We have a portable closet-sized temperature chamber that I want to use to cool an external mass to -40C, which will be used to provide cooling to a larger chamber. I envision a heat exchanger external to the larger chamber that I clamp the smaller chamber to, with lines running from it to the mass inside of the larger camber. A separate set of lines and fan would be used to transfer cold from the mass (perhaps an insulated box of several tons of sand?) to the large chamber.My question is, what might be some choices for the fluid and pump used in the heat exchanger? I don't expect air will give sufficient performance? Again, the operating temperature will be -40C. Experienced refrigeration answerers only, please. Thanks.
Liquid Nitrogen is good options
If you are using a closed circulation system then you need a fluid like an alcohol or a water/antifreeze mixture that will still be liquid at that temperature. There are also some hydraulic fluids (used in aircraft) that should also meet the -40 degree requirement. I would think a simple centrifugal pump would be fine to circulate the fluid. I have used them to circulate several -40 deg fluids at different times. To comments, at -40 degrees your metals can be subject to brittle fracture unless they are properly selected. Normal steel, cast iron or cast steel are not used at these cold temperatures unless the stress levels are kept very very low. There are some specific carbon steels that can be used for these applications (see ASME and ASTM codes). Stainless steel will work to temperatures much below -50 To insulate the box, I would think you would use perlite, foamglass or maybe even fiberglass as insulating materials. I'm not sure sand will be a very practical choice.
