I finally got a bit of time to start with my fridge project and decided to share a few details on the progress.
The concept behind is pretty simple:
Use a small bar fridge as a proof of concept model.
Replace the evaporator plate with a cold finger and add a little heater and fan.
Of course stupid me went a bit over board with the cold finger as you can see in the pic.
Plan was to have it long enough so it will work for both the storage area above and the drying compartment (to be added) below.
Problem with fridges is that they won’t come with anything to connect your gauges to, except for service port you have to add on the low side.
The idea was that this oversized finger would also act as a reservoir for the refrigerant so any over-filling would not result in compressor damage or much higher power consumption.
A quick test however showed that the boiling refrigerant collecting at the bottom result in a rather strong cooling effect.
After just half an hour there was a massive ice cube growing in the cup.
Takes only about 4 minutes to get the copper tube to freezing temps.
Of course I couldn’t call it quits there and wanted to see how well this cold trap would work without any pipe or such around to guide the airflow.
Just a simple computer fan and a 40W heat lamp - aimed at some soaking wet paper below.
The paper did dry (eventually) the cold finger iced up from the released moisture…
But who would have thought that a salvaged bar fridge with 2 start energy rating would struggle to retain the heat the heater generates…
I had the compressor turn on for 5 minutes every 10 minutes.
But the inside temp never made the thermostat turn off the heater.
Never made it past the 34 degree mark - with the ambient temp sitting at 19.
With the fridge totally off there was a huge improvement, the temp up to a whopping 38 degrees - great incubator, not so great to dry filament.
Sure, I COULD use a 100 or 200W heater but it would not solve the issue of loosing more heat than what is generated.
How to move forward after this unplanned disaster?
I need a good sized storage ‘box’ able to hold a bunch of rolls.
The keep the humidity out it needs to be mainly air tight, hence the fridge idea.
The proof on concept worked well enough for me to continue.
But it needs a different approach.
The size of the box does not really matter as long as the humidity levels can be kept low enough.
Silica gel alone only goes so far here.
Recycling the air in the box to go through/around the cold finger as an outside solution seems to be the most feasible option now.
Meaning I will have to abuse the fridge one last time to get the compressor out and make a new, compact sized condenser and a much shorter cold finger, if I can with some fins or such added.
As then all the hard work is done outside the box it is quite easy to just divert the air flow between storage box and drying box.
Like that some large sized reptile heating mats should be sufficient to provide the required heat for the rolls - after removing the temperature limitations of course.
Biggest issue now is to find a suitable box and enough copper tubing for free…
I was trying to do something similar because I have a faulty fridge.
Yet, I didnt plan to use the refrigeration circuit (faulty anyway); even if I did, I would invert the circuit (Heat Pump). I planned to use a heater and forced convection. Despite not being so efficient, it will spare a large amount of work. In other words, make an oven in a fridge without exceeding the minimum temperature of the fridge materials. I´ve seen users using Peltier, but never something like a cold finger and heater. Can you explain it?
With the original evaporator plate the refrigerant went through the entire channel system of the plate.
The increases the ability to cool a bit faster in return for a less than ideal performance to reach very low temperatures.
A bit of icing up is fine as this SHOULD drop off as water during the off cycle.
A cold finger acts as the evaporator while also doubling up as a reservoir for the coolant.
So rather than spreading the effect over the largest possible area it is concentrated on a very small footprint - the pool of refrigerant at the bottom the keep boiling and the cooling from the refrigerant coming from a tiny outlet (the metering capillary pipe) to go into a rather oversized low pressure environment.
Like that and if kept mostly ice free the cold finger can reach temps as low as minus 42 degrees Celsius using just propane.
With the finger being made from a copper pipe it conducts heat very well, resulting in this cooling effect quickly getting the entire finger down to below minus 30.
The idea behind the use is that a liquid will condensate on a surface if the temperature of this surface is below the dew point.
We dry filament at 55 or more degrees, so based on the hygrometer reading we could calculate the required dew point.
Under normal conditions and just considering the water in the air the ideal dew point for highest condensations levels is usually between 4 and 6 degrees.
We however dry filament, which means the moisture is released rather slowly and the rate goes down the dryer the filament gets.
Dry, hot air does not really like to take on much moisture.
Rather than fine droplets forming it more a matter of individual water molecules.
In those conditions and cold finger or cold trap would not be very efficient in terms of attracting this moisture.
You would require a rather huge surface area with a very slow air flow rate.
Convection would also be a problem as you want to limit and control the cold air exiting the cold trap.
Ideally you have the heating on side and at the bottom while cold trap is on other side and high up.
This way natural convection does all the hard work and takes the guessing out of the flow rate requirements.
The intentionally way too low temperature for the cold finger has a dual purpose:
First to ensure best performance and then to allow for a much longer off cycle to save energy.
You want to slightly overcharge the system:
During the start up phase the compressor has little bit more work to than before the mod - it is just harder due to the excessive pressure on the high side.
But once the finger reaches operational temperatures more and more of the refrigerant will fail to boil off or evaporate inside the finger.
The load on the compressor goes down with the rising refrigerant levels in the finger.
The low side then goes down from the initial 35-40 PSI to operate close to zero.
Like this the boiling temp of the refrigerant is much lower and the temperature of the finger reaches the maximum low.
In a working model it would go like this:
Drying is activated and the heaters turn on to get the chamber temp up to specs.
A hygrometer switch/controller is used to determine when the humidity levels inside the box get above, let’s say 20%.
If so the compressor is turned on to cycle in the required intervals to allow for a good water drainage.
Once levels are below of what these sensors can handle (around 10%) the cycle is kept for specified time to be determined based on testing and filament requirements.
A bit like freeze drying just without the vacuum.
I did some really basic performance tests.
Using a cheap mug warmer and a gel ball…
You know, those tiny beads that soak up water until they have the size of a ping pong ball or bigger…
Note to self: Those things don’t go well directly on a heated plate use some baking paper next time…
The goal was simple:
Turn the mug warmer on - giving around 60 degrees Celsius on the plate surface.
Check the hygrometer every now and then and once above 40% turn the fridge on for about 10 minutes to let the moisture condense and freeze.
No fan, no nothing else used for this test.
As expected the ball got smaller and the cold finger collected the moisture.
Had two near identical balls, one 32g the other 34g.
Compared to using the hotplate in the fridge and opening the door once the humidity went up enough:
With the cold finger I made it down to 10g for the ball 4 hours faster…
So the principle works, even a bit better than what I hoped for.
Next was grabbing a very old and very brittle roll of PLA that I left out in the rainy weather for a few days.
After a day of air drying inside it had 418g.
For the heating I abused a cheap reptile heating mat for 12V.
Ran on a lab supply at about 15V in order to get close to 60 degrees.
Then as above - wait until the humidity was high enough run the fridge for a moment and wait for the humidity to go down again.
Not to self: Cheap hygrometers suck and react way too slow…
6 hours of doing this resulted in 407g for the roll.
Did another 3 but the weight only went down another g or so.
Can’t tell how much this test worth using rubbish filament but noticed a severe issue.
A small and cheap bar fridge does not have the insulation values you want
It might be perfectly fine if it runs where the ambient temps are way above the dew point but here and in the carport where I had the fridge it was barely 10 degrees.
The inside of the fridge had quite a bit of fine condensation on all walls.
So a fan and some meaningful ducts to evenly warm the inside is a must.
Same story for my oversized cold finger - it needs to be replaced with a much smaller one inside a fan forced CPU heatsink or such.
Conclusions at this point?
The theory works and that surprisingly well without having to use desiccant.
A bigger and much better insulated fridge or small chest freezer (camping type or such) would be ideal if one wouldn’t like the idea of building a custom box…
Is this direct fridge conversion as good of an idea as I thought it would be?
As a proof on concept idea it certainly was and provided a lot of insight.
But in terms of being a practical solution not so much.
So what’s the next step then?
Next probably is going back to the original idea of having the drying chamber separated from the filament storage.
Meaning a well insulated and fan forced heated box where the air is going through an outside (the box) drying chamber with the cold finger.
The later will be more like a tube with the finger inside.
Air goes in at the top and out on the lower side with a bit to spare below to act as a reservoir for the water.
As the step after this one is already obvious I am not looking for some cheap and large enough vacuum chamber that allows me to replace the lid with something more suitable.
A vacuum strong enough to lower the boiling point of water below the 20 degree mark and the cold finger can just stay active with only minimum heating of the chamber required to prevent condensation on the metal walls.
Probably even a cheat heating mantle and some insulation wool on the outside will do just fine.
Based on experiments I did in the past with vacuum chambers I guesstimate that the average drying time a normal dryer like those Sunlu ones has would go down by over 50%.
And in a high enough vacuum as in freeze drying we can things basically bone dry and that very fast…
What are the potholes on this road?
Me being rather stubborn is probably the worst one LOL
Don’t like the idea of buying a small, preferably 12 or 24V powered compact freezer unit it I have to replace the evaporator with a cold trap anyway…
Means, once I can squeeze out more time again or s a few nice but lonely weekends come I will rip this bar fridge apart for parts.
Custom and fan forced condenser to actually fit the volume of the cold trap and reduce the energy demands for the compressor.
Not that the power consumption matters much with those tiny things…
Once I reach at least minus 40 Celsius on the cold trap I have to take it apart again to fit the cold trap into the vacuum chamber.
Oh how I love soldering stainless steel to seal aluminium tubing - real fun…
Major design considerations for the final vacuum assisted model:
Pressure…
No problem having a refrigerant circulating through a HVAC/heat pump system - ambient pressures…
But once those tubes and joints are in a deep vacuum the pressure difference has to be considered.
While in the range is less than micrometres the resulting expansion stresses can quickly weaken things.
Hight vacuum, opening - normal pressure, and then this repeats probably hundreds of times per year…
Last thing you want if those lines or joints failing while the chamber has a high vacuum…
So I have to replace the crappy aluminium tubing with decent copper tubing and braze rather than solder all connections and tube joints inside the chamber.
Safety third…
A vacuum chamber large enough to hold 1 to 3 rolls has a huge energy potential if this usually thick acrylic or polycarbonate lid fails.
Those things don’t just develop some sign of failure like a tiny crack to give you a warning…
I had them fail on me like a bomb with no prior warning LOL
With refrigerant lines going trough the lid it is a recipe for disaster.
So those have to go through the metal chamber.
The wires for the required heating and sensors come with additional hassles…
There isn’t really much out there that makes sealing them good and secure enough to last in this type of application.
So pogo pins are the way to go and they probably have to go through the lid.
A lot of work, a lot to consider early in the build process and a lot that can go wrong or turn out to not work as expected.
Keeping a good vacuum in terms of keeping things sealed can be much harder than you think…
Those Chinese guys might never be able to provide English use manuals one could understand but they do get creative with electronics…
It seems that for whatever idea you might have, they already have some of circuit board or chip waiting for you.
A fully programmable dual channel fridge/heater controller is your for under 20 bucks - including the temp sensor of course.
A single channel one usable for bother heating or cooling is your for the same price only that you can get up to 10 of them for it…
Of course there is also all sorts of hygro switches and sensors available and all this not that for mains power but also for 12/24V applications.
Needless to say that a random search for something else resulted in me spending way more time than planned looking for thing I wasn’t looking for…
Like something that, according to the pics provided, seems to be targeting ‘indoor home growers’…
A fully fledged hydroponics controller…
Funny enough, be it mistake or intentionally free from limits to restrict the use to hydroponics.
The humidity control can be set fully within in the sensor range - 12 to 98%.
The temp goes from zero to 100 degrees celsius.
According to the specs in the listing that is.
Currently waiting on a reply from the seller to clarify if those ranges are indeed fully usable and whether or not those outputs the controller has are really individual and not sharing a common ground or such.
My main concern though is the humidity control of this thing.
Nothing in the listing or specs explains where the corresponding sensor is.
In the pcis is only the external temp sensor.
Last thing I need is having found a near ideal controller for the purpose to find out the humidity sensor is inside the thing and can’t be relocated…
But for just over 50 bucks I am happy to accept this stacked circuitboard thing and print a housing for it if suitable.
10 freely adjustable presets should be enough to cover the filament types I work with…
Let’s see what the seller replies…
