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What makes solid CO2 better for the purpose than liquid nitrogen or just the high-pressure air or maybe air mixed with super-chilled water? I'm sure the folks i

What makes solid CO2 better for the purpose than liquid nitrogen or just the high-pressure air or maybe air mixed with super-chilled water? I'm sure the folks involved have thought about alternatives, but I'm most curious what properties solid CO2 has that make it especially good.

robbedpeter 5y ago
It's cheaper and safer using co2 pellets propelled with ordinary air than liquid nitrogen. It gets sufficiently cold, and sufficiently abrasive to freeze, shatter, and disperse the sap and plant fats and other sticky stuff. Co2 cleaners are used in high end auto cleaning and detailing, so there's established use, safety, and equipment out there. The use is novel, the tech is decades old.
- arcticbull 5y ago
  The temperature alone - dry ice is only like -109F and sold at most Safeways for a few bucks a pound (and safe enough to ship via all standard postal carriers with nothing more than an optional sticker) while LN2 is -320F. That requires a completely different storage and handling protocol.
  - robbedpeter 5y ago
    Interesting rabbit hole, I can't find any detailed reasons why co2 is better or not to use LN2. LN2 might also then be too cold and affect the temper of the steel, or it might evaporate too quickly, requiring a lot more mass than co2. Or maybe it could be a perfectly fine alternative but the co2 pellet systems have market advantage.
    - flaviut 5y ago
      CO2 density: 1562 kg/m^3 Sand density: 1520-1680 kg/m^3 LN2: 806 kg/m^3 CO2 is just disappearing sand as far as these systems are concerned. They probably use the same equipment that they use for sand. Plus LN2 is way colder and harder to handle and also it's a liquid.
  - adhesive_wombat 5y ago
    Temperature and injuries aside, a dewar of LN spilled in a room temperature enclosed space will boil and evaporate into an asphyxiating cloud very, very quickly. A box of dry ice dumped on the floor will take quite a bit longer to sublimate enough, partly because the phase change temperature is much higher, and also because thermal contact between a very low viscosity liquid and surfaces is very much higher than a solid pellet sitting on the surface. Nitrogen will also kill you silently, while you will definitely know when CO2 levels are getting high (because it really really hurts).
scratcheee 5y ago
liquid nitrogen would require better insulation at the least, since its much colder. I suspect they're also taking advantage of the fact that dry ice is solid, so they're effectively sand-blasting the track (rather than merely pressure-washing it), but they're using sand that helpfully sublimates into the atmosphere without leaving residue (or poisoning anyone). I imagine blasting sand or chemicals has the downside of leaving residue that can build up, meaning regular cleanup is required, and care required to use enough but not too much, with this you can spray as much as you can afford to buy/carry on the train and you don't have to worry about over-doing it. Dry ice blasting is already a thing, so I guess they just realised it was a particularly promising technology for this application.

robbedpeter 5y ago

It's cheaper and safer using co2 pellets propelled with ordinary air than liquid nitrogen. It gets sufficiently cold, and sufficiently abrasive to freeze, shatter, and disperse the sap and plant fats and other sticky stuff. Co2 cleaners are used in high end auto cleaning and detailing, so there's established use, safety, and equipment out there. The use is novel, the tech is decades old.

arcticbull 5y ago
The temperature alone - dry ice is only like -109F and sold at most Safeways for a few bucks a pound (and safe enough to ship via all standard postal carriers with nothing more than an optional sticker) while LN2 is -320F. That requires a completely different storage and handling protocol.
- robbedpeter 5y ago
  Interesting rabbit hole, I can't find any detailed reasons why co2 is better or not to use LN2. LN2 might also then be too cold and affect the temper of the steel, or it might evaporate too quickly, requiring a lot more mass than co2. Or maybe it could be a perfectly fine alternative but the co2 pellet systems have market advantage.
  - flaviut 5y ago
    CO2 density: 1562 kg/m^3 Sand density: 1520-1680 kg/m^3 LN2: 806 kg/m^3 CO2 is just disappearing sand as far as these systems are concerned. They probably use the same equipment that they use for sand. Plus LN2 is way colder and harder to handle and also it's a liquid.
- adhesive_wombat 5y ago
  Temperature and injuries aside, a dewar of LN spilled in a room temperature enclosed space will boil and evaporate into an asphyxiating cloud very, very quickly. A box of dry ice dumped on the floor will take quite a bit longer to sublimate enough, partly because the phase change temperature is much higher, and also because thermal contact between a very low viscosity liquid and surfaces is very much higher than a solid pellet sitting on the surface. Nitrogen will also kill you silently, while you will definitely know when CO2 levels are getting high (because it really really hurts).

scratcheee 5y ago

liquid nitrogen would require better insulation at the least, since its much colder. I suspect they're also taking advantage of the fact that dry ice is solid, so they're effectively sand-blasting the track (rather than merely pressure-washing it), but they're using sand that helpfully sublimates into the atmosphere without leaving residue (or poisoning anyone). I imagine blasting sand or chemicals has the downside of leaving residue that can build up, meaning regular cleanup is required, and care required to use enough but not too much, with this you can spray as much as you can afford to buy/carry on the train and you don't have to worry about over-doing it. Dry ice blasting is already a thing, so I guess they just realised it was a particularly promising technology for this application.