Graduate Can high air velocity in vacuum system create "freezing" of the pipe?

[kaigoss69]

TL;DR

Can high air velocity in vacuum system create "freezing" of the pipe?

Hi folks, very simple question, but have not been able to find an answer: Assume a vacuum pump pulls air through a pipe. Also assume that within the pipe run, there is an area with reduced cross section, creating a temporary increase in air velocity. Also assume the air is very moist, and is at room temperature. Question: Does the air in the section where the cross section is reduced have a possibility to "freeze" given a high-enough air velocity? If yes, at what velocity of air would "freezing" occur?

 

[Lnewqban]

It is possible, under favorable conditions.
Please, see:
https://en.m.wikipedia.org/wiki/Carburetor_icing

 

[boneh3ad]

Sure, though you parameters are all wrong. A few things to consider:

• The system will also be highly compressible so an increase in velocity could require an area increase or decrease depending on the specific conditions.
• Temperature and pressure are the relevant quantities to determine whether you get any fun non-gases that form. They both decrease when velocity goes up.
• It's not just water that can get interesting like this. High-speed wind tunnels at Mach 5 or higher have to heat their reservoir air because it expands so much that the temperature gets low enough for oxygen to liquefy. Preheating prevents this from happening, making sure the test gas remains a gas.
• Given the above, the air must also be dried to avoid liquid water or ice crystals forming in the test region.

In short, it's a very real phenomenon.

 

[Keith_McClary]

I learned recently this is an issue going from high pressure NG lines to low pressure:

Heaters: The reduction of compressed Natural gas pressure causes a drop in temperature. In terms of physics, this is known as the Joule Thompson effect. This drop in temperature can create operational and material quality problems in the distribution networks. The water vapor dew point falls and causes icing within and outside the pipelines.