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

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

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

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

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

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