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Homework Help: Electrical power out of R134a phase changes

  1. Dec 30, 2012 #1

    My 2'nd year of engineering doesn't quite let me to find an answer to the following problem:

    In a nutshell:
    I want to use rankine cycle to extract 5kW worth of electrical energy from a system utilizing R134a as working fluid.

    The properies of R134a are as follows:
    Tag R134a
    Compound CH2FCF3
    MolMass 102.03
    Critical T 101.08
    Critical P 4060.3
    Critical D 515.3
    Critical V 0.00194
    Gas Cosnt 0.0815

    http://www.pandikas.ee/kool/r134a/propertiesR134a.jpg [Broken]

    Available heat source is 70C and cooling 20C
    The evaporator needs to be of X size, the Condenser needs to be of X size and I am sorry to say the turbine size isn't preset either - they just have to be big enough. No real idea on how to go a bout it.
    Now, working with handy materials I figured the standard 2.54cm coiled copper tube will do nicely as evaporator emerged in the 70C water.
    This assumption gives me the force value of 1072.91 kg*m/s - the other values I dare not to put down in here as they seem way too unrealistic...

    Could someone just guide me to right direction on how to go about those multiple problems I've encountered here?


    My previous work can be found in attachment area.

    Attached Files:

    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Dec 31, 2012 #2

    rude man

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    What exactly are you trying to determine? No. of kg of required R134A? Speed (frequency) of the cycle? Amounts of heat extracted from the 70C reservoir and dumped into the 20C reservoir per cycle?
  4. Dec 31, 2012 #3
    Yea it's not really clear what exactly you are looking for....

    You already know the operating temperatures and specified power output but beyond knowing what you are looking for we don't know if you are idealizing the cycle either or if you are counting in isentropic efficiencies
  5. Jan 3, 2013 #4
    Thank you for your answer and a happy new year to you all.

    I am not trying to idealize at all.

    To put it simply, I am trying to figure out the turbine size needed to extract the 5kW.

  6. Jan 3, 2013 #5

    rude man

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    Question is, what you mean by "size".

    The important parameters to be deduced are the amount of R134 and the frequency of the Rankine cycle. If you were to specify one, the other could be computed based on your inlet and exhaust temperatures, and electrical power requirements. We could compute an answer based on a reversible (ideal) cycle and also take a shot at a realistic efficiency for your power plant.
  7. Jan 3, 2013 #6
    The size of the turbine: I would like to calculate the approximate height, depth and width of the blades. To visualize, here is the turbine project.
    http://www.pandikas.ee/kool/r134a/turbine191212.jpg [Broken]

    This is what I have so far - but aside from the general idea - for the moment at least, I don't have much else to work with.

    As for the amount of R134a - this is interesting: This is something that should be possible to calculate from the evaporator outlet diameter and pressure. However, this would again lead me to the point of where I'd need to know the rate of flow - which I don't. =/

    Last edited by a moderator: May 6, 2017
  8. Jan 3, 2013 #7

    rude man

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    Jonks, I finally looked at your work and I think you're looking for some pretty advanced and detailed info which I doubt you'll get on this forum, but maybe you will.

    I looked at this from a pretty basic, theoretical viewpoint. In this view the salient features of the Rankine cycle are the two temperatures and the type of working fluid (specifically its 'steam tables' from which enthalpies and entropies are either looked up or calculated. A p-v diagram for R134 would be a big help also.) This enables one to compute the efficiency of the cycle (assuming reversibility) = work output/heat input at the higher temperature. The latter two are computed from the aforesaid steam table entries on a per-unit-weight of working fluid basis ("specific" entropies and enthalpies). Then, you'd have to make allowances for the fact that no part of the cycle is really reversible. That drops the efficiency by at least half.

    So to get output power you'd need to know the amount of fluid which would give you the work output per cycle. Then the work output per unit time, which is your power, is just that work times the frequency of running the cycle (set by the pump adiabatically pushing the pressure up from p2 to p1 and corresponding temperature up from T2 to T1.

    You'd need this info in any case, on top of your other design details you're looking for, seems to me. Anyway, I hope you find what you're looking for, here or elsewhere.
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