Lecture 3 8-26-11 - 1 Problem Solving Units and Dimensions...

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Unformatted text preview: 1 Problem Solving Units and Dimensions Unit Conversions Difference Between Weight and Mass Gravitational conversion g c Dimensional Consistency 1. Read and understand the problem 2. Draw a Sketch and Specify the System Boundary 3. Place labels (symbols, numbers, units) on the diagram 4. Obtain any data missing data. 5. Determine the number of unknown variables. 6. Determine the number of independent equations and carry out a degrees of freedom analysis. 7. Write the equations to be solved in terms of the known and unknown variables. 8. Solve the equations and calculate the quantities asked for in the problem. 9. Check your answer(s) Example: How many months have 30 days? January 31 July 31 February 28 August 31 March 31 September 30 April 30 October 31 May 31 November 30 June 30 December 31 1. Read and understand the problem You must develop an understanding of the problem, before you are ready to solve it. This means that you read the problem carefully so that you know what is given and what is to be accomplished. Understand the problem and the goals Himmelblau, Basic principles and calculations in chemical engineering. Prentice Hall, 1996. 2. Draw a Sketch of the Process and Specify the System Boundary It is always good practice to begin solving a problem by drawing a sketch of the process or physical system. Example: A high jumper clears 10 ft 3 inches. If he weighs 200 lbs. What is his potential energy ((N-m) at maximum height? 2 3. Place labels (symbols, numbers, units) on the diagram for all of the known variables. For the unknown variables insert symbols and units. Add any other useful relations or information. By putting data on the diagram you will avoid having to look back at the problem statement repeatedly. You will be able to clarify what data are missing. Mass = 200 lbs PE=? 10 ft 3 in. When you review a problem, you may immediately notice that some essential detail, such as a physical property (MW,Density, etc.) is missing in the problem statement. 4. Obtain any data you need to solve the problem, but are missing. Use Simulation Software Look up Physical Properties Estimate a value! Example: An evaporator costs $34,700. How much did it cost per pound? How do you pronounce the name of the capital of Kentucky: Loo-EE-ville or Loo-ISS-ville? 5. Determine the Number of Variables whose values are unknown (the unknowns) If you put symbols on the process diagram or make a list of them, determining the number of unknowns is easy....
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Lecture 3 8-26-11 - 1 Problem Solving Units and Dimensions...

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