Retrofitting a thermoelectric refrigerator with a conventional compressor system

A couple years ago, I built a nice wooden table that housed a small refrigerator. The idea is that it keeps drinks handy in the living room for parties and guests. It's also cool, and fits with my idea of what "functional furniture" should be.

I chose a refrigerator meant for storing wine because it had a nice glass door, was pretty small, was very quiet, and could operate facing upwards because the thermoelectric device doesn't care about its orientation. I knew that thermoelectric refrigerators generally suck at actually refrigerating, and this one was not exception.
On really hot days, the fridge would get up to 5o-60*F, which is cool, but not nearly as nice as drinking 40*F beverages. The other problem is that it draws 70W, essentially constantly. This comes out to 613 KWh/year. Most small conventional refrigerators use around 300 KWh/year. Also, I like to tinker and wanted to mess around with a refrigerant system.



I bought a small 1.8 cu ft Haier fridge off craigslist for $15. Geez, can't beat that! My first task was to pull out all of the system components from the insulated box. This required draining the system of refrigerant through a small hole that I drilled in the compressor fill tube. Before doing this, I checked the label, and it indicated the system used 1.6oz of R134a. I can buy R134a at the auto parts store, so I will be able to refill the system.


I de-soldered the tiny capillary line from the accumulator / dryer and de-soldered the suction line on the compressor. I used an oxy-acetylene torch to heat the joint, then I just pulled the tubes apart when the solder became molten.

Here is the evaporator liberated from the fridge.



Luckily, with one flap unfolded, the evaporator fit perfectly in the thermoelectric cooler (TEC) fridge.


I mounted the compressor underneath the table (also getting very lucky that there was enough clearance). I cleaned the copper tubes carefully, then re-soldered them with silver solder and some paste flux. I had to extend the suction tube by a few inches, and so I just got some tube that fit around the existing line and soldered it to the outside.


I soldered the capillary tube back into the dryer, and put a valve on the compressor fill tube. The system was sealed at this point. Now, I connected my vacuum pump (Welch 1400) with a micron vacuum gauge and a tee that connected to a can of R134a. I positioned the can on a sensitive scale so that I could meter out 1.6 oz of refrigerant.

I pulled a 375 micron vacuum in about 15 minutes or so. I even ran the compressor while the system was under vacuum. It raised the pressure just a couple hundred microns, then it settled back down quickly, so I felt the system was dry and sealed.


After I metered out 1.6oz, which only took a few seconds with the valve just cracked open, I tightly closed the valve that I added to the fill tube, and was very pleased to see feel the evaporator getting very cold.

Tomorrow, I will re-insulate the TEC fridge and hopefully give it a final test.

Experimenting with a "liquid lens"

A couple months ago, I started to investigate building an auto-focus mechanism for cameras that could operate in high magnetic fields. This was for a work project (where I build MRI-compatible devices). Inside an MRI machine, the base magnetic field is usually 1.5 or 3 Tesla. At these fields, any motor or solenoid that relies on manipulating a local magnetic field will not work. Unfortunately, manipulating magnetic fields is one of the easiest ways to convert electricity into physical movement, so most auto-focus assemblies are built this way. There are some
alternatives, though:

Piezo "ultrasonic" ring motors. Many high-end DSLR (and SLR) lenses have silent ring motors that vibrate at ultrasonic frequencies to create rotation. Great! The only problem is that they are built from steel, and the steel cannot be used in the MRI environment. They are also too big -- I need a tiny lens about 1/4" in diameter, not 2" like a DSLR lens

Piezo wiggler motors. Looks interesting, but getting a sample is near impossible. The screw assembly is probably steel, and the system requires a fairly rigid and complex slide system

Liquid lenses. These lenses are built by sealing a small amount of water and oil inside a shirt button-shaped capsule. The walls of the capsule are metal with glass windows on the faces of the button. The water and oil will not mix, and the boundary between the fluids will bend light because the two liquids have different refractive indices. If the walls of the capsule are charged with electrical current, the shape of the water droplet will change because of electrostatic attraction. (check this out: http://www.youtube.com/watch?v=p1f6zLysilU ) The oil is made up of non-polar molecules, so it does not share this same attraction. By controlling the voltage on the capsule walls, the shape of the water can be controlled and thus the focal length of the lens formed by the water-oil interface. Neat!


The liquid lens is the metallic button-shaped thing on the left. I built a holder for the lens out of 1" squares of copper clad board.

There are only one or two manufacturers of liquid lenses: Varioptic, Philips, and maybe Siemens. Philips and Siemens would not discuss the liquid lens and would not sell any samples. Varioptic would send samples -- only $340 for each lens! What?! The evaluation kit was a mere $4000, while the full developer's kit was $12,750. I kid you not. What kind of company would charge a potential customer $12k for the privilege of testing their products? Insane!

Anyway, since I couldn't deal with any of the manufacturers directly, I set about finding an off-the-shelf product that contained a liquid lens. After many hours of searching, I found an actual product that had the Varioptic lens in it: Digitus DA-70817. There were many other prototypes and press releases for other liquid lens products, but it seems the Digitus camera is the only mass-produced device right now. Unfortunately, it is not available in the USA. It's not on eBay. Amazon.de has it, but will not ship to the USA. After many more hours of searching, I got in touch with a German distribution company called Assmann (no joke). I exchanged a few emails with a friendly person there, and she sent me five of these cameras (about 25 euro apiece with the usual outrageous international shipping charge).

I removed the lens from one of the cameras and mounted it between two pieces of copper clad board. To make the lens change focal length, I need to supply 0-70V AC to the lens. AC is required to prevent the liquid in the lens from polarizing over time and losing effectiveness. The voltage needs to be fairly high to deform the water drop sufficiently, and also needs to be changed rapidly for quick changes in focal length. Fortunately, there are two companies that make ICs specifically designed to directly drive liquid lenses. Unfortunately, one of the companies, Maxim, will not sell samples of their chip, and they will not even give out datasheets for it. Take a look: http://www.maxim-ic.com/req_full_ds.cfm?action=request&id=5949 How rude is that? WTF?! The other company that makes liquid lens drivers is Supertex. They will sell samples of their HV892 at a reasonable price, but the chip is only made in a 10-lead 4mm x 4mm DFN package with .65mm pitch. Apparently, this SMD footprint is so unusual, there are no DIP converters available from anywhere. Geeeez! I ended up designing my own PCB for the chip and having them made at http://www.expresspcb.com/. Here it is:

I interfaced the chip to an Arduino Duemilanove. The Arduino sends a value between 1 and 255 to the HV892 via the I2C bus. The value controls the HV892 output voltage, and hence the lens's focal length. I am not sure what the mathematical relationship is





I used a laser pointer to show the different focal lengths that the lens can achieve.

Sharpening planer / jointer knives

In short, sharpening planer and jointer blades is more difficult than it looks, and is probably not worth it. Get them sharpened or buy replacements if you value your time. I originally was inspired by this article:
http://www.ablett.jp/workshop/oak/doug_abbott.htm

I built a similar jig that looked like this:

I had used the sandpaper-on-glass technique for other projects, so I felt it could work well for sharpening knives. It works, but it's SLOW. My knives were in admittedly bad shape (very bad shape -- see picture) and needed a lot of material removed. I resorted to using a power sander to take off the initial material. Also my jig is not as good as the one shown in the link because tightening the screws will make the jig not be perfectly straight anymore.


Before and after

Yeah, it's sharp! The blades appear to be bi-metallic. I am not sure if all planer blades are like this, but there are definitely two different types of steels joined together.

Compressed air dryer / nitrogen generator

In my last post, I described designing and welding a pressure vessel for a compressed air dryer. I'll describe the completed dryer in the this post.

The purpose of this project is to take dirty, oily, damp air from a compressor, and provide very clean, very dry nitrogen to the liquid nitrogen generator.


The airflow path is as follows:

Air inlet -> mechanical filter -> carbon filter -> air dryer #1 -> air dryer #2 -> carbon filter -> mechanical filter -> humidity sightglass -> nitrogen membrane -> flow valve -> output

Oil vapor is a big problem when using conventional compressors with systems that rely on having clean air. Mechanical filters are not able to capture all of the oil odor, so that is why I also included activated carbon filters. I made these filters out of Harbor Freight pneumatic oilers. I gutted the oilers, added some filter material, rearranged the airflow path and threw in some activated carbon. Even though the photo shows the two filters joined together, this is just a mechanical connection -- each filter is actually plugged off. The flow goes between the bottom of the bowl and either the left or right outlet.

The whole system is fitted with 1/4" compression fittings.

Machining and welding an aluminum pressure vessel (air dryer)

I recently decided to build a compressed air dryer for use in my liquid nitrogen generator project. I will be using silica gel beads (the same stuff as found in those ubiquitous "do not eat" packets) to soak up moisture from the air. In order to do this, the silica gel must be contained in a vessel that will withstand the pressure of the compressed air. In this case, I designed the cylinders to have a working pressure of 150 psi. The system will normally operate around 100 psi. The vessel should also be fairly long and narrow to ensure the air flowing through it has enough time to make good contact with the silica gel.


I started with some basic engineering equations for a thin-walled cylindrical pressure vessel.
http://en.wikipedia.org/wiki/Radial_Stress

I already had some aluminum pipe that I felt would be suitable and checked it with these equations. The pipe is 3" in diameter and has a .0625" wall.

The tangential stress is = (150) * 3)/(2 * 0.0625) = 3600 psi
The axial stress is = (150 * 3)/(4 * 0.0625) = 1800 psi
The radial stress is = -(150) / 2 = -75 psi (negligible, the negative indicates compression)

In order to determine if this amount of stress is going to break my aluminum cylinder, I used the Von Mises stress calculation for multi-axial loading:
http://en.wikipedia.org/wiki/Von_Mises_yield_criterion

The Von Mises stress in the walls of my cylinder is:
= sqrt[ ( (3600-1800)^2 + (1800-3600)^2 + (3600 - -75)^2 ) / 2 ] = 3161 psi

In this case, the Von Mises stress is actually lower than the tangential stress component alone. This is because the walls of the cylinder are being pulled in two orthogonal directions, thus reducing the amount of shear that would be produced if the cylinder wall were being pulled in only one axis. In order to be as conservative as possible, I'll use 3600 psi as the load stress.

The cylinder is made from aluminum alloy 6061, which has a yield stress of at least 8000 psi. It's likely much higher with T4 or T6 heat treatments, but I will be welding this material, and I'm not sure what effect that will have on the yield stress, so I'll be very conservative and stick with 8000.

Clearly the 3600 psi load is much less than 8000, and this design has a safety factor of 2.2. Working backwards, the tank will hold 330 psi before suffering permanent damage. Again, these figures are likely to be very conservative.

I also calculating the plate deflection for cylinder's end caps, and it was insignificant.


I cleaned up the cylinder by turning it on the lathe and running some sandpaper over it.


The end caps are 1/8" thick and have a step turned on their edge to make placement and welding easy.


I made some bosses that will be threaded later. Luckily, the diameter of my horizontal belt sander drum matched the cylinders' diameters perfectly. The boss will sit flush up against the cylinder wall for easy welding.

Welding!


I milled a flange for the pressure vessel to hold an O-ring the in groove and the holes will be tapped for 1/4-20 bolts. The flange will be bolted to a 1/2" solid aluminum plate. This was done so that the vessel could be removed from the plate, and the silica gel could be replaced easily.



I tested the vessel, and....... it leaked! I had a tiny pinhole leak in one of my welds. It was so tiny, I could barely see the imperfection. I repaired the leak, and pumped the tank up to about 220 psi. Nothing was leaking or breaking, so I considered it a success.