Friday, March 25, 2011

ENGR 224 - Thermodynamics at GRCC

Welcome to the Thermodynamics course at Green River Community College in Auburn, WA, USA.

Feel free to post any general questions about the course as comments on this blog entry.

I have created a blog entry for each chapter we will cover in this course. If you think your question applies to a specific chapter, then please post your question as a comment on the appropriate blog entry.

I have created a blog entry for each of the 7 HW assignments in the course. If you have a question about a problem in the HW, please post it as a comment on the appropriate blog entry. Be sure to state which problem number your question refers to !

You can post your questions using your Google ID or a crazy fake name or your real name or you can post anonymously. Whatever makes you happy. Just don't hesitate to ASK !

"It's not a miracle, mother, it's thermodynamics !"
- Harrison Ford in Mosquito Coast


Anonymous said...

Hi I am an undergrad and was wondering if you could give any hints how to start off this question:

A rigid vessel contains 5 kg of a mixture of air and water. The relative humidity of the mixture is 0.9 at 20 C. The energy required to raise the temp of the mixture from 20 C to 40 C is 73 kJ. Treat the air and water vapor as perfect gases. Find for a temp of 40 C
1 - mass of air
2 - volume of mixture
3 - pressure of mixture
4 - dew point temp
5 - relative and specific humidities...

I can get the pressure of the vapor at 20 C and have tried writing and energy balance for the overall change in U. Help would be greatly appreciated!!

Dr. B said...

Anon 5:23pm
Step 1: check out my lesson about humidity on my website:

Step 2 - Look up Cv for air and Cv for water vapor. Remember that in terms of molar heat capacity, Cp = Cv + R.

Step 4 - From an energy balance and the given value of Q you can now determine the mole mole fractions of air and water in the gas. Q = Cp,avg * DT and Cp,avg = ya*Cpa+yw*Cpw

Step 2 - use the mole fraction of water in the tank and the initial relative humidity and the info you find on my website to determine P1.

Step 3 - Calculate the avg molecular wt, the total moles in the tank, the moles of air in the tank, and then the mass of air in the tank.

Step 4 - Apply the IG EOS to determine the volume of the tank.

Hopefully you can take it from here. Best of luck.