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Could an air compressor be used to compress steam? What would be practical problems if any?

Say I have saturated steam at 1 barg and I need to compress it to 2 barg. In theory could I just pipe to the inlet of a conventional air compressor (reciprocating type).

Sounds dicey to me but when someone came up with this idea during brainstorming for a DIY project I couldn't come up with cogent reasons why this would be a bad idea.

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  • $\begingroup$ Steam is a gas. Maybe the question is: what is the input pressureof your steam, and what output pressure and temperature do you want? $\endgroup$ Feb 20 '17 at 15:48
  • $\begingroup$ Input is saturated steam at 1barg and the output needs to be at 2 barg. Temprature can be whatever it will end up being at the end of an isotropic compression. $\endgroup$ Feb 20 '17 at 15:52
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One practical problem, if it's saturated steam, is that by increasing pressure, you're increasing the condensation point, so a lot of the steam will keep turning to liquid water without any considerable pressure growth. If you take 10l of dry air at 1 bar and compress it to 2 bar, you get about 5l of the compressed air. But if you take saturated steam - you'll get a glass of water at a little more than 100C and probably less than a liter of steam of 2 bar.

There will be a lot of other minor problems, like a compressor that operates at 100+C in high moisture, safety and so on, but it all boils down to: WHY?

It's very easy to increase pressure of steam by superheating it. This is the primary method how all turbines are propelled. Sometimes superheated steam will even be used to propel turbocompressors of other materials, it's so easy and ubiquitous. What purpose would compressing the saturated steam with a compressor serve? You won't even get the work back, you'll just get a lot of hot, compressed liquid water (that can be obtained by much easier means).

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  • $\begingroup$ The goal is mechanical recompression. To create a proof of concept pilot system. Say you wanna concentrate a solution. Normally you'd need a lot of steam. But if you can pressurize the evolved steam to boost its Temperature then you can continue to evaporate with only an input of the electrical energy of compression. $\endgroup$ Feb 20 '17 at 16:53
  • $\begingroup$ @curious_cat: wait, how can you concentrate a solution using a lot of steam? $\endgroup$
    – SF.
    Feb 20 '17 at 17:00
  • $\begingroup$ Evaporation. Use a heat exchanger to boil off water. Recompress the boiled off steam to boil off more water. $\endgroup$ Feb 20 '17 at 17:47
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    $\begingroup$ @SF. few quibbles: 1) Saturated steam is basically vapor, theoretically speaking and by looking on a T-S diagram, compressing saturated steam (ignoring all practical difficulties) will always take place in superheated region as the saturation dome narrows down as the condensation pressure increases. so why would there be any condensation? 2) How would you increase the pressure of steam by superheating it? $\endgroup$
    – Algo
    Feb 20 '17 at 17:52
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    $\begingroup$ The first paragraph is wrong or misleading, any real compression process raises the temperature. $\endgroup$
    – mart
    Apr 19 '17 at 14:35
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You are describing Vapor-compression Evaporation. According to wikipedia, since any real compression will also heat the gas "The compression work will deliver the steam superheated if compared to the theoretical pressure/temperature equilibrium." IMO this should be verified with the actual figures from your process design etc., you don't want liquid water in a compressor.

This document by GEA gives an overview over the process and the compressors types usually used. Not that this helpful selection diagram is marked in degree temp. diff, not pressure, since this is what you ulimately want:

enter image description here

Most likely you will want a rotary fan.

So, to sum it up, yes, you can pressurize vapor mechanically and this is an established process.

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  • $\begingroup$ This document (only skimmed) seems to go into more depth on compressor selection: dwa.nl/wp-content/uploads/2014/09/Gea-Wiegand-MVR.pdf $\endgroup$
    – mart
    Apr 19 '17 at 11:12
  • $\begingroup$ also, doing a quick chek it appears even an isentropic compression heats the gas up more than the dew line rises with the pressure, so condensation inside the compressor should not happen. $\endgroup$
    – mart
    Apr 19 '17 at 14:34
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Practically speaking, it would be very inefficient. Just thinking about its application, the pressure would not increase since the steam will just turn into water in a liquid state.

Now I have found this link that might be helpful, depending on your specific application, and they mention that the compressor is:

"Capable of recovering steam even at atmospheric pressure.**Requires the use of a COS or CV-COS control valve."

They also mentioned that the different application areas include

  • Removing vapor clouds above tanks of boiling water.
  • Pressurizing and reutilizing flash steam from tanks.
  • Pressurizing and reutilizing excess steam used in power generation.
  • Pressurizing and reutilizing low pressure steam generated after use in processes.

I'm assuming the third application is the one suiting the description and question you asked.

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  • $\begingroup$ The device in the link is an eductor - it requires a small flow of high pressure steam to induce a flow of low pressure steam, producing a combined flow at a pressure somewhere in between (though a lot closer to the lower pressure). If the OP had the higher pressure steam to power this device, he wouldn't need it. $\endgroup$
    – Mark
    Apr 19 '17 at 13:52
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    $\begingroup$ The first paragraph is wrong or misleading, any real compression process raises the temperature. $\endgroup$
    – mart
    Apr 19 '17 at 14:36
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Holcroft "Condensing by Compression" was a development on the British railways to increase efficiency by pushing the expended steam back into the boiler. It is the definitive study.

http://www.douglas-self.com/MUSEUM/LOCOLOCO/holcroft/holcroft.htm

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    $\begingroup$ I didn't check the document but might it be that this was also motivated by the desire to reuse the water for non-stop long-distance journeys to avoid water top-up stops. There were various other schemes using scoops in long troughs between the rails on level sections of track. A scoop was lowered from the locomotive and the water effectively rammed into the tender. $\endgroup$
    – Transistor
    Apr 11 at 9:06

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