Compressor theory--is there an ME in the house?

[WetCell]

Please be tolerant. I'm working from memory.

When I learned to dive back in the 1960's, someone was marketing a single cylinder scuba compressor. It would only pump to about 2,500 psi. It was tiny and fairly inexpensive.

If I recall correctly, it had a piston about half an inch in diameter and took several hours to fill a steel 72. Mako comes to mind, but I wouldn't swear that they had marketed them.

I actually saw one once. It wasn't in serviceable condition. Evidently, the single piston and cylinder would get outrageously hot. The heating and cooling cycles would crystallize the piston and cylinder and in a few hundred hours, it would go bang. It was a great idea, but with affordable materials, it simply wouldn't work. I guess that something similar could be made today, using materials like inconel, 9 nickel cobalt, and beryllium copper alloys, but the cost would be insane.

The point here is that anything can be done, given sufficient money. If there was a way to build a small, inexpensive compressor to fill scuba tanks, someone would be all over it. The market is ready.

Technology is available to do virtually anything that can be imagined. In the end, it will always come down to cost. The military compressors cross the cost benefit line by a substantial margin. They are vastly better made than the commercial units of similar size. Cost was a consideration in their design, but only in a secondary sense. Longevity, reliability, and weight took top honors.
In many cases, economy of scale will drive costs down but not with the materials I mentioned. Some things are so difficult to machine that very few people on the planet can even do it (9 nickel cobalt is one of those materials). In those cases, economy of scale is just a dream.

You can have it good, cheap, quick. Pick two.

I work in the aerospace industry. We face these decisions on a daily basis. We do not resort to exotic materials if any reasonable alternative exists. This is just reality and the same reality that compressor designers and manufacturers have to consider.

Bill.

I'd like to see that circa 60's system. That is exactly the idea. As you no doubt know, many great ideas have had to wait for the technology or science to catch up. Based on what I have seen, I think the time is ripe for the system out in the garage puttering away for a few hours--especially given the fact that the NG systems are coming online, which use the same concept at the same pressures.

Of course, many ideas that finally get into production die on the vine of the marketplace because no one picks them. I think a reliable, easy to use, inexpensive to maintain system that came in at under $1K might be well received in the scuba market.

 

[mhinagoya]

Even if weight wasn't a consideration, you would need some pretty impressive materials in such a compressor to make it reliable. The old military units used inconel valves, and I understand that the Walter Kidde units used an inconel third stage piston. Inconel is a form of stainless steel, but outrageously expensive and extremely difficult to machine. Just drilling a hole in inconel is a real chore. If it matters, that was what the government used to skin the X15 because nothing else would take the heat and stress.

It could be done, of that, there is no doubt.

Cheap? I seriously doubt it. Just having a piston machined from something like inconel is going to cost close to $1,000 (or more). Ceramics might be a possibility, but they are so brittle that I doubt they would last long. 9 nickel cobalt would probably work but who would pay $3,000+ for a piston? Titanium is far too susceptible to galling.

Small and slow aren't part of the formula for an inexpensive compressor. Good design and common (easily cast and/or machined) materials are necessary. The smaller you make it or the higher the compression ratio each stage has to achieve, the more difficult and expensive it will become.

If you want an inexpensive compressor, buy a surplus military unit that has been reworked to provide breathing air.

Bill.

 

[pescador775]

The inconel alloy used in the Kidde appears to be heat treated. I know that Inconel is hard but this material is really hard. IMO, that stuff could not be drilled unless an electron beam were used. I guess that the machinists hardened the stuff after machining or precision grinding. However, the pistons (plungers) still had to be select fitted to a couple tenths (0.0002 inch interference). This may have been due to various things, like changes after heat treatment, cylinder tolerances, or just because it was the most practical method from the start. Anyway, funny story, Peter in Taiwan took a piston from the three stage unit to a shop with the intention of grinding it down to fit a fourth stage cylinder. The machinist burned up a grinding wheel and said it was the hardest stuff ever, but he got the job done.

 

[Gunther49]

Here is your answer, work on this design, Diaphragm Compressors and Diaphragm Compressor systems for cylinder filling, Hydrogen refueling technology, and other gases.

 

[frogman62]

Wet cell,
If you want a scuba compressor that is $1000 go to my web site <sheldensportinggoods.com>
I have one that will give you grade E breathing air at 3500 PSi and fill your "80" tank in 25 minutes. I only have one and it is gas engine powered and weighs about 80 pounds
Jim Shelden