Nicely done. Good work catching the error. Thank you.Look at your denominator for you losses for each branch (62.4 ft/s^2).
I am getting (D) 0.4 lbm/hr.The manufacturing process of thin films on microcircuits uses a perfectly insulated vacuum chamber whose walls are kept at -320°F by a liquid nitrogen bath. An electric resistance heater is embedded inside a 1.5 feet long cylinder of 1½ inch diameter placed inside the vacuum chamber. The surface of the cylinder has an emissivity of 0.25 and is maintained at 80°F by the heater. The nitrogen enters the chamber bath as a saturated liquid and leaves as a saturated vapor. Neglecting any heat transfer from the ends of the cylinder, the required flow rate of nitrogen (pounds-mass per hour) is most nearly (the latent heat of vaporization for N2 is 53.74 Btu per pound):
(A) 0.05
(B) 0.1
(C) 0.2
(D) 0.4
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They may not be posting much, but they're lurking here for sure...Is there not many TFS exam takers this round?
I was going to say...The april test takers were MUCH more active...Not a good sign for these October test takers....I hope.
The level of activity on this thread (and others) now pales in comparison to what we had for the April exam.
I would like to see the solution for this... I must be making this more difficult than what it is.Correct.
The way I solved it: Qrad=(emissivity)(Stefan-Boltz Constant)(Area)(delta T^4), once you have Q then Q=(m-dot)(hfg). Solve for m-dot.I would like to see the solution for this... I must be making this more difficult than what it is.
This is the correct approach.The way I solved it: Qrad=(emissivity)(Stefan-Boltz Constant)(Area)(delta T^4), once you have Q then Q=(m-dot)(hfg). Solve for m-dot.
It's because I retook the exam so many times they don't need to sign up any more people.They may not be posting much, but they're lurking here for sure...
No idea. But then again, in my circle of mechanical friends I'm the only TFS taker. Literally everyone around me took MDM once and passed (not looking at you @SacMe24). Any person who I heard about anecdotally who took TFS usually had to retake it. I definitely know zero HVAC'ers.Is there not many TFS exam takers this round?
So we were to ignore the convective heat transfer portion of the problem? The flashing of the Nitrogen should have induced a velocity in the chamber and enhanced the heat transfer.
If in that example they had specified the inlet and outlet conditions of the air in the duct, then you wouldn't need to calculate the heat loss with a heat transfer analysis. A simple energy balance for the air in the duct should give you the heat loss.So we were to ignore the convective heat transfer portion of the problem? The flashing of the Nitrogen should have induced a velocity in the chamber and enhanced the heat transfer.
Similar to example 37.1 in MERM... On page 37-4 thru 37-6.
I just think problem needs to state, neglect the convective heat transfer...If in that example they had specified the inlet and outlet conditions of the air in the duct, then you wouldn't need to calculate the heat loss with a heat transfer analysis. A simple energy balance for the air in the duct should give you the heat loss.
In our problem, the inlet and outlet conditions of the nitrogen are given, therefore the heat transfer rate into the nitrogen can be calculated with an energy balance on the nitrogen alone.
The nitrogen is changing phase because it is being heated by convection het transfer occurring from the chamber walls to the nitrogen. It cannot be neglected. It is just not necessary to calculate it, because you can do an energy balance on the nitrogen.I just think problem needs to state, neglect the convective heat transfer...
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