Final Report DL Spring 2009

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Solar Autoclave with Sterilization Indicator

ECE/MEE 432L, Design and Manufacturing ClinicApril 2009

Sponsor: ETHOS

Team Members:Staci Grey

Christopher McGuinnessRyan SmolikCorey Vossler

Kyle Zeller

Industry Mentor:Susan Kinne

Faculty Mentors:Margaret PinnellJohn Hageman

Don SchenkEric Lang

Phil Doepker


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NOTICE

This report represents the results of a student project at the

University of Dayton. Because of the matter of this project, thefaculty and student team members do not warrant or guaranteethe accuracy of the results nor that they are suitable for anyparticular purpose. The sponsor agrees that if ideas, concepts ordesigns from this project are implemented, the sponsor is solelyresponsible for the reliability, performance and safety of theconcepts and designs, and shall indemnify and hold harmless

the University of Dayton, employees, students and any otherrepresentatives from and against all claims, losses or damagesarising out of scope of this agreement.

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Table of Contents

Summary 71.1 Purpose 7

1.2 Results 71.3 Conclusions 71.4 Recommendations 7

Background and Research 82.1 Solar Autoclave 82.2 Pressure Release Valve 92.3 Sterilization Indicator 10

Specifications 113.1 Functional Requirements 113.2 Design Requirements 113.3 Design Criteria 11

3.4 Deliverables 12Procedure 13

4.1 Establish Need 134.2 Proposal and Specifications 134.3 Research 164.4 Conceptual Design 164.5 Embodiment Design 164.6 Initial Calculations and Testing 164.7 Engineering Calculations 164.8 Final Design 174.9 Prototype Fabrication 17

4.10 Prototype Testing/Data Analysis 174.11 Cost Estimation 174.12 Final Presentation 174.13 Final Report 17

Embodiment Design 185.1 Pressure Vessel 185.2 Outlet Valve 195.3 Sterilization Indicator 20

Calculations 216.1 Sterilization Indicator 21

6.1.1 Variables 21

6.1.2 Calculations and Equations 216.1.3 Analysis 22

6.2 Pressure Release Valve 226.3 Water Amount 22

Prototype Development and Testing 237.1 Pressure Vessel Testing 23

7.1.1 Temperature Testing 267.1.2 Air Pressure Testing 28

7.2 Pressure Release Valve 29

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7.2.1 End Cap as Regulator 297.2.2 Nut and Bolt Fixture as Regulator 317.2.3 Safety Valves 337.2.4 Check Valve 35

7.3 Sterilization Indicator 36

Results 388.1 Pressure Vessel 388.1.1 Pressure Seal Testing 388.1.2 Temperature Testing 388.1.3 Air Pressure Testing 38

8.2 Pressure Release Valve 398.2.1 Verification of Final Design 40

8.3 Sterilization Indicator 41Manufacturing Instructions & Nicaragua Test Plans 42

9.1 Manufacturing Procedure Pressure Vessel 429.1.1 Materials 42

9.1.2 Tools 429.1.3 Procedure 429.2 Manufacturing Procedure Sterilization Indicator 459.3 Nicaragua Test Plans 46

Cost Estimation 48Conclusions 50Recommendations 51References 52

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Appendix Table of Contents

Appendix A Design Project AgreementAppendix B Contact Information

Appendix C Project ProposalAppendix D Weekly Status ReportsAppendix E Final Gantt ChartAppendix F First Oral PresentationAppendix G Second Oral PresentationAppendix H Final Oral PresentationAppendix I Meeting MinutesAppendix J Individual Conceptual DesignsAppendix K Decision Analysis Sheets (Team)Appendix L Embodiment DesignsAppendix M Learn, Lead and Serve Grant Application

Appendix N Miscellaneous Calculations & MATLAB CodeAppendix O Miscellaneous Engineering DrawingsAppendix P Project ExpensesAppendix Q Purchase ReceiptsAppendix R Miscellaneous ResearchAppendix S Structural & Sealant Test DataAppendix T Oven Air Pressure Test DataAppendix U Project Description FormAppendix V Solar Autoclave Final ReportAppendix W Solar Autoclave Technical ReportAppendix X Design of a Sterilization Indicator

Appendix Y Rough Draft Graded John HagemanAppendix Z Rough Draft Graded Don Schenk

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List of Tables and Figures

Figure 4.1 Gantt ChartFigure 4.2 Gantt Chart (Continued)

Figure 5.1 Embodiment Design Assembled Pressure VesselFigure 5.2 Embodiment Design Deconstructed Pressure VesselFigure 5.3 Embodiment Design Outlet ValveFigure 5.4 Embodiment Design Sterilization IndicatorFigure 6.1 External Temperature as a Function of Indicator Height and DiameterFigure 7.1 Valve Assembly with Hydraulic Pump AttachedFigure 7.2 Strain Gages Side of CanFigure 7.3 Strain Gages Bottom of CanFigure 7.4 Pouring Hydraulic Oil into CanFigure 7.5 Hydraulic Oil Pressure Test SetupFigure 7.6 Bottom of Can and Hoop Stresses

Figure 7.7 Temperature Test Setup with Plastic TubingFigure 7.8 Melted Plastic TubingFigure 7.9 Temperature Test Setup with Copper TubingFigure 7.10 Air Pressure Testing with Plexiglas ShieldFigure 7.11 End CapFigure 7.12 End Cap Regulator Final Cap with PlateFigure 7.13 End Cap Regulator ConceptFigure 7.14 End Cap Regulator TestingFigure 7.15 Nut and Bolt 1/2"Figure 7.16 Nut and Bolt 1/4"Figure 7.17 Nut and Bolt Pressure Regulator System

Figure 7.18 Safety Valve Rubber PlugFigure 7.19 Safety Valve Rubber PlugFigure 7.20 Safety Valve Rubber PlugFigure 7.21 Purchased Check ValveFigure 7.22 Sterilization Indicator Initial Heat Transfer Verification TestFigure 7.23 Airless Sterilization Indicator ConceptFigure 7.24 Airless Indicator Post ExperimentFigure 7.25 Vented Sterilization Indicator ConceptFigure 8.1 Oven Testing of Solar AutoclaveFigure 9.1 Marking the Corner HolesFigure 9.2 Cap after Drilling Holes

Figure 9.3 Assembled Autoclave and Valve AssemblyFigure 9.4 Sterilization Indicator for Manufacturing Procedure

Table 8.1 Force (Weight) Required to Maintain Pressure for Fabricated Vent PipeTable 10.1 Manufacturing Cost Per Solar Autoclave

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1.0 Summary

1.1 Purpose

The primary objective of this project has been to develop a solar autoclave, or medical instrument sterilizer,with an integrated sterilization indicator to validate each process to be used in rural communities in Nicaraguaby local nurses. The objective for this semester was to continue the development of the open-ended cylindersolar autoclave design and to integrate a working sterilization indicator that will serve as a reliable post-process verification. The solar autoclave was to be taken from an initial design, reevaluated, improved,prototyped, tested, and turned into a final reliable design. The final design must consistently sterilize surgicalinstruments to the standards practiced, including holding a specified minimal temperature and pressure acrossa period of time. The incorporation of the sterilization indicator is a necessity as it provides a visual way forthe local nurses to validate each process.

1.2 Results

Research and testing performed in previous semesters allowed the team to continue in the current semester toimprove the open-ended cylinder design for the solar autoclave. Conceptualization from the previoussemester led the team to continue the phase-change material indicator design. Re-evaluating therecommendations left from last semester as well as developing new conceptual designs for both products ledto testing of the new ideas. The team performed cycles of testing, implementing improvements at each phase.At the conclusion of the testing the final design contained an aluminum end plate, a new sealant, a pressurerelease valve, and easy-to-access assembly. The final design of the sterilization indicator was vented in asimple way to ensure stability and to make it easier to use but was found to be unreliable as a final design.

1.3 Conclusions

By improving and adding components to the previous semesters activity, a final design of the solar autoclavesystem has been delivered. Necessary improvements to the system developed from the extensive testingperformed throughout the semester on all three main components. The end-opening pressure vessel has beenproved to be able to withstand the pressure loads as well as the high temperatures of the sterilization cycle.The sterilization indicator has been prototyped and tested to ensure accuracy. The pressure relief valve hasbeen designed and implemented into the final system. The final solar autoclave design meets the designrequirements and criteria. Engineering drawings have been made, testing is completed, and manufacturinginstructions and further testing procedures have been written for the final design.

1.4 Recommendations

The final design has been verified through extensive testing, but further investigation will involve testing theentire autoclave system to verify sterilization. The entire system including the solar cooker, the pressurevessel, sterilization indicator, and pressure release valve must be tested together and verified to work in theNicaraguan climate. The amount of water needs to be studied using various amounts to make sure that theoptimum level is being used. In addition, further testing must be conducted to verify endurance of the designacross its lifespan for safety and maintenance purposes. The financial cost of the entire system also needs tobe further considered to keep production costs at a minimum.

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2.0 Background and Research

The primary objective for this semester was to continue the development of the solar autoclave and integrate aworking sterilization indicator that will serve as a reliable post-process verification. In order to accomplish

this objective the team used the completed work of groups from the previous semesters, finalized their designsand integrated them together into working form. The team had recommendations provided from the previousteams to guide the testing and design process. As a result of extensive previous work on this project, thebackground and research section contains material compiled only from this semester. Previous team reportsand background information will be provided in the appendices. Specifically, this semester the research wasdivided into three sections: pressure vessel testing and improvements, pressure relief valve design, andsterilization indicator design.

2.1 Pressure Vessel Research

The majority of the research into the solar autoclave was conducted last semester. This information was

utilized along with additional research conducted on pressure cookers and autoclaves this semester. There isvery little research on solar autoclaves as this is a relatively new idea. A patent search was performed and itwas found that no patent exists for such a design. There is significant information on solar cookers available;however the information on solar cookers only pertained to temperatures of 100C. This lower temperature isall that is required to boil water, which is a main function of a solar cooker in third world countries.

A pressure cooker is a relatively inexpensive pan with a lid that is placed on top and locked into position. Amore traditional pressure cooker has two outlet valves: a safety valve and a pressure regulator. The safetyvalve usually consists of a rubber grommet or plug that will expel from the lid above a certain pressure, shouldthe regulator not be functioning properly. The pressure regulator allows the user to know that the desiredpressure has been reached, indicated by the weight jostling and the audible hissing sound. In more modernpressure cookers, there is no safety plug instead the gasket seal in the lid will blow out if the pressure exceedsa safe amount. The pressure regulator on more modern pressure cookers is a spring valve which acts as aregulator and a check valve, ensuring that pressure is maintained during operation and that no vacuum occursafter the cycle.

The end-opening pressure vessel design was given to the team for further development and testing. Afterinitial meetings and reading through the previous semesters report, the team developed conceptual designs andideas about how to improve the current design. Additional research was performed on potential seals, end capdesign, conducting pressure testing, and adhesives. After the team ran an initial hydraulic oil pressure testingof last semesters design, many design changes were considered. The initial test showed the designweaknesses and where the team should focus more attention. The ring-like seal was replaced with a fullycovered seal with a small hole drilled in the center that will be glued to the top end cap. The seal material wasalso changed from the butyl rubber seal recommended from the previous semester to a silicone pie pan seal.The butyl rubber seal was researched and found to emit hazardous gases during its initial heating cycle. Otherseals such as EPDM and a silicone baking sheet were also researched and tested through pressure tests. Theresults of these tests will be discussed in the Results section of this report.

The type of wood for the end caps from the previous team was Caribbean Pine. The wood was researched tobe locally available in Nicaragua without environmental concerns of deforestation that some third-worldcountries are experiencing. The top end cap proved to be unable to be sealed effectively in hydraulic pressure

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testing. The team then moved to a new design utilizing an aluminum top end cap in which pipe threads couldbe machined into a thin aluminum plate to prevent leakage. Research showed that at this Nicaragua location, ametal shop was available to support this machining. The aluminum plate adds significant cost to the systembut is necessary for the pressure vessel to contain the steam and control the pressure.

Additional research was performed to conduct effective pressure testing. Labview programs were written andstrain gage application and theory was researched to improve the knowledge base of the team and gatheraccurate data. Adhesives were also researched to glue the silicone pie pan to the bottom of the aluminum endcap. Contact cement was found to be ineffective at adhering to the surfaces and silicone based adhesives wereresearched to achieve results desired.

2.2 Pressure Release Valve Research

The pressure relief system that is required for a standard pressure cooker was researched in order to gainknowledge on the overall purpose and operation of a pressure release valve. It was discovered that there arevarious types of valves, as well as several operations of the valves. The three most common valve types

researched were: check/spring valves, weighted valves and ball valves. After thorough research of themechanical operations and the ease at which they could be duplicated, a weighted valve was chosen as thedesired pressure regulator valve. It was the one that could most readily be manufactured from off the shelfproducts and would be intuitive to operate. A check valve was researched, and it was determined that thesafest way to ensure a vacuum did not occur was to purchase a pre-fabricated check valve.

The main purpose of a pressure regulator valve is to act as a control valve and our system requires a controlled15psi of internal pressure. It is initially sealed, allowing no air to enter or escape the can, until the insidepressure reaches the force at which the weight will begin to move. The weight will begin to move and jostleallowing any excess pressure above the desired amount to escape. Should the pressure in the autoclave forsome unforeseen reason increase to a pressure much greater than 15psi, for example 25psi, the weight will liftoff of the outlet valve and release the pressure from the can emphatically.

A third purpose of outlet valves is safety. In most pressure cookers and autoclaves there is a safety valvewhich will blow out, allowing for an instant release of pressure. This particular type of valve, while veryimportant in the operation of a pressure cooker was believed to be impractical for this specific application.The seal of the solar autoclave is an inherent safety valve, in that the seal will break when the pressure exceeds30psi. This type of seal is common in today's pressure cookers and research suggests that it is replacing theolder blow out grommet safety valve. A secondary purpose of the regulator is to act as a safety valve that willblow off the weight for pressures that exceed 25psi.

Research was conducted on spring valves and their operations. However, due to the complexity of the springand determining the spring force, it was concluded that it would not be practical for the Nicaragua application.It is not known conclusively if any of the local people would be able to perform the calculations, nor wherethey could purchase materials to construct the spring valve. Consideration was given to importing the part infrom the United States. However, further research suggested given that cost is one of the main factors in thisproject that the part would be too expensive.

The most productive research came from the weekly trips to Lowes and Home Depot and speaking with theiremployees. The experts in the Design Clinic also provided insight into the operation of the various valves andassisted with the construction in a few of the iterations.

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2.3 Sterilization Indicator Research

Work completed in the previous semester, including background research, the conceptualization of variousideas, and early prototyping of a phase change material design were the starting point for the work done thissemester. The recommendations provided at the end of the semesters worth of work identified the key areas

the team needed to pursue while working toward the final design of the sterilization indicator. After a phasechange material design was chosen last semester, research for the sterilization indicator has primarily revolvedaround phase change materials. Research information that has been used this semester includes meltingtemperatures of different phase change materials, material properties of different types of materials such asacrylic and lexan, and the bonding strengths and properties of glue.

Acrylic was chosen as the material used to manufacture the hourglass because in comparison to lexan it ismore transparent, it has a higher temperature tolerance, and it has a slightly better thermal conductivity. Also,Polywax2000 was chosen as the phase change material because its melting temperature is theoretically statedto be 121C. The advantage of a phase change material is its ability to quickly change from liquid to solidover a small temperature change. Lastly, IPS Weld-On adhesive was chosen as the glue since it is designed to

be able to hold together acrylic surfaces under high temperatures given at least 24 hours of curing time.

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3.0 Specifications

3.1 Functional Requirements

The functional requirement for this project is to design and develop a turn-key solar autoclave system tosterilize medical instruments using only solar energy as an energy source. The system will include the solarcooker, pressure vessel, pressure relief valve, and sterilization indicator.

3.2 Design Requirements

The autoclave must:

Satisfy the required conditions to destroy bacillus subtillis spores.

Require no fuels or electricity to operate and must be easily constructed using commonly availabletools and processes known to the locals (such as using hammers, drills, etc).

The pressure vessel must:

Be limited to 65.3cm x 64.3cm x 18.7cm; however, must be large enough to accommodate the traysthat the medical instruments are place on, as well as the sterilization indicator and the largerinstruments.

The sterilization indicator must:

Be able to verify that the sterilization process was or was not successful and is complete.

Not indicate any false positives for any thermal cycle that lasts less than 10 minutes and also must notinterfere with the operation of the solar autoclave or solar cooker.

Fit inside the solar autoclave, while leaving room for the instruments.

Be environmentally and biologically safe to handle without protection.

Cost less than $10.00 per unit if it is reusable or less than $0.01 per unit and constructible on-site ifnon-reusable.

Conform to Nicaraguan importation laws if non-reusable.All project tasks must:

Be constructed using materials locally available in Nicaragua, except the sterilization indicator.

To try to meet minimize cost, the overall cost of parts must be minimized as long as safety is notcompromised.

3.3 Design Criteria

Using the Innovation and Design Clinic manual as a guide, we chose ten criteria that we felt were the mostimportant consideration in our design of the sterilization indicator.

Customer: As with any new design, the first concern is the customer.

Environment: Neither the autoclave nor the indicator must be environmentally and biologically(unsafe?) safe for the user and the constructor.

Safety: Due to the nature of the sterilization process, warning labels are required to be placed on thesolar autoclave to warn of the potential hazards involving steam and high temperature.

Ease of Use: Also, for the autoclave and the indicator, simplicity is the key. Both should be intuitivelyeasy to use.

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Maintenance: The construction and repairs of the solar autoclave should be uncomplicated. Theindicator may not be easy to repair if constructed of material not available locally in Nicaragua;however, it should be intuitively easy to use.

3.4 Deliverables

Throughout this project there have been administrative, engineering and business deliverables that wererequired.

Administrative Deliverables:

Project Proposal: The project proposal was one of the first administrative deliverables to be completed.The project proposal was submitted to Dr. Margaret Pinnell and provided the project's functional anddesign requirements, deliverables and the Gantt Chart.

Weekly Status Reports: Each week, weekly status reports provided updates showing the week'sprogress, group activities from the week and immediate future project plans.

Oral Presentation: After the generation of conceptual designs for the pressure vessel, an oralpresentation was given, allowing the sponsors, mentors, group members and design clinic faculty to

provide feedback on the prospective designs. Interim Briefing: During the middle of the semester, there was an interim briefing with the primary

mentor and design clinic faculty.

Final Report and Oral Presentation: The final administrative deliverable is the final report and finaloral presentation, which are to be completed by the end of the semester.

Engineering Deliverables:

Analyze Recommendations: The first engineering deliverable was to weigh the recommendations fromprevious project efforts and determine what project items need revised.

Improve the Pressure Vessel Design: After analyzing the previous group's recommendations, makechanges to the pressure vessel design.

Determine the Autoclaves Structural Integrity: Through pressure testing with strain gages, thestructural limitations of the autoclave is determined.

Determine the Indicator's Structural Integrity: Through temperature testing with the autoclave and theoven, the indicator's durability in high-temperature environments is determined.

Verify System Performance: Once complete, the systems sterilization capabilities are verifyied usingbacillus subtillis cultures.

Create Final Drawings, Assembly Instructions, and Parts List: Upon verification of the systemsperformance, final drawings, assembly instructions, and parts lists are created.

Detailed Cost Estimate: A final deliverable is creating a detailed cost estimate for materials andconstruction per autoclave.

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4.0 Procedure

This section illustrates how the design process was performed by the ETHOS team following the ProductRealization Process (PRP) from the Design Manual. The Gantt chart in Figure 4.1, continued onto Figure 4.2

and also included in Appendix C, illustrates the timeline followed. The black bars indicate the estimatedduration of each activity, and the yellow bars indicate the actual duration of activity. It can be seen that whilemost tasks were completed within the intended timeframe, some activities ran over due to complexity andissues that arose during prototyping and feasibility testing.

4.1 Establish Need

The need for the solar autoclave was initially established during ETHOS trips in the summers of 2006 and2007. Interviews conducted among healthcare facilities throughout Nicaragua indicated that the need for asolar autoclave was the most urgent in small rural clinics without electricity. Of these there is nearly athousand. The need for the sterilization indicator was brought to the attention of the original solar autoclave

group during a presentation to the Ministry of Health in Nicaragua. The need for an indicator to verifysterilization is a necessary component to the overall system of the solar autoclave. During this semester, thesolar autoclave with sterilization indicator is to be completed and sent to Nicaragua for further testing.

4.2 Proposal and Specifications

The proposal and specifications from the solar autoclave project in the fall of 2008, MEE-432L, and thesterilization indicator project also in the fall of 2008, ECE/MEE-431L, were modified to incorporate the newdiscoveries and decisions that had been made through the course of the semester. The modifications weremostly an affirmation of which designs would be pursued, as well as an endeavor to incorporate the twoindividual projects, solar autoclave and sterilization indicator, into one more definitive project. The team metwith the sponsors at the start of the semester in order to begin making the necessary modifications to the solarautoclave design from the previous semester and to make various decisions about how to complete the projectwithin the given time frame. The sterilization indicator was only modified slightly in design, by incorporatinga vent hole.

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ETHOS Gantt Chart

Task (Person)

Functional Requiremen

Identify Project & Tea

Develop Questions for Sp

Meet Sponsor

Establish Need (ALL

Figure 4.1 - Gantt Chart

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4.3 Research

The research findings, combined with the previous work by both the solar autoclave team and the sterilizationindicator team, allowed the team to focus on completing the pressure vessel and the sterilization indicator inorder to have a complete solar autoclave unit for testing this summer in Nicaragua. In order to further develop

the pressure vessel design, research was conducted on pressure vessels, valve types and operations, pressurerelief devices, abundance and availability of materials needed in the construction of the solar autoclave andsterilization methods.

4.4 Conceptual Design and Decision Analysis

Six conceptual designs were drawn, four for the pressure vessel and two for the sterilization indicator. Thedesigns incorporated the teams prior research, previous developments of the prior semester's team andconversations with the sponsors and advisors, including Lori Hanna and Dan Hensel from the previousautoclave projects. The team then came together and went through a decision analysis in order to determinethe final design for both portions of the project. It was found that no one pressure vessel design was superior

and instead the group as a whole came up with a final design that integrated improvements that were felt tohave the most positive impact on improving the pressure vessel.

4.5 Embodiment Design

With the completion of the decision analysis and a final design drawn up by the group, the team split intothree separate entities to concentrate on the three pressing areas of the project: sterilization indicator, pressurevessel improvements and testing. As testing began on the original pressure vessel, improvements were madealong the way that had not been considered during the design stage, such as trading the threaded rods in for 10inch bolts and replacing one wood end cap for an aluminum end block.

4.6 Initial Calculations and Testing

Initial calculations were performed on the original can design to determine feasibility. Even though it hadpreviously been determined that the can would be able to withstand a pressure greater than the ASMEstandards of 45 psi, the sponsors were skeptical. Therefore, a pressure test was conducted on utilizinghydraulic oil. During this phase, the testing team took the opportunity to begin to improve the pressure vessel.The pressure vessel improvement team supplied various sealants that were researched and determined to beadequate for the functions required for the solar autoclave. These various sealants were tested during thehydraulic oil pressure tests.4.7 Engineering Calculations

The maximum stresses on the can were determined during the previous group's work. Through initialfeasibility testing, these stresses were experimentally measured and tabulated. Calculations were alsoperformed for the pressure outlet valve to determine the theoretical weight required to seal in pressure andregulate the system. For the sterilization indicator, calculations were done to determine the thickness of thehour glass container, the amount of material, the size of the aperture and the slope of the sides of the container.

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4.8 Final Design

Based on feedback from the sponsors and advisors during an oral presentation, the team continued with thefeasibility testing of the can and began to look at alternative methods for the sterilization indicator. As well, itwas determined at this time that a check valve would be required in addition to the pressure regulator valve in

order to prevent a vacuum from forming in the can. The outlet team gathered parts from Lowes and HomeDepot and began to make various prototypes for testing. The sterilization indicator team began the new designof a bi-metallic strip indicator with a solar photocell as the source of power. The testing team began to domore strain gauge testing and oven temperature testing. Individual components of the solar autoclave wereimproved upon based on research conducted, deliberations among the team and through the feedback of thesponsors and advisors.

4.9 Prototype Fabrication

The prototype pressure vessel from the previous semester was redesigned throughout the entire process.Design iterations were made as new information was discovered through testing and conversations with

advisors. Each improvement was tested as the fabrication process went in order to determine its effectivenessin the overall system. If an improvement was found to be faulty or less than desirable, the team would resortback to the previous design that was functional.

4.10 Prototype Testing/Data Analysis

The Final Design Prototype, an end-opening cylinder design made with a coffee can, one wooden end block,one aluminum end block and 10 inch bolts, was tested. Strain data was taken and visual observations weremade in order to confirm the strength of the design under pressure. The prototype outlet valves were eachthoroughly tested with compressed air in the design lab. Improvements were made during the testing to try andovercome any complications that arose. Prototype sterilization indicators were fabricated and tested in anavailable electric autoclave to determine their functionality.

4.11 Cost Estimation

Using the direct costs of the materials purchased for the prototypes and improvements, as well as productioncosts estimations gathered from the previous work completed in Nicaragua, a cost estimate was developed.This gives the estimated cost per unit for the solar autoclave and the sterilization indicator.

4.12. Final Presentation

The final presentation was developed with the cooperation of the entire team. Each team member wasresponsible for certain portions, given his or her roles throughout the process. The presentation wascompleted with the objective to convey all the information that had been gathered over the semester as well asto exhibit the final solar autoclave design unit with improved and completed pressure vessel and verifiedsterilization indicator.

4.13 Final Report

The final report is an accumulation of the work conducted this semester. It will include the background,research, embodiment designs, tests and results as well as recommendations and conclusions.

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5.0 Embodiment Design

After the decision analysis and brainstorming session, refer to section 4.4, the group decided on a final designthat incorporated elements believed to better enhance the functionality of the solar autoclave. The sterilization

indicator design was already in place from last semester's work, as a precaution it was determined that asecond back-up indicator should be designed.

5.1 Pressure Vessel

The original solar autoclave design consisted of a 34.5 ounce coffee can measuring 7" tall and 6" in diameter.The can is open on one end. Both ends of the can were capped with 8"x8"x2" pine end blocks. The caps wereheld in place with 12" threaded rods and wing nuts. The sealant utilized prior to this semester was butylrubber. From the team's final conceptual design and the brainstorming session, we incorporated newmodifications to the existing solar autoclave design from last semester. It was determined that the wood endblock on the open end of the coffee can would be replaced with an aluminum block, 8"x8"x1/2", to prevent the

seeping of moisture into the wood. The 12" threaded rods were replaced with 10 inch hex bolts, allowing forthe autoclave to have only stationary end block (the wood end block on the closed end). Various sealants weretested in order to determine the best possible seal for the pressure vessel. The outlet valve was researched andseveral prototypes were constructed for testing. It was determined that the overall dimensions of the pressurevessel would be no larger than originally specified. The current design dimensions are 8"x8"18". Figure 5.1shows the pressure vessel complete and Figure 5.2 shows a blown apart image of the pressure vessel and thecomponents.

Figure 5.1

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Figure 5.2

5.2 Outlet Valve

The outlet valve consists of 1/2" copper pipe, a 1/2" T-fitting, a 1/2" male couple, a 3/4" to 1/2" malereduction couple, a 3/4" check valve, a vent pipe from a pressure cooker with a weighted regulator, and a 1/2"

copper end cap. A 1/4" hole is drilled into the top of the 1/2" end cap and the vent pipe is securely fastenedthrough the hole. Three pieces of copper pipe are cut and affixed to the three T-bracket openings. The checkvalve and the reduction couple are screwed together with pipe tape. The right side of the T-bracket is solderedto the reduction couple. The copper end cap with the vent pipe is soldered to the top of the T-bracket and the1/2" male couple is soldered to the left side of the T-bracket. Figure 5.3 shows the complete outlet valve.

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Weighted Regulator

Check Valve

Copper T-fittingMale copperconnector

Copper Pipe Vent PipeCopper End Cap

Male copperconnector


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Figure 5.3

5.3 Sterilization Indicator

For the sterilization indicator, the physical design is focused on how it can passively indicate whether or notconditions for sterilization have been met. To avoid pressurization of the indicators interior, end caps withholes were created to allow pressure to escape. Inorder to prevent leaking, a plug cap was designedthat is removable such that both ends of theindicator can be used. Also, the inside of theindicator is sloped to help the liquid wax flow tothe bottom. The slope is very shallow; however, itis steep enough to facilitate movement.

Intended to be a reliable and sustainable indicator,the phase-change material hourglass has anundeniable appeal. Its simplicity in operationmakes it an attractive solution to the sterilizationindication problem. However, it became clear thatthe hourglass approach has its faults and some ofthese faults may be fatal for its use in thisapplication. In order to prepare for this possiblepitfall, a second embodiment design was createdfor an electronic sterilization indicator.

Figure 5.4

This electronic indicator was originally proposed during the first semester of sterilization indicator design butwas forgone for the hourglass design. Similar to the passive material-based hourglass, the electronic indicator

passively determines the temperature and, once the temperature exceeds 121Celsius, a timer is activated.After the timer reaches 15 minutes, a light is activated to indicate that the conditions have been met. Itstemperature sensor is a PTC thermistor that would need to be inserted into the pressure vessel through a sealedhole. A thermistor functions as a resistor that correspondingly changes its resistance with temperature. As thetemperature increases, for a PTC thermistor, the resistance increases.

Aside from the thermistor, the rest of the electronics need to be outside of the pressure vessel such that thepower source, a small photovoltaic cell array, can be powered. Similar to the cell arrays found on solarcalculators, some cell arrays can provide 5V and upwards of 100mA while being reasonably sized. Once thecircuit is connected and organized on a protoboard, it can be mounted onto one of the sides of the pressurevessel such that it will be easily seen.

The electronic indicator's theory of operation is very simple. Once the temperature reaches 121Celsius, thethermistor's resistance will increase to a predicted resistance. Since the thermistor is in a voltage divider forbiasing a transistor, the voltage at the base of the transistor will increase until the transistor is activated.Voltage at the emitter will be seen by a CN555 timer's enable pin. With voltage at the enable pin, the timerwill begin to create pulses that have a period of 3.5 seconds. The output of the timer connects to the trigger ofa counter and, once started, the counter will count up 256, which is 10000000. Once this value is reached, anLED will be attached to the 28 pin and will be activated. Thus, the LED will be activated once the 15 minutesat 121Celsius has been reached.

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6.0 Calculations


Calculations are performed to attempt to predict how long it will take for the material to melt under a given set

of circumstances. These calculations are shown and explained in this section.

6.1.1 Variables

t = Thickness; T1=Temperature of outside plastic; Tw=Temperature of Inner Surface; tm=time interval;hsf= latent heat of fusion; p=density; V=volume; k=thermal conductivity of hourglass, A=Surface area.

6.1.2 Calculations and Equations

(6.1)

(6.2)

(6.3)

(6.4)

(6.5)

(6.6)

12

34

56

0.51

1.52

2.53

115

120

125

130

135

ZA

xis:RequiredTemperature(C)

External Temperature as a function of Indicator Height and Diameter

X Axis: Indicator Height (cm)Y Axis: Indicator Diameter (cm)

Figure 6.1 - External Temperature as a Function of Indicator Height and Diameter

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As can be seen in Figure 6.1, a range of different heights and diameters will melt the necessary amount ofPolywax 2000 in 15 minutes at 121 Celsius. By inspection, the plot is very intuitive; in order to have aconstant melting temperature over various dimensions (each contour), one dimension will have to decrease asthe other increases. That is why the height decreases as the diameter increases and vice-versa. Since thehourglasss phase and encasement material has been selected, these are the only remaining variables.

Once the plot was created, it was used to determine the dimensions of the initial prototypes.

6.1. 3 Analysis

From Equation 1, the melting time can be calculated through the ratio of the product of the waxs mass, latentheat of fusion, and the thickness of the hourglass over the product of the hourglasss surface area, thermalconductivity, and the temperature difference between the wax and the external temperature. Using the samevariables, the required external temperature can be calculated for a given time and initial conditions by using arearranged version of Equation 6.5 Equation 6.6.

From Equation 2, the required temperature outside the indicator can be calculated through the ratio of theproduct of the waxs mass, latent heat of fusion, and wall thickness over the product of the hourglasss surfacearea, thermal conductivity, and the time. This is then added to the initial temperature of the system.

The assumptions made in the equations were accepted because a certain amount of prototype testing wasneeded. This testing proved how the assumptions affected the final design. These assumptions include evenheating of the phase change material, the phase change material has reached melting temperature at thebeginning of the 15 minutes, and the slope in the hourglass can be approximated by averaging it with the restof the container.

6.2 Pressure Release Valve

Once it was concluded that weighted valve design would be utilized, calculations were done to determine theamount of weight required to maintain 15 psi and up to 18 psi. The simple Newtonian law of F=W/A wasused, where F is the force of the pressure in psi, W is the weight required in pounds to keep the seal on theregulator intact and A is the cross sectional area of the outlet valve on which the weight will rest.

6.3 Water Amount

The question of how much water is required to create enough steam for the entire sterilization cycle is aproblem worked on by the team. Calculations were performed by the previous semesters group and thesewere used to provide a starting point for our group when performing oven testing. These calculations can beseen in Appendix. The amount of water needed is a very important calculation and experimental problem totest because too much water would cause for a longer heating up time and not enough water will result in nosteam creation and a lack of pressure.

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7.0 Prototype Development and Testing

There were many prototypes developed throughout the semester of the pressure vessel, pressure relief valve,and sterilization indicator. Feasibility tests were performed to check the designs and to ensure they worked

properly. The development of effective testing methods and data capturing techniques of the three maincomponents of the solar autoclave was a large portion of time spent on this project, and results of these testsproved very valuable to further the design.

7.1 Pressure Vessel Structural and Seal Testing

Verification of the previous teams data was necessary for further advancement of the design and sealing ofthe system. Three different sealant types were put into testing to determine the most feasible and favorable.The pressure vessel was modified after our first test to allow for quicker setup and testing. To test the systemand the sealants, hydraulic oil was pumped into the vessel at a controlled rate until the seal broke and thedesign reached its maximum pressure

The previous design utilized a 34.5 ounce can served as the pressure vessel with two end caps held together bythreaded rods and 8 wing nuts. To ensure a better seal and less stress on the end caps, threaded bolts were usedin conjunction with machined aluminum end plates. To pump fluid into the system and gauge pressure, anassembly was made as shown in Figure 7.1.

The can had 6 strain gauges applied to measure various strains on the can when put under pressure. 2 gaugeswere places on the bottom of the can, 2 were placed in the vertical axis of the can to measure longitudinalstress and the last 2 strain gauges were placed along the middle of the can to measure the hoop stress. SeeFigures 7.2 and 7.3

The tests began by placing the vessel into a bin, to allow capture of any leaking oil, with the front plate andvalve assembly removed. The strain gauges were then wired into the strain gage measuring equipment andwere ordered in a manner to allow identification of where the strain gages were placed. The can was thenfilled with hydraulic fluid until it was near the rim, as shown in Figure 7.4. The front plate and assembly wasthen placed onto the can and then tightened down by hand and then by a wrench.

Once completely sealed, hydraulic oil was pumped in at a slow rate and measurements were taking every 5psi. Readings were taken on the stresses and then the pumping continued. The seal proved to be the failurepoint of all systems and as such, the maximum pressure and strains were recorded once the seal gave.

After testing, strain gauge data was analyzed and verified the ability of the can to hold the pressures. Inaddition the maximal pressures and seal failures were compared to determine the best type of seal. Thefollowing table details the maximal pressures and strain measurements for each sealant type.

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Figure 7.1 - Valve Assembly with Hydraulic Pump Attached

Figure 7.2 - Strain Gages - Side of Can

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Figure 7.3 - Strain Gages - Bottom of Can

Figure 7.4 - Pouring Hydraulic Oil into Can

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Figure 7.5 - Hydraulic Oil Pressure Test Setup

7.1.1 Temperature Testing

Upon completion of the initial air pressure testing, the system underwent temperature testing to see if theproper temperature could be attained in a reasonable amount of time. A 1/8 hole was drilled into the

aluminum end cap to allow a K-type thermometer to be inserted directed into the can. Epoxy was used to sealup and enclose the hole. Another K-type thermometer was attached to the outside of the can so that the endwould measure the ambient air temperature.

Water was added to the can and then sealed in the same manner as the oil pressure tests. Based off ofcalculations, one cup of water was required to create the pressure needed to sterilize the medical equipment.A tube was connected to the vessel through the valve assembly to allow the gauging of pressure inside of thecan. At first, a thick plastic tube was utilized but failed under the high temperature, shown in Figures 7.7and7.8. A thin copper tube system was attached in place of the plastic tube as shown in Figure 7.9.

At periodic intervals, temperatures were read off from the ambient air temperature and the internal

temperature. The maximum temperature allowed for the ambient air temperature was set at 160Celsius. Thistemperature simulated the maximum temperatures reached by the solar autoclave.

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Figure 7.7 - Temperature Test Setup with Plastic Tubing

Figure 7.8 - Melted Plastic Tubing

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Figure 7.9 - Temperature Test Setup with Copper Tubing

7.1.2 Air Pressure Testing

Initially, air pressure tests were done to check the leak rate of the system and conclude the design was safe foroven testing. Also, due to a lack of pressure readings from the temperature testing, further air pressure testswere conducted to find leak locations. The pressure vessel was attached to an air outlet with a pressureregulator to allow pressurization of the system at a controlled rate. A sheet of Plexiglas was placed beside thesystem to provide a safe barrier in case of catastrophic failure of the system, detailed in Figure 7.10.

The pressure vessel was sealed in the same manner as the other tests. The air pressure was increased andbrought up to around 15psi. Then it was cut off so that a calculation of the leaking rate could be determined.

By making an accurate calculation, a comparison could be made to determine the required water needed in thesystem to hold 15psi for the required time frame. The leaking rate was high and it was found that the threadsof the system were bleeding pressure.

Figure 7.10 - Air Pressure Testing with Plexiglas Shield

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Materials were purchased from Lowes and Home Depot once it was determined that there is a Home Depot inNicaragua where materials can be purchased. All materials are off the shelf products. The original thoughtwas that off the shelf products would be easier to obtain in Nicaragua and therefore the purchase and

construction of the outlet valve could be completed in Nicaragua by the clinics. Parts were chosen from theplumbing section and the hardware section. Two different designs were constructed using two different sizesfor a total of four designs.

7.2.1 End Cap as Regulator

Research into pressure regulator valves led to a weighted valve design for the outlet valve of the solarautoclave. The initial design was a copper pipe from the aluminum end block with a vent to release thepressure out of the can. In order to build and maintain pressure, it was decided that an end cap with a loadattached (weight) would suffice as the weighted valve. A 1/2" copper pipe was utilized and a 1/2" end capwith weights was attached. First holes were drilled in the end cap to allow for excess pressure to escape. The

1/2" cap with holes is shown as Figure 7.11. The cap had a plate attached to it to hang the weights from. Thisis shown in Figure 7.12. The theory was that as the pressure builds in the can, the force begins to push on theweighted cap on the vent pipe. The cap would remain in place and sealed up to the 15psi threshold. It wasdesired that the valve would regulate the pressure, meaning that as the pressure increased above 15psi, the capwould slightly raise up, allowing the excess pressure to escape through the drilled holes. When the excesspressure was relieved, the cap would slide back into position, again forming a seal to keep the pressure in thecan at 15psi. Utilizing the calculations performed previously, for a 1/2" outlet valve, 3 lbs would be requiredfor 15psi. Since we wanted the system to maintain pressure at 15psi and the cap to remain intact at thispressure, we began testing the cap design with 3.5 pounds and gradually increased the weight. The cap wasdesigned to maintain a seal for a pressure of 20psi. The outlet valve concept is shown as Figure 7.13.

Figure 7.11

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Figure 7.12

Figure 7.13

During a team meeting, it was brought to the pressure vessel improvement team's attention that the 5 poundsrequired for 20psi on the 1/2" weighted valve may be enough to actually tip the solar autoclave over. It wasdecided that the outlet valve could be reduced to 1/4" outlet via a reduction couple. This in turn would resultin a weight requirement of only 1 pound. Figure 7.14 is resultant end cap design. A soup can was used to holdthe weight under the end cap. It was thought that wet sand or water could be placed in the can to a specifiedline that indicated one pound.

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Figure 7.14

Feasibility testing on both end cap designs with various weights proved to be unreliable in maintaining a seal,as well as the proper blow off pressure. It was determined at that time to move on to a second design concept.

7.2.2 Nut and Bolt Fixture as Regulator

Through research it was found that some pressure regulators actually have a cylindrical weight that isphysically in the outlet valve. This concept was adapted by using a nut, bolt and rubber gasket to simulate thedesign seen in the research. Figure 7.15 and 7.16 show the two nut and bolt configurations that wereprototyped. The first is for the 1/2" outlet valve and the second is the for the 1/4" outlet valve. A proper sealwas not obtained in initial test runs and therefore aquarium sealant was utilized to try and make a better sealon the vent pipe. This sealant is shown on only the smaller nut and bolt, Figure 7.16.

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Figure 7.15

Figure 7.16

The weight was held in the same manner as the first two designs, with a metal plate and soup can. Figure 7.17is the pressure regulator system for the nut and bolt configuration.

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Figure 7.17

While the concept seemed to have some merit, it did not maintain a seal for even low pressures and the designwas abandoned for a weighted outlet valve off an existing pressure cooker. At this point in the semester, itwas decided that while the idea of constructing a pressure regulator from off the shelf material was appealing,it was not realizable given the materials purchased of the shelf were not sufficient for creating a high-qualityseal which resulted in a lack of maintaining pressure.

7.2.3 Safety Valves

During the interim meeting, it was suggested that a safety valve be incorporated into the design through aseparate hole in the aluminum end cap. Plastic and galvanized steel 1/2" nipples were utilized along withvarying sizes of rubber plugs. The various safety valves were constructed and tested. Figure 7.18, 7.19, and7.20 show the various designs that were tested.

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Figure 7.18

Figure 7.19

Figure 7.20

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A major factor in the functionality of the safety valve was the force at which the plugs were inserted into theshafts. When a mallet was used, the plug would not budge. If only human brute force was utilized, then it wasnot consistent on when the plug would pop. At this same time, the sealants were being tested and it was foundthat none of the sealants that were being considered could withstand more that 30-35 psi. It was decided atthis point that this would act as the safety valve, similar to modern pressure cookers. While the safety valve

plug design was feasible given a prescribed force on each use, there is no way to verify that this exact forcecould be accomplished by the nurses in Nicaragua.

7.2.4 Check Valve valvula de descarga

The check valve was required to prevent a vacuum from forming inside the can. Research into check valvesshowed that Home Depot had a check valve for less than $10. It was decided that one could not be fabricatedfrom off the shelf parts for less than this. Also, the check valve was known to work. Figure 7.21 is a pictureof the check valve purchased for the solar autoclave. This is a 3/4" check valve and the piping from the endblock was 1/2", therefore a reduction couple had to be soldered to the copper pipes.

Figure 7.21

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The sterilization indicators prototype development process began with the choice of materials andverification of concept. A suitable housing material was identified as extruded acrylic from USPlastic and a

good phase-change material was identified as Polywax

2000. Inaddition to being a good fit for the requirements, Polywax 2000 wasreadily available.

The first test that was performed was creating tubes out of theacrylic and sealing phase-change material inside. The prototypes wereapplied to a fast high-temperature cycle. Basically, this kind of cyclequickly heats up to the maximum temperature of 121 Celsius andholds that temperature for 15 minutes. It then rapidly depressurizesand cools down. So, the focus of this specific test was to see whetheror not enough heat would conduct through the housing to melt thePolywax 2000, so the length and the amount of Polywax2000

in each tube were arbitrary. Figure 1 shows the experimentalsetup for this test. Although only a small amount of the material melted, the test revealed a major flaw with thedesign that was not previously considered: pressurization of the contents. As the interior of the indicator heatsup, the fixed mass of air heats up. Thus, a heated fixed mass of air in a fixed volume results in a large pressurebeing exerted on the walls of the indicator. It was seen with this initial test that the end caps sprung leaks. Weconcluded that a method for venting the indicators was needed.

Designing the vented indicators was decided to be important enough to have theteam apply the decision analysis process to the designs. The concept that waswidely accepted as being a good solution was to have a single hole drilledthrough the top and bottom of the indicator. Whenever the indicator is inoperation, a removable plug will be placed into the bottom such that no wax willleak out; however, the top will still be open in order to allow venting. Asprototypes for this concept were being fabricated through UDRI, a weight-drivenindicator concept was designed, fabricated, and tested.To try to avoid the venting issues presented by the other design, a quick

prototype was created to test an airless indicator. The concept, shown in Figure2, is to have a cylinder completely filled with Polywax2000 and a washersuspended in the solidified wax. The theory of operation was that it would indicatesterilization when the washer dropped to the other end of the indicator, showingthat all of the contents melted. Three of these were created of various size, 0.75cm,1cm, and 1.25cm tall. They were put into the autoclave in a similar experimentalsetup as Figure 1 and were exposed to a fast high-temperature thermal cycle. Theresults were surprising. Two of the three indicators had a sealing failure while theother indicator bulged indicating that it still had a large amount of internal pressureThese prototypes shows how important it is to take into consideration the thermalexpansion of the phase-change material as it changes from solid to liquid.

Initially, three vented indicators were machined according to the concept shownin Figure 4. One-half of each indicator contained the Polywax 2000 while the other half remained empty.Once the Polywax 2000 melts, it is intended to travel through the material aperture and into the secondchamber. Only when all of the material has been transported to the other side of the indicator can the cycle can

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Figure 7.22 - Initial test to verify heat transfer.

Figure 7.23 - Airless design.

Figure 7.24 - One of the

failed airless indicators.


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be considered complete. These were machined to be 0.75inches, 1 inch, and 1.25 inches tall. Similar to theother tests, they were initially applied to a fast high-temperature thermal cycle.

The results of this test were very encouraging. Although theexpansion of the solid material into its liquid state was seen through havingmaterial flowing out of the top venting hole, some material did flow down

into the empty half of the indicator and was held there by the stopper. Thus,the next prototypes require the top chamber to be only half filled and to findthe necessary amount of material to melt over a thermal cycle.

To accomplish this, a test grid was created that had indicators ofvarious sizes and amounts of Polywax 2000. So, six indicators of the kindshown in Figure 4 were made and once filled with their appropriate amountsof material, were tested under a slow high-temperature thermal cycle. Theslow high-temperature thermal cycle slowly increases the temperature fromambient to 200 Fahrenheit holds the temperature at around 225 Fahrenheitfor 20 minutes. Once those twenty minutes have elapsed, the temperature is then increased to 250 Fahrenheitand held at 250 for 15 minutes. The chamber is then quickly cooled and the test is complete. Such a cycle is

intended to replicate the conditions that would be found inside the solar autoclave.

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Figure 7.25 - Vented Sterilization

Indicator Concept


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8.0 Results

Research and testing performed in previous semesters allowed the team to continue in the current semester toimprove the open-ended cylinder design for the solar autoclave. Conceptualization from the previous

semester led the team to continue the phase-change material indicator design. Re-evaluating therecommendations left from last semester as well as developing new conceptual designs for both products ledto testing of the new ideas. The team performed cycles of testing, implementing improvements at each phase.At the conclusion of the testing the final design contained an aluminum end plate, a new sealant, a pressurerelease valve, and easy-to-access assembly. The final design of the sterilization indicator was vented in asimple way to ensure stability and to make it easier to use but was found to be unreliable as a final design.

8.1 Pressure Vessel

Extensive testing has been performed this semester in an effort to develop a final working system. Theresearch performed at the beginning of the semester and the recommendations left from the groups that

worked on the project in the previous semesters have allowed us to pursue defined projects. The team workedto improve upon the open-ended cylinder design for the solar autoclave. Testing was performed on thepressure vessel through hydraulic oil structural testing, air pressure testing, and oven testing. The results ofthe tests examined the structural integrity of the design as well as its ability to heat and gain pressure. Afterthe final testing on the pressure vessel was completed, conclusions and recommendations were drawn as tohow to improve the design further and how to continue testing.

8.1.1 Pressure Seal and Structural Testing

The seal and structural testing phase developed as a need to check the structural integrity of the design anddetermine which seal was the most effective. Conclusions drawn from this series of tests were that the canwill safely be able to withstand the pressures of the sterilization cycle. The sealant located between the canlip and the top end cap was proved to be an effective safety relief at 32-40 psi through failure. The siliconepie pan was selected as the most desired seal as it performed the best and is proven to be safe at oventemperatures. The final conclusion drawn from the hydraulic oil testing was that it would be safe to start airpressure testing.

8.1.2 Air Pressure Testing

The results of the air pressure testing concluded that the 10 psi of pressure were lost over a 10 minute period.After the ten minutes, the system steadied at 5 psi and did not lose further pressure. This test proved that thecurrent design is not completely air tight and that there might be difficulty getting the pressure up to 15 psi andmaintaining this pressure. The air pressure testing also concluded that the design was safe to start oventesting.

8.1.3 Oven Testing

Oven testing was performed to simulate the environment that the pressure vessel and relief valve willexperience when placed in the solar autoclave. Six oven tests were performed and measurements taken. Thechart below shows the recorded temperatures of the oven and inside of pressure vessel during the test. Many

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conclusions were drawn from this testing phase of simulated heating. First, all the system component wereable to withstand the high temperatures of the oven, including the wooden end cap. The pine wood was thecause for concern because of the evaporation of its moisture content which could result in cracking.

From oven testing, the temperature of 121C was determined to be reachable in the pressure vessel given

simulated oven temperatures of 150C over a time period of approximately 50 minutes. With copper piping tothe outside of the oven, no pressure was able to be recorded from the gage, but steam could be seen leaving themanually triggered outlet. Water and steam leaks were visible with the setup.

The final oven test resulted in the pressure vessel incorporating the relief valve. During this test, the inner cantemperature steadied at 109C, which from steam tables pressure would be about 8 psi. The removal of thepressure relief valve resulted in 30 seconds of steam out of the vessel.

Figure 8.1 Oven Testing of Solar Autoclave


The pressure vessel system completed at the end of last semester that was improved upon did not include arelease valve that is necessary to an autoclave. An effort was made to develop a valve that could bemanufactured by the same people who will manufacture the pressure vessel. Unfortunately, due tounreliability in these manufactured parts, it was determined that this would not be possible. A commercialvent pipe and weighted regulator were purchased and implemented in our final design. Also for safetyconcerns a check valve has been included to prevent a lower pressure vacuum inside of the pressure vessel.

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8.2.1 Verification of Final Design

The improved pressure vessel required a pressure release/regulator valve, as well as a check valve and a safetyvalve. A reasonable effort was made to manufacture an outlet valve from off the shelf components that couldbe easily assembled by the solar autoclave construction team in Nicaragua. Due to the randomness andunreliability of the off the shelf copper components, it was decided that this design be abandoned for a morereliable pre-fabricated outlet valve and check valve.

The outlet valve obtained from a commercial pressure cooker is fabricated with more precision than thecopper caps and copper piping. The copper pipes, couples and cap were soldered together and used in the finaldesign as part of the overall structure. The testing on the final outlet valve and check valve design provedsuccessful in forced air tests. It was determined that the jiggler for the vent pipe would activate at 15 psi. Itwas desired to have an outlet valve that would maintain the pressure at 15 psi; therefore weight was added to

increase the psi at which the outlet valve would regulate. Various weights were added and the nominaladditional weight found was to be 0.015 lbs, which results in a total weighted regulator of 0.215 lbs. Thisweight will jiggle at 16 psi, therefore maintaining and regulating 15 psi. Without the addition of the 0.015 lbs,the jiggler maintained pressure at 14 psi, by jiggling at 15 psi. This additional weight is critical to sustain therequired 15 psi in order to sterilize the medical instruments. Table 8.1 shows the experimental testingcompleted on the final outlet valve/check valve.

Table 8.1

Force (Weight) Required to MaintainPressure for Fabricated Vent Pipe

AdditionalWeight (lb)

TotalWeight(lb) (psi)

0.000 0.2000 15

0.005 0.2050 15

0.010 0.2100 15

0.015 0.2150 16

0.035 0.2350 18

The safety valve for the overall system was addressed and it was concluded that the sealant of the solarautoclave, in addition to the weighted regulator was adequate as a safety valve. The silicone sealant was tested

and verified to break its seal at 30-35 psi. The known operating levels of the solar cooker, from previousexperiments conducted during the summer of ***, conveyed that the autoclave would not be able to reach orsustain elevated pressures above what the sealant would break down at. It was also concluded that the drillingof an additional passage in the aluminum end block would result in a weakness in the integrity of the block.

The check valve was incorporated into the system in order to prevent a vacuum from forming in the solarautoclave during the cool-down cycle. The swing check valve will stabilize the pressure inside the can to thatof the pressure outside the can. The testing results of the check valve prove that it is operating correctly andwill prevent the can from collapsing in on itself, as is the case with a vacuum.

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The sterilization indicator testing performed during this semester has provided the team with insight into the

nature of phase change material. The failure of the airless, sealed approach provided a better understanding ofthe expansion properties. The vented, hourglass design proved to be functional but relies on an extensiveheating procedure. Also, the current vented design relies on an end cap to prevent leakage and therefore alsowill need to be placed on a flat, stable and dry surface during the entire process. These factors, while notruling out the design, have made the final design not a reliable indication device. The electrical embodimentdesign appears to be a much more reliable approach to the problem. The following sterilization indicatorsection details the findings after testing.

After comparing the two embodiment designs on paper, the simplicity of the hourglass is still appealing;however, disadvantages to its design became apparent after testing. First, the amount of heat required toincrease the indicator's temperature is very large and may inhibit the autoclave's ability to produce steam.

Second, the phase-change material is very slow and will not be able to respond to rapid changes intemperature. Also, it requires a rather large amount of flat space in the pressure vessel to operate. Additionalconsiderations will have to be made to ensure that the pressure vessel does not become rocked or bumped suchas to not disturb the indicator. If the pressure vessel uses its top-loading design and then needs to be set ontoits side, this indicator will be very difficult to keep upright.

The electronic indicator has its appeals as well. It has a much smaller thermal footprint and does not interferewith the contents of the vessel. There will be no trouble in disassembling or reassembling the pressure vesselwith the electronics. It also will operate more responsively to temperature variations, unlike the hourglass.Also, the electronics will operate more predictably than the hourglass as there is a sense of uncertainty relatedto the operation of the hourglass. However, there will be a much larger cost in creating an electronic indicatorthan a phase-change hourglass. Also, the electronic indicator may be more fragile and susceptible to damagethan the hourglass.

Being the more developed concept, the hourglass design was chosen to be refined and, unfortunately, notenough time remained to complete any additional work on the electronic indicator. By the end of the semester,it was determined that the amount of inconsistencies and unknowns related to the hourglass indicator will keepus from recommending it for implementation. It is currently not a reliable indication device. If future effortwill be placed in developing a sustainable indication device, it is recommended that the electronic indicator bedeveloped. The benefits from the electronic indicator outweigh its flaws and, when compared to how thehourglass's flaws outweigh its benefits, the electronic indicator clearly is the better choice.

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9.0 Manufacturing Instructions & Nicaragua Test Plans

9.1 Manufacturing Procedure Pressure Vessel9.1.1 Materials

a. Tin plated steel can (coffee can), 6 diameter x 7 longb. Pine board, 1-1/2 x 8 x 8c. Four (4) threaded bolts, 3/8 x 10 longd. Plumbers Tape, 1/4" wide [is there an alternative? This acts as a sealant to the threads. Makes

it water tight].e. Eight (8) 3/8" flat washerf. Four (4) wing nuts, 3/8 threadg. Aluminum Plate, 8 x 8 x [could possibly be thinner]h. Silicone pie plate

Pressure Relief Valve System parts [complex systems. We need to find a place that sellsplumbing systems. If was difficult for Daniel to find a place that sells piping]

i. Copper pipe diameter x 9j. Copper end cap diameterk. Copper T-bracket l. Check Valve

Pressure gauge.

9.1.2 Toolsa. 12 inch (min.) rulerb. Pencilc. Sharp pointed marker

d. Power drille. Hand sawf. 1/4" drill bitg. 1/2" drill bith. 5/16 drill biti. Adjustable wrenchj. Medium-sized locking pliers (min. 3/8 jaw opening)k. Utility knife or a box cutterl. Soldering Toolsm. Hack Saw

9.1.3 Procedurei. Forming the wooden block:1. Using the hand saw, cut the pine board into two equal sections, 8 inch by 8 inch

square.2. The cap needs to be drilled to accommodate the threaded rods. Make one mark

in each corner of the block, one-half inch from the adjacent faces, as shown inFigure 9.1

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3. Fit the 1/2 inch drill bit into the power drill and plug in the drill, if necessary.

Make sure to secure the drill bit firmly.4. Drill one hole at each of the four marks made. It is very important that these

holes be as near perpendicular to the surface as possible. Clear the holes of anysplintered wood.

5. Cap should now look as shown in Figure 9.2.

ii. Adding the valve assembly:

1. Cut a 3 long piece of copper tubing and solder with a male adapter.2. Solder a T-bracket onto the assembly.3. Cut two (2) 3 long pieces of copper tubing and solder to the other sides of the

T-bracket.4. Using a drill bit, drill a hole into the copper end cap.5. Assemble outlet valve through the cap.6. Solder cap to top of the pipe of the assembly.7. Solder the male adapter reduction to .

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Figure 9.1 - Marking the Corner Holes

Figure 9.2 - Cap after Drilling Holes


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8. Couple to the right side of the t-bracket.9. Using plumbers tape and attach the check valve. IMPORTANT: Make sure the

Arrow on the side of the check valve points towards the pressure vessel.

iii. Creating the seals:

1. Lay the silicon pan out on a flat, firm surface--preferably on a cutting board.2. Cut off any sides to silicon so that you have a flat piece.3. Using the ruler and a marker, make a rough circle of 8 diameter putting marks

down at each measurement including the center.4. Using the marks as a guide, use the shears or the utility knife to cut out the

silicon.5. Using the utility knife or sharp object, pierce a hole into the center of the silicon

iv. Producing the metal end cap1. Cut out an 8 x 8 block of aluminum, if not already done.2. Using the marker, mark locations from corner. See Figure 9.2

3. Using a drill bit, drill at each mark the entire way thru.4. Mark the center of the plate with the marker by measuring 4 from both sides.5. Drill a pipe threaded (NPT) hole into the center of the plate.

v. Applying the seal to the metal end cap1. Take metal cap and apply a coat of silicon glue.2. Place the silicon cutout onto the glued side, making sure that the center hole is

aligned with the center hole.

vi. Assembling the Solar Autoclave1. Place wood cap on-end, on a flat surface.2. Place 1 washer on each bolt and insert the bolts into the holes of the wooden

cap.3. Place the current assembly on a flat surface, allowing the bolts to be upright.4. Place the 6 inch diameter can on the upper face of the wood cap, in between the

threaded rods. The closed end of the can should be facing the wood.5. Slide the instrument tray into the can.6. Place aluminum cap and sealant onto the can, allowing the bolts to go through

the machined holes.7. Once Cap A is settled onto the can, slide a flat washer down each end of the

threaded rod followed by a wing nut.8. Adjust the can such that it is centered on the silicone seal, and then tighten the

wing nuts down by hand.9. The Solar Autoclave should now look as shown in Figure 9.3

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9.2 Manufacturing Procedure Sterilization Indicator

To machine a phase change hourglass indicator, perform the following steps

assuming that d equals 1":

1.) Drill a 1/8" hole as deep as the desired indicator is tall (h) into the center

of the acrylic rod. Continue to drill an additional 1/8" deep.

2.) Cut two 1/16" cross-sectional slices from the newly drilled rod. These

two slices will be the vented caps for the indicator.

3.) Cross-sectionally cut the acrylic rod once at length "h" from the end of

the rod. This resulting piece will be the indicator.

4.) Bore out a 7/8"-wide hole into the newly cut piece a distance of

(h/2+1/32)".

5.) Repeat step 4 on the other end of the indicator. If completed successfully,there should be a 1/16"-thick partition in the center of the indicator.

6.) To create the plugging cap, insert the rod into a lathe and lathe-down 1/8" of the rod until a 1/8" wide plug

is created.

7.) Cross-section the rod 1/16" below the base of the 1/8"-tall plug. This will produce the cap.

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Figure 9.3 - Assembled Autoclave and Valve Assembly

Figure 9.4


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8.) Apply a thin coat of IPS Weld-On glue onto one of the caps with a hole as well as onto the edge of one of

the ends of the hourglass. Firmly press the cap onto the hourglass and maintain pressure between the cap and

the hourglass with a c-clamp. Let sit for 48 hours.

9.) Insert the plug cap into the hole of the newly capped indicator. Melt an amount of Polywax2000 and pour it

into the open end of the indicator.

10.) Repeat step 8 for the last open end of the indicator.

11.) Heat the indicator in an oven at 125 Celsius for 30 minutes. This should transfer all of the phase-change

material into one end of the indicator.

12.) The indicator is complete.

9.3 Nicaragua Test Plans

After completion of the teams final testing on campus, the test plans for Nicaragua have been developed toguide the team heading to Nicaragua this summer. The proposed testing is very similar to tests performed oncampus with integration of the solar box cooker. The supplies to conduct these tests will be made available tothe team and recommendations of testing procedures follow.

Solar Autoclave Testing:

Supplies:

All materials and tools listed in manufacturing instructionsPressure GageCopper PipingSleevesConnecting inserts for copper pipingManual triggering outletT-valveThermocouplesMultimeterAluminum plate with hole for thermocouples

Procedure:

The Nicaraguan test plans are to perform the same oven testing procedure, but use the solar autoclave as theheating source. The pressure vessel design should be built according to the manufacturing instructions and thealuminum plate should have a small hole be drilled in it to allow for a thermocouple to be inserted into thesystem. Use epoxy to seal the hole and keep the thermocouple in place right above the water level.

The silicone seal should then be glued to the aluminum top end can using the high temperature siliconesealant. A small slice or hole in the silicone pie pan will need to be made to let the thermocouple wire into theinside of the can. A t-valve should be tightened into the top end cap to attach both the pressure relief valveand the copper piping to the outside air. The copper piping setup with pressure gage and pressure relief valve

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will be intact on arrival and should be connected to the T-valve. The pressure relief valve cap should pointvertically when the can sits on one of its sides to ensure of its performance.

After the pressure vessel is constructed, the solar box cooker should be placed under the sunlight to achieve itsmaximum temperatures. An open container of water should be placed in the cooker to begin heating the

system. Once the water is boiling, approximately one cup of the water should be poured into the coffee can.The system should then be tightened down using the wing nuts. The bolt heads should be held with a wrenchand the wing nuts tightened down as much as possible by hand.

Another thermocouple should be tied to a bolt on the pressure vessel to capture the actual oven temperature.The thermocouples can then be connected to the multimeter, which should be set up to read temperature inCelsius. The pressure vessel should be inserted into the solar box cooker and the copper piping guided out ofthe box cooker. The system is then ready to begin testing to check the pressures and temperatures of thepressure vessel.

It is recommended that the temperatures and pressures be recorded every five minutes of the heating process.

The oven temperature, can temperature, and can pressure should be monitored checked for any abnormaltrends. Oven testing took almost an hour to achieve sterilization temperature, so the solar autoclave may takea considerable amount of time to heat up.

When taking the pressure vessel out of the oven, extreme caution should be kept as the contents will be veryhot and take a long time to cool down. If no pressures are recorded, the copper piping setup can be checkedfor leaking and re-soldered to prevent the leak. The test could also be run with just the pressure relief valveremoving the T-valve. This test would only provide for temperature readings, but the pressure can becalculated from steam tables.

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10.0 Cost Estimation

The design requirements of the solar autoclave and the sterilization indicator included the cost to manufacturesaid products. The solar autoclave was to be low cost both in materials in production. The sterilization

indicator was set to a cost of $0.01 for non-reusable and $10 for reusable per unit for materials andmanufacturing. The cost of a low end pressure cooker runs approximately $19.95 for a 4 quart model and aupwards of $97.98 for a 23 quart model. These figures will be used to make a comparison between the finaldesigned solar autoclave and a pressure cooker from Wal-Mart (which has a several hundred stores inNicaragua). The final design cost estimate including the sterilization indicator was found to be $211.16 andcan be located at the bottom of Table 10.1.

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Ite

Direct Labor (PrAssemblers

Skilled LaborOverhead

Total Dir

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11.0 Conclusions

There were three main components that were designed and tested this semester for our project. The solarautoclave was taken from a previous pressure vessel prototype and improved, the sterilization indicator was

taken from an embodiment design and finalized and the pressure release valve was developed to add to thepressure vessel. During this the Product Realization Process (PRP) was followed. The solar autoclave wasimproved in many areas including the components used in the final design, for example the seal, and theassembly process. The structural integrity of the final design was verified and shown to be pressure stableprior to the inclusion of the pressure release valve. Ultimately, the autoclave system takes an extensiveamount of time to heat in a normal home oven under warm, stable conditions. Therefore it is likely that theconditions in a solar cooker in Nicaragua will also lead to a very lengthy time to reach the necessarytemperature. The pressure release valve developed for the autoclave was initially designed and built withcomponents that could be easily found off the shelf. Unfortunately, it was determined that they were notmanufactured within the necessary tolerances to allow them to be consistently used in a final design.Therefore we purchased an outlet valve and a check valve that work reliably. Also, in our final design the seal

of the pressure vessel functions as a secondary safety system as it has been shown to fail between 32 and40psi. The vented, hourglass design of the sterilization indicator has been shown to require a lengthy heatingtime before it is ready to indicate the critical temperature across time. Like any phase change material, itrequires a substantial amount of energy to change from solid to liquid and this will require a lot of inputenergy. The current design requires an end cap in the bottom to keep the material within the bottom chamber.This creates a requirement that the device be placed on a flat, stable and dry surface in order to work and beprepared to be flipped over for the following test. Currently it is not a reliable indicator for the sterilizationprocess.

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13.0 References

1. Megyesy, Eugene F. Pressure Vessel Handbook. Tenth Edition. Pressure Vessel Publishing, Inc. Tulsa,

OK. 1995.

2. Zappe, R. W. Valve Selection Handbook. Fourth Edition. Gulf Publishing Company, Houston, TX.1999.

3. Malek, Mohammad A. Pressure Relief Devices. McGraw-Hill Publishing, New York, NY. 2006.

4. Young, Warren C. Roarks Formulas for Stress and Strain. Sixth Edition. McGraw-Hill, New York.

1989.

5. Green, David W., Winandy Jerrold E., Kretschmann, David E., Mechanical Properties of Wood. Wood

handbookWood as an engineering material

6. Michael Vaughen, Manager of Central Supply, Akron Children's Hospital.Michael served as a contact for obtaining medical instruments to be tested.

7. Jim Seemore, Manager of Sterilization Department, Akron Children's Hospital.

Jim helped the design team .

8. Doepker, Philip E. The Design Project. The Application of the Product Realization Process.University of Dayton

9. HALA. Lexan Data Sheet. Raw data. Dec. 2005.

10. Hanna, Lori, Daniel Hensel, and Pete Kolis. Solar Autoclave/Solar Cooker Technical Report. Tech.NoSummer 2008.

11. Hanna, Lori, Daniel Hensel, Matt Pinger, Adam Ryba, and Chris Weiss. Solar Autoclave. Vers. Final. 24Apr. 2008. University of Dayton. 7 Oct. 2008.

12. Incropera, Frank P. Introduction to Heat Transfer. Wiley. 2001. Fourth edition.This book was used as a reference to perform the heat transfer calculations through the housing units.

13. Kerr, Barbara P., and James Scott. Use of the Solar Panel Cooker for Medical Pressure SteamSterilization. Tech.No. Kerr-Cole Sustainable Living Center.This document provides their methodology for sterilization indication, which turns out to be differentfrom our own.

14. Lemieux, P. "Destruction of Spores on BDR in a Commercial Autoclave." Applied and EnvironmentalMicrobiology.

15. "Periodic Table-Melting Point." 2006. Sept. 2008.

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16. Colavito, C.J. Solar Cooker Box Construction Manual. Nicaragua: Grupo Fenix, 2

Final Report DL Spring 2009

Documents

Transcript of Final Report DL Spring 2009