Tim and myself may have. And at the varying levels of products/boats?

You've worked with them, you tell us ;)

 

But in all fairness I think Wheels is referring to some play panels I made just after I got the pump and I was a rough as high speed bastard as I did them so there is a high chance it was me rather than the pump. I've since done many others and they have been sweet but then a lot more care has been taken.

 

Great pump for it's small size. You'd never do a whole boat with it but I have done some 1400 x 2900mm carbon panels (on a big table I made) and they came out real good. Not at all fast in any way when sucking volume but once the excess air is out it sucks like a $10 K Rd early morning special :lol:

 

We do run the pump at near on maximum even If I'm not totally sure how many sucks that is, but do know it's a lot of suck as it flattens out 25mm conduit. I'm getting a vacuum gauge today to calibrate the gauge on the machine. That will be interesting..... or maybe scary

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Ah the amount of vacuum pump experts I've met over the years.

Its funny how people always try to re invent the wheel.

Unfortunately I have little knowledge of the exact method of vac bagging (there must be 1 way that is better than others), but I do know a bit about the pumps. We have probably 10 or so pumps down stairs of varying types and sizes that we use as loaners when our customers have problems with their own units.

About the only similarity between the different types is that they all suck.

Also be very aware that you get what you pay for in quality.

If you want to hold a continuous suck at what we would call rough vac (0.1 mbar), you can't go past a good quality rotary vane. To get better suck after that you need to go multistage, but that's very high end and I wouldn't think necessary.

 

Sure they get hot, but the lower the vacuum the less work they do and less heat generation. We've had installations where the pumps have eventually been outgrown by the facility, so they are no longer large enough to pull down far enough we've had to put synthetic oil in them to stop the oil from burning off. If the pump is quality enough, they can handle it.

 

In the case of large jobs where the bag may take a long time to pull down, it may be a case of using a blower 1st to pull the bulk air out, then swap over to a rotary vane.

There are probably a dozen other types of pump which could still do the job, but some are really expensive, some are just too complicated while others will just eventually melt.

Even a compressor piped backwards will work, but remember that it will get very hot and bothered. When a compressor pumps out air, it always gets a mouthful of fresh cool air to replace it. When going backwards, the idea is to reduce the air intake down to zero.

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Just to de-confuse a slightly confused me.

 

Is it OK to use any sort of pump to hold the bag down as long as it can hold the ideal perfect vacuum?

 

And the size of the pump needed would be in straight relation to the size of the job being sucked?

 

My pump runs at around 55 degrees when it's doing it's business.

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I guess if its pulling the correct amount suck, which I don't know cause I 'm not a boat builder and its not going to catch fire, then it should be fine.

 

By the way. Is there a preferred pressure, or does it vary according to the job at hand?

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for infusion we try and get 99.98% of an atmosphere, but the bag can't leak either. I'm typically not happy under 95% for a wet laminate vac job

 

I can only assume km means the people very close to him that make real flash wood stuff for the super yachts, so vacuuming down a decorative veneer, of teak deck panels is a whole different story.

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2 questions for our more learned posters -

 

1 - Can you have too much of a vacuum or is there an ideal vacuum or do you just suck hard as the pump will go?

 

2 - Once the excess air is out of the bag does it matter whats sucking as long as you can suck the required amount of suck? Just asking as my 'Wheels XLS Special' vane pump isn't good with volume but once the bags down i8t has no problem holding it there.

 

Nah you can only suck 1 atmosphere of vacumn no matter how much you pay KM, or how much you suck! Don't worry it won't turn inside out.

And as long as you can hold the vacumn it doesn't matter how you do it.

The rotary vane pumps are designed to run for hours, and the pump bits don't touch the housing so they are much happier than vacumn cleaners, milking pumps etc. most of which need some bypass air to stay happy & cool.

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