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by: Hudson Weimann Jr.


Hudson Weimann Jr.

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Class Notes
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This 8 page Class Notes was uploaded by Hudson Weimann Jr. on Friday October 30, 2015. The Class Notes belongs to GEEN 1400 at University of Colorado at Boulder taught by Staff in Fall. Since its upload, it has received 25 views. For similar materials see /class/232200/geen-1400-university-of-colorado-at-boulder in General Engineering at University of Colorado at Boulder.




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Date Created: 10/30/15
The Design Loop INTRODUCTION Obtaining a viable solution to an engineering problem requires thought and action coordinated in a speci c iterative pattern The method enables engineers to evaluate and refine potential solu tions Each step de nes a speci c action that propels the project forward to the next step This pro cess is shown in Figure 31 Recognize De ne the p the need Evaluate prObleIIl test results Plan the project Test Gather Build the information 7 prototype Generate alternative Select the concepts Evaluate the most promising alternatives concept Analysis rlgure 3 I acnemauc UT me cleslgn process The process begins when a design engineer recognizes the need for a solution to a problem A brief examination of the problem leads to defining and determining the requirements of the project Although the full scope of the solution is still undefined the design team begins to plan the project by Introductory Engineering Design Chapter 3 The Design Loop estimating how much time each stage of the project should take The team then gathers information to refine the objectives of the project Throughout these first stages of the design process ideas for potential solutions representing alternative concepts will be generated These concepts must be analyzed so that an informed deci sion selecting the most promising concept can be made These important analyses take the process out of the realm of pure trial and error into an engineered solution Next a prototype or first model is constructed The prototype is then tested to see if it fulfills the requirements The outcome of the test is evaluated to determine whether modifications are needed or if new problems have been created If so the loop is started over iterated If not the original problem has been solved This is an idealized model of the design process In reality events rarely proceed so smoothly Instead stages occur simultaneously shortcuts around the loop are often taken and sometimes the process goes in reverse The following example illustrates some possibilities the column on the right identifies the design process stages as they occur in the example EXAMPLE DESIGN OF A CUP HEATER Suppose that your small design team at Sunbeam Problem statement Inc was given the following task Design a better cup heater to heat 250 ml of tap water to boiling in 2 minutes First notice how simple yet general the problem Problem definition statement is There are no guidelines for the size or type of heater except that it must fit inside a cup Also the shape of the heater is unclear Your team also asks the questions To what is the heater compared Better than what Your supervisor says that the heater should per form better than a microwave oven and plug into an elec tric wall socket Then your supervisor mentions that you have only one week to complete the design One week Your team quickly realizes that the pre Planning liminary design must be completed today and tested and refined tomorrow These first observations and questions are written in Information gathering your design journal but they remain unanswered What is the next step in assessing the problem A quick look in a physics textbook helps to further define the problem Your team finds that it is possible to generate heat by passing a current through a wire Introductory Engineering Design Chapter 3 The Design Loop Now the ideas start to flow One team member remembers that her mother uses a small electric heater to heat cups of water You remark that electric space heaters basically are made of long sections of thick wire too A suggestion is made to purchase a tea cup heater Many team members speak at once Someone suggests obvi ously injest building a pocketsized nuclear weapon Another idea is to use the chemical packet that warms up feet in ski boots All of these ideas are documented in your journal Your supervisor then requests that the heater be con structed from specialty wire located in the warehouse You wonder what is so special about this particular wire You wonder if the supervisor will continue to add more parame ters Since heating wire is now required your team decides to evaluate that conceptfurther A sketch is drawn in your design journal The potential setup looks some thing like the drawing in Figure 32 Plug Thermometer Figure 32 Design journal sketch A parts list prepared from the sketch includes wire beaker water and thermometer Two team members state that they are ready to build the heater but are they Some members of the team want to build the heater immediately but you say that the team needs more infor mation before beginning Another member returns from Brainstorming generating alternative concepts Additional problem definition Selection Information gathering Introductory Engineering Design Chapter 3 The Design Loop the warehouse with a spool of the specialty wire The label states that the material is NiCr that the wire diameter is 22 gauge and that the resistance is 16 2 per foot One team member notes the thinness of the wire Another team member says that the symbol stands for the word ohm How does the value of 16 2 per foot relate to the heating of water Your team realizes that it must perform an engineering analysis of the problem to understand the relationship between the heating wire and the temperature of the water Together you discuss how a heater performs Current flowing through the wire causes the wire to heat The heat is transferred to the water Your team estimates that the temperature of the water must be raised a maximum of 100 C if the temperature comes out of the tap at close to 0 C as a worst case scenario How much heat is required to perform this feat What equations apply to heating water Someone digs out a firstyear chemistry book and the team finds that the heat absorbed by a vol ume of water is related to its heat capacity according to the following energy equation Q Cp AT where Q is the energy or heat absorbed by the water in watts and AT T2 T1 the difference between the beginning and ending water temperatures The reference tables in the back of the chemistry book give a value for Cp of water as approximately 1 call cc C Joule Q lcal loooci00Cl 41868J0ules1Waitesec C ml cal 50C Waffsi sec 41869 Q ml This is the amount of energy needed to heat one ml of water 100 C Heating a cup of water 250 ml in 2 minutes 41868250m1 m willtake 873Watts 2min60amp mm J Information gathering cont d Engineering analysis Introductory Engineering Design Chapter 3 The Design Loop This value Q if 873Watts is the energy per unit time or power required At this point you perform a reality check If this were a light bulb it would be pretty bright Some hair dryers are only 800 watts Some micro waves are around 1000 watts but it does take a lot of energy to heat water Your next move is to determine how much wire is needed The physics text states that for any material con ducting electricity Ohm s Law is given by the following for mula V IR where V is volts l is current in amps and R is resistance in ohms You also read that the power P dissipated by the wire is given by this formula P 12R IV Now by using the above power equation your team finds the current that is carried by the wire The heater is to be plugged into a socket supplying common household electricity that has a voltage of 110 volts From the power equation the following expression is obtained for l I I 873Watts 8am S V 110V P The required wire resistance is calculated from Ohm s Law 110V 7K R71 8amps 14g Now the length of the wire can be estimated The length is found from the resistance characteristic stated on the label or 16 2 per foot and the required resistance value for the heater or 14 Q Length 14 2 16 2 per foot 875 feet This seems like a reasonable amount to coil up in a coffee cup since the wire is so thin Your team decides that the analysis supports the orig inal design concept shown in Figure 31 and chooses to build it The team gathers in the lab and locates a glass beaker an alcohol thermometer a pair of wire ships and an unwired electrical plug One team member suggests using a 10amp fuse just in case The wire is wound Engineering analysis cont d Introductory Engineering Design Chapter 3 The Design Loop neatly around a 1inch rod and he ends of the wire are attached to the plug and the fuse After filling the beaker with water the coil of wire is submerged You are ready to record the amount of time to heat the water while another team member prepares to record the temperature rise and a third snaps a photograph of the experimental setup The plug is placed into the outlet Everyone watches as the plug smokes and they hear the snap of the fuse What happened to heater Why has it shorted out The team evaluates the test procedure The bare wire was placed in the beaker of water The fuse is attached to one end of the wire The other wire end and the fuse end were connected to the plug A team member comments Doesn t water conduct electricity All of you simultaneously recognize that the water created a short circuit around the wire because the wire was not insulated from the water Your team decides to modify the configuration so that the wire is electrically insulated from the water Scouting around the supplies in the warehouse the team notices thin small diametertubing called shrink tubing The tubing slides over uninsulated wire A heat gun is used to shrink the tubing until a snug fit is obtained You apply the shrink tubing and then start the test again After four minutes the temperature of the water reaches 50 C After eight minutes the temperature is only 90 C It takes 11 minutes to heat the water to boiling The team evaluates the data by plotting the tempera ture rise as a function of time see Figure 33 Why does it take so long to heat the water to boiling One team member muses on the possibility of the insulation affecting the rate of heat dissipating from the wire You suggest that the plastic tubing acts as electrical and thermal insulation The team decides that only electri cal insulation is necessary How can the design be changed to obtain the correct amount of heat transfer Engineering analysis cont d Testing Evaluation of test results Problem redefinition Concept selection Prototype fabrication Testing Evaluation of test results Introductory Engineering Design Chapter 3 The Design Loop 100 2 93 E U 5 50 Q E 9 0 0 5 10 15 Time min Figure 33 Plot of temperature vs time One person proposes that thinner shrinktubing be Generating alternative found Another team member suggests using a longer concepts wire How much longer should the wire be Two team members suggest using at least four times as long while another says it should be six times as long After much discussion yourteam decides to try using Engineering analysis more analysis to determine an appropriate length for the wire You reviewthe equations and realize that you need a heat transfer model to take the thermal insulating effect of the shrinktubing into account Conclusion What do you thinkwill happen in the next iteration of the design loop The heat transfer analysis could lead to a new prototype with a much longer length of wire Testing could showthe same tem perature response Additional information gathering could reveal that a thick rod of NiCr could better heat the water Additional brainstorming could lead the design team at Sunbeam Inc to a completely different design Many possibilities exist most requiring substantial planning problem definition engineering analysis and testing to justify advancing a particular design choice The preceding example demonstrates the iterative process that a typical engineering design solution can follow In this course your design team will find that many solution pathways can exist Practicing the steps in the design loop will help determine the optimal solution for your design prob lem Introductory Engineering Design Chapter 3 The Design Loop Introductory Engineering Design


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