April 29, 2009
Below are the three largest challenges faced by the design clinic team in developing a design for the solar autoclave. We welcome you to contribute your thoughts and suggestions to these challenges in the comments section!
Pressure Release Valve
The team wanted to build the pressure release valve of easily-found materials, however each of their designs proved to be too insensitive. In the end, the team decided to use the release valve off of a commercial pressure cooker. Fortunately, these are available to purchase without requiring the purchase of a whole pressure cooker. Its precisely machined weight and aperture is necessary.
Sterilization Indicator
For much of the semester, the team worked to improve a cylinder/washer design, in which a small plastic cylinder would be filled with a phase-change material, Polywax2000 and a washer. The idea is that the washer starts on top of the solid wax, but as the autoclave heats up and the wax melts, the washer moves to the bottom. There is a calculated amount of wax inside such that when the washer lies on the bottom (i.e. all of the wax has melted), at least 15 minutes have passed, the minimum for sterilizing surgical instruments at 121 degrees Celsius and 15 psig. Ultimately, this design was abandoned because too much pressure built up inside the cylinder.
The final design is a two-compartment cylinder, with one compartment filled with Polywax2000. It functions as does an hourglass, and is manufactured such that when all of the wax melts and flows into the bottom compartment, the solar autoclave operator can rest assured that sterilization is complete. It has a hole in both ends, for ventilation, and a cap that blocks the whole at the bottom.
This design requires further testing, so the team also offered a contingency plan-an electronic timer, using a thermistor and powered by a small solar cell. Currently, the price of one sterilization indicator stands at $2.00, which complies with the team’s goal of under $10 if reusable and about $0.01 if non-reusable.
Pressure Vessel
For the solar autoclave pressure vessel, the team has replaced one of the wooden end caps with an aluminum end cap, used a silicone pie pan as the seal, and used hex bolts with wing nuts to hold it all together. Upon testing with hydraulic oil, they successfully brought the pressure up to 32psig – well above the 15psig required for sterilization – before it started to leak at the seal again. The stresses were well below the yield strength for the can. It is currently thought that the leaking seal, once validation testing is complete, could provide the emergency safety release, because it always fails well above the operating pressure, but fails before rupture of the vessel.
Testing with air revealed that the prototype does not consistently maintain pressure-so, tests run in an oven were not able to achieve 15psi because the system is not completely air tight. Further development is necessary to ensure a good seal. The oven testing also revealed that the time to heat up is very lengthy, but that steps can be taken to decrease this time, such as adding already-boiling water in the autoclave instead of cold water.
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