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TonyC

Freeze Dryers and Freeze Dried Food (Part 2)

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All I had been able to find out till now is that the heat should come on at ~500 mtorr and go off again at ~600 mtorr, at which point the pressure increases, the mtorr drops till it again goes below 500 etc. I was also told that the temp would go up and down from -50 to -5F during the cycling till the end when it will slowly rise to +50F. I had no real 'confirmation' of what exactly was happening. I am not sure what you saw was what I understood either but thank you for your work.

My unit turns the heaters on at 500 mt, and off at 620 mt.  In theory, you can get the temperature based on the pressure.  With mtorr being roughly equivelent to microns,  you can get your temperatures from this chart:

 

Boiling Temperature Vacuum Pressure

(oF)     (in. Hg)     Microns

212     0               762000

192     10.24        500000

152     22.05        200000

125     25.98        100000

101     27.95        50000

84       28.74        30000

72       29.13        20000

63       29.33        15000

52       29.53        10000

39       29.69        6000

29       29.76        4000

15       29.84        2000

1         29.88        1000

-12      29.90        500

-21      29.91        300

-28      29.91        200

-33      29.92       150

-40      29.92       100

-50      29.92        50

-          29.921      0

 

However, I'm sure the pressure reading isn't completely accurate because the pump is only good to 25 microns, and if you were to drop below 50 microns you would begin to draw water vapor off the ice on the chamber walls.  

 

...And the forum totally slaughtered my chart.  I tried to clean it up.  hopefully it's better this time.


Edited by Pipsqueak (log)
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Thank you, Pipsqueak. Your chart makes perfect sense (for the drying cycles) even to this physics challenged mind - and it seems to be roughly in line with what I was told was happening. It doesn't however seem to correlate very well with Tony's actual readouts.

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My understanding of the process is pretty limited, so anything I say should be regarded as suspect, but here is what I think is happening.  

 

In a vacuum the heat transfer from the heaters to the food is primarily radiant, rather than convection, because you can't have convection in a vacuum.  It is probably hard to get an accurate temperature reading from a sensor because the chamber walls are much colder, and the shelves are much warmer, and the radiant heat from the shelves will be affecting the sensor temperature.  

 

The heaters come on at 500 mtorr, and start to heat the food past the vaporization point, which produces water vapor which then condenses on the chamber walls.  The heaters can produce more heat than the refrigeration unit can carry away, so the pressure starts to rise as the surface of the chamber walls warms up.  Once the pressure rises to 620 mtorr, the heaters shut off and the refrigeration unit cools the chamber enough to condense the water vapor down to 500 mtorr, and then the cycle repeats.

 

I have my freeze dryer in the garage, and the ambient temperature is greatly affected by the outdoor temperature.  When the ambient temperature in the garage drops to around 35F the freeze dryer kicks into the final dry cycle after only about 2 hours, but I always have to increase the dry time a lot.  I think this is because the cold air makes the refrigeration unit more efficient, so it can condense the water vapor much more quickly, therefore the heaters can run longer without raising the pressure.  When the ambient temperature is around 65F the freeze dryer doesn't kick into the final dry cycle for about 8 hours, but I don't have to run the final cycle as long.

 

The heaters appear to have a thermostat that keeps the temperature from going too high on the final dry cycle, because my power draw drops down later in the cycle.  I need to watch it run more though before I can be certain of this.

 

My background is in electrical engineering, and I have a little experience in refrigeration, so when I see a machine like this, I immediately start thinking of what I would do different to make it better.  Here are my suggestions for anybody from HarvestRight who may be listening.  :)

 

1  A time out for the vacuum pump if vacuum doesn't drop to 500 mtorr within 15 minutes.

2  A way to set the final dry cycle time before it actually kicks into the final cycle.

3  A way to adjust the heat power based on what you are drying.  Things like ice cream could use a more gentle heat cycle.  This could be done by adding a solid state relay and pulsing the heaters.

 

Those are the basics.  Now here is my dream list.  :)

 

1  A touch screen.  This is 2015 after all...  ;)

2  A strip chart on the touch screen showing the pressures and times and any another information you could want.

3  Internet.  I want to be able to control this thing from my phone at work.

4  Open source hardware and software.

 

Don't take this the wrong way, this is already an incredible machine.  The engineer in me just sees things I would like to change.  

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Tony - Welcome to eGullet and to our FD thread. What a great first post! Thank you so much for doing all that temperature/cycle work.

 

All I had been able to find out till now is that the heat should come on at ~500 mtorr and go off again at ~600 mtorr, at which point the pressure increases, the mtorr drops till it again goes below 500 etc. I was also told that the temp would go up and down from -50 to -5F during the cycling till the end when it will slowly rise to +50F. I had no real 'confirmation' of what exactly was happening. I am not sure what you saw was what I understood either but thank you for your work.

 

You are very kind Deryn,

 

My thermometer's low range limit is -50 so it is possible that the temperature dropped even lower.  But it would not have stayed there long as within 30 minutes it was right at 0.  

 

I worried most about the severe cold weakening the batteries enough to stop the radio  transmission to the receiver, but ti work very well.

 

My wife and I both love the machine,  It has been running non stop since we installed it.

 

Tony

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"I Wish" Functions:

 

In talking with the HR owners When I bought my machine about a year ago.... I had all kinds of ideas for improvement... (the Process Eng in me wanting to improve the machine-process).. it basically comes down to Cost.

 

HR. "could" add all kinds of options... and drive up the cost.   Their objective is to have a machine that is as inexpensive as they can make and have a repeatable, reliable freeze drying process. 

 

They (HR) incorporated some of my ideas as the improvement recommended really didn't add too much cost, but did improve basic function or safety.

 

Mr. Mike


Edited by Mr. Mike (log)

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"I Wish" Functions:

 

In talking with the HR owners When I bought my machine about a year ago.... I had all kinds of ideas for improvement... (the Process Eng in me wanting to improve the machine-process).. it basically comes down to Cost.

 

HR. "could" add all kinds of options... and drive up the cost.   Their objective is to have a machine that is as inexpensive as they can make and have a repeatable, reliable freeze drying process. 

 

They (HR) incorporated some of my ideas as the improvement recommended really didn't add too much cost, but did improve basic function or safety.

 

Mr. Mike

I agree with that.  Maybe a deluxe model in the future...  It's easy to stand back and say I would have done this or that different, but if I could make a better unit, I would be doing it, not talking about how somebody else should make theirs different.  :)

 

I may still try to re-control mine though.  I ordered a Raspberry Pi to play around with, but I have no experience programming in python, so I have a lot of learning before I could even think about running the freeze dryer on it.  Most of my programming experience is from Arduino, which is basically C, as well as Kuka Robot Language, which is similar to Pascal.  

 

Theoretically, for about $150 and countless hours of programming, I can add everything on my wish list.  The $150 I can swing, not sure about the countless hours...

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My wish list would include a scroll vacuum pump - no more oil changes!

Just a few thoughts to throw out there on the oil issue.

 

All the options for a vacuum pump that's impervious to water vapor seem to be too expensive to be practical.  So I have an idea, not sure if it's even practical, but I'll throw it out there anyway.  Somebody will probably explain why it wouldn't work.

 

The issue with the oil is entirely caused by the water vapor.  If you could keep the water vapor out of the oil, it would probably last a really long time.  One way to keep the water vapor out of the pump is to have a cold trap that the water condenses in before it gets to the pump.  The problem with using this method is that you have to keep it really cold to condense the water.  You would need to use liquid nitrogen or dry ice to keep it cold.  Both options are expensive, especially compared to just buying more oil.  

 

So what would happen if you used a second vacuum pump, attached to the exhaust of the first vacuum pump, and used a gas ballast on the second pump to leak air into the line between the two?  If you could keep the vacuum level on the exhaust from the first pump at 5 torr or less, the water should never condense in the pump because the pump would be to warm at that pressure for condensation.  The second pump would have enough air blowing through it to blow the water vapor through.  You can get a rotary vane vacuum pump fairly cheap that would probably work for the second pump.  

 

Not sure if this is practical or not, or if you can even run the vacuum pump with a vacuum on the exhaust port.  

 

Could we possibly put a valve on the vacuum line and shut off the vacuum pump after the air is removed?  Once the air is removed, all that's left is water vapor so as long as no more air gets in, you dont need to pump it out right?


Edited by Pipsqueak (log)

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Thanks for all the good info.. I haven't been on for a while, and wanted to let you guys know that we have eaten several (freeze dried raw, rehydrated then cooked) pork chops, chicken, steaks (Now a several months freezed dried and stored in mylar w/o2 absorbers)... The results are perfect. But the ones I tenderize first with a blade tenderizer are wonderfully tender and rehydrate faster. There are a lot of vegetable chips on the market where they freeze dry then deep fry raw veggies. I've found that roasting freeze dried veggies, lightly misted with olive oil, or canola then seasoning (lightly) with the seasonings of your choice make fantastic snack foods... Also, for green beans you actually have to cook them first or the chips are too bitter (or just use canned beans). Have fun freeze drying.... I won't be freeze drying for a while because we're moving. I can't wait to see more of your freeze drying experiences (and I can't wait to get the new place set up so I can get back to it).

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I'm looking for a source of No. 10 metal cans for storage of freeze dried foods. Any ideas on a source?

Thanks!

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Thanks Kerry! I spent hours searching and that is the only source I came up with also. Can't imagine there is only on source out there?!?

I'll keep looking and hope someone else has ideas also.

Thanks again!

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You could try calling the nearest LDS home storage facility and see if they will sell you some. (http://providentliving.org/self-reliance/food-storage/home-storage-center-locations?lang=eng ). Not all will sell to non-church members but some have, at least in the past, been very friendly to 'outsiders'. I used to buy my #10 cans from the facility in Greensboro, NC.

 

I don't think All American sells cans to be used with their sealers but if you call them (614-564-9817) perhaps they can tell you where you might purchase some.

 

On another note - a tip I read today that I think is worthy of being mentioned here: Someone posted on another board that he is 'greasing' the chamber of his HR FD (Pam and a polish cloth is what I think he is using) so that he can stop the process mid-stream if necessary and defrost in only a few minutes because the ice will fall right off when he hits it with his heat gun, shortening the drying time for wet or large loads significantly (and eliminating the necessity to remove the food and cutting down on re-freezing time associated with a mid-stream shutdown).


Edited by Deryn (log)

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Tried something new, it was time to drain the oil and I decided to pull the cover off and look at what's it like inside the pump. Well draining the oil does NOT get the goo out. There was a lot of crud which was water and rust filled. There was quite a bit of it. It was easy to get off and the gasket on the pump is heavy duty and should last many many times of coming on and off. I think it will be worth the effort.  

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Dregs:

If you read one of my previous posts about rocking the pump end to end while draining the oil and adding more oil to continue the float the "dregs" out.. that should help clean the pump of debris. 

 

When I say rock back and forth.. I mean tilting-rocking the front of drain area to rear of motor direction.  I tilt the motor housing at least 30- 45 deg from horizontal back and forth several times then open up the drain, tilt foward at least 30 deg. and let'er flow.  Close the valve, add more oil.  Rinse.. Repeat.. 

 

Each time-(rocking) adding more oil to float, wash, clean the particulate out.  It looks like the small pieces of hamburger left in the bottom of the pan after browning. .. small little chunky stuff.  I add oil (2-3 Tbls) and rock unitl very little particulate comes out.

 

My experiences..."Your mileage may vary."

 

Mr. Mike


Edited by Mr. Mike (log)
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Here in the frozen north (2 harsh nor'easter blizzards dropped about 4 feet of snow here this week), I would like to employ an (oil changing) cabana/pool 'boy' to service my machine. He can keep in shape by shovelling snow between oil changes.

 

Seriously though, I wish you guys hadn't told me about all the gunk in there because this 65 year old lady just can't see herself lifting this pump up and tipping it back and forth for hours trying to clean it all out like that every few days. There has to be a better way to get this stuff out, or a way to trap the stuff so it can't get in there in the first place or a better pump, perhaps with an easy to open valve under the gunk trap area? Right now I have to pretty well ignore the fact it is there and cross my fingers that, long term, that doesn't affect much or that the buildup will be very slow over time.


Edited by Deryn (log)

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I take it there's no oil filter in this unit? It sounds like something HR should consider.


Nancy Smith, aka "Smithy"
HosteG Forumsnsmith@egstaff.org

"Every day should be filled with something delicious, because life is too short not to spoil yourself. " -- Ling (with permission)

"There comes a time in every project when you have to shoot the engineer and start production." -- author unknown

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there is no way to add a filter without bringing the cost up.

the cover comes off easy, just 6 allen bolts.

Another option to clean it out would be to first drain the oil then use something like mineral spirits for a short time, say 2 minutes. that would more than likely not damage the pump.

I'll take a pic next cleanout so you can see the junk left after draining. I was very surprised (I bet the pump manufacture would be too lol)

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Finishman, we already have our units and have already paid for them so I don't expect a filter 'included' in the original price. Cost is therefore not an issue (within reason, of course - and as long as it doesn't void my warranty with HR either but I cannot see that as an issue if we are improving the system/saving the pump).

 

So, is there any way for current owners to retrofit an in-line filter of some kind between the main machine and the pump that might reduce or completely stop this gunk from getting to the pump? If so, perhaps we should begin investigating what kind of filter would fit the bill, and where it can be bought.

 

My point was that I am not lucky enough to have a handy guy around the house to take on these dirtier, more mechanical jobs. There is only me - so I have to find the best way to take care of it all alone. I am not about to take a chance on opening up the housing every time I change the oil (which could be as often as once a week if I am using the machine a lot) - since I might ruin it doing just that. And yet, now that I know there is junk in there that could be either reducing the efficacy of the drier, or perhaps even harming the pump long term, I have to do something about it.


Edited by Deryn (log)

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If I'm not mistaken a vapour trap in line with the vane pump would involve liquid nitrogen or other very cold system to precipitate out the water so it won't contaminate the pump.  I suspect that's not practical for a home based system.  

 

I'm still in search of a cheap scroll pump to replace my vane pump.  

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well if you feel you can't take the cover off (I bet you can) you may want to try the mineral spirits washout.

 

Oh and I did try a filter, a very good one from Germany, but as the vacuum lowed it leaked a slight amount of air in (wouldn't go below 550) so it kinda stopped the FD process and was removed. Most filters are designed for pressure and not vacuum.


Edited by finishman2000 (log)

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Are you all changing the oil when the pump is hot? According to what I read, if you allow the pump to cool off, the particulates and water separate and will, of course, precipitate out into the lower space available. Sounds logical. If one changes the oil while the pump is hot, it is supposed to minimize this buildup, no?

 

(Now, how long must one run the pump to ensure it is sufficiently 'hot' if doing this after the pump has been idle/cool for several days would be my next question - is a few minutes enough to agitate and warm up the oil mixture again?)

 

http://www.jbind.com/technical/faq-pumps.aspx (see #4)


Edited by Deryn (log)

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My vacuum pump came with a booklet that gave instructions on removing the cover and cleaning inside the pump. I haven't done it yet, but it didn't look that hard. I'll probably try it next time I change the oil. The instructions said it was designed to be field serviced so I don't think you need to worry about ruining the seals or anything.

I'm trying to find a way to recycle the oil so I can just change it more often. If you could get the moisture out of the oil and filter any dirt out of it, you could probably dump it right back in. Getting the moisture out is the hard part. If you could recycle the oil, the cost wouldn't be an issue so you could probably just change the oil every few cycles and the pump wouldn't get as dirty.

I've been running mine constantly for almost two months and I've only changed the oil twice. It's probably pretty nasty inside by now.

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