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Throttling Saturated Steam

  1. Jan 31, 2015 #1
    Upstream Conditions: Saturated steam, 230 PSI, 399 degree F.
    Downstream conditions: 170 PSI
    Assume 98% quality steam
    We throttle steam to our distribution system for use by hospitals for sterilization, heating and autoclaves.
    I have a theory of what is happening after throttling and would like to know if this is correct.
    After flow through the pressure reducing valve, any moisture in the steam will "flash" because its temperature is greater than its corresponding saturation temperature at that reduced pressure (170 PSI). Then it is possible that the temperature drop may stop before getting to saturation of 170 psi because all of the moisture has flashed to steam. (assume mass flow rates of 100 mlb/hr)
    A lot of people (coworkers) are under the assumption that the steam temperature after the valve will be saturation temp of 170 PSI because it is at 170 PSI, but I disagree for the previous stated reason. I ask them where did the energy go besides losses due to noise and velocity changes, they don't have much of an answer. that's why I am here.
    I understand we can measure the temp diff across the valve, but I would like to understand why.
    Thank you
  2. jcsd
  3. Jan 31, 2015 #2
    The throttling part is right. Actually in a throttling calorimeter, that is the reason throttling is done-to remove the moisture. The moisture at 230 psi will get converted to steam after pressure reduction. But I didn't exactly get the part after that. What are you wanting to know after pressure reduction?
  4. Jan 31, 2015 #3

    jim hardy

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    energy is conserved.

    Can we put a number on it?
    I dont have my steam table handy.
    What's enthalpy of incoming steam , 230 psi and 98% quality ??
    from this online calculator http://www.tlv.com/global/TI/calculator/steam-table-pressure.html
    i get
    enthalpy of dry 230 psi steam hv = 1201.57 BTU/lb
    and of liquid hl = 374.196 BTU/lb
    so 98% of former + 2% of latter = about 1185.02 BTU/lb ?

    What are temp and quality of 170 psi steam with same enthalpy ?
    at 170 psig i get, from same online calculator

    enthalpy of dry 230 psi steam hv = 1197.91 BTU/lb
    and of liquid hl = 348.536 BTU/lb
    Clearly there's moisture in that 1185.02 BTU/lb exit stream.
    So the question becomes 'What mix of those two enthalpies works out to 1185.02 BTU/lb ? '
    and that's your exit quality.

    Check my arithmetic , i 'm prone to calculator typos.
  5. Jan 31, 2015 #4
    I can do the calculations from the data in the steam table and really don't need to put a "number on it" (Sorry)
    jim hardy, Would you use the saturation table for the downstream side? We know the pressure, but we (I) don't know if it is saturated, correct?
    I suppose my question is:
    Is it possible to have superheated steam downstream of a pressure reduction valve. I am not saying we do, I am saying it is possible.
    siddharth23, the second part is basically saying if there isn't a lot of moisture that needs flashed into steam (lets say the quality is 99.9%)
    then the steam would be super heated because there is nothing (no work needed) to reduce temp after the .01% moisture is removed.. If it isn't, then back to my question to my coworkers is, where did the energy go?

    Let's take extremes here, lets say 99.9% quality steam upstream, and we reduce pressure from 230 to 15 psi, once the moisture is all flashed (converted to steam) would there be superheat?
    If not, where did the energy (temp reduction to 15 psi saturation) go?

    Thank you for your time.
    Last edited: Jan 31, 2015
  6. Jan 31, 2015 #5

    jim hardy

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    I'm having troubles this morning with my rusty old thermodynamics... and have misplaced my old ASME Steam Tables book

    Throttling doesn't change the enthalpy because as you observe energy is conserved.
    this and countless other links state that, and it's what i was taught.
    sadly i'm getting frustrated trying to find references in English units.

    If at a given pressure the enthalpy is more than for saturated vapor at that pressure, you're in superheat region
    if it's less then you have a mix of vapor and liquid....
    and since your enthalpy downstream was less than for saturated vapor at that pressure yes i'd use the saturated table.
    and solve for the mix.

    Let's go one better and start with dry steam at 230psi , zero moisture. Saves us an interpolation.
    Hv = 1201.57 upstream and downstream.
    At 15psi hv for saturated steam = 1164.16
    so it looks like we should be into superheat.by 37.41 BTU/lb
    so what's temperature of 1201 BTU/lb steam at 15 psig ?

    I cannot find online a decent steam table in BTU's and degF.
    i'm extrapolating from page 395 here
    and getting temperatures around 320F which i don't believe because we only started at 299F.

    edit found my mistake see post 8

    maybe you or @SteamKing can straighten me out ?

    sorry to let you down.

    will keep after it a while longer......but for the moment i admit defeat.
    time for breakfast...

    breakfast helped a lot

    old jim
    Last edited: Jan 31, 2015
  7. Jan 31, 2015 #6
    Yes, it is possible to get superheated steam by throttling. If you throttle steam of high dryness fraction. And the energy does not go anywhere. It remains constant. If you measure the initial temperature (before throttling) and the degree of superheat (after throttling), you'll see that enthalpy is the same on both sides.
  8. Jan 31, 2015 #7
    Thank you both!

    I appreciate your time and efforts!
  9. Jan 31, 2015 #8

    jim hardy

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    @Dennis C

    ohh, duhhhhh ! ---- found my mistake

    we started at 399 degf not 299 :DD:)):s:oops::rolleyes:o_O

    so temperature of 320 is reasonable,, and we could interpolate that steam table to get a closer number
    or look further for a better table..

    so YES you can go into superheat and the tables demonstrate it.. Since the energy isn't used to evaporate moisture, you'll experience less cooling.

    Mollier chart should show the same but i cant find one in English units anymore.
    Last edited: Jan 31, 2015
  10. Jan 31, 2015 #9
    According to my steam tables, the saturation pressure of water at 400F is 247 psi, not 230 psi. Was that 230 gage pressure? It must have been. So the final absolute pressure after the throttle must be 185 psi. From my steam tables, I get an enthalpy of the mixture of 1185 BTU/lb (this matches Jim's result). At 185 psi, the saturated vapor enthalpy is about 1197 BTU/lb, at a temperature of about 374 F. So the quality would be less than 100 % coming out of the throttle.

  11. Jan 31, 2015 #10

    jim hardy

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    Dennis i re-read your questions... and Chester's concise post i think made me understand your original question better.

    The energy went into turning part of that 2% moisture into steam. That moisture requires substantial 'heat of vaporization' to become steam.
    If your incoming steam is dry enough that all its entrained moisture can become steam, then you have enough energy in the downstream side for temperature to be above saturation.

    So if the incoming steam is dry enough, you're right
    and you could calculate that quality.
    Were your incoming steam above Chester's 1197 BTU/lb(what quality is that?) , it could arrive downstream superheated at 184.7psia(your 170 psig).
    But 2% moisture is too wet for that to be the case.

    Thanks Chester !

    old jim
  12. Feb 1, 2015 #11
    Thank you all again.
    Yes Chester, PSI always means gauge (in the steam industry) you are suppose to specify A for absolute.
    Another mistake on my end, I should of used PSIA........
    Thank you
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