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Session 2478

A Material Processing Cell Utilizing Black-water Hydrostatic Pressure: A Student Project

James S. Burns Department of Mechanical Engineering San Diego State University

Abstract:

Autoclaves and hydroclaves are devices that contain and control moderate to large hydrostatic gas or fluid pressure. These devices are used with hot fluids (gas for autoclaves, water for hydroclaves) to heat and squeeze polymers, metals and/or ceramics during diffusion or pressure gradient controlled solid and liquid-phase materials processing. Autoclave and hydroclave technology are limited by traditional steel construction to a range of pressure- volume performance that may hinder visionary designers of objects and structures composed of advanced materials. This paper will examine some practical limitations of autoclave and hydroclave technology with trend data from an ad hoc survey. A new concept for autoclave design will be introduced, one that employs blue or black-water hydrostatic pressure. A student team’s experiences in the construction and field test of a working prototype will be documented.

Autoclaves are typically large steel cylinders that contain and control moderate to large hydrostatic gas or fluid pressure. This pressure is used to promote material flow and/or diffusion during some controlled manufacturing process. The most common use for autoclaves (outside the medical field where they are used in equipment sterilization) is in the processing of polymeric matrix, fiber reinforced composites. Applications for these materials abound in aircraft systems and are found increasingly in marine infrastructure and surface ship due their high specific structural performance and resistance to corrosion. An autoclave under construction at San Diego State University’s Facility for Applied Manufacturing Enterprise [1,2] is shown in concept drawing form in Figure 1. The steel shell for the autoclave was donated by a commercial gas company and has a length of 15’, a working diameter of 28” and a thickness of 3”. Working diameter, shell thickness, pressure ratings and estimated purchase cost for an informally surveyed list of commercially available autoclaves are shown in Figure 2. The underlying limitation evident in the figure is that the autoclave shell’s design stress in tension is not permitted to exceed 10,000-12,000 PSI. Given that the stored energy in a 3’ diameter, 6’ long vessel operating at 500 PSI is on the order of 1 MJ (Sorry for the unit change), conservative design is the only prudent course[3]. A material change to exotic alloys or composite materials might increase the not-to-exceed stress by a factor of five, but factors of safety related to the increased energy storage became the limiting concern. Indeed, hydroclaves are employed at pressures above 500 PSI. These devices use water as the internal pressurizing medium to reduce the energy stored in material compressibility.

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Burns, J. S. (1998, June), A Material Processing Cell Utilizing Black Water Hydrostatic Pressure: A Student Project Paper presented at 1998 Annual Conference, Seattle, Washington. 10.18260/1-2--7273