How can my system do what I need??

[Thisisausername]

Hi there!

Okay, So I am a student who has been working on building a freeze drying machine from scratch for about a year. I have actually had reasonable success with most of the components inside the machine, but believe it or not, all this time and I am still having much difficulty with the "freeze" part of the machine.the amount of info on refrigeration continues to make my head spin. I am really hoping to make a sellable machine, so I am trying to avoid a cascade system, due to extra price and failure mode. There is one other somewhat similar system on the market from Harvest Right who manages to do the deed with a single compressor. the refrigeration coils will be used to condense and freeze water from the air onto the coils. The machine will be well-insulated, and will have a capacity of several pounds of food, containing possibly a half gallon or so of water.

With all this in mind, here are the requirements I need to meet in the refrigeration system:
Be able to freeze product to close to -40 with proper insulation,
Be single stage,
Have a coil big enough to support over a gallon of moisure and still cool.
Have a coil temp greater than 10 degrees lower than the product temperature.

I do not myself have an EPA license, but I do have an aquaintance who does, who is willing to assist my efforts. I am really stuck on this, and would REALLY appreciate assistance designing a system which can do this.

Thank you,
Ryan.

[Magoo]

Are you building a vacuum freeze drier.?

[Thisisausername]

I am, yes :).

[Magoo]

What vacuum level are you achieving, are you using heated beds for product trays. Generally vacuum freeze driers are rated on water removal rates per cycle. A cycle could be between 18 and up to 24 hours. I have worked on few. Local manufacturers here are Cudden Industries google there site, their tech people may help with your project.

[mad fridgie]

Are you trying freeze the product in the chamber.
Then you need to understand the various stages of sublimation of the product (and how this effects the refrigeration system)
"note the product has to be keep frozen to be a freeze drier"

[monkey spanners]

Also, be aware of what is happening with refrigerants being phased out etc, no point designing something that will be obsolete in 2016.

[Thisisausername]

What vacuum level are you achieving, are you using heated beds for product trays. Generally vacuum freeze driers are rated on water removal rates per cycle. A cycle could be between 18 and up to 24 hours. I have worked on few. Local manufacturers here are Cudden Industries google there site, their tech people may help with your project.

I am reaching something like 20 or 30 pascals, somewhere in that range.
I will be using heated beds, yes.
Thanks, I'll be sure to drop them a line. I need all the help I can get, good info on the subject is hard to come by!


Are you trying freeze the product in the chamber.
Then you need to understand the various stages of sublimation of the product (and how this effects the refrigeration system)
"note the product has to be keep frozen to be a freeze drier"

Yessir, it will be frozen in the chamber, however, the freezing will happen before the vacuum. I.E. start cycle>lower temperature to -40 or so>hold so produc may reach eutectic point>lower pressure to about 150 pascals... and so on. as for how this part affects the refrigeration system, if anything it should make it easier with sublimation cooling I think, but if you have any insight, I'd love to hear it.


Also, be aware of what is happening with refrigerants being phased out etc, no point designing something that will be obsolete in 2016.

Thanks, I'll keep that in mind.

Thank you all for the response, if anyone has input on how to reach the desired temps, I would be greatly appreciative.

Regards to all,
Ryan

[mad fridgie]

Without knowing the product and if freezing time is not an issue, then you can freeze within the chamber.
But when you get into the drying process, you generally have a very high initial flush due to the release of free water (water not held within the cell structure). Load then dramatically drops, as the moisture vapour has to diffuse through the cells and product directly, plus during the original flush we have large amount of ice forming on the evap coil (ice is a thermal insulator), this means we require a lower refrigerant temperature to achieve low ice surface temperatures. With these combinations is where your refrigeration equipment is working outside design envelope.
Many fail with freeze dries because they do not understand the variability of the process.
Personally if i were building a small one I would cascade a conventional refrig system with peltiers. There would also be enough load to keep refrig system satisfied, and peltiers are not damaged by minimum or no load.

[Thisisausername]

Thanks for the response, fridgie. The product is indeed variable. The plan is simply "food", which obviously covers a fairly broad spectrum. Freezing time shouldn't be a main concern, I think. One thing I've thought of in regard to ice insulating evap coils is to increase their surface area via a sort of heat sink of some kind (aluminum flashing, fins, wire branches soldered on, what have you) to increase convection. In addition, hopefully the initial flush could be slowed by controlling temperature and pressure, as well as air flow in the chamber. I kind of like the peltier idea, but it would substantially increase both cost of production and power consumption. Perhaps it would be better to start with what it would take normally to freeze food this way, without the freeze drying part of the story. Any thoughts, anyone?

[mad fridgie]

Depending on the product freezing time may effect the product, due to the size of the ice crystal formed
faster freezing smaller crystal, less cell damage.
Put a fan inside the chamber!

[Thisisausername]

There is one :). there may even be several.

[Magoo]

Pre frozen product, sublimation happens in deep vacuum, separate frozen coil accumulates moisture, the colder the quicker cycle happens. Average -20' C bigger systems -70'c with compounded refrigeration systems. FANS, no point in a deep vacuum.
magoo

[mad fridgie]

Pre frozen product, sublimation happens in deep vacuum, separate frozen coil accumulates moisture, the colder the quicker cycle happens. Average -20' C bigger systems -70'c with compounded refrigeration systems. FANS, no point in a deep vacuum.
magoo
He wants to freeze the product in the chamber, hence the need for fans

[Magoo]

Hi MF.
a good catch up today, cheers magoo.

[Thisisausername]

I wouldn't call the chamber a deep vacuum. It is about 100-200 pascals, which is considered fairly rough. On top of that, there is a water vapor inside which needs to be moving about in order to deposit on the coils, and circulate the system, even under vacuum.

With all this in mind, gentlemen, I would sincerely appreciate any input at all as to how to get anything that cold. I can't begin tweaking the process of freeze-drying if I can't even get the freeze part down. Any input would be very welcome.

[mad fridgie]

I wouldn't call the chamber a deep vacuum. It is about 100-200 pascals, which is considered fairly rough. On top of that, there is a water vapor inside which needs to be moving about in order to deposit on the coils, and circulate the system, even under vacuum.

With all this in mind, gentlemen, I would sincerely appreciate any input at all as to how to get anything that cold. I can't begin tweaking the process of freeze-drying if I can't even get the freeze part down. Any input would be very welcome.
The vapour will naturally move from the product to the freeze coil, check density of water vapour at your design vacuum and that of ice, so a fan is not need during the vacuum process,

Refrigeration is not about getting things cold, it about getting things cold reliably
And this can only be achieved by understand the product/process to be cooled.

[mad fridgie]

Hi MF.
a good catch up today, cheers magoo.
It was magoo, but your feet up old fellow!!!

[Magoo]

As suggested previously pre freeze the product, prior to sublimation process . Uncomplicates the process your are trying to achieve.
I was servicing an actual system the other day, pre frozen product on to shallow trays, heated beds under trays, 0.3 Torr vacuum and product temp in trays +60' C , generally input product weight reduce by 90% , the water content removed and accumulated on freeze coil in vessel. Freeze coil at -22' C
maggo

[sterl]

Autocascade. Per lab freezer. Not expensive but lots of experimentation required to get the geometry and accumulation straightened out....