We need super insulation for a cylindrical container of approximately 10 cm in diameter and 10 cm height to be kept at -40 ° Centigrade for less than 24 hours (outside=room temperature). The cylinder cools itself actively through a phase transfer process with total heat capacity of 35,000 Joule.
2 Comments
Dr. Glen McIntosh
August 1, 2008The area of your cylinder, including ends, is 0.04712 sq. meter. Allow about 50% out to the mean insulation area for a working area of 0.07 sq. meter. Maximum heat leak is 35 j/ (24)(3600) s = 0.0004051 W. This makes Q/A = 0.0058 W/ sq. meter. This value is more than 10 times lower than the best multilayer insulation. I do not think that your desired heat leak can be realized with conventional multilayer insulation. It may be possible to achieve by very carefully preparing a series of concentric surfaces surrounding the test cylinder. Each surface must have an emissivity of around 1% (gold plated) to be successful with an acceptable number of shields. This is a standard multiple shield radiation calculation.
Dr. Ralph Scurlock
August 1, 2008I think this might be the answer to your problem…it is not super insulation you need. My calculation suggests an all round thickness of 70cm of polystyrene foam, which is not very practicable. Think instead of a Dewar’s dewar and use an ice bucket thus: the bucket must consist of a double-walled, silvered glass container—not a modern stainless steel version. Then use solid carbon dioxide to pre-cool and to provide a purge gas. Put your cylinder at -40 ° C and seal the bucket with a lid containing a Bunsen valve (one-way valve) to keep out air and water vapor. This arrangement should keep your cylinder cool for 24, if not 48 hours.