Wednesday, September 17, 2008

TE 303 - HW #3, P5 - Clapeyron & Clausius-Clapeyron Equations - 16 pts

Estimate the latent heat of vaporization, in Btu/lbm, of ammonia at -10oF using:

a.) The Clapeyron equation
b.) The Clausius-Clapeyron equation
c.) The ammonia tables

In your analysis, be sure to include a comparison of these three methods and propose a reason for any significant differences. (4 pts)

13 Old Comments

12 comments:

Anonymous said...

I've been working on this for a while and cannot seem to figure it out. Do we have to solve for the vapor pressure using antoine equations? And when I do, my LogP is about 19, so inverse log is huge. This doesn't seem to be right. AHHHH

Anonymous said...

Dear anonymous student,

You are allowed to use the thermodynamic data tables (i.e., steam tables) for this problem. So no, you do not need to use the Antoine equation (although you can). Just like we did in class, pick a temperature range around our given temperature, and with those values, look up the vapor pressures (or calculate them with the Antoine equation). You can those use those values to calculate deltaP* and deltaT.

I suspect that your error with using the Antoine equation is that you didn't convert the Temperature to Rankine, so check your units!

Anonymous said...

I am looking at the tables and I was looking at -10 and -15 temperature pressures but what one would we put for initial and what one would we use for final?

Anonymous said...

For these problems I am not sure how to convert from the units given to Btu/lbm. I am trying to use the back of the book but it doesn't seem to help.

If i am correct the until from part a. are ft^3*psia/lbm. I have no idea what that is equal to.

Anonymous said...

for part b. where do you get a,b, c from the equation Log10P*= A-(B/T+C)

on the nist webbook the temperature starts at 298K- 1400K for those values to be acceptable. For part a. we used the values 0 and -20 F which is less than 298K so I don't know where to find these values.

Anonymous said...

For part c. when you use the ammonia tables do you just look up the values at -10F at Sat. Vapor or is the an equation required to solve for this?

Anonymous said...

Dr. P's biggest fan:

First, thanks for making me smile :-)

Secondly, what you use for initial and final doesn't really matter in this case, as long as you're consistent with both the pressure and temperature. (The signs will cancel.) I recommend converting to RANKINE prior to using the equation.

Anonymous said...

Dear unitless,

These conversion factors might help you:

1 psia = 144 lbf/ft^2

1 Btu = 778.17 ft*lbf

Anonymous said...

Dear values?,

You do not need to use the Antoine equation for part B. But, you can. I believe that you are not looking at the right constants -- the antoine constants exist at these temperatures. (Remember to convert units!!)

Anonymous said...

Dear Anonymous,

You just need to look up Hsat at liquid and vapor to calculate deltaH.

Anonymous said...

If you use the Antoine equation, the constants are given for T(K), how do I convert these for T(R)?? I cant find it anywhere :(

Anonymous said...

Dear Anonymous #2,

If you use the Antoine equation (which I feel is the much harder way!), you will need to either convert all your values to SI, or find a calculator online via google that will convert the Antoine constants to the English system (or has the constants in that unit system). To solve it without Antoine, see the previous postings in this blog.