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Calculating compressed air expansion when heated

  1. Oct 23, 2013 #1
    Can anyone help me with this calculation? I'm trying to figure out how much compressed air (generated from screw compressor) will expand by a ratio of what when fed into a wood furnace.

    1) Wood Furnace Fuel Input: 30kg dry wood fuel/hr
    2) Compressed Air Pressure: 8 bar
    3) Compressed Air Flow: 5m3/min or 300m3/hr

    If compressed air at 8 bar is pumped into an enclosed furnace at rate of 300m3/hr, what air flow xxxm3/hr, do I expect to come out from the other end after heat expansion? My normal atmospheric air temperature is about say 24C.

    Thx in advance
    Last edited: Oct 23, 2013
  2. jcsd
  3. Oct 23, 2013 #2


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    Staff: Mentor

    This does not compute. If the system is closed, there can be no expansion. And the pressure can't rise through the furnace; it can only drop as it flows. Or is it open to atmosphere?

    And is the compressor flow in actual or standard (corrected to atmospheric pressure) units?

    We'll need more detail about what your system looks like and what exactly you are trying to accomplish if we are to help.
  4. Oct 23, 2013 #3
    Sorry, there is a 6 inch outlet for the hot gases to escape. I'm trying to model a biomass wood furnace, to feed it with highly compressed air, and through a special mechanism, the forced air will be able to drive 2 shafts. One shaft will power a screw compressor, which feeds the compressed air into the furnace. And the other shaft will power an electrical alternator to generate electricity. Works similar like a gas turbine based on brayton cycle but parameters different as the heat energy output of wood is less than natural gas. The back work ratio of the compressor and alternative is split about 50:50. i.e. the shaft work is divided equally among these 2 components. Of course, there is will some lost shaft power due to heat loss.

    Hence if the biomass furnace consumes 30kg of dry wood per hour. And in conventional green power generators, 1 kg of wood would roughly generate about 1 kwh of electrical power based on ~30% efficiency. Hence a 30 kwh alternator is needed. And since the ratio of the alternator and air compressor is 50:50, I deduce that a 30 kw screw compressor would also be needed. And from manufacturers specs, a 30 kw compressor would output about 4.5m3/min of compressed air of 0.8mpb/ 8 bar pressure.

    So I'm gather to gather enough calculation for a computer simulation program. And I need to find out how much the compressed air expands by when it exits the gas outlet and the calculation to do this. Hope you can help.
    Last edited: Oct 23, 2013
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