Fittings for tubing under low pressure (vacuum)

[Orionstar]

Does anyone know if there is a flexible polymer tubing (in the 1/2" OD range) that can handle fluid flow inside at 80C under 27" Hg vacuum (about 50 Torr). If so, how to connect it (metal compression fitting, flared, barbed)? Most charts I see give vacuum ratings at 72F. Also, don't know the effects of vacuum inside on the reliability of the fitting connection.

 

[moltenmetal]

How is this vacuum tubing being kept hot? A fluid at 50 torr isn't going to transfer much heat to the tubing, even if the velocity is quite high- the density is just too low. If the tubing itself is in a hot ambient environment, then you have an issue.

The issue with polymeric materials isn't the end connections, it's the permeability of the material itself as well as its structural strength against inward collapse.

A teflon hose with a crimped hose barb assembly will probably do the job. I would have no hesitation using the teflon hose with stainless overbraid that we use for pressures up to 3,000 psig, in a modest vacuum application at 80 C.

 

[Orionstar]

Thanks for the comment. The tubing, at different times, will see 60-80C fluid (similar to water in viscosity and density) at pressures between -7 and + 15 psig, and 80C vapor at pressures as low as 50 Torr (absolute). Oh, by the way, diameter is in the 1/2" range for the most part. I don't think permeability is a problem as it is not a deep vacuum application, nor is it sensitive to contamination/particles as in deep vacuum applications. There are going to be many short runs of this tubing (between valves, tees, etc). I want to stay away from the assembled hoses (like PFA lined with silicone over and SS braid and build in ends) as there are so many and it would add up to lots of cost. I have used PFA and/or FEP tubing in the past, either with PFA flare fittings (ultrapure) or SS compression - but never with vacuum, and am not sure of the vacuum rating at 80C with the material softened. I am guessing that I can just use a hard nylon (rated at 209" hg at 72F) over a metal barb. But I want to get experience and advice from others. Default, I think, is SS tubing and compression fittings. I am also asking the client if we can use copper (easier to bend) tubing with brass compression fittings).

 

[moltenmetal]

Flaretek-style solid PFA should do the job. That's with the tubing heat-flared to go over the ends of the fittings, not using an OD compression type joint using external ferrules- those are notoriously poor with PFA. It is used in high purity applications all the time, and the mechanical strength will not be a problem for vacuum resistance because your temperature is very modest. I'd be concerned if you go above 1/2" OD though- at 3/4" OD you will have to go with a fairly heavy wall.

 

[danw2]

Temperature ratings for Parker's thermoplastic tubing are in the guide here: