Vac Pump not pulling as high as it should?

[xtc444]

Hi,

I've had a vac former for a few years which has run fine on my 'shop vac' but I recently decided to upgrade to a tank and vac pump system.

I purchased 2 x 90 litre tanks along with a 3 CFM vac pump which was advertised as capable of pulling 29HG. After trying it, it seemed incapable, only managing 21 or 22HG before running out of energy.

I decided it might be the pump so sold it and purchased a new Robinair 15400 (4 CFM pump capable of 20 microns). However, when I start the pump it only manages to reach 22HG before the pump just stops pulling any more vacuum.

The entire system is made from 15mm copper pipe and there are definately no leaks in the system (the oil level in the Robinair is at the correct level too) - it just seems incapable of pulling 28 or 29HG.

Am I missing something? I'd really appreciate any help or pointers.

Thanks

[martinw]

Hi,

I've had a vac former for a few years which has run fine on my 'shop vac' but I recently decided to upgrade to a tank and vac pump system.

I purchased 2 x 90 litre tanks along with a 3 CFM vac pump which was advertised as capable of pulling 29HG. After trying it, it seemed incapable, only managing 21 or 22HG before running out of energy.

I decided it might be the pump so sold it and purchased a new Robinair 15400 (4 CFM pump capable of 20 microns). However, when I start the pump it only manages to reach 22HG before the pump just stops pulling any more vacuum.

The entire system is made from 15mm copper pipe and there are definately no leaks in the system (the oil level in the Robinair is at the correct level too) - it just seems incapable of pulling 28 or 29HG.

Am I missing something? I'd really appreciate any help or pointers.

Thanks

Dear xtc,

I had the same kind of problem with a much more modest vac. bag set-up a month ago. The problem was traced to a dodgy non-return valve between the pump and the rest of the system. Worth a check....IMVHO

Best wishes

Martin

[widgits]

First of all what altitude are you at?
I knew a guy up in the mountains of Colorado at 8000 ft who wondered why his vac system wouldn't go above 23" HG
It's almost physically impossible to do this with a standard rotary piston
or vane pump

How do you know there are no leaks?
Everytime I built a vac system there was always a leak
Even though I tried diligently to prevent this
Theres a product called Kinseal made by the kinney vacuum pump company
A small can of this stuff works wonders on the pipe fittings and I once
used it to seal a pinhole leak in a tank weld (good stuff)
Just brush it on as the system pumps down
Dont be afraid of the pump housing itself
These also have been known to cause leaks

Even though the pump is full of oil doesnt mean that it can reach higher vacuum
Dirty oil contaminates the whole process, Change it
Better yet drain it and refill the pump with an oil called Texaco R&O 32
(It's a thin Hydraulic oil)
Run the pump for about 2 hours on this stuff
This will help draw out the bad impurities that are outgassing from
the cast iron parts inside the pump

Drain this oil and replace it with a good quality vacuum pump oil
(Maybe a Shell Ursa 68)

Believe it or not people have problems with their pump not working correctly
so the sell them to the next guy who has the same problem
first thing I would replace was the shaft seal and flush the system
This solved 50% of all the repairs relating to the pump itself

Hope this helps
Widgits

[kayaker43]

Its very possible you are seeing an altitude problem. For instance at sea level on a "standard" day you could get 29.92 max. at 10,000 feet you wont get much over 20 in.hg. Also barometric pressure has a direct effect. Some days even at seal level you can only get 27 inches, or on a god day over 30? And yes oil purity matters when going for that last tiny bit, but not so much till you are past 29 in.

Having said that, I'll bet you are seeing normal effect of connecting a too small pump to a too large tank. You have 4 cfm "open flow" but much less at any other vacuum level. I'm too lazy to look up the litres/gallons conversion but it sound like your pump should be much larger for that tank capacity. As soon as the vacuum builds, the flow decreases until it reaches zero flow at max rated vacuum. That means it goes really fast for the first 10 or 20 inches but at higher levels the flow will be a tiny fraction of its open flow. It may take a whole day for a small pump to reach high levels on a big tank, and that's only if it has zero leaks! Scroll down this page to get some sizing guidelines http://www.build-stuff.com/vacuum_pumps.htm

The real test is to stick a gauge directly on the pump with no tank and see what it says.

Here's another fun test to see how important leaks are. With a gauge hooked up, put a piece of tape over the end of an open fitting, a 1/4 inch opening should be easy to seal with electrical tape. Now run the pump and note the max vacuum. then poke a tiny hole in the tape with a sewing needle and see how much vacuum you lose. Again the flow at max vacuum is miniscule, so even the slightest leaks hurt.

[drcrash]

If xtc444 is in England (as his "location" says) I'm guessing that altitude is not the problem.

Sounds like a leak to me. I agree with Doug that the first thing to do is to see what the pump can pull, connected to nothing but a gauge.

If the pump is fine, and there really are no leaks, one possibility is damp tanks. If you use old hot water tanks, for example, they often have damp sediments in them, which outgas water until they're completely dry. (The fix for that is to keep evacuating the water vapor until the tanks are really dry.)

[kayaker43]

I think dampness and low oil purity can mean the difference between mid 29's and high 29's, but doesn't make any real difference in the low 20's.

Leaks are always bad and can surely cause this problem, but I think you are seeing the normal results of trying to evacuate a 48 gallon tank with a 3-4 CFM pump. Past 20 inches, the flow would be so low it would seemingly take forever? Combine that with even a tiny leak and its no surprise.

Check vacuum directly at the pump (no tank) and then close the dump valve and see if the tank eventually leaks down. A tank that size can have a pinhole leak and not show a vacuum loss for quite a while?

Sound like a mismatched system with the possibility of leaks?? Let us know the results so we can all learn from it.

[drcrash]

I think dampness and low oil purity can mean the difference between mid 29's and high 29's, but doesn't make any real difference in the low 20's.

I hadn't realized you had to get the pressure nearly to zero before water would boil at room temperature, but you are quite right. (About .025 atmospheres, or one third of one PSI or .75 in. Hg, absolute.)

Thanks for the correction.

I wonder, though, if evaporation might be considerably faster at low pressures, even if they're a few inches of mercury short of zero...?

I've heard of people noticing that damp water heater tanks made a difference, and now I'm trying to figure out how they'd make a difference big enough to notice with a normal vacuum forming setup.

(It could have been just a slow loss of vacuum due to evaporation---overnight, say---rather than noticeably decreasing the vacuum level they got with the pump on. Then they might think they had a very slow leak, and look for it. Or maybe they had a pump that could draw the tank down to very nearly nothing, and did that while testing, even though you wouldn't normally bother over that last inch of mercury.)

[kayaker43]

drcrash

I'm not at all knowledgable about deep vacuum applications, but in real life moisture doesn't seem to matter for our simple use.

The reason I can confidently make a sweeping non-technical statement like that, is because I used to use a converted air compressor as a vacuum pump up to 28 in.hg. I also used the tank it came with and still used it as an air compressor when not vac forming. The tank often had a gallon or more of condensation water in it, and I always got the same vacuum level whether I drained it or not.

Even when using laboratory or refrigeration pumps, its rare that we will see enough vacuum to boil water. You need to run the pump a long time against a small volume that is very leak free,... we hardly ever find these conditions in vacuum forming.

[drcrash]

Thanks, Doug. That is good to know.

[Geof]

I hadn't realized you had to get the pressure nearly to zero before water would boil at room temperature, but you are quite right. (About .025 atmospheres, or one third of one PSI or .75 in. Hg, absolute.)....

If you do have a good enough vacuum to boil water then what you are doing is evaporating water. This needs energy, it is called the latent heat of vaporization, and the only place the energy can come from is by the remaining water cooling down. With a really good vacuum pump and a few pints of water is does not take very long before the remaining water has cooled down to zero degrees and starts to freeze. It keeps on evaporating from the ice; going directly from solid to water vapor which is callled sublimation.

Another name for the process is 'freeze drying'.

But it is very unlikely you are going to have a pump that is good.

[xtc444]

Thanks for the replies everyone.

I tried removing the one way valve and it managed to pull 23HG but then stopped pulling. I also don't think altitude is the problem, Dr Crash is right - I am from the UK and after checking my altitude - I'm only 80 feet (or thereabouts) above sea level.

I had a load of problems finding tanks for my former, water tanks were just far too expensive so I went for 2 x 90 litre gas propane tanks and there are definately no leaks in them (I did have them emptied professionally before I used them). I also went through my entire connection system to the plattern and I'm 100% there are no leaks whatsoever. I pulled the tanks down to 20 and left it for over an hour and the gauge hadn't moved at all.

I think kayaker43 may have hit it on the head with the tanks being too large?

Both tanks together equals approximately 47.5 US Gallons. They do evacuate quickly to 19 inches - about 4 minutes - but then it takes another 6 minutes to pull another 3 inches before the pump starts to sound like it's really struggling. I connected my vac gauge directly to the pump and it reads approx 28inHG.

Seeing as its 2 x 23 US gallon tanks connected together, I'll try removing a tank tomorrow and giving it another shot to see if it can pull a better vacuum.

I suspect one tank would be fine for my current 24" x 24" plattern but I'm currently building a 32" x 24" dual vac machine based around the over under design and I'm just concerned that 23 gallons may not be enough for a plattern that size?

[Geof]

Where are you measuring the vacuum, at the pump or at the tank?

Also what size hose are you using between the pump and the tank and how long is it?

There will be a vacuum gradient between the tank and the pump; there has to be or the residual air would not flow toward the pump. This could possibly account for one or two inches of pressure. In addition if you have a smallish hose that is long the gradient will be increased. I would use a hose at least 15mm ID and would keep it below a copuple of meters in length. Preferable I would use 3mm ID or larger and have the pump sitting right at the tank.