Seeing how R-134A boils at 10 to 15° below 0 @ zero psi
Nope not gonna work

 

you need a high pressure refrigerant that can get to low temp without going into a vacuum hopefully, such as R-502 in the past. i would consider a 400 series refrigerant, 410a has really high head. I dont know how long you want the process to take? TXV fed Cu coil wrapped around SST won't give you very good heat transfer. Carbon steel cracks around the temp you want (yeah , they blew up a crack plant that way dumping ethelyene into a carbon steel HX) You could see the fire for miles. What you want is a flodded cooler, similar to shell and tube but you only have one big tube. Get an 18" tube, weld eccentric plates on the ends, suction on top, Mcdonnald miller float to feed (like a boiler).

 

To be more specific about the tank, if it makes a difference. It a new, never been used, 100 lb propane tank. Ethelyene would be a pretty rapid cooling of a steel tank. I am shooting for going from ambient to -20F or lower over a period of roughly 9 hours.

 

what are you freeze drying?

 

To the OP . . .

Many questions . . .

Are you in the trade? Meaning, have you ever worked on a freeze dryer? From the refrigeration aspect?

Let's start from the beginning. Or at least one beginning. And what follows is by no means complete, just a starting point.

I have done a lot of work on freeze driers in another life. All for medical purposes. Well, there was one animal freeze dry chamber, think I told the guy I couldn't help him. Can't remember for sure, but I think he fell into the hack category.

So, let's cover some basics before we go further.

All freeze driers I worked on had the product stored and frozen in a ~ -70*C freezer before the freeze drier was ever turned on.

Freeze driers are over driven. For example, there was this one that had two 3 hp compressors for a 3'x3'x3' space.

I've worked on others that had minimal refrigeration, because the space was smaller, and a vacuum down to 5 to 10 microns could be achieved quickly. No problem transferring frozen product to freeze drier and no problem having a chance on getting above something like -40* by the time a very low vacuum was achieved.

What you describe as wrapping copper tubing around a cylinder for cooling would not be a normal freeze dryer thingy. That copper tubing would be for the moisture to condense (freeze) on after leaving the product. All freeze driers I worked on had a shelf or shelves inside the chamber that were cold. Then brought up very slowly in temp while the 'exiting' moisture collected and froze on the cold evap you mentioned.

Depending on the product, bringing up the temp on those shelves was carefully controlled. Some very small samples of human tissue or bones could easily take over a week. A small animal could take well over a month. A pet dog could easily take three months.

How much money do you have to pay for all of this electricity.

Back to the vacuum thingy. Previously frozen product at -70*C put on shelves in vacuum chamber that are -40 or colder. Turn on vacuum pump. Once vacuum reaches 5-10 microns, then the shelf temp is VERY slowly increased.

Each increase of shelf temp will show a rise of microns. No other shelf temp is increased until that same low vacuum is achieved. Used to all be manual adjustments, but now there are controllers to do that.

Are we still on the same page? Let me know if you really want to go further.

Okay, time to open my third beer . . .

 

The things I was never aware of, TKS,

 

The intent would be food

 

Regarding refrigeration, I'd go with a low temp compressor, using 410A (no, you won't find one spec'd out for that, my guess anyway), evap fed by a TEV in parallel with a cap tube, CPR to keep compressor from over amping on start up.

TEV and cap tube in parallel for a faster pull down. TEV will end up closing, and system will meter at low temps on the cap tube.

How you gonna heat the product in a controlled fashion with no heat transfer in a vacuum?

 

How you gonna heat the product in a controlled fashion with no heat transfer in a vacuum?

Each of the shelves will have electric heat strips attached and regulated externally.

 

My experience is as a tech doing repair and maintenance on residential and light commercial HVACR. No design. No industrial. No work specifically on freeze dryers. This is a trial and error process for me with the goal being food preservation. Medical, laboratory, or industrial freeze drying is probably out of my scope. The freeze dryer I’ve been referencing for my build is a Harvest Right.

 

Just did a Googler search on Harvest right. See those images, with all of the shelves? That's what I was referencing earlier. There is no direct refrigeration or heating on those shelves. I've never heard of Harvest Right, but I can tell you how they are most likely doing it.

They run a fluid through piping which is attached to the shelves. When I first got into the trade, almost 40 years ago, the most common fluid was TCE. Then it was found to cause cancer, so other fluids began to be used. You can have extremely precise control over the shelf temp this way; by controlling the temp of the fluid. Both heating and cooling.

The condenser, not for the refrigerant, but for the moisture you are going to be boiling out of the product, can be the tubing around your cylinder as you mentioned. Or it could be (for example) another cylinder next to the one you are already planning. The two connected by say a 4" connector tube. That way, the ice that will collect will not interfere with your shelves and/or product.

Here's another little tid bit, it was only told to me once by someone who operated these machines for a living. I won't get it perfectly right, but I'll be close, and you'll get the idea. There are three different 'types' of moisture that you are removing. Said better, the moisture is in three different places.

The easiest and fastest moisture that is removed would be the liquid you could squeeze out with you hands, like if you squeezed an orange. The most difficult moisture to get out is the moisture inside of each cell, which is why proper freeze drying takes so long. Then there's an intermediate moisture, just can't remember where it is.

My experience is as a tech doing repair and maintenance on residential and light commercial HVACR. No design. No industrial. No work specifically on freeze dryers. This is a trial and error process for me with the goal being food preservation. Medical, laboratory, or industrial freeze drying is probably out of my scope. The freeze dryer I’ve been referencing for my build is a Harvest Right.

 

This is the best image I have of the compressor that I’ve been referencing. I know they are using 7.5 oz of 410a according to their scarce info.

 

The shelves have electric heat applied to the bottom of each shelf. The tubing is exterior in direct contact with the chamber and insulated with foam. As the water is extracted from the food, it is drawn to the interior walls of the chamber. It runs over a period of 20-32 hours. From what I’ve read, it takes about 9-10 hours for the freeze cycle, then the vacuum is pulled. Once the vacuum is reached the electric heaters are cycled to remove the water.

My chamber is built and will pull the appropriate vacuum according to their info. My heater rack is built and working. I know that they are using 410a in their system which I have on hand. My biggest issue is finding the correct compressor. They have several service videos on their site. There are numerous videos online for different issues. But the refrigeration system is not addressed for obvious reasons.

On another note, it has been mentioned to use TEV in parallel with the cap tube. However, I don’t see this in the photo of the compressor. I know it’s not the best photo and it may be there.

 

Believe it or not, your choice of compressor will not be as critical as you think. Why? Because you will be operating at low back pressures, therefor, not over loading the compressor. Your most critical concern will be getting enough cooling back to the compressor so it just purrs along. I'd probably choose a low temp compressor and run 410A.

You could even have a little liquid injection to keep the compressor cool. Maybe a stat on the discharge, operating a solenoid, to a small cap tube injecting liquid into the suction.

 

You know, I've just been dredging up old memories. And this will contradict something I said earlier, but follow my explanation afterwards.

I think I'd choose a high temp refrigeration compressor. High temp refrigeration is the temp range of your refrigerator. Anything warmer is usually not considered refrigeration.

Now here's why:
Back in the day, you'd go out and buy a refrigeration compressor. If you wanted high temp, like for cooling beer, you'd run R-12. If you wanted a freezer, you'd run R-502. If it was a two ton beer cooling compressor, you'd use a two ton TEV. If that same compressor was used for a freezer application, you'd use a one ton TEV (with a different power head).

If you had a freezer at -30*F running R-502, and you wanted it 10* colder, you'd dump the R-502 and charge with 13B1.

So, you see, the compressor is not as critical as many will have you believe. It's the refrigerant and the associated piping that really makes things happen.

The way it works is this:
The colder you go, the less capacity the compressor has.

I can promise you I could take an air conditioning compressor, and make it run at -100*C. Just wouldn't have much capacity.

 

One of the guys I worked with years ago started and owns Harvest Right. The idea is not new nor is the equipment. The controller and the algorithm to control everything is the key and it is very proprietary. There is nothing really new with what they do. As BBeerme said, just SCR driven heater plates, relay for the vac pump, and relay for the compressor, all fed by a very intuitive vacuum sensor. Chamber is first pulled down to temp, then placed under vacuum for a specific amount of time, then heaters are slowly brought up to release moisture at a very controlled rate. The rate is dictated by the climb in microns thus if they climb too high the heaters are backed off if they climb too slow, the heaters are ramped up. That was about all I cared to learn from my visit. I was there to buy some already freeze dried food for a long hiking trip I had with some friends, this is where I ran into my old co-worker the owner and he showed me a few thing. It is not as easy as anyone thinks and takes a verrrrry long time just to do a batch of your own stuff.

 

The very first freeze drier I worked on, had a simple algorithm. As I think I mentioned, each morning when the guy showed up for work, he'd look at the microns. If back down to 5-10, he'd bump up the shelf temp a few degrees. LOL. He had a special feel for the machine; about how much to bump it up each time, and where they were in the cycle.

Of course, that would be unheard of these days. Unless a home built unit. Wonder what the pay back would be for a controller on a home built unit versus the extra electricity required to make the manual adjustments?

 

Seems like a whole lot of extra effort for a couple extra years shelf life.

Why not just dehydrate the food?

Im no expert, but according the interweb, your gain is a couple extra years shelf storage time.

Like 20-25 years for dried goods vs 25 to 30 for freeze dried.

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I've never heard of dehydrated foods lasting 20+ years. I do dehydrate too. But I'm also after the reconstitution of the food and nutrition. Dehydrated food can not be reconstituted to close to original and the hydration process depletes the nutrition of the food. Freeze dried foods can be reconstituted to near original with minimal nutrition loss.

 

Let's pretend that my question about what compressor to use has nothing to do with the freeze drying process. Let's pretend it has to do with modifying a regular upright freezer.

I have a Fridgidaire 20 cu ft upright freezer Model #LFFH20F3QM2. It easily maintains a temp of -10 charged with 134a. That's not good enough for me because I like the extreme in everything. I want this thing to hit -50. Can I evac the 134a, flush the system and drop in 410a without issue? Or do I need to change out the compressor? If a change out is required. What would be your recommendation for compressor that would not require any more, or not much more, space in the cabinet?

 

The bore and stroke are different on compressors made for those refrigerants. I think the 410 compressor has a larger bore and shorter stroke, but it's been many years since I learned which was which. Since you'll be dealing with the extreme low end of things, probably wouldn't make much difference in the life of the compressor. Meaning, compressors made for 410 are typically going to be made for air conditioning, which means they are designed to work with 80*F return air temp and in ambients of 100*F which would be pushing 450-500 psig.

If you were to put 410 in a compressor designed for 134, and keep the suction around 10-15 psig, your head would probably be in the range of 200-225. It would take some tinkering around with evap and cap tube sizing to get the proper cooling back to the compressor. Remember, every time you drop further in temp, compressor capacity goes down as well. And you will need cooling back to the compressor in order to have a long compressor life.

You will want the suction line temp at compressor inlet no warmer than 65*F. Cooler is better, but that is the design threshold.

 

Id be most concerned about start up / initial pull down pressures using 410 in a 134a compressor. Metering device sized for ultra low temp could easily create significant high side pressure at start up.

Im not certain but I imagine they may use thinner wall thickness and such in a 134a compressor cause they can get away with it.

Might get away with it, I just dont know.


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Pretty sure I mentioned earlier that a CPR would be required.

Been thinking about his cap tube sizing problem, gonna take a lot of experimentation to get it right. Not to mention the cold coil sizing. Then it occurred to me that to eliminate all of the time sizing the cap tube, to just use an AEV (automatic expansion valve). Then you could dial in whatever suction pressure you wanted.

 

I have worked on a lot of freeze dryers over the last thirty years. What you're building sounds like a thirty year old design.

 

Sounds like something from patent info.

 

These Harvest Right controllers may be of interest to you.

Unused HARVEST RIGHT FREEZE DRYER ?? CPU Harvest Right # 102214H Purchased 2020

Sorry, I cannot post the entire link.

ebay.com/itm/Unused-HARVEST-RIGHT-FREEZE-DRYER-CPU-Harvest-Right-102214H-Purchased-2020-/373050182224?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l49286&mkrid=711-127632-2357-0

 

What size cooper tubing did you use for the coil that was around the chamber. And did you put anything on the chamber before you put the foam on?

 

What size cooper tubing did you use for the coil that was around the chamber. And did you put anything on the chamber before you put the foam on?