Freeze drier

Discussion in 'General Survival and Preparedness' started by Airtime, Sep 1, 2015.

  1. Airtime

    Airtime Monkey+++

    My wife saw a freeze drier advertised in Backwoods Home and expressed interest in that machine and in freeze drying. There was a nice 3 part product review on that unit on another prepper blog website but when I checked out the cost.... holy smokes, I am not excited about 4 grand for a small freeze drier.

    My question to the group here is, does anyone have first hand experience with freeze drying?

    I have read a bit about the technical details of the process and understand the concepts. Of course the devil is in the details and still trying to uncover some of those. But, I am pondering this out of the box notion that employing a rather unconventional approach with the vacuum system, the cost could be cut way down and a DIY system might be rather straight forward and not very hard.

    The freeze drying process employs a vacuum that pulls the moisture out of the frozen food (the pressure is taken way down and the water boils at much lower temps). The biggest challenge (that seems to drive much of the cost) is that as the water vapor goes through the vacuum pump and reaches the ambient pressure side of the pump, it condenses into water. The water then mixes/dilutes the oil in the vacuum pump messing it up.

    The problem is condensing water vapor but what if it just never condensed in the pump? It seems that running the water vapor coming out of the freeze dry chamber through a heat exchanger pushing its temp to 250-300 degree F and separating the vacuum pump section itself from the motor driving it and putting the pump into a heated chamber of 250-300 degF as well, then the vapor will still be above the condensation point as it goes through the pump and would never condense inside. (Most oils can handle those temps so not a problem there.) So, it seems a cheap pump, simple heat exchanger, maybe a small countertop oven (to hold the heat exchanger and pump), a good deep freeze (hold the drying chamber) and a fabricated vacuum chamber with maybe a shelf heater could be put together and possibly to do the job. Maybe a couple temp and pressure sensors to monitor things. Not that hard really (if the idea would indeed work.)

    Before spending more time or any money on this hair-brain idea, has anyone experimented with any thing like that or have familiarity with commercial equipment that might function that way?

    Last edited: Sep 1, 2015
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  2. Dont

    Dont Just another old gray Jarhead Monkey

    I read up on these and toyed with the idea of building one.. Theres plenty of examples on the net of using dry ice, however I question how effective that process is in removing all the moisture. Industrially this is accomplished at temp. at -50 to -70 deg. F. .. Also, one needs to introduce heat back to the product and is accomplished by use of two stage reffer pumps.. The vacum prevents the moisture from returning to a liquid state, from ice to a vapor. The vapor is circulates over frozen metal plates that collect the vapor. If done correctly, one is rewarded with a product free of moister at room temp... If done wrong you have a rotting mess..

    This is the book that I was studying up in.. It cost over a 100.00 dollars..
    Industrial Refrigeration Handbook: Wilbert Stoecker: 9780070616233: Books
    Last edited by a moderator: Sep 5, 2015
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  3. Airtime

    Airtime Monkey+++

    Yeah, the key is getting the pressure and temp low enough that the ice sublimates straight to vapor. I'll look at sublimation temperatures some more.

    Is the system you are thinking about a single chamber design? In single chamber designs, the chamber is taken to those really low temperatures and the product inside the chamber sits on heated shelves warming it back up so it is not as cold (but still below freezing). So, that is where much of the really low temperature is needed, to drop the vapor out of the air and create frost deposition in a moisture separation chamber (or the walls of a single chamber design) so that the vacuum pump is spared condensation inside it. But my goofy idea is if the pump is hot (well above condensation temperatures at 14.7 PSI gauge) and the vapor is heated before getting to the pump, then vapor would go right thru the pump with no ill affect, its just another gas like the nitrogen and oxygen. This would also suggest, maybe incorrectly as this is a detail I'm working on, the really low temps (-50 to -70 F) might not be needed as much, just the temp the product is taken to ( or heated to) at in the freeze drying chamber.

    Thanks for the book link, I'll check that out.
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  4. kellory

    kellory An unemployed Jester, is nobody's fool. Banned

    When you compress a gas it get hot, and when you release that pressure it gets cold, if I recall correctly. If you are running it through a pump, this heating and cooling is going to happen anyways, and you are still going to get water and frost issues, I think.
  5. Airtime

    Airtime Monkey+++

    Yeah, Kell, you are talking the ideal gas laws (Charles and Boyles laws) or the combined version of PV=nRT. The fundamental challenge and why moisture traps are a big deal is keeping moisture out of the vacuum pump. The system needs to operate at a pressure less than about 600 Pascals (Pa) to freeze dry and probably more like below 60-100 Pa. If moisture is allowed to go into a vacuum pump, when that moisture comes out of a it the pressure will be ambient which is 100 kPa. Note, the gas is being compressed and will heat, that won't cause frost and actually works in our favor.

    If you look at the phase chart below, you see that at 100 kPa pressure, the water will be a liquid between 0 and 100 deg C. So, if the pump is at room temp, 20-25 deg C. the water vapor will condense into liquid and that adversely affects the oil in the pump. And you pretty much need oil filled vane pumps to get down to those vacuum levels.

    My thought was to not have a room temperature pump. Have a pump at say 150 deg C. As the vapor comes out of the vacuum chamber heat it up (but pressure is still low.) You essentially follow the 100 Pa line across on the chart from say -30 deg C to the 150 C temp, then the pressure goes up until you hit ambient at 100 kPa, but you will see the gas was never out of the yellow vapor zone on the chart, it would never be in the liquid phase where it messes with the pump oil.


    I think I could separate the actual pump from its motor and put it inside a small heated chamber, probably an insulated box with a small resistive heater.

    I am concerned about the drying process and also keeping the food frozen as the moisture sublimates. The sublimation process (which is like evaporation but is going from solid to gas directly as opposed to going from a liquid to a gas) will have a cooling affect on the food. I've seen some DIY systems that kept the food warmer, room temp, but I think it needs to always be well below 41 deg F while drying to prevent bacteria growth and needs to be colder still, actually frozen and then dried to maintain cell wall structure and the food texture.

    The conventional FD process is to trap the moisture on chamber walls or on cooling tubes that are well below the deposition temperature (similar to freezing temperature but going from a gas straight to ice without going through the liquid phase) and keep the moisture out of the pump that way. This is why the really cold (-50 F and below) temperatures are needed. Then when the food is removed, the ice is melted off and then a new batch is done.

    My other concern with this idea is vaporization of the pump oil and any back-streaming into the chamber. If the oil out-gases, some may travel back into the chamber and taint the food. Not sure filtration would could catch that. An isolation chamber would certainly help. Possibly using a food grade oil (we ran vegetable oil in our scuba air compressor years ago) in the pump could work. Or maybe best flowing a tiny bit of gas like nitrogen or argon through the whole set-up would cause a molecular flow that would prevent back-streaming.

    Still pondering.
    Last edited: Sep 1, 2015
  6. kellory

    kellory An unemployed Jester, is nobody's fool. Banned

    Beyond me. Another approach might be to increase the oil supply, and run it through a loop heater to remove the water from the oil before going back to the pump. Can your oil be heated to 212f to boil off the water, without damaging or igniting the oil? Simply handle it as two stages? Though in truth, the freezing coils trapping water would be easier and safer. Maybe use a chest freezer as the dehydration chamber?
  7. Ganado

    Ganado Monkey+++

    Last edited: Sep 1, 2015
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  8. DarkLight

    DarkLight Live Long and Prosper - On Hiatus

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  9. Airtime

    Airtime Monkey+++

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  10. Brokor

    Brokor Live Free or Cry Moderator Site Supporter+++ Founding Member

    If you're not going into business freeze-drying food, it's not worth the time and money, even for a "budget" freeze-dry setup, which is by no means guaranteed to yield positive results. But, I would love to be proven wrong.
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  11. Airtime

    Airtime Monkey+++

    It might be kind of fun to prove you wrong. If I can work out the details, and not encounter a flaw in the concept, depending upon how much scrounging I do, I think I could have a freeze dryer that works in a conventional freezer, costs only a couple hundred bucks and have higher thru-put than the $4k Harvest Right unit. We like freeze dried fruit a lot at our house. Then if that system can FD just 30-40 pounds of berries, I'm ahead money wise.
  12. Brokor

    Brokor Live Free or Cry Moderator Site Supporter+++ Founding Member

    That would be awesome...just don't blow anything up!
  13. Airtime

    Airtime Monkey+++

    Heck, that's half the fun. Arrr....Arrr.... Arrrr (best attempt at text version of Tim the Toolman manly grunt)
  14. Dont

    Dont Just another old gray Jarhead Monkey

    The vacuum pump would not need to run continuously to have the system function as intended. By pulling the vacuum prior to the heating and sublimation cycle and having the evacuated air pass through a settling tank prior to the vacuum pump, with an in line, replaceable air dryer. With adequate seals and latching on the door you could maintain a vacuum for a longer period and allow any moisture to settle. The pump would only need to run periodically.

    To heat the trays, you could pull hot gas from the High pressure side of the refrigerant pump with a return line to a collection tank after the condenser.. The refrigerant from the warming tubing would possibly be a liquid after leaving the cold box.. If this liquid where to return to the reffer pump, it could damage the pump..

    The evaporator would be bonded to metal plates with fin's for the moisture from the food product to condense on in the secondary chamber. And of course we would need to have a fan or maybe two to circulate air inside the box..

    I am not to sure of what refrigerant would be the ideal. However, if this is a home constructed unit, then you would use what you have. Possibly supplement the evap. cooling on the plates with an addition of dry ice to reach the desired effects..
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  15. Airtime

    Airtime Monkey+++

    What you describe is not unlike small commercial FD units, though their moisture "dryers" are even colder surfaces for H2O to deposit on as frost/ice. Liquid water does not exist at these low pressures, just vapor or solid. Moisture won't settle. Desiccants generally won't work useless they are chilled well below the deposition/sublimation line on the graph above, otherwise the H2O just boils out of the desiccant (well technically it sublimates) just like it does from the food. And if we have the refrigeration system to chill a desiccant, we don't really need a desiccant because frost will form on the chillers trapping the moisture there.

    However, I am exploring an idea, maybe incorrectly, that a much simpler approach could be implemented if we change the base assumption that every FD seems to make about the vacuum pump's operating temperature because keeping liquid water out of the pump seems (I may be missing something, hence discussion here) to be the whole point of the extra complexity of freeze dryers. I'm exploring the idea that we can pull moisture right thru the pump with no problem if we keep it in the vapor phase the entire time, then it's no different than pulling N2, O2, CO2, Ar, He, or most other gases thru the pump.

    Thanks for the thoughts and thinking about this. It all makes me think it through more.
  16. Dont

    Dont Just another old gray Jarhead Monkey

    The real challenge isn't keeping the moister from the vacuum pump. It is building this much cheaper than buying one and from locally obtainable material. I have my doubts that a household freezer could supply the temperatures required, the box may be used if you can seal it sufficiently to maintain the vacuum. and add extra insulation to increase the so so R-value of the case.. Hmm, Maybe take two freezers combined with both evaperoters in the freezer box?? Still a long stretch to get -70 deg. F. from a home freezer..
  17. azrancher

    azrancher Monkey +++

    R12 which is what the old freezers use, which I believe is much more efficient than what is used now has a boiling point of -29.8, the new freezers use R134a which boils at -26.3, adding two in series doesn't do any good.

    You probably need to be using Methane -162 degrees, good luck with that.

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  18. Dont

    Dont Just another old gray Jarhead Monkey

    That is correct. Was just a random thought..
  19. melbo

    melbo Hunter Gatherer Administrator Founding Member

    I really wanted to play with one of these Freeze Dryer – White | Harvest Right
    I understand what Brokor said about cost effectiveness but if I had my own, I could control the quality and type of food that I stored. My current food needs are buckets of meat - retail freeze dried meat is pretty cost prohibitive so I'd rather invest the $$ upfront and do it myself and perhaps for a few local friends.
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  20. Airtime

    Airtime Monkey+++

    I concur you can't get -70 F with a home freezer. The point of my crazy idea is I don't need it. Those temps are only to trap moisture as ice/frost. I might need some kind of chilled chamber to condense pump oil to reduce oil back streaming that could taint the food but quite likely would not need real cold temps for that. I am still assessing vapor pressures and lubricity of various oils seeing if that can be controlled without a chilled chamber. Note the point of my goofy concept is such that other than the initial freezing of the food product, and maybe some initial chilling during early stages, it seems plausible to freeze dry with NO refrigeration. It mostly depends upon the pump and it's oil.

    Still pondering how to add heat to the food as it dries (heat of vaporization will further reduce the food temp and given there is a vacuum, convection heat transfer will not occur meaning the food will just get colder, below the sublimation temperature and drying will stop. Radiant or conduction are the leading methods but micro/radio waves are a consideration to raise the food temp back up.

    Melbo, the Harvest Right is the one that caught my wife's eye and caught mine for the same reasons you noted. I could build a copy for a lot less than 4k, but wondering if rethinking the basic premises employed in the design might yield a very different and cheaper approach. I can see some nice aspects of the Harvest Right. Maybe combine some ideas.
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