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A steam injector effectively raises fresh water pressure above boiler pressure, otherwise it would not work. Replacing the steam by compressed air and condensation heat by a combustion, can an ejector be able to compress more air than consumed to a higher pressure?

The main objective is emulate an steam boiler feedwater injector. Take a bit of compressed air from a pressurized bottle and use it in some way to inject more air in the same bottle at a same (a little higher) pressure. In other words, an air compressor running from a jet pump.

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  • $\begingroup$ When you say you want it to compress air, do you mean that the suction fluid (usually water) is being replaced with air? $\endgroup$
    – Carlton
    May 17, 2015 at 23:15
  • $\begingroup$ @Carlton Yes, I am asking about an air-air jet pump. Feed fluid must be air. If it is impossible to compress air in this manner, driving fluid can be replaced by another fluid (water, steam, etc.). The objective is to fill a volume with air at higher pressure than atmospheric air. $\endgroup$
    – user3368561
    May 17, 2015 at 23:50

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I'm not sure if an injector will work in this situation. In the case of a steam injector, the specific volume of the mixture is lower than that of the inlet steam, which is partly how the outlet pressure is able to increase above the inlet pressure. If you have a combustion reaction between the air and some kind of fuel, the specific volume of the reaction products will be much higher than the inlet air, so you may not be able to build up enough pressure in the outlet stream to make it work.

You may want to instead look into turbo pumps like the kind used on liquid fueled rocket motors.

If both the motive fluid and the suction fluid are air, then this will work, though the efficiency may not be as great as a steam-water injector. The inlet air will induce flow in the suction air, and the resulting outlet flow will have pressure and velocity somewhere in-between that of the inlet and suction streams. So, if the suction air is at atmospheric pressure, the outlet flow will be higher than atmospheric. Whether or not the pressure will be high enough to meet your needs depends on the pressure and flow rate of the inlet air. A pneumatic vacuum uses this same principle, you may want to look into those as well.

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  • $\begingroup$ It is not necessary that the combustion takes place in the mixing section. It can take place at an earlier stage, heating compressed air before feeding it to injector, for example. This way, the specific volume of mixed air will be lower than inlet compressed air. Can it work this way? $\endgroup$
    – user3368561
    May 16, 2015 at 14:01
  • $\begingroup$ I still think it will be difficult. Steam injectors work well because the steam goes from 100% vapor to 100% liquid, thus a big drop in specific volume. Air and combustion products only liquify at very low temperatures, so you probably won't get the extreme change in density that you need. $\endgroup$
    – Carlton
    May 16, 2015 at 15:22
  • $\begingroup$ if the working principle of pressure raising effect is the reduction of specific volume, it can be accomplished injecting a cold gas jet instead of a hot one. Would a cold jet of air be able to compress ambient air above pressure of jet itself? $\endgroup$
    – user3368561
    May 17, 2015 at 20:01

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