from Hacker News

A centrifugal spin on a 500-year-old air compressor design

by joshwa on 5/6/21, 5:13 PM with 61 comments

• by teuobk on 5/6/21, 5:46 PM

  Interesting. A few questions that come to mind:

  1. What is the dew point of the compressed air? Many applications using compressed air require very dry air so as to avoid corrosion issues (e.g., air tools) as well as problems related to heating of water vapor (e.g., tires). For small applications, the dew point is reduced using simple traps, silica gel beads, or molecular sieves, but for even-drier air or larger applications, refrigeration-based dehumidifiers are needed. If the use of water in the compression stage leaves the dew point significantly higher than what you get from reciprocating or Roots-type compressors, that added complexity could be a deterrent to adoption.

  2. What's the energy efficiency of the process? Does the claimed "20% lower cost of ownership" account for the difference in efficiency (whatever it is)?

  3. What sort of pressures can be achieved with a tabletop or shop-sized compressor? Presumably, this is related to the speed at which the drum is spun, but what are the practical limits?

  I mean, I use a decent amount of very dry air at 300 bar (yeah...), and I'd love to replace the noisy beast of multi-stage compressor with something quieter and more efficient.

• by FiatLuxDave on 5/6/21, 7:02 PM

  I suspect that Carnot is aiming to compete with liquid ring compressors https://en.wikipedia.org/wiki/Liquid-ring_pump, which have much of the same advantages and disadvantages (near isothermal compression, water saturation). I'm guessing the article didn't mention this because every home compressor user wants a less noisy one and they get better propagation of the article around the internet that way. I considered building a centrifugal trompe a while ago for a project of mine, mainly because its really hard to find sub-horsepower liquid ring compressors, but frankly my experience with DIY centrifuges has been explosively scary.

  You can run a pump like this with kerosene instead of water, in order to reduce the amount of saturated vapor that comes out with the compressed air, but that can also cause certain flammability concerns. I once had one of my homemade vacuum pumps blow kerosene all over me when a gas bubble entrained a lot of fluid. This is one of those reasons I don't smoke - it can kill you.

• by magicalhippo on 5/6/21, 8:36 PM

  Grady over at Practical Engineering did nice a video[1] on the trompe, which they claim this design is inspired by. As usual with some nice history and a working prototype.

  [1]: https://www.youtube.com/watch?v=uvf0lD5xzH0

• by pengaru on 5/6/21, 7:40 PM

  Am I the only one twitching and reaching for Close Tab when the video labeled atmospheric air intake as "o2"?

• by swypych on 5/6/21, 8:11 PM

  The legacy design is called a trompe, here is a video of how it worked from Ragged Chutes Air plant near Cobalt, Ontario Canada. https://www.youtube.com/watch?v=UtYLVLkWyGc

• by owenversteeg on 5/6/21, 9:34 PM

  From the diagram, this seems like it wouldn't be terribly hard to DIY: you can take an off the shelf heat exchanger and a fan, then all you'd need is the centrifuge part and tubing for the pressurized gas.

  Would there be any special considerations in DIYing this that I'm overlooking? The only part that I can see being tricky is the centrifuge mechanism and sealing the bottom of that to a compressed air tube... not quite sure how to build that (presumably it'd have to withstand both high RPM and high pressure.)

  Obviously a trompe is pretty easy to DIY if you have a steady stream of water and some serious elevation change, but the vast majority of places don't have that.

• by metalman on 5/6/21, 10:25 PM

  Very interesting machine. The target market is for food and other processes that require food grade ingredients or just very clean air. If they can get cleaner air with less power and maintanance then its a go. And the initial model is slated to be 10 to 20 hp ,so the size of a big deepfreez,hundreds of pounds. 100% duty cycle,air and water filtration,clean outs,drains,etc,all has to be assumed to fit into an industrial environment.

• by londons_explore on 5/6/21, 8:35 PM

  Air compressors need not be noisy.

  Your fridge at home likely has a cylinder air compressor in it, pumping gas up to 30 bar (400 psi) and it only makes a quiet humming.

  The trick is that air compressor is attached to a big vibration damping weight, mounted on big springs, which are mounted into an oil filled heavy steel box, which is itself mounted onto rubber feet.

• by joneholland on 5/6/21, 10:05 PM

  I think they need to do far better than 70db to make this viable compared to existing technology.

  My home shop has a California air tools compressor in it that I chose specifically for its <70db volume. You can easily have a conversation when it’s running, and compared to my dust collector or other tools, it’s nearly silent.

• by tomkinstinch on 5/6/21, 8:50 PM

  I wonder if a pump like this could be made smaller using a perfluorocarbon liquid like FC-70 (Perfluorotripentylamine)—nearly double the density of water, higher boiling point, non-flammable, and would not impart water vapor to the compressed air.

• by 1970-01-01 on 5/6/21, 9:29 PM

  A very common failure mode for machines that use water is freezing. No information for the eventuality of the water becoming ice. Maybe they just won't sell these in freezing climates.

• by ChrisArchitect on 5/6/21, 8:39 PM

  (2020)

  some previous discussion: https://news.ycombinator.com/item?id=24760625

• by londons_explore on 5/6/21, 7:39 PM

  So by this same reasoning, adding a little water spray to the input of my air compressor, and a water seperator on the output, would reduce energy consumption since the water would absorb heat during the compression stroke making the compression closer to isothermal rather than adiabatic?

  Do any commercial products do this?

• by kumarvvr on 5/7/21, 3:10 AM

  Isnt this how water ring vacuum pumps work, used for vacuum generation, instead of air compression?

• by abakker on 5/7/21, 3:03 AM

  Can this model be used in a closed loop, with a different fluid for refrigeration?