Building a Freeze dryer

[kennedyb4]

Hello. I browsed this forum a bit and it seems more oriented to engineers and refrigeration experts, but I was hoping someone could direct me to some resources to help me with a planned project.

I would like to build a small freeze dryer, approx 4 to 8 cubic feet. I will need a way to cool this volume to approx -40F.A vacuum pump will have to reduce pressure to near zero.

Apparantly vacuum pumps cannot tolerate moisture well so the water vapour in the chamber will have to be condensed out fully prior to reaching this pump.

Can anyone direct me to a resource that will give me an idea of how to set up a compressor and condensor that could meet the above requirements?

Any help would be greatly appreciated.

Thanks BK

[US Iceman]

HI kennedyb4,

Welcome to the RE forums.

Here are a couple of links that may help you.

http://www.freezedry.com/r_process.htm

http://www.niroinc.com/html/chemical...eze_drying.pdf

http://www.ald-vt.com/home/pdf/bibel...eezedrying.pdf

The actual refrigeration system part of this should not be too difficult. The vaccum chamber may be the most difficult to design to prevent an implosion due to the vacuum.

Perhaps one of our other members has had some experience with these and could offer some details.

[Peter_1]

We service some vacuum chambers for metalizing machines (foil fo capacitors).
These vacuum chambers have huge vacuum pumps (posted some pictures last Friday under Chit Chat Service story) and in this vacuumroom, there's a Meissner coil.
This coil is there to speed-up the vacuum process. It traps water on it's coil (coil temperature -90°C)

[kennedyb4]

Hi. Thanks for the info and responses.

Can a standard fridge compressor running near full time acieve these low temps in a small volume?

If I can build this with readily available parts, so much the better.What cooling system would be adequate?

If I bend some tubing and hook up the compressor, will a local firm charge it with coolant?

Where will the ice form?

[The MG Pony]

it forms on the coil surface, I am actualy going to try and make my self one, possibly by using a cascade system and using a couple rotery vane A/C compressores in series as they pull good vacuums.

The compressors I'll be targeting will be the type with oil coolers to keep them happy during heavy duty cycles and the roteries I'll braze on some piping to run water through to cool them.

My hang up is how to make the chamber its self. 6" PVC pipe with a front and back flange with two Ts, the Tees go to another pipe below them that houses the coil, was my idea.

Where in Canada are you?

[Peter_1]

Kennedyb4, I don't see how you can do it with a domestic freezer.
The temperature will not be the problem but the high vacuum you need will be the problem.
Your fridge will implode.
You need a very strong casing (steel), in fact a round shape is the best form, with preformed ends on it.
And two vacuum pumps after each other to achieve large vacuum flows and low vacuum pressures.
The lower you go, the faster the drying process will go and the more water will be evaporated.

[The MG Pony]

Peter what do you think of the design I layed out?

[kennedyb4]

Kennedyb4, I don't see how you can do it with a domestic freezer.
The temperature will not be the problem but the high vacuum you need will be the problem.
Your fridge will implode.

Yes of course you are right. I was talking about getting a used fridge compressor, bending some tubing appropriately and recharging the system with coolant.

I am probably showing much ignorance by saying this. Would a local refrigeration firm recharge such a makeshift system with coolant?

What kind of tube? Where does one get the fittings?

I was thinking a short section of sewer pipeing with one end welded over with plate and some form of "hatch" with gaskets at the other end for loading and unloading trays when done.

PS I am from Thunder Bay Ontario for the member who asked.

Thanks for all your replies....

[US Iceman]

Here is a link to a file that has some helpful information on the Meissner coil and benefits.

http://www.helixtechnology.com/files...lycold/PFC.pdf

I was thinking a short section of sewer piping with one end welded over with plate and some form of "hatch" with gaskets at the other end for loading and unloading trays when done.

What type of sewer pipe? This vessel is the reverse of a pressure vessel. With the external pressure being greater than the vacuum inside, this vessel/tank should be designed carefully. A flat plate may be acceptable, if it is thick enough. A domed head offers much better distribution of stresses, than a flat plate.

I would not suggest anyone use PVC, etc, for the containment vessel.

[Peter_1]

The main goal for a Meiisner coil is catching and holding the lastwater molecules, 'flying' around in he chamber.

I posted under chit/chat a picture of A Meissner coil we made in a metalizing machine connected to a 2-stage cascade.
The link USIceman gave is an autocascade from Polycold. We service 2 of those.

Same remarks as USICeman, I shouldn't use a plastic pipe because it can collapse. Give it a try. Pressure difference is only 1013hPA.

The small domestic system can work but it will be a small coil. I don't see why a tech shouldn't charge it for you.
If you don't find anyone, call me
The Meissner coil must be installed as much as close to the opening (even better in front of the opening) leading to the suction of the vacuumpump.

May I ask, for what will you use it? Drying books or valuable materials..?

USIceman, 'domed head' ...that was the expression I was searching for when I mentioned 'preformed ends...'

Why don't you use an old gas bottle for this you search on a scrap yard?
Like the one my son used for making his BBQ (see section hobbies)

[The MG Pony]

for my system it was to be very small 6Inch with a rather thick style of pipe, I have messed with low vacuums at school and found it very hard to impload any thing with a wall greater then 5mm

[kennedyb4]

Hi. the idea is for drying food for long term storage for expeditions.

There are lots of really good ideas here for a pressure chamber.

But how do I cool it with DIY available equipment? I can get fridge compressors easily enough. Is it possible to take a fridge compressor, fit custom made tubing to it, and have it charged by a local company?

What kind of temperatures could be achieved this way in a small volume/well insulated 4 cubic foot volume?

[The MG Pony]

Fridge compressors in a multistage cascade is the only way to achieve such low temps how ever I am afraid to say most are not powerful enough, One that is would be the NFL11 from Dan foss you'd need three or so.

Secondly you are not cooling the chamber per say, you are subliming the Ice so only the Coil needs to be ultra cold.

Now say with a single Fridge compressor with a highly restrictive cap tube with a very high duty cycle running R-290 (Identical to R-22 in most respects with lower discharge pressure n temp) you will get -46C, most likely warmer how ever. You need to do allot more learning on the refrigeration end mate as this is the heart of the system.

As for the Vacuum you can use two rotary van compressors from an A/C unit in series as they tend to pull good vacuums, it may or may not work but very high probability it will just going to take longer and eat power like nothing ells.

There are places that sell freeze dried foods cheaply did you know? may be a better option as you can not guarantee the quality of your final product for long term storage, which will be of utmost importance, the reason I am making my system is to freeze dry batches of 12 Ice cream sandwiches with a total expected shelf life of 2 weeks or so.

[flyboy3b]

I would like to build a small freeze dryer, approx 4 to 8 cubic feet. I will need a way to cool this volume to approx -40F.A vacuum pump will have to reduce pressure to near zero.


Any help would be greatly appreciated.

Thanks BK

i realize this is an old thread, but im wondering if youve had any progress or if anybody else has successfully set one of these up.

there is a company called manitowoc aluminum (http://www.wafco.com/) that makes a cast alluminum pressure cooker. with some slight modifications to the pressure relief outlets, it would make a good vessel.

[wbeatty]

Here is how I did it. I used a Harmsco stainless steel filter housing for a pressure vessel. It stands vertical and has a removable domed lid. there are two ports in it at the bottom, one is a drain at the bottom that I capped off. the other has a tube that runs up inside to just below the lid. this one I attached to the vacuum pump that I scrounged off a piece of scientific equipment. It gets it down to 10-4 pa.....yep you heard me correct. Just inside the lid, I mounted a set of coils from a small lab freezer I picked up. I use it to freeze dry meat pices... (dog treats) I don't sue a moister trap at all... I just sip some alcohol through the pump at the end of each cycle to clean and dry it out. It has worked fine for almost 3 years now.

[Andy W]

I repaired one for a museum a couple of years ago, it was used for taxidermy, the stainless steel well which was also the evaporator had a glass dome on top, and it also had a vacuum pump on it, cant remember the manufacturer but it was apparently a common machine used in science etc.