Difference between revisions of "DIY Freeze Dryer"

Revision as of 13:18, 28 February 2012

Overview

This is a project to try and build a reasonably priced, reasonably functional, freeze drying machine at home.

Purpose

Commercially available freeze drying units, even relatively small ones, are really expensive. We're talking in the thousands of dollars. This got me to thinking. In theory at least, the physics behind freeze drying and how it works aren't terribly complicated, so I figure I'll give a crack at making a decent one from scratch and save myself a few thousand bucks, and if it works other people on the internet will be able to follow my instructions and build one for themselves as well.

Info

Progress

Step 1: Acquire Vacuum Pump

• Found a vacuum pump on sale at harbor freight for $110. It pumps 3cfm and is rated to pull a vacuum down to 3 Pascal. I believe it is supposed to be used to pull a vacuum to clean out AC systems and refrigeration coils.

Next Steps

Vacuum Chamber Construction

Door

It may be overkill, but I'm thinking of using a 1/2" sheet of Polycarbonate (Lexan) from eStreetPlastics for the door. With more careful analysis and design I could probably use a thinner sheet, but I've never built a vacuum chamber before so I'm overdesigning the crap out of this one. Also, you could skip this material altogether and just use the side of the pressure vessel that you cut off as the door, but I want to be able to see into the chamber.
I chose polycarbonate (Lexan) over acrylic (Plexiglass) because the polycarbonate is much stonger, more resiliant, and has a higher working temperature.
Polycarbonate:

• Melting temperature (Tm) 267 °C*
• Glass transition temperature(Tg) 150 °C
• Upper working temperature 115–130 °C
• Lower working temperature −40 °C

• Young's modulus (E) 2.0–2.4 GPa
• Tensile strength (σt) 55–75 MPa
• Compressive strength (σc) >80 MPa
• Elongation (ε) @ break 80–150%
• Poisson's ratio (ν) 0.37

Chamber Body

After some consideration, I'm thinking of using some sort of pre-constructed pressure vessel as the body of the vacuum chamber. Something like a propane tank, or a compressed air tank. My reasoning is that, at least for medium scale, if something is designed to hold 140 psi positive pressure, then it should maybe probably be able to hold 14.7 psi negative pressure. Yeah yeah yeah, I know that the structural design for the two things is not exactly the same, but I'm building in a factor of safety of nearly 10x, so I'm modestly confident that this won't implode.
I'm thinking that an old 10 to 20 gallon auxiliary compressed air tank will be a good place to start with this design. My plan is to use a torch,plasma cutter, or angle grinder cutoff wheel to cut off one end(one of the rounded ends). That will leave a round, flat opening on that end. I'll then weld on some sort of flange, add some sort of o-ring / seal, and then the Lexan door should just sit flush against it.

Vapor Deposition Cryo-coils/Condenser Plates?

Still looking into this to see if it's really necessary

• UPDATE(2/28/12): After playing with the prototype mason jar vacuum chamber I have concluded that this element will, in fact, be necessary. Even doing small slices of apple, the water vapor destroys the vacuum completely in less than 10 minutes. So unless I want to run the vacuum pump continuously, I'm going to have to put in Vapor Deposition Cryocoils.
  • I need to do some more research on this subject, but initially I'm thinking that pulling the coils out of a mini fridge, portable/mini freezer, window A/C unit, or something like that might do the job.
    • May have to consult with a HVAC proffesional about cutting & reattaching the lines, since I'll need some way to get them trough the wall of the vacuum chamber.
    • Might could just make notches at the seal to do this more easily.

Ideas

Resources

Research

Basics of Vacuum Technology
Using Vacuum Freeze-Drying to Salvage Water-Damaged Archival Materials
HowStuffWorks.com: Freeze Drying

About.com Inventor: Freeze Drying & Freeze Dried Food