1. ChemE 260 1st Law of Thermodynamics, Problem Solving Procedure
Dr. William Baratuci
Senior Lecturer
Chemical Engineering Department
University of Washington
TCD 4: C & D CB 4: 1 & 2 + Supplement
April 12, 2005

2. 1st Law of Thermodynamics
Definition
Energy is neither created nor destroyed.
General Conservation Principle
1st Law, Integral Form
1st Law Differential Form
This statement of the 1st Law applies to systems that do not undergo nuclear reactions.
Many quantities are conserved.
The integral form of the 1st Law will help you analyze a process that takes place between two given times.
These are often BATCH processes.
The differential form of the 1st Law will help you analyze continuous processes.
This form is sometimes called the RATE form of the 1st Law because the terms are all rates.
The heat transfer rate, the rate at which work is done, the rate at which the energy of the system changes.
Baratuci
ChemE 260
April 12, 2005

3. Special Cases Special Case: No change in Ekin or Epot
Ekin = 0 and Epot = 0
and
In many problems changes in potential energy are negligible
Equipment isn’t usually so LARGE that changes in potential energy are significant
Changes in kinetic energy are often negligible when …
Pipe diameters do not change and
No phase change occurs
We need velocity data or flow rate and pipe diameter to calculate Ekin
When we don’t have this data, we usually assume changes in Ekin are negligible.
Baratuci
ChemE 260
April 12, 2005

4. Total Work and Heat Work : Heat :
There are MANY different types of work, but only one type of heat
There are three mechanisms for heat transfer
Conduction: Fourier’s Law
Convection: Newton’s Law of Cooling
Radiation: The Radiation Equation
Ws = shaft work
This is very important. It is work that involves rotating machinery and is the most common form of work in continuous processes
Baratuci
ChemE 260
April 12, 2005

5. Problem Solving Procedure
Read Carefully
Draw a Diagram
List Given Information
List All Assumptions
Write Equations and Lookup Data
Solve Equations
Verify Assumptions
Answer Questions
The problem solving procedure is a strategy to help organize your thoughts and your work so you can avoid mistakes and communicate effectively with other engineers and scientists.
Read
Get all the important info out of the problem statement and onto your paper.
Draw a Diagram
Draw a process schematic
Draw the system boundary
Draw a phase diagram
List All Given
List all given values
Assign variable names, including units
List objectives (what are you trying to find)
List All Assumptions
State your sign convention
You will add to this list as you work through the problem
Write Equations and Lookup Data
Work backwards from the answer you are looking for.
Count the unknown variables and the INDEPENDENT equations
When # of equations = # of unknowns, the problem is well-posed and you can solve it !
Solve Equations
This usually turns out to be the EASY part !
Repeat the last two steps until you have gotten all the answers you need.
Verify Assumptions
In this course, the main assumption you may have to verify is that a gas is ideal.
Answer Questions
Hard to believe, but students routinely forget to answer the question they were asked !
INTERPRET the results, what do they MEAN.
Baratuci
ChemE 260
April 12, 2005

6. Next Class … Special Types of Processes Thermodynamic Cycles Isobaric
Isochoric
Isothermal
Adiabatic
Thermodynamic Cycles
Application of the 1st Law to Thermo Cycles
We have seen all of these types of processes before and we have seen cycles before as well
Now, we will apply the 1st Law to them !
Baratuci
ChemE 260
April 12, 2005