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.

Comparing different brands of acrylic and cement to get the best joints

Getting really clear, bubble-free joints in thick acrylic is not easy. I've done quite a lot of practicing with various methods and eventually did some testing on the materials and cements that I was using. The link below shows the results. Since McMaster plastic is a lot less expensive than TAP plastic, I tend to use McMaster to supply the plastic and I use TAP green label cement on it. I've never tried IPS #3, since I've been told it's identical to TAP green label. I'm not convinced about this and may have to try it some day.


http://www.magconcept.com/acrylic/

Replacing a Honda Civic condenser fan without draining the freon

My 1992 Honda Civic recently developed a strange problem where the engine would idle badly and sometimes stall when the air conditioning was being used. Oddly enough, the problem turned out to be a non-functional condenser fan. Presumably, the compressor would pump the high side of the system up to an abnormally high pressure because the system was unable to dissipate heat from the condenser. Eventually the mechanical load from the compressor was so great it would cause the engine to stall. I would have guessed there was a high pressure limit switch that would deactivate the compressor, but maybe not.

In any case, this is an R12 system, and I did NOT want to open any of the freon lines. It had been working great ever since I bought the car (until this recent problem), and I did not want to mess with it. Luckily, it's possible and not too difficult to change the fan motor without opening the freon lines.
The fan shroud is bolted to the condenser with two 10mm bolts near the top facing into the condenser. There is also a small bracket on your left that has two 10mm bolts to hold the bracket to the condenser and shroud. I removed all four of these bolts, two bolts that held the shroud to the car's frame at the top, a bolt that holds a freon hose to the shroud, and also a bolt that holds a relay to the frame (on your right).



The shroud can be lifted up and out of two pockets that are formed into the metal of the condenser. It cannot be fully removed from the car because there is a freon hose in the way, and the hose cannot be moved. Instead, I removed the plastic rock guard on the underside of the car, and slipped the shroud out through the bottom. There are a few wire-guides and connectors on your right that need to be removed before the shroud will clear the car. I didn't even need to jack the car up.



Once the shroud (and fan) were out of the car, I switched the motor with the new one. Do NOT forget to also move the tiny washer from the old motor to the new one, which tends to stick to the old motor shaft and fits so tightly it almost looks like part of the shaft itself. I made this mistake and it nearly ruined my day. Without the washer in place, tightening the nut will cause the metal hub of the fan to be pushed in a bad way so that it is no longer engaged with the shaft. Since there is no other way to grab hold of the shaft, you are left with trying to get the nut back off and no way to stop the shaft from turning. After fiddling with it for a long time, I remembered my dad recommended the use of a pneumatic impact wrench in situations where a nut must be removed from a free-turning shaft. The idea is that the impact wrench hits the nut so hard and so fast, it is able to back it off while the inertia of the shaft holds the part still. Lo and behold, it zipped the nut right off. Thanks, dad!


The rest of the job was pretty straight-forward, and now my air conditioner works like a champ and doesn't stall the engine.

The old motor appears to have died of old age. There aren't any catastrophic problems, but the brushes and commutator show heavy wear. The motor case was full of carbon dust from the brushes.

Repairing a wireless keyboard

Last night, I sat down to use my home theater (a PC connected to a DLP projector) and had some problems typing in the name of a movie into Netflix. I thought I had mistyped the word, but I soon realized that my wireless keyboard had some non-functional keys on it. The letter 't', numbers 1 and 3 among other keys were not working. Since the failure affected more than one key, and pressing harder did not help the situation, I reasoned the problem was one of the matrix lines in the keyboard's circuit and not a bad connection in the button itself. The rest of the keys worked fine, so the microprocessor and transmitter were probably OK.

I took the keyboard apart and located the matrix lines between the microprocessor and the array of buttons. I used an oscilloscope to see which lines were pulsed (outgoing from the microprocessor) and which were switched (incoming to the microprocessor). I also compared the signals from the working "2" key to the broken "1" key. The signal was present but weak for "1". I tested the total circuit resistance for "1" and "2". "2" (working) was a couple hundred ohms, but "1" was 10k ohms. The circuit had a bad connection somewhere.
I used my meter to trace the bad connection to this spot on the flexible circuit board beneath the keys. It looks fine visually, but there is an electrical discontinuity right in the very center of this photo where the trace becomes narrow and passes above the middle rectangular cutout.

I used some conductive paint (marketed for repairing automotive defrost grids) to cover the bad area of the trace.

I was tempted to use conductive epoxy for the repair, but it can be very brittle and has low adhesion qualities. We'll see how the defroster repair product holds up.