Pressure pot question

[VisExp]

I got my pressure pot set up using the HF pot and vacuum pump. I tested it and it works great on the pressure side. However when I am trying to pull a vacuum it seems as if the vacuum pump starts to gain on my compressor.

It develops a vacuum but then reaches a point when the compressor is running full speed, the air is pouring out of that little hole in the vacuum pump but the vacuum gauge stops dropping. If I disconnect the compressor, let it fill the tank again and then reconnect it the vacuum gauge starts to drop. It is tedious and I'm not really pulling much of a vacuum.

My compressor does not have big tanks, 4 gallon I think. I'm guessing that I would not have this problem with a bigger compressor. Short of getting another compressor is there anything else I can try? Would a different vacuum pump work better?

I really would like to be able to use a vacuum to stabilize some wood and also to dye some wood.

Thanks in advance for any advice.

 

[skiprat]

Sounds like you are using one of those venturi effect vac pumps?
If so, there is a minimum compressed air flow through it required to achieve a good vacuum. The Vac should say how much constant flow is required, could be in litres per minute or cubic feet per minute.
Your compressor should have some indication of the same about it's output. Unless your compressor is a very good make, I'm sure you will only get about 70-80% of the rated output.

Loads of people here have had good success with those venturi vac's, but I much prefer the quieter noise of a proper vac pump to the racket of a compressor running at full tilt.

 

[skiprat]

I've just had another thought that may or may not help. I haven't tried it.
If you turn down the output from the compressor a bit, it could still be enough to draw a vac and maintain some pressure in the tank.

My theory is that if the vac requires say a constant 10 cfm to work, you could be blasting it with say 15 cfm and thus emptying your tank before it gets to a vac.

Of course it would just take a bit longer. Worth a try though, before you go out and buy more kit.

 

[VisExp]

You are right Skiprat. It is a venturi effect vac pumps. That is a good idea about turning down the compressors output. I will experiment with it. Unfortunately I have no idea what the cfm required to run the venturi effect vac pump is. I took it out of the metal case it came in and through that away along with the literature [:I]

 

[holmqer]

The HF venturi vacuum pump claims that it draws 4.2 SCFM

 

[gketell]

Its claims lie through their sheet metal teeth. People have measure over 8SCFM through them.

My compressor is a 20 Gallon, 6.5SCFM@90psi and it will barely pull a good vacuum and I'm only at 400' MSL elevation. That last bit is also an important factor. The amount of vacuum you will be able to pull is directly proportional to how high up you are. The higher you are, the less vacuum you will get no matter what pump you use.

If you are going to spend money to make this work I would do like Skip says and invest in a real vacuum pump before you spent money on a new compressor (unless you have the need for a bigger compressor). you can get a really good vacuum pump for $80 or so.

GK

 

[VisExp]

Greg, at 400' you are up there in the ozone layer compared to us. Our elevation is about 25' ASL.

I had a feeling that the answer would be a good vacuum pump. I don't really need, or have room for, a bigger compressor. However I have been wanting to get a veneer press and vacuum pump for a long time. Seems like I know have three reasons to get a vacuum pump, veneering, stabilizing and a vacuum chuck for my lathe. I will have to start saving my pennies.

Thanks for the replies folks.

 

[Blind_Squirrel]

Anyone have a good recommendation for a vacuum pump?

 

[RONB]

You're in the ozone layer yourself at 25'. Mine is -6' here in New Orleans! I takes a lot of compressor to run the HF vacuum pumps.I thin my regulator down to 70psi and it keeps up.

 

[skiprat]

Originally posted by Blind_Squirrel

Anyone have a good recommendation for a vacuum pump?

Scott, I use two portable ones for some of the work I do. First is a Robin Air Vacuum Master. They come in slightly different variants. They come wired for the voltage you request, but can be swapped. 220/110V. The next is the Rolls Royce of vac pumps and comes from Dilo. The Dilo one is virtually silent but is not really any better than a Robin Air.

I use both of these pumps to withdraw ALL of the air from High Voltage equipment prior to filling them with inert gasses. The Robin Air one will evacuate a phone box sized tank in about 5-6hrs. When I use it on my pot, it get there in about 2.5mins. They are designed as continously rated and will never be damaged if you let it run and run. (the oil inside will get hot though)

The Robin Air vac is available in the UK for just over £100, but I have seen the same pump on USA web sites for much much less.

Someone JUST sold one here on IAP, but if you go back a few days you can see a pic.

 

[dbriski]

If you wanting to get into veneer pressing, check out www.joewoodworker.com It has lots of great information about veneering and vacuum pumps and building vacuum systems.

The HF Venturi vacuum pump requires 90PSI to be flowing to build any vaccum, so if your tank falls below that you are just wasting air. You will have to disconnect and allow your compressor to fill back up(putting a ball valve in there will make that process much easier just turning the air on and off). Also if you put a check valve in your piping after the venturi pump it will build vacuum easier because it won't allow any air to go back in once the the venturi looses efficency.

 

[Glass Scratcher]

Originally posted by holmqer

The HF venturi vacuum pump claims that it draws 4.2 SCFM

No it states:

Air consumption: 4.2 CFM

Which is how much air your compressor must output for the venturi to generate a vacuum.

Notoriously the HF venturi piping needs to be resealed to make it work "better".