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Name: Rob
Status: educator
Grade: 9-12
Country: Australia
Date: Fall 2013


Question:
I have been working with vacuum recently, and upon the multitude of questions that have come about, one has really been bugging me.

We have a vacuum pump that depressurises a chamber with a liquid inside. The chamber is not full. Anyway, eventually the liquid boils and creates vapor. There is a line between the chamber and the vacuum pump that carries the harvested vapor. The pump then discharges air/vapor through an output line.

The question is, due to not having sufficient 4" vacuum line (it is expensive!) we split the output line and joined 2 x 3" lines to a tee, which then ran to a tank. One line was slightly (0.5m) longer than the other line. What we observed was that the steam would always travel completely through the shorter line. We took the lines off the tank and ran them straight to the ground to check it out, and the same result. So why is this?

I have a couple hypotheses. Obviously, the longer the line the more friction and hence slight back pressure there would be on the 'steam'. Given that it will always take the easiest path, this could explain why one is chosen, however why is there nothing coming out of the other one? Is it that perhaps we could not detect the flow because it was so small? I understand that 2x 3" lines have only slightly greater area than 1x 4" line. Could it be that we were not utilising full capacity of the 4" line, therefore it only needed 1 3" line to discharge?



Replies:
Hi Rob,

Thanks for the question. There is no need to lose sleep over vacuum problems. Otherwise, I would not have slept in years. I presume you are speaking about mechanical (or rough) vacuum pumps and not diffusion or turbo-molecular pumps. When evacuating a chamber using a vacuum pump, two hoses can be treated mathematically like a set of parallel resistors. The larger the cross-sectional area of the hoses, the greater the rate of vapor transport. The longer the length of the hose, the lower the rate of vapor transport. There will be vapor transported through both the hoses. However, the rates of transport will be different if the length and cross-sectional areas are different. I will mention that it may not be easy to detect vapor in hoses. Additionally, the exhaust lines of a vacuum pump will have oil vapor/mist in them. You may be fooled by this mist into thinking it is steam.

On a practical side, I strongly recommend against pumping vapors (such as water or flammable vapors) through a mechanical pump. Water will corrode the metal inside the pump. Flammable vapors will decrease pump performance and present a serious explosion hazard due to sparking motors.

I hope this helps. Thanks Jeff Grell






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