pneumatic tubing swelling
 

http://imgur.com/MLbRg18

We have this piece of tubing going from the compressor to a T-fitting. We noticed last night that it had swollen and had it replaced. The new one has now swollen. Our system isn't going above 115 psi, so I don't think the pressure is an issue.

Does anyone have any ideas, or experience with a similar problem?

Re: pneumatic tubing swelling
 

Could the compressor be getting hot and melting the tubing?

Re: pneumatic tubing swelling
 

I suppose. If the tubing is touching the compressor anywhere it would be on the opposite side of where the heatsink is. I didn't think that part of the compressor got very hot, but I could be wrong.

Re: pneumatic tubing swelling
 

We used a computer fan to blow air across our compressor last year and it kept it nice and cool all season. Before this it was getting extremely hot.

Re: pneumatic tubing swelling
 

Is this tubing being connected directly to the compressor output?
If so, I would recommend adding some metal fittings before the plastic tubing to dissipate heat from the hot air coming out of the compressor. Placing one of the large fans from the KoP next to the compressor will also help.

Re: pneumatic tubing swelling
 

I'll confirm the suggestion to add a fan, blowing across the compressor, to solve this problem. The duty cycle of the compressor is only 9%, which means you need to let it cool for eleven times as long as you run it. This time of year, we typically are working on the robot, and testing stuff...running it for five minutes at a time is typical, and doing this will over heat the compressor, unless you add active cooling. It doesn't take much to keep it cool, one of those square fans at least 100mm across should do it.

Re: pneumatic tubing swelling
 

I was amazed last year at how cool the fan kept it, especially with how hot it was getting prior to adding it. I don't think the compressor ever got hotter than warm to the touch.

Re: pneumatic tubing swelling
 

In addition, remember that (from R76) the pressure relief valve must be directly connected to the compressor, with only rigid fittings in between (you can't put plastic tubing between the compressor output and the relief valve). If you currently are not doing this, the extra length from the valve might be enough.

Air leaving the compressor is hot because it is compressed rapidly without much time to transfer heat to the surrounding environment. By adding the valve (or additional metal fittings if you already have the valve in place), you give the air more time to cool off before it reaches the plastic tubing.

Re: pneumatic tubing swelling
 

Not only does the compressor get hot from the friction of the piston but the Ideal Gas Law states that increasing the pressure of a gas increases its temperature. The compressed air from the compressor is actually hotter than ambient. If you run it for a long time it would be conceivable that the tube gets heated by the compressed air to a point where it can no longer withstand its designed working pressure and you see the bubbling that you showed.

A short metal pipe attached to the compressor would help to dissipate this heat.

Or let your system coll a bit in between runs.

EDIT: looks like John beat me to it :)

Re: pneumatic tubing swelling
 

There are a few metal fitting before the tubing is attached. We'll see about mounting a fan to cool it this evening.

Thanks everyone.

Re: pneumatic tubing swelling
 

Would you mind PM me a picture of how you attacked the fan to the compressor? We are just using zip ties at the moment. The fan keeps the compressor WAY cooler but worried about the zip ties possibly being melted.
-Ronnie

Re: pneumatic tubing swelling
 

I would not be concerned about the zip ties melting. A quick Google search shows that most zip ties are made out of Nylon 6-6, which has a melting point of ~500° F. Even if the fan somehow fails, I don't think the compressor will get anywhere near that hot.

Re: pneumatic tubing swelling
 

Lets hope not right? :eek: lol. Thank you, will stick with simplicity and stay with the zip ties.
-Ronnie

Re: pneumatic tubing swelling
 

If you're using the Viair compressor, it has a relatively low thermal mass and heats up much quicker than the old Thomas compressors. There used to be a rule that you had to have solid brass fittings connected to the compressor output for the pressure relief valve, and this extra metal provided some amount of a buffer for a sort of heat sink. A few things to try is maybe adding several of your compressor components directly to the compressor through solid fittings and see if that helps. Another option would be to add a fan to provide some forced convection across the compressor head to cool it down.

More speculative answers: the tube you have may not be rated for the pressure you're using (not likely, but worth checking) or could be old and have lost some of its stiffness if stored improperly for an extended period of time.

[edit: bah, took too long to write and missed the discussion!]

Re: pneumatic tubing swelling
 

Keep in mind that there are other legal compressors, too! The commonly used Viair compressor has a bigger brother in their industrial grade line. It has a cfm below 1.05, which makes it legal to use, and has a much bigger thermal mass. This allows it to be rated for continuous use, not just the 9% of the normal Viair teams use. It also comes with a steel mesh leader hose - this is considered part of the compressor (per last year's Q &A, at least... I haven't seen it asked this year yet), and can be used to help ensure the tubing is well away from the hot parts of the compressor!