In a Giffard type steam injector, boiler feed water is added to the boiler by passing steam at around 10 bar/180C through a converging/diverging nozzle, collecting cold water in the vacuum at the vena contracta. The combined steam/water mix is then returned to the boiler at a higher pressure but lower temperature than when the steam left the boiler. The quality of the steam is lower, therefore energy in the form of heat needs to be added to the boiler to restore the status quo.

My question is this: If instead of the boiler, I took a cylinder of refrigerant (vapour pressure of say 10 bar@20C) and the vapour from the top of the cylinder passed through a similar nozzle, with liquid refrigerant from the bottom of the same cylinder being allowed to vaporise at the vena contracta (say 0 bar, -50C) to mix with the high pressure vapour. Would the resulting pressure increase/temperature decrease in the same way as a steam injector?

If so, whilst the resulting vapour/liquid mix which might be at -20C would still need to be restored to its original state by a heat input, this would now be from the addition of heat from ambient air, not by adding fossil fuels

If it does work, would such a closed system produce a 'free' useful temperature drop for refrigeration/air con etc at the point where the liquid vaporises.

I apprciate latent heat of vaporisation plays a part in this process and in water it is high - but then some refrigerants like ammonia are pretty high too.

  • $\begingroup$ Your posting must contain errors. The highest pressure steam that can be used for the steam injector must be the boiler exit pressure. When this steam goes through the injector to entrain boiler feed water, it experiences pressure drop. Due to this, there is no way for the combined steam and boiler feed water to get back into the boiler. $\endgroup$ – David White May 17 at 21:22
  • $\begingroup$ Injectors, also known as eductors, are somewhat inefficient. Due to this, compressors are used on refrigerants. And note, your question seems to be seeking a way to make a perpetual motion machine, which is a physical impossibility because it violates the law of conservation of energy and it violates the 2nd law of thermodynamics. $\endgroup$ – David White May 17 at 21:24
  • $\begingroup$ Hello David. Thanks for your reply. Though it seems counter intuitive steam injectors do return a water/steam mix at a pressuregreater than the boiler exit pressure. Wiki has a description en.wikipedia.org/wiki/Injector The resulting mix when returned to the boiler needs some heat added to the system to restore the steam/water mix to dry steam. All of this happens at about 100C of course. $\endgroup$ – Colin Smith May 18 at 8:03
  • $\begingroup$ My question is if vapour from a cylinder of refrigerant were introduced into the nozzle (at say 20C) and liquid from same cylinder allowed to vaporise at the throat (say -50C) and the resulting mix could be returned to the cylinder (at say -20C), then heat will still need to be added to the the cylinder to restore the quality of the vapour. But now, instead of burning fossil fuel the heat would come from the ambient air all around us. So not perpetual motion, but heat/energy input from ambient air - and with a 'free' deep freeze element where the liquid vaporises. $\endgroup$ – Colin Smith May 18 at 8:17
  • $\begingroup$ Colin, for your steam injector to work, you need a steam supply that is running at a pressure greater than atmospheric pressure, or greater than 100 deg C, in order to get water into a boiler that is running at atmospheric pressure and 100 deg C. If Wikipedia says otherwise, then Wikipedia is in error. Regarding the refrigerant injector, the 2nd law of thermodynamics forbids refrigeration without doing work on the system. You can't get your refrigeration "for free". $\endgroup$ – David White May 18 at 13:21

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