ME200 Division 5, Spring 2014 Review Concepts and Important Points for Exam 2

Important points from lecture

1. You must use absolute temperature (K or R) and absolute pressure with the ideal gas equation

Pv RT= .

2. /R R MW= , where R is the universal gas constant (8.314 kJ/kmol-K in SI units) and MW is the molecular weight of the gas in kg/kmol.

3. Do not use bar as a pressure unit in Pv RT= or in ( )@ @ @f T f T sat Th h v P P + . Convert bar to kPa.

4. Be very careful about units when using Pv RT= with English units. 5. 1 kJ/kg = 1000 m

2/s

2. Use this to make KE and PE units consistent with units of enthalpy from

the tables.

6. When finding enthalpy or internal energy of an ideal gas, you have 2 main options:

a. Use the ideal gas tables, and find h or u as a function of temperature. If you find a

change in h or u, such as h2 h1, using the tables, you have accounted for variable

specific heat.

b. Use the ideal gas relations

2

1

2 1

T

p

T

h h c dT = and 2

1

2 1

T

v

T

u u c dT = . Normally when we

take this approach, we do not know specific heat as a function of temperature. We

therefore simplify the integrals as 2 1 2 1( )ph h c T T = and 2 1 2 1( )vu u c T T = by

assuming constant specific heat.

New Assumptions for Exam 2

1. Steady state, steady flow (SSSF). Unless specifically noted, we will always assume this for an

open system.

2. behaves as an ideal gas. If you have information to check this

assumption, do it! How do you check? Do one of the following:

a. Find the compressibility factor, Z, using the Generalized Compressibility Chart. You will

need to calculate two of the following to use the chart:

R

crit

PP

P= , R

crit

TT

T= , R

crit crit

vv

RT P =

*Must use absolute temperature and absolute pressure!*

i. If Z 1.0 (0.95 < Z < 1.05), you may assume ideal gas.

ii. Otherwise, use Pv = ZRT where Z is the value you found.

b. Check the following conditions:

0.1RP < OR ( 2.0RT > AND 1.0RP

ME200 Division 5, Spring 2014 Review Concepts and Important Points for Exam 2

3. 1-dimensional flow at measured state points. We always assume this in order to use our

forms of the basic equations for open systems.

4. Open system. If mass crosses the boundary of the system, it is clearly an open system.

Things to Practice/Review

1. Know how to draw Pv and Tv diagrams by hand. Your drawings will be qualitative, but the

relative positions of lines and points should be correct. You have the skills and knowledge to do

this.

2. Review general concepts of cycles and cycle efficiency from lecture 6. We did not revisit them in

class.

Exam-taking Tips

1. If state points are not given in the sketch, define your own to keep your work organized.

2. If values of properties at certain states are given in the problem statement but are not shown on

the sketch, add them to the sketch.

3. Watch out for the hfg column in the saturated properties tables. The columns are hf, hfg, hg.

Many people accidentally write the value of hfg instead of hg.