DIY freeze drier revisited

[peltier]

Hi all,

Read this topic, but it seems old though:
http://www.refrigeration-engineer.com/forums/showthread.php?5214-Building-a-Freeze-dryer

I’ve been reading up on different sites about freeze drying, I’m trying to figure out if a freeze drier for food can be made for something around 1000$. Just toying with the idea basically, so I was wondering if someone can correct me on the basic idea of how it works and if it’s doable in the way I imagine it.

Thinking of something like 2-3 cubic feet volume chamber for the chamber.
First off the high-strength pressure vessel part I totally understand, so what`s needed is a strong steel casing with an airtight lid, good gas-tight tubing etc, this part is clear. I`m thinking stainless or there abouts.
Looking around at eBay I found a number of vacuum pumps for HVAC, some of them in the 200$ range with two stage rotary vane pumps that deliver 0.05mbars at 3CFM.
The two questions I have about those kinds of pumps is this: can they handle moisture well, and does anyone know of a stainless version perhaps?
Second, what CFM should I be looking for, the chamber size of 2-3 cubic feet taken into consideration?

I`ve read that the super-cool coils in freeze driers are to condense\freeze water molecules sucked out of the dried materials, and they are mainly to keep the pump moisture-free. Is this a “must” just against corrosion? I`m asking because I`ve seen a picture of a freeze drier that has an external moisture trap, like a modular design where the trap, pump, and chamber are separate units. And also, will moisture in the pump reduce its efficiency significantly during the drying?

Now for the products being dried; at what temperature would they have to be frozen, and kept frozen during the process? This is a puzzling thing for me, because like I said before some driers have external moisture traps.

I`m probably missing something here, mind you I am no HVAC tech myself (work with electrical systems and automation), so forgive me for any misunderstandings.

Ps:
Thinking of the the moisture trap, I’ve read that even using common refrigerator compressors won’t quite do it as far as temperature goes. Some say in series maybe. I though of a compressor with a heat exchanger that would pre-cool the second (final) compressor’s condensed coolant fluid before it reaches the evaporator. Does that sound plausible or what?

Any replies are highly appreciated.

[mad fridgie]

You are mis understanding how freeze drier works.
The object is to sublime the water in the product. The product must be kept under the eutecic point (which changes during the process) To cause vapourization you need a heat source, the heat injected must be controlled to ensure the product does not defrost.
The vacuum pump is NOT there to remove the water vapour, it is there to remove "non condensables".
The vapour produced (which can be many 1000s cubic meters/kg at the working pressures) is basically compressed (ice on a refrigeration tube) The refrigeration evap temperature must below the freezing point of the water vapour at that pressure, and kept there. The size of the refrigeration unit must equal + a small% of that of the heat element. (this covers the first flush of vapour, which is the free water on the product and around the internal cells of the product) Then the vaporization rate reduces, so you then need to control the refrigeration capacity which drops off,. Could you actually build a freezer drier for under 1000, NO, but you could build a pretend one which really would just be a vacuum drier. (Product would defrost)

[peltier]

By "non condensables" you mean compounds other than water, like oil?

"Then the vaporization rate reduces, so you then need to control the refrigeration capacity which drops off,"

I dont understand this. Does it drop off because of the eutetic point?
Also, what do you think about pre-cooling refrigirant, would that do any good or would the final coil temperature be too high?

[mad fridgie]

By "non condensables" you mean compounds other than water, like oil?

"Then the vaporization rate reduces, so you then need to control the refrigeration capacity which drops off,"

I dont understand this. Does it drop off because of the eutetic point?
Also, what do you think about pre-cooling refrigirant, would that do any good or would the final coil temperature be too high?

Non Condensables, basically air components. (air within the chamber, and dissolved in the product)
Your product has basically 2 types of water "free water" and "water held within the cells", the first is removed easy, the second not so. An extreme example, free water removed, water in the surface cells removed, the water in the centre cells vapourize, this vapour has to pass through all the outer cells, slowing the process down.
The issue is controlling the refrigeration at max load and min load. (issues with the mechanics of refrigeration)
On small systems "look at your name" for another possible method.

[peltier]

He he, yes! I do have a peltier idea! But I can`t find the peltier element limitations/performance graph. I KNOW it`s in use in high-end CPU cooling where it is cooled by a conventional compressor, and that in turn results in even cooler peltier element. However I`m almost certain it`s not the magical "~50C difference" when it gets down to say -15 on the "hot" side, though I may be wrong.

But still, on the idea of using off-the-shelf parts, would a pre-cooled coolant do the job well? Or does anything below -20C involve exotic coolants? (N2, He2 CH4 etc?)

[mad fridgie]

your product and time line determines the pressure (vacuum) required, hence the refrigeration temp.
Bog standard refrigeration is down to -40C, a bit colder with a bit more knowledge

[mad fridgie]

I ran test cabinets to -75C with peltier/comp combo

[peltier]

That sounds interesting. What cooling power and power consumption are we talking about there? Eventually, what efficiency?

But I assume having a peltier element in a vacuum chamber would be quite a challenge, since wiring would have to go trough an airtight seal, plus on-the-element evaporator.

But alright, how about pre-cooling coolant?

[mad fridgie]

Cascade always an option for LT systems. But why? whats the product? some form of drug?

[peltier]

No, food. I want to freeze dry&vacupack food.

[mad fridgie]

Do not need that low a pressure, unless you need real speed.

[peltier]

Well that`s the idea. I`m not in need to freeze something small like a medicinal sample to a high qiality result, but food in decent quantities (I am guesstimating in that chamber size) around 2kg "Wet" product.

So what do you think is more advantageous - cascade or peltier? Peltier seems tempting with the lower size and temp, but cascade seems more easier and less costly in both materials and operating costs. Peltiers are around what, 1/3-1/5 efficient maybe?
Found a peltier on eBay, for 400W input QCmax is 177W. Does not exactly sound very efficient. Also did a search on peltier-compressor CPU coolers, just out of curiosity, they seem to be rare these day, plus last time I saw one it was around 1000$ for the whole unit.

[mad fridgie]

Advice go buy a purpose built unit.

[peltier]

If i had the money, I obviously would have done so. Now how about answering what kind of temp I can get with staged compressors?