Towards a better tinyhouse

Inventing to freedom?

Home freeze Drying machine

with 3 comments

Okay, this has relatively little to do with tinyhouses, but a bit.

If you think dried food has to taste poor, you’ve never tried freeze dried food. It’s actually quite amazing the sort of fidelity with which the fresh taste is preserved. I recommend buying some from a camping store to try it if you don’t believe me.

But it’s way too darn expensive. Freeze dried food is 3, 4 or more times the price of fresh food.

So what we want is a way to make our own. Once we have a freeze drying machine we can dry all kinds of stuff, including prepared entrees, meats, maybe some soup for delicious and healthy powdered soup, all kinds of stuff. This post inspired by one of the news from serenity valley videos from the Laptop And A Rifle blog, in which he notes that he doesn’t like dried food.


So basically take a pressure cooker, that will be our vacuum chamber. I’ve tried using an aluminum pressure cooker as a vacuum chamber before and it worked perfectly. Just put some silicone oil, which you can buy at the pharmacy as lip balm, around the rubber seal.

Then I recommend food grade RTV silicone sealant as sealant and glue for the rest of the sealing and gluing in this project. It is used in making aquariums, but I had bought mine at an electronics store, active electronics. Look on the back and there are numbers referring to various standards, one of them is one for use in food-contact applications, so you know it doesn’t contain anything too toxic. You might want to clean surfaces off with acetone before applying.

In case you didn’t have a look at the freeze drying article, basically you want to remove the air, then keep the pressure below a certain level, and radiant heat gets to the food, causing the water to sublimate out of the food and it’s dry. The water has to go somewhere obviously, and you’ll see below that it is going to condense in a separate area.

The first step is to set the valves to the right positions and turn on the main pump(I remember seeing a cheaper one on nextag for $62). This puts enough water through the aspirator pumps, drawing out air. The aspirators can be the inexpensive plastic ones for $2 each, and you might want 15 or more of them because they don’t pump very much (100 ml/sec at 1.2 atmosphere input water pressure iirc). It could take 5 minutes or more to pump down adequately, I don’t know exactly. In case it is not clear, the pumps are inside the chamber, the water goes in, the air and water comes out as a single stream. They can continue pumping down to a suitable pressure because the working fluid is antifreezified water at a low temp. You might want a very small container of pure water inside the drying chamber to provide some water vapor to flush out the air a bit better, maybe there is no need.

When it’s pumped down, you would flip the valves quickly, turning to dry mode, and maybe turn the throttle valve down.

Okay, a tupperware container, indicated by the green line in the figure, is where the water vapor will travel and collect. It will collect here because the cooling coil has some water+antifreeze mixture at the appropriate temperature flowing through it, somewhere fairly well below 0 deg C . The cooling coil can just be a piece of copper or stainless steel tubing. You can buy copper tubing at hardware stores in the plumbing section, it is used for the last few feet of plumbing under sinks and to toilets so is quite common. Use a mini pipe cutter to cut it.

The antifreeze mix on the outside of the coil is for the purpose of interfacing this coil to the atmosphere inside the drying chamber. If there was no antifreeze fluid, water would deposit as a solid on the cooling coil, causing problems, with the antifreeze the coil cools the antifreeze, and the water vapor condenses at the surface of the antifreeze pool, then mixes with it, staying liquid. When I say antifreeze it could be sugar-water or maybe polypropylene glycol + water or something else suitable. For the coolant it can be alcohol if that would be practical, but inside the chamber you don’t want to use a solute that would increase the vapor pressure of the antifreeze.

The coil is shaped so that it breaks through the surface of the pool no matter what level the pool is at, so convection can always occur adequately in the pool. If at any time it was completely covered with water, an inversion layer would form i.e. the water above the coil would be hoter than what is below, therefore less dense and more buoyant, and would stay on top. That would be bad because the heat would have to be conducted through this still layer of water to get to the coil and be pumped away, which would be very slow compared to this way of doing it.

The hat shaped thing there is just to try to prevent radiant heat from getting in your pool of antifreeze. Just a sort of hat made from Al foil would do, but leave lots of room for water vapor to get by, because at these low vapor densities some room is needed.

The pink stuff indicates some insulation, again to reduce the amount of unwanted heat entering the antifreeze pool, could be made from a sheet some expanded polystyrene stuff, cut a ring or 2 out of it and stack them, then put a layer of foam beneath them, and the tupperware container inside the rings and glues everything together (might as well use the rtv sealant).

Then you got some shelving to put the food on, then a suitable vacuum gauge (ebay, make sure you get one that works in the appropriate pressure range, not those automotive ones, I got one for $16 once and simple ones are not inherently expensive, they are aneroid and very cheap to make (google that term)). You can use fittings to make the connections between the outside of the drying chamber and the inside, but when I did that thing to using a pressure cooker as a vacuum chamber last time, I used flex copper tubing from the hardware store, and 5/8 inch PVC tubing fits over that about right.

The coolant obviously needs to come and go somewhere fairly below 0 deg C, so a bucket of anti-freeze ice water, maybe sugar or alcohol-ice water would do fine. It takes about 10 kg of ice to remove 1 kg of water from the food. However this whole thing is still much, much more energy efficient than hot air drying. Plus you can dry protein containing and other stuff without bacterial growth being a problem, and there are a bunch of other perks too: Tastes way better, shape of food does not have to be a sheet, no oxidative browning, might be faster and need less food prep

The rate of food drying will mostly be determined by how fast energy can get into the food. Unfortunately freeze dried food is probably a pretty good insulator, so the middle of a piece might take some time to dry. A heating coil around the outside of the chamber or putting the chamber in the sun would help, add some mirrors for better effect. The food also should be arranged with this in mind, the source of heat is radiant heat from the walls of the chamber, food that receives little heat would take longer to dry. So maybe experiment with chopping the food into smaller pieces and see if that speeds things up.

Radiant heat from the walls of the chamber is by far the most predominant way heat is transferred to the food, almost none gets transferred in by convection because with the gas pressure and therefore density at one seventieth of atmopheric you would need gas velocities 70 times as high for the same amount of heat transfer rate, and that means 70 squared as much kinetic energy which is hard to provide…. You could however heat up the chamber with an electric heating coil of some sort or something to try to inject some extra power there.

A food safety note: if there is a leak in the chamber, allowing the air pressure to get too high, the temperature of the food could now rise above freezing, so bacteria could be a problem. Gotta seal it fairly well. Also, like with all food drying techniques, you need to verify that the food is dried all the way through before putting in storage.

The food temperature of food which still contains water (and we don’t have to worry about the dry stuff) was previously more or less locked to whatever temperature results in the vapor pressure of the water in the food being the same as the vapor pressure of the antifreeze in the cooling chamber. If the vapor pressure of the (frozen) water in the food was higher, vapor would escape and preferentially travel to the cooling chamber. Which is the sublimation, and condensation, which transfers heat from the food to the antifreeze area quite effectively. This sort of effect is used in so-called heat pipes usually with liquid methanol rather than frozen water (googlable term).

I guess this system has the additional perk that any flavor compounds that do escape the food would be trapped in the antifreeze, so you can taste it to see what you’re missing out on. Or you could use this to produce flavor extracts for molecular gastronomy or something.

If anyone builds this, please let me know.

edit: I was just wondering what the water removal rate would be, and it looks like from the stefan-boltzman law, if you take the amount of heat radiated by the walls of the chamber (293 deg K) and the 273 deg k (0 deg c sinc eit is frozen, it might be a bit colder than that actually) then you get a difference of about 102 watts per sq meter. Of course that is for a blackbody but just as a rough estimate.

The surface are of concern would be the total of the surface area of the food that intercepts the full light (so after accounting for shading). I guess you could approximate that by thinking about dividing the inner wall of the chamber into a grid and seeing how much light that is coming from each area of the grid gets intercepted. Or you could just approximate the mass of the food as a convex polygon shape and add up the surface area of the polygon, since the incident light on the surface of each polygon face would be uniform, I think, the cross section of a light emitting plate appears to have the same intensity so matter which angle it is viewed from, so if you think about looking through a hole in the polygon face outwards, towards the chamber wall, it would be a uniform 180 degrees view of uniform radiation emitting surface so matter where in the chamber the plate was located. So if it is not shaded the intensity at a surface in the chamber is always the same. We know what it is right at the surface of the inner wall of the chamber so we know what it is everywhere.

So obviously to want to space out the food to be dried close to the walls and try not to have any of the food shaded much, and if the 42 liter chamber was a sphere it would have an internal surface area of 0.58 square meters. So the maximum amount in kg of water that could be removed per hour if the food was at 0 deg c and room temp was 20 deg c would be 102W*0.58m2/92Wh+636Wh=0.081

92.5+636 is the number of Wh needed to melt then vaporize a liter of water
oh. hm so only 80 ml per hour max. Well that’s 2 liters per day. It’s useable. To increase it you would increase the temperature differential (and the food would be colder than 0, it should be about the temperature of the coolant which could be -18) either with the coolant or adding heat in some way heating the walls of the chamber or with a heating coil inside the chamber to increase the surface area involved and/or if electric coils using higher temps.

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Written by gregor

November 18, 2010 at 19:50

Posted in Uncategorized

3 Responses

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  1. This is something I am interested in. I started collecting parts in the last couple years to make a vacuum dehydrator as I figured that was easier than freeze drying. The delay mostly came in trying to do it on a budget. I bought a vacuum pump and also got a spare All-American pressure canner that I am going to use. The pump I got should be sufficient to reduce the boiling point of water to 66F, though it remains to be seen if my vessel can sustain that level of vacuum.
    My goal is to concentrate the flavors of fruit without having to add heat as I don’t want a cooked fruit taste. I was hoping to be able to separate the water from the aromatic compounds to be able to readd that to the concentrate, but this is probably the trickiest bit. Maybe I can do the opposite of moonshine distillers and only keep the heads?
    I haven’t quite wrapped my head around your design, but our plans are probably incompatible.. Still having a working prototype is better than an unknown one, so I may give it a try.


    December 12, 2010 at 01:02

    • hey eightball, that sounds great! could you keep us posted on if it works? BTW air conditioner compressors make great vacuum pumps, they have a much lower ultimate vacuum than a refrigersaator compressor for sone reason, I think they are rotary cpompressors, while the refrigerator compressors are otto (auto?) type.

      if you read my design, you may see that it is more practical than using a vacuum pump to pump the vapor away, and the only reason it is a freeze dryer rather th\an vacuum dehydrator is th e lower ambient prwsessure maintained, the waterr can’t be a liquid rweally when exposed to a pressurwe below a certain amount.


      December 12, 2010 at 12:11

  2. good web site for freeze drying information


    February 23, 2013 at 02:23

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