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Dryness factor / vapor quality relation to Pressure

  1. Aug 27, 2009 #1
    Hey guys! Just wanted to inquire about the following relationship. Given a boiler and all the water content inside is above boiling point. Would the dryness factor (vapor quality) decrease or increase with pressure reduction.

    Example we start at 500 kPa then move down down to 400 kPa. Would the dryness factor increase or decrease.

    From the little calculation I know I estimated that the dryness factor would increase as pressure decreases. Indirect relationship. IF this is true what scientific explanations can be given for it.

    The only explanation I can think of is that high pressured gasses tend to liquefy. Thus if the pressure drops the dryness factor should increase until the temperature and pressure drops to a certain level.
  2. jcsd
  3. Oct 7, 2009 #2
    The quality of steam is very important in boilers. If you have a boiler at 500 kPa (gauge) (72.52 psig) the saturation temperature is 159 C (318.1 F). You have a saturated-vapor mixture and you can calculated a quality, x, for that mixture. If you are above 159 C (318.1 F) then you have a superheated vapor with a quality of 1.

    Now decrease the pressure to 400 kPa (g) (58.1 psig) your saturation temperature is now 152 C (305.6 F) and again you can calculate a quality.

    Now compare those two qualities and you can see how steam will behave with a pressure decrease. Or you can just look at a temperature vs. specific volume (or other property of steam) chart and see the relationship.

    For more details on these relationships see any thermodynamics text.

    Remember that all gases have a critical point. For steam this critical pressure point is about 22.09 MPa (3204 psig) in this range the properites of the steam are dificult to quantify.

    However, don't assume that all gases behave like steam. Before any calculation involving a gas or gas mixture is undertaken, always determine the critical pressures and temperatures.

  4. Oct 8, 2009 #3
    Increasing the pressure beyond the pressure corresponding to the saturation temperature will decrease the quality.
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